ALBERT

Description: The Canadian urban and land surface external modeling system (known as urban GEM-SURF) has been developed to provide surface and near-surface meteorological variables to improve numerical weather prediction and to become a tool for environmental applications. The system is based on the Town Energy Balance model for the built-up covers and on the Interactions between the Surface, Biosphere, and Atmosphere land surface model for the natural covers. It is driven by coarse-resolution forecasts from the 15-km Canadian regional operational model. This new system was tested for a 120-m grid-size computational domain covering the Montreal metropolitan region from 1 May to 30 September 2008. The numerical results were first evaluated against local observations of the surface energy budgets, air temperature, and humidity taken at the Environmental Prediction in Canadian Cities (EPiCC) field experiment tower sites. As compared with the regional deterministic 15-km model, important improvements have been achieved with this system over urban and suburban sites. GEM-SURF’s ability to simulate the Montreal surface urban heat island was also investigated, and the radiative surface temperatures from this system and from two systems operational at the Meteorological Service of Canada were compared, that is, the 15-km regional deterministic model and the so-called limited-area model with 2.5-km grid size. Comparison of urban GEM-SURF outputs with remotely sensed observations from the Moderate Resolution Imaging Spectroradiometer (MODIS) reveals relatively good agreement for urban and natural areas.

Description: A classification of rainfall seasonality regimes in Africa was derived from gridded rainfall and land surface temperature products. By adapting a method that goes back to Walter and Lieth’s approach of presenting climatic diagrams, relationships between estimated rainfall and temperature were used to determine the presence and pattern of humid, arid, and dry months. The temporal sequence of humid, arid, and dry months defined nonseasonal as well as single-, dual-, and multiple-wet-season regimes with one or more rainfall peaks per wet season. The use of gridded products resulted in a detailed, spatially continuous classification for the entire African continent at two different spatial resolutions, which compared well to local-scale studies based on station data. With its focus on rainfall patterns at fine spatial scales, this classification is complementary to coarser and more genetic classifications based on atmospheric driving forces. An analysis of the stability of the resulting seasonality regimes shows areas of relatively high year-to-year stability in the single-wet-season regimes and areas of lower year-to-year stability in the dual- and multiple-wet-season regimes as well as in transition zones.

Description: Weather Research and Forecasting (WRF) model predictions using different boundary layer schemes and horizontal grid spacings were compared with observational and numerical large-eddy simulation data for conditions corresponding to a dry atmospheric convective boundary layer (CBL) over the southern Great Plains (SGP). The first studied case exhibited a dryline passage during the simulation window, and the second studied case was used to examine the CBL in a post-cold-frontal environment. The model runs were conducted with three boundary layer parameterization schemes (Yonsei University, Mellor–Yamada–Janjić, and asymmetrical convective) commonly employed within the WRF model environment to represent effects of small-scale turbulent transport. A study domain was centered over the Atmospheric Radiation Measurement Program SGP site in Lamont, Oklahoma. Results show that near-surface flow and turbulence parameters are predicted reasonably well with all tested horizontal grid spacings (1, 2, and 4 km) and that value added through refining grid spacing was minimal at best for conditions considered in this study. In accord with this result, it was suggested that the 16-fold increase in computing overhead associated with changing from 4- to 1-km grid spacing was not justified. Therefore, only differences among schemes at 4-km spacing were presented in detail. WRF model predictions generally overestimated the contribution to turbulence generation by mechanical forcing over buoyancy forcing in both studied CBL cases. Nonlocal parameterization schemes were found to match observational data more closely than did the local scheme, although differences among the predictions with all three schemes were relatively small.

Description: Three decades of weather records at meteorological stations near Death Valley National Park are analyzed in an attempt to gauge the frequency of conditions that might form and erase the famous trails of wind-blown rocks in the mud of Racetrack Playa. Trail formation requires the playa to be wet, followed by strong winds and/or freezing conditions. Weather records are compared with a limited set of meteorological data that were acquired in situ at the playa over three winters and that indicate freezing on 50, 29, and 15 nights during the winters of 2007/08–09/10, respectively, as well as with the hydrological condition of the playa as determined by time-lapse cameras that observed flooding over ~1, ~5, and ~40 days, respectively, during those winters. Measurements at the nearby Panamint and Hunter Mountain stations are found to be a useful, if imperfect (~50%), indicator of Racetrack Playa conditions and give some features of Racetrack Playa’s micrometeorological behavior. Wind speed probability distributions suggest that winds that are fast enough to cause unassisted rock motion are rare and therefore that freezing of water on the playa has a role in a significant fraction of movement events.

Description: A single-layered square prism urban canopy (SPUC) scheme for the Japan Meteorological Agency nonhydrostatic model (NHM) was developed. This scheme considers the urban canopy layer with square prism–shaped buildings. The basic concept of this scheme is similar to those of the conventional energy balance models for an urban canyon structure. The scheme presented here differs slightly from them in its representation of the bulk resistance circuit and some treatments of radiation processes, however, as well as by considering the water reservoir on building surfaces. A comparison between the SPUC scheme and the existing slab scheme of the NHM was made on summer days in the Tokyo metropolitan area. The SPUC run more accurately reproduced the expected behavior of the urban canopy effect than did the slab run. The effective albedo was smaller in the SPUC run than in the slab run (the upward shortwave radiation of the SPUC run was smaller than that of the slab run). The forecast heat fluxes in the SPUC run, however, showed worse performances. Adequate parameter settings (especially concerning latent heat fluxes) are needed in the SPUC run. The mean bias errors of the surface air temperature during nighttime were less negative and slightly improved in the SPUC run than in the slab run.

Description: A mountain air chemistry observatory has been operational on the summit of Whistler Mountain in British Columbia, Canada, since 2002. A 1-yr dataset of condensation nuclei (CN) concentration from this site has been analyzed along with corresponding meteorological data to assess the frequency and patterns of influence from the planetary boundary layer (PBL). Characterization of air masses sampled from the site as either PBL influenced or representative of the free troposphere (FT) is important to subsequent analysis of the chemistry data. Median CN concentrations and seasonal trends were found to be comparable to other midlatitude mountain sites. Monthly median number concentrations ranged from 120 cm−3 in January to 1601 cm−3 in July. Using well-defined diurnal cycles in CN concentration as an indicator of air arriving from nearby valleys, PBL influence was found to occur on a majority of days during spring and summer and less frequently in late autumn and winter. Days with PBL influence were usually associated with synoptic-scale weather patterns that were conducive to convective mixing processes. Although more common in the warm season, vertical mixing capable of transporting valley air to the mountaintop also occurred in February during a period of high pressure aloft. In contrast, an August case study indicated that the more stable character of marine air masses can at times keep the PBL below the summit on summer days. Considerable variability in the synoptic-scale weather conditions at Whistler means that careful analysis of available datasets must be made to discriminate FT from PBL periods at the observatory.

Description: The Snowy Precipitation Enhancement Research Project (SPERP) was undertaken from May 2005 to June 2009 in the Snowy Mountains of southeastern Australia with the aim of enhancing snowfall in westerly flows associated with winter cold fronts. Building on earlier field studies in the region, SPERP was developed as a confirmatory experiment of glaciogenic static seeding using a silver-chloroiodide material dispersed from ground-based generators. Seeding of 5-h experimental units (EUs) was randomized with a seeding ratio of 2:1. A total of 107 EUs were undertaken at suitable times, based on surface and upper-air observations. Indium (III) oxide was released during all EUs for comparison of indium and silver concentrations in snow in seeded and unseeded EUs to test the targeting of seeding material. A network of gauges was deployed at 44 sites across the region to detect whether precipitation was enhanced in a fixed target area of 832 km2, using observations from a fixed control area to estimate the natural precipitation in the target. Additional measurements included integrated supercooled liquid water at a site in the target area and upper-air data from a site upwind of the target.

Description: Ground-based polarimetric weather radar is arguably the most powerful validation tool that provides physical insight into the development and interpretation of spaceborne weather radar algorithms and observations. This study aims to compare and resolve discrepancies in hydrometeor retrievals and reflectivity observations between the NOAA/National Severe Storm Laboratory “proof of concept” KOUN polarimetric Weather Surveillance Radar-1988 Doppler (WSR-88D) and the spaceborne precipitation radar (PR) on board NASA’s Tropical Rainfall Measuring Mission (TRMM) platform. An intercomparison of PR and KOUN melting-layer heights retrieved from 2 to 5 km MSL shows a high correlation coefficient of 0.88 with relative bias of 5.9%. A resolution volume–matching technique is used to compare simultaneous TRMM PR and KOUN reflectivity observations. The comparisons reveal an overall bias of

Description: A new automated daytime cumulonimbus/towering cumulus (Cb/TCu) cloud detection method for the months of May–September is presented that combines information on cloud physical properties retrieved from the Spinning Enhanced Visible and Infrared Imager (SEVIRI) on board Meteosat Second Generation (MSG) satellites and weather radar reflectivity factors. First, a pixel-based convective cloud mask (CCM) is constructed on the basis of cloud physical properties [cloud-top temperature, cloud optical thickness (COT), effective radius, and cloud phase] derived from SEVIRI. Second, a logistic regression model is applied to determine the probability of Cb/TCu clouds for the collection of pixels that pass the CCM. In this model, MSG-SEVIRI cloud physical properties and weather radar reflectivity factors are used as potential predictor sources. The predictand is derived from aviation routine weather reports (METAR) made by human observers at Amsterdam Airport Schiphol for 2004–07. Results show that the CCM filters out 〉70% of the “no” events (no Cb/TCu cloud) and that 〉93% of the “yes” events (Cb/TCu cloud) are retained. Most skillful predictors are derived from radar reflectivity factors and the COT of high resolution. The derived probabilities from the combined MSG and radar method clearly show skill over sample climatology. Probability thresholds are used to convert derived probabilities into derived group memberships (i.e., yes/no Cb/TCu clouds). When comparing verification scores between the combined MSG and radar method and either the radar-only method or the MSG-only method, the combined MSG and radar method shows slightly better performance. When comparing the combined MSG and radar method with the current Royal Netherlands Meteorological Institute (KNMI) radar-based Cb/TCu cloud detection method, the two methods show comparable probability of detection, but the former shows a false-alarm ratio that is about 8% lower. Moreover, a big advantage of the newly developed method is that it provides probabilities, in contrast to the current KNMI method.

Description: Snow cover plays an important role in the climate system by changing the energy and mass transfer between the atmosphere and the surface. Reliable observations of the snow cover are difficult to obtain without satellites. This paper introduces a new algorithm for satellite-based snow-cover detection that is in operational use for Meteosat in the European Organisation for the Exploitation of Meteorological Satellites Satellite Application Facility on Land Surface Analysis (LSA SAF). The new version of the product is compared with the old version and the NOAA/National Environmental Satellite, Data, and Information Service Interactive Multisensor Snow and Ice Mapping System (IMS) snow-cover product. The new version of the LSA SAF snow-cover product improves the accuracy of snow detection and is comparable to the IMS product in cloud-free conditions.

Description: The annual cycles of upward and downward longwave fluxes at the earth’s surface are investigated by use of the NASA Global Energy and Water Cycle Experiment (GEWEX) Surface Radiation Budget Dataset. Principal component analysis is used to quantify the annual cycles. Because of the immense difference between the heat capacity of land and ocean, the surface of the earth is partitioned into these two categories. Over land, the first principal component describes over 95% of the variance of the annual cycle of the upward and downward longwave fluxes. Over ocean the first term describes more than 87% of these annual cycles. Empirical orthogonal functions show the corresponding geographical distributions of these cycles. Phase-plane diagrams of the annual cycles of upward longwave fluxes as a function of net shortwave flux show the thermal inertia of land and ocean.

Description: Measured 94-GHz reflectivity in midlevel, stratiform ice clouds was compared with reflectivity calculated from size distributions determined with a particle imaging probe. The radar and the particle probe were carried on the same aircraft, the Wyoming King Air, ensuring close spatial correspondence between the two measurements. Good overall agreement was found within the range from −18 to +16 dBZ, but there is an important degree of scatter in the results. Two different assumptions about particle density led to calculated values that bracket the observations. The agreement found for reflectivity supports the use of the data for establishing relationships between the measured reflectivity and ice water content and between precipitation rate and reflectivity. The resulting equation for ice water content (IWC vs Z) agrees with the results of Liu and Illingworth within a factor of 2 over the range of overlap between the two datasets. The equation here reported for precipitation rate (PR vs Z) has a shallower slope in the power-law relationship than that reported by Matrosov as a consequence of sampling particles of greater densities. Because the radar and the particle probe were collocated on the same platform, errors arising from differences in sampling locations and volumes were minimized. Therefore it is concluded that the roughly factor-of-10 spread in IWC and in PR for given Z is, primarily, a result of variations in ice crystal shape and density. Retrievals of IWC and PR from cloud radar data can be expected to have that level of uncertainty.

Description: A sudden increase in temperature during the nighttime hours accompanies the passages of some cold fronts. In some cold front–associated warming events, the temperature can rise by as much as 10°C and can last from a few minutes to several hours. Previous studies suggest that these events are due to the downward transport of warmer air by the strong and gusty winds associated with the cold-frontal passages. In this study, a climatology of nocturnal warming events associated with cold fronts was created using 6 yr of Oklahoma Mesonetwork (Mesonet) data from 2003 to 2008. Nocturnal warming events associated with cold-frontal passages occurred surprisingly frequently across Oklahoma. Of the cold fronts observed in this study, 91.5% produced at least one warming event at an Oklahoma Mesonet station. The winter months accounted for the most events (37.9%), and the summer months accounted for the fewest (3.8%). When normalized by the monthly number of cold-frontal passages, the winter months still had the most number of warming events. The number of warming events increased rapidly from 2300 to 0200 UTC; thereafter, the number of events gradually decreased. A spatial analysis revealed that the frequency of warming events decreased markedly from west to east across the state. In contrast, the average magnitude of the warming increased from west to east. In contrast to control periods (associated with cold-frontal passages without nocturnal warming events), warming events were associated with weaker initial winds and stronger initial temperature inversions. Moreover, the nocturnal temperature inversion weakened more during warming events than during control periods and the surface wind speeds increased more during warming events than during control periods. These results are consistent with previous studies that suggest the warming events are due to the “mixing out” of the nocturnal temperature inversion.

Description: Using observational data from coherent Doppler light detection and ranging (lidar) systems situated at the Hong Kong International Airport (HKIA), the authors extract Lagrangian coherent structures (LCS) intersecting the flight path of landing aircraft. They study the time evolution of LCS and compare them with onboard wind shear and altitude data collected during airplane approaches. Their results show good correlation between LCS extracted from the lidar data and updrafts and downdrafts experienced by landing aircraft. Overall, LCS analysis shows promise as a robust real-time tool to detect unsteady flow structures that impact airplane traffic.

Description: In-flight microphysical measurements in classical freezing-rain conditions were used to study the vertical and horizontal characteristics of the precipitation and associated low-ceiling, stratiform clouds, which are usually present as overcast in freezing-rain conditions. The low overcast is usually based in the surface cold layer but may extend up into the inversion, or transition layer, between the overrunning warm air and the surface cold layer. This gives the cloud an unusual temperature-inverted structure—supercooled in the lower half and warmer than freezing in the upper half. The low cloud is also subject to wind shear and turbulence that is due to the warm overrunning. The apparent effects of this are 1) increased cloud droplet concentrations in clusters up to a few hundred meters wide that occur sporadically in the cloud layer, 2) possible forcing of cloudy air upward from lower levels against the resistance of the temperature inversion and into the transition layer, and 3) highly variable air temperatures during level flight in the inversion layer.

Description: Urbanization affects near-surface climates by increasing city temperatures relative to rural temperatures [i.e., the urban heat island (UHI) effect]. This effect is usually measured as the relative temperature difference between urban areas and a rural location. Use of this measure is potentially problematic, however, mainly because of unclear “rural” definitions across different cities. An alternative metric is proposed—surface temperature cooling/warming rates—that directly measures how variations in land-use and land cover (LULC) affect temperatures for a specific urban area. In this study, the impact of local-scale (

Description: The regional coupled climate–chemistry/aerosol model (RegCM3) is used to investigate the difference in the spatial distribution of aerosol optical depth (AOD) between a strong summer monsoon year (SSMY; July 2003) and a weak summer monsoon year (WSMY; July 2002) under the actual- and same-emission scenarios. It is shown that the intensity of the Asian summer monsoon is primarily responsible for the AOD spatial distribution anomaly in midsummer over East Asia. Specifically, the AOD over southern China, upwind of the Asian summer monsoon, is greater in WSMY than in SSMY, but the opposite is observed for the AOD downwind over northern China and the Korean Peninsula. The AOD spatial distribution patterns simulated on the basis of the actual emission inventories for the SSMY and WSMY do not substantially differ from their counterparts that are based on the same emission inventory, confirming that the monsoon circulation, rather than local emissions or dry and wet deposition processes, is the predominant factor determining the regional AOD distribution. These modeling results are consistent with the analyses based on the Moderate Resolution Imaging Spectroradiometer (MODIS) products, NCAR–Department of Energy wind fields, and air parcel movements according to the 7-day trajectories of air parcels determined by the Hybrid Single-Particle Lagrangian Integrated Trajectory model.

Description: One major concern of climate change is the possible rise of temperature extreme events, in terms of occurrence and intensity. To study this phenomenon, reliable daily series are required, for instance to compute daily-based indices: high-order quantiles, annual extrema, number of days exceeding thresholds, and so on. Because observed series are likely to be affected by changes in the measurement conditions, adapted homogenization procedures are required. Although a very large number of procedures have been proposed for adjustment of observed series at a monthly time scale, few have been proposed for adjustment of daily temperature series. This article proposes a new adjustment method for temperature series at a daily time scale. This method, called spline daily homogenization (SPLIDHOM), relies on an indirect nonlinear regression method. Estimation of the regression functions is performed by cubic smoothing splines. This method is able to adjust the mean of the series as well as high-order quantiles and moments of the series. When using well-correlated series, SPLIDHOM improves the results of two widely used methods, as a result of an optimal selection of the smoothing parameter. Applications to the Toulouse, France, temperature series are shown as a real example.

Description: Observations made during the Meteor Crater Experiment (METCRAX) field campaign revealed unexpected nighttime cooling characteristics in Arizona’s Meteor Crater. Unlike in other natural closed basins, a near-isothermal temperature profile regularly develops over most of the crater depth, with only a shallow stable layer near the crater floor. A conceptual model proposed by Whiteman et al. attributes the near-isothermal stratification to the intrusion, and subsequent detrainment, of near-surface air from outside the crater into the crater atmosphere. To quantify and test the hypothesis, a mass flux model of the intrusion process is developed. It is found that the observed temperature profile can be reproduced, providing confirmation of the conceptual model. The near-isothermal stratification can be explained as a result of progressively cooler air entering the crater and detraining into the atmosphere, combined with the finite time of ascent in the compensating rising motion. The strength of detrainment largely determines the characteristics of the cooling process. With weak detrainment, most of the cooling arises from an adiabatic rising motion (“filling-up” mode). Stronger detrainment leads to reduced rising motion and enhanced cooling at upper levels in the crater (“destabilization” mode). Of interest is that the detrainment also reduces the total cooling, which, for a given intrusion mass flux, is determined by the temperature difference between the intruding air and the crater atmosphere at rim height.

Description: Finescale simulations (with 500-m grid spacing) using the Weather Research and Forecasting Model (WRF) were used to investigate impacts of urban processes and urbanization on a localized, summer, heavy rainfall in Beijing. Evaluation using radar and gauge data shows that this configuration of WRF with three-dimensional variational data assimilation of local weather and GPS precipitable water data can simulate this event generally well. Additional WRF simulations were conducted to test the sensitivity of simulation of this storm to different urban processes and urban land-use scenarios. The results confirm that the city does play an important role in determining storm movement and rainfall amount. Comparison of cases with and without the presence of the city of Beijing with respect to the approaching storm shows that the urban effect seems to lead to the breaking of the squall line into convective cells over the urban area. The change of precipitation amount depends on the degree of urbanization (i.e., the change over time in the extent of Beijing city). Model results show that an early urbanization prior to 1980 decreases the maximum rainfall, whereas further urbanization in Beijing is conducive to bifurcating the path of rainfall. According to sensitivity results with a single-layer urban canopy model, the thermal transport (sensible and latent heating) induced by the presence of an urban area apparently is more important than associated momentum transport, with latent and sensible heating apparently having equally important roles in the modification of simulated precipitation. Urban surfaces tend to cause the rainfall to be more locally concentrated. High-rise urban cores may bifurcate the path of rainfall as well as increase the area percentage of heavy rainfall.

Description: The climatological characteristics of drought in South Korea were investigated using daily precipitation data for 1777–2008. The effective drought index was used to quantify the drought intensity. As a result, five characteristics were discovered. First, South Korea can be divided into four drought subregions (the central, southern, and east coastal regions and Jeju Island) using hierarchical cluster analysis. Second, a map for long-term drought conditions in the four subregions is created that allows identification of the spatiotemporal distribution of droughts for the 231 yr at a glance. Third, droughts in South Korea have time scales that depend on the onset season. Spring (March–May) droughts tend to be short (≤200 days) because the summer (June–September) rainy season follows. Summer droughts tend to be long (〉200 days) because the dry season (October–February) follows. In the dry season, droughts tend to be sustained or become severe rather than being initiated or relieved. Fourth, 5-, 14-, 34-, and 115-yr drought cycles were identified by spectral analysis. The 5-yr cycle was dominant in all of the regions, the 14-yr cycle was observed over the southern and east coastal regions, and the 34-yr cycle was observed over the central region. Fifth, the most extreme drought occurred in 1897–1903 (return period: 233 yr) and was associated with the 115-yr drought cycle. After this drought, severe droughts (return period of 〉10 yr) occurred in 1927–30, 1938–40, 1942–45, 1951–52, 1967–69, and 1994–96; they were caused by the consecutive shortage of summer rainfall for two or more years.

Description: The 850-hPa synoptic flow patterns over eastern Texas and adjacent states during the 2005/06 Second Texas Air Quality Studies (TexAQS-II) period were classified into six groups using a two-stage clustering method. This study identifies synoptic weather patterns that are conducive to creating high-ozone events and reveals potential emission source regions leading to ozone exceedances through backward trajectory analysis. Clusters with dominant southerly synoptic flows and higher average wind speeds (C1 and C2 clusters) were related to low mean daytime ozone levels. Easterly (C3) and northerly (C5) clusters showed a higher probability to exceed the 8-h ozone standard than the other clusters. The dry and sunny postfrontal days with weak northerly or easterly weather patterns were often associated with ozone exceedances in the Houston–Galveston–Brazoria (HGB) area during September 2006. The majority of backward trajectories on ozone episode days were from the east, which brought polluted air from the Houston Ship Channel resulting in high ozone in the southwestern part of the metropolitan area. The shifting trajectories of the C3 and C5 weather patterns were associated with surface wind turning from weak northerly to southeasterly/southerly because of the development of bay/sea breeze. Pollutants were shifting inside the metropolitan area and high ozone built up in the afternoon or early evening. A small portion of trajectories of C3 and C5 that caused high ozone passed over a short distance from the south of Houston, picking up emissions from the industries along the coast.

Description: In northwestern North America, which is a large area with complex physiography, Climatic Research Unit (CRU) Time Series, version 2.1, (TS 2.1) gridded monthly mean 2-m temperatures are systematically lower than interpolated monthly averaged North American Regional Reanalysis (NARR) pressure-level temperatures––in particular, in the winter. Quantification of these differences based on CRU gridded observations can be used to estimate pressure-level temperatures from CRU 2-m temperatures (1901–2002) that predate the NARR period (since 1979). Such twentieth-century pressure-level temperature fields can be used in glacier mass-balance modeling and as an alternative to calibrating general circulation model control runs, avoiding the need for accurate boundary layer parameterization. In this paper, an approach is presented that is transferable to moisture, wind, and other 3D fields with potential applications in wind power generation, ecology, and air quality. At each CRU grid point, the difference between CRU and NARR is regressed against seven predictors in CRU (mean temperature, daily temperature range, precipitation, vapor pressure, cloud cover, and number of wet and frost days) for the period of overlap between CRU and NARR (1979–2002). Bayesian model averaging (BMA) is used to avoid overfitting the CRU–NARR differences and underestimating uncertainties. In cross validations, BMA provides reliable posterior predictions of the CRU–NARR differences and outperforms predictions from three alternative models: the constant model (24-yr mean), the regression model of highest Bayesian model probability, and the full model retaining all seven predictors in CRU.

Description: A study of the Israeli summer coastal boundary layer (BL) with a 1290-MHz wind profiler located 3.5 km from the coastline has revealed four main features associated with the sea-breeze onset: 1) by midday the boundary layer height drops by an average value of 250 m, 2) a sea-breeze calm zone (SBCZ) develops near the top of the onshore BL, 3) persistent strong downdrafts, with average values of 0.35 m s−1, invigorate from the BL top downward, and 4) a combination of the SBCZ and strong downdrafts may cause an elevated plume released from 250–300-m-tall coastal stacks to reach the ground only 3.5 km downwind. Results from the Weather Research and Forecasting (WRF) numerical weather model confirmed the development of a sea-breeze downdraft zone up to 4 km onshore. Such a zone, which was observed as a cloudless strip in satellite images [Moderate Resolution Imaging Spectroradiometer (MODIS)], may be a result of persistent large BL eddies that develop to horizontal transverse rolls. Because the WRF-simulated downdraft magnitude is weaker by a factor of more than 5 than the profiler measurements, the magnitude of the downdrafts is inconclusive.

Description: The spatial and temporal variations of aerosol loading over eastern Asia specified in terms of the aerosol optical depth (AOD) at the 550-nm wavelength during July are examined in conjunction with the intensity of the Indian summer monsoon. AOD derived from Moderate Resolution Imaging Spectroradiometer (MODIS) observations, gridded reanalyses, and ground-based measurements are used in the analysis. Two contrasting years, 2002 and 2003, which represent weak and active Indian summer monsoon events, respectively, are selected for the study, with a focus on an eastern Asian southern subregion (SR; 23°–32°N, 105°–120°E) and an eastern Asian northern subregion (NR; 35°–44°N, 115°–130°E). It is shown that the interannual variation of July mean wind intensity is a major factor in regulating the midsummer spatial pattern of aerosols over eastern Asia when the Indian monsoon index is anomalously large. The AOD anomalies in the NR and SR are positive and negative, respectively, during an active monsoon year, whereas the opposite is observed during a weak monsoon year. The variation patterns of less cloudy-day visibility, observed at four meteorological stations in the SR and NR subregions, also show spatial–temporal aerosol variability evident in the MODIS AOD data. Relative to the case of a weak monsoon year, meridional winds and convection are stronger and more clouds and precipitation are observed in the NR subregion during the active monsoon year. The opposite pattern is observed in the SR subregion. The spatial–temporal variation pattern of aerosols over eastern Asia illustrates the nonnegligible role of transport and dispersal mechanisms associated with the Indian summer monsoon in the region.

Description: Clear and cloudy daytime comparisons of land surface temperature (LST) and air temperature (Tair) were made for 14 stations included in the U.S. Climate Reference Network (USCRN) of stations from observations made from 2003 through 2008. Generally, LST was greater than Tair for both the clear and cloudy conditions; however, the differences between LST and Tair were significantly less for the cloudy-sky conditions. In addition, the relationships between LST and Tair displayed less variability under the cloudy-sky conditions than under clear-sky conditions. Wind speed, time of the observation of Tair and LST, season, the occurrence of precipitation at the time of observation, and normalized difference vegetation index values were all considered in the evaluation of the relationship between Tair and LST. Mean differences between LST and Tair of less than 2°C were observed under cloudy conditions for the stations, as compared with a minimum difference of greater than 2°C (and as great as 7+°C) for the clear-sky conditions. Under cloudy conditions, Tair alone explained over 94%—and as great as 98%—of the variance observed in LST for the stations included in this analysis, as compared with a range of 81%–93% for clear-sky conditions. Because of the relatively homogeneous land surface characteristics encouraged in the immediate vicinity of USCRN stations, and potential regional differences in surface features that might influence the observed relationships, additional analyses of the relationships between LST and Tair for additional regions and land surface conditions are recommended.

Description: A statistical simulation framework is developed to explore the future frequencies of threshold-crossing events, focusing here on low seasonal rainfall totals. Global change (GC) is represented by a trend on the seasonal mean rainfall total. Natural decadal to multidecadal variability (MDV) is represented by an autoregressive process. Interannual variability (IV) of seasonal totals is represented by white noise with either a normal or skew normal distribution consistent with parameters observed in the historical record at the location being modeled. Monte Carlo simulations are undertaken for various combinations of the above components, and the authors evaluate the extent to which future event frequencies can be estimated from the statistics of previous years. The sample of four study locations used to illustrate the approach is drawn from the Millennium Villages Project in Africa, where the potential of index insurance as a development and adaptation tool has been considered, thereby bringing a targeted problem setting to the analyses. The simulations highlight a number of general principles. For example, it is shown that a 10% change in the mean rainfall can lead to a change of order times 2 in the number of threshold-crossing low seasonal rainfall totals, even without invoking any change in the characteristics of the IV. The magnitudes of change are also shown to be sensitive to the threshold studied, as well as to site-specific climate features (here, coefficient of variation and skewness). The framework developed permits quantification of how, especially in the near term (of order 30 years), MDV can strongly add to uncertainty about future event frequencies. Therefore, statistical treatment of estimated MDV magnitudes will often be a key input to optimal risk management, with further enhancements expected through explicit MDV forecasts. The results highlight the importance of finding optimal ways to update climate statistics such as event thresholds, in the presence of GC and MDV.

Description: On 1 July 1961, the climatological day was redefined to end at 0600 UTC at all principal climate stations in Canada. Prior to that, the climatological day at principal stations ended at 1200 UTC for maximum temperature and precipitation and 0000 UTC for minimum temperature and was similar to the climatological day at ordinary stations. Hutchinson et al. reported occasional larger-than-expected residuals at 50 withheld stations when the Australian National University Spline (ANUSPLIN) interpolation scheme was applied to daily data for 1961–2003, and it was suggested that these larger residuals were in part due to the existence of different climatological days. In this study, daily minimum and maximum temperatures at principal stations were estimated using hourly temperatures for the same climatological day as local ordinary climate stations for the period 1953–2007. Daily precipitation was estimated at principal stations using synoptic precipitation data for the climatological day ending at 1200 UTC, which, for much of the country, was close to the time of the morning observation at ordinary climate stations. At withheld principal stations, the climatological-day adjustments led to the virtual elimination of large residuals in maximum and minimum temperature and a marked reduction in precipitation residuals. Across all 50 withheld stations the climatological day adjustments led to significant reductions, by around 12% for daily maximum temperature, 15% for daily minimum temperature, and 22% for precipitation, in the residuals reported by Hutchinson et al.

Description: Surface energy fluxes, at averaging times from 10 min to 1 h, are needed as inputs to most state-of-the-art dispersion models. The sensible heat flux is a major priority, because it is combined with the momentum flux to estimate the stability, the wind profile, and the turbulence intensities. Because of recent concerns about dispersion in built-up downtown areas of large cities, there is a need to estimate sensible heat flux in the midst of tall buildings. In this paper, the authors work with some high-quality and relevant but arguably underutilized data. The results of analysis of urban heat flux components from 10 locations in suburban and built-up downtown areas in Oklahoma City, Oklahoma, during the Joint Urban 2003 (JU2003) field experiment are presented here. At street level in the downtown area, in the midst of tall skyscrapers, the ground heat flux and the sensible heat flux are relatively large and the latent heat flux is relatively small when compared with concurrent fluxes observed in the upwind suburban areas. In confirmation of measurements in other cities, the sensible heat flux in the downtown area is observed to be slightly positive (10–20 W m−2) at night, indicating nearly neutral or slightly unstable conditions. Also in agreement with observations in other cities is that the ground heat flux in the downtown area has a magnitude that is 3 or 4 times that in suburban or rural areas. These results should permit improved parameterizations of sensible heat fluxes in the urban downtown area with tall buildings.

Description: The ratio of record highs to record lows is examined with respect to extent of time series for monthly mean temperatures within the continental United States for 1900–2006. In counting the number of records that occur in a single year, the authors find a ratio greater than unity in 2006, increasing nearly monotonically as the time series increases in length via a variable first year over 1900–76. For example, in 2006, the ratio of record highs to record lows ≈ 13:1 with 1950 as the first year and ≈ 25:1 with 1900 as the first year; both ratios are an order of magnitude greater than 3σ for stationary simulations. This indicates a warming trend. It is also found that records are more sensitive to trends in time series of monthly averages than in time series of corresponding daily values. When the last year (1920–2006, starting in 1900) is varied, it is found that the ratio of record highs to record lows is strongly correlated with the ensemble mean temperature. Correlation coefficients are 0.76 and 0.82 for 1900–2006 and 1950–2006, respectively; 3σ = 0.3 for pairs of uncorrelated stationary time series. Similar values are found for globally distributed time series: 0.87 and 0.92 for 1900–2006 and 1950–2006, respectively. The ratios evolve differently, however: global ratios increase throughout (1920–2006) whereas continental U.S. ratios decrease from about 1940 to 1970. Last, the geographical and seasonal distributions of trends are considered by summing records over time rather than ensemble. In the continental United States, the greatest excess of record highs occurs in February (≈2:1) and the greatest excess of record lows occurs in October (≈2:3). In addition, ratios are pronounced in certain regions: in February in the Midwest the ratio ≈ 5:2, and in October in the Southeast the ratio ≈ 1:2.

Description: The forthcoming broadband radiometer (BBR) on board the Earth Clouds, Aerosols, and Radiation Explorer (EarthCARE) will provide quasi-instantaneous top-of-atmosphere radiance measurements for three different viewing angles. The role of BBR data will be to constrain the vertical radiative flux divergence profiles derived from EarthCARE measurements. Thus, the development of an instantaneous radiance-to-flux conversion procedure is of paramount importance. This paper studies the scientific basis for determining fluxes from radiances measured by the BBR instrument. This is an attempt to evaluate a possible solution and assess its potential advantages and drawbacks. The approach considered has been to construct theoretical angular distribution models (ADMs) based on the multiangular pointing feature of this instrument. This configuration provides extra information on the anisotropy of the observed radiance field, which can be employed to construct accurate inversion schemes. The proposal relies on radiative transfer calculations performed with a Monte Carlo algorithm. Considering the intrinsic difficulty associated with addressing the range of atmospheric conditions needed to determine reliable ADMs, a synthetic database has been thoroughly constructed that considers a diverse range of surface, atmospheric, and cloud conditions that are conditioned to the EarthCARE orbit and physical constraints. Three inversion methodologies have been specifically designed for the BBR flux retrieval algorithm. In particular, an optimized classical inversion procedure in which the definition of an effective radiance leads to derive fluxes with averaged errors up to 1.2 and 5.2 W m−2 for shortwave clear and cloudy sky and 1.5 W m−2 for longwave radiation scenes and a linear combination of the three instantaneous radiances from which averaged errors up to 0.4 and 2.7 W m−2 for shortwave clear and cloudy sky and 0.5 W m−2 for longwave scenes can be obtained.

Description: An empirical scheme for predicting the meteorological conditions that lead to summer forest fire severity for Canada using the multivariate singular value decomposition (SVD) has been developed for the 1953–2007 period. The levels and sources of predictive skill have been estimated using a cross-validation design. The predictor fields are global sea surface temperatures (SST) and Palmer drought severity index. Two consecutive 3-month predictor periods are used to detect evolving conditions in the predictor fields. Correlation, mean absolute error, and percent correct verification statistics are used to assess forecast model performance. Nationally averaged skills are shown to be statistically significant, which suggests that they are suitable for application to forest fire prediction and for management purposes. These forecasts average a 0.33 correlation skill across Canada and greater than 0.6 in the forested regions from the Yukon, through northern Prairie Provinces, northern Ontario, and central Quebec into Newfoundland. SVD forecasts generally outperform persistence forecasts. The importance of the leading two SVD modes to Canadian summer forest fire severity, accounting for approximately 95% of the squared covariance, is emphasized. The first mode relates strongly to interdecadal trend in global SST. Between 1953 and 2007 the western tropical Pacific, the Indian, and the North Atlantic Oceans have tended to warm while the northeastern Pacific and the extreme Southern Hemisphere oceans have shown a cooling trend. During the same period, summer forest fire exhibited increased severity across the large boreal forest region of Canada. The SVD diagnostics also indicate that the El Niño–Southern Oscillation and the Pacific decadal oscillation play a significant role in Canadian fire severity. Warm episodes (El Niño) tend to be associated with severe fire conditions over the Yukon, parts of the northern Prairie Provinces, and central Quebec. The linearity of the SVD manifests opposite response during the cold (La Niña) events.

Description: The radar observation operator for computation of polarimetric radar variables from the output of numerical cloud models is described in its most generic form. This operator is combined with the Hebrew University of Jerusalem cloud model with spectral microphysics. The model contains 7 classes of hydrometeors and each class is represented by size distribution functions in 43 size bins. The performance of the cloud model and radar observation operator has been evaluated for the case of a hailstorm in Oklahoma on 2 February 2009. It is shown that the retrieved fields of polarimetric radar variables at C and S microwave bands are generally consistent with results of observations. The relationship between microphysical and polarimetric signatures is illustrated.

Description: In this paper, a convective initiation event from the International H2O Project (IHOP) field experiment is used to demonstrate the potential utility of a future geostationary advanced infrared (IR) sounder for severe storm nowcasting applications. An advanced IR sounder would provide detailed stability information (e.g., lifted index and other parameters) with high temporal resolution useful for determining favorable locations for convective initiation. Atmospheric data from a high-resolution Weather Research and Forecasting model simulation was used to generate simulated Hyperspectral Environmental Suite (HES) and Advanced Baseline Imager (ABI) stability products. Comparison of these products shows that the ABI [or the current Geostationary Operational Environmental Satellite (GOES) Sounder] provides limited stability information before the storm development as a result of the limited spectral IR information for temperature and moisture profiling. The high spatial and temporal geostationary advanced IR sounder, however, can provide critical information about the destabilization much earlier than the current GOES Sounder or ABI.

Description: To evaluate the impacts of the urban heat island (UHI) effect on precipitation over a complex geographic environment in northern Taiwan, the next-generation mesoscale model, the Weather Research and Forecasting (WRF) model, coupled with the Noah land surface model and urban canopy model (UCM), was used to study this issue. Based on a better land use classification derived from Moderate Resolution Imaging Spectroradiometer (MODIS) satellite data (the MODIS case), it has significantly improved simulation results for the accumulation rainfall pattern as compared with the original U.S. Geological Survey (USGS) 25-category land use classification (the USGS case). The precipitation system was found to develop later but stronger in the urban (MODIS) case than in the nonurban (USGS) case. In comparison with the observation by radar, simulation results predicted reasonably well; not only was the rainfall system enhanced downwind of the city over the mountainous area, but it also occurred at the upwind plain area in the MODIS case. The simulation results suggested that the correct land use classification is crucial for urban heat island modeling study. The UHI effect plays an important role in perturbing thermal and dynamic processes; it affects the location of thunderstorms and precipitation over the complex geographic environment in northern Taiwan.

Description: Many wave propagation applications depend on the local, instantaneous structure parameters of humidity and of potential temperature . This study uses a large-eddy simulation to explore and compare the variability of and in the shearless, entrainment-drying convective boundary layer (CBL). The predicted horizontal mean profiles of these quantities are shown to agree with corresponding observations. The results in the bulk CBL suggest that the largest occur in the entrained tropospheric air whereas the largest are within the convective plumes. There are distinct correlations between the vertical velocity and and between the vertical velocity and . It is shown that these correlations can significantly contribute to the mean vertical velocity biases measured from radars and sodars. A physical interpretation for these contributions is offered in terms of the CBL dynamics.

Description: The accurate real-time detection of turbulent airflow patterns near airports is important for safety and comfort in commercial aviation. In this paper, a method is developed to identify Lagrangian coherent structures (LCS) from horizontal lidar scans at Hong Kong International Airport (HKIA) in China. LCS are distinguished frame-independent material structures that create localized attraction, repulsion, or high shear of nearby trajectories in the flow. As such, they are the fundamental structures behind airflow patterns such as updrafts, downdrafts, and wind shear. Based on a recently developed finite-domain–finite-time Lyapunov exponent (FDFTLE) algorithm from Tang et al. and on new Lagrangian diagnostics presented in this paper that are pertinent to the extracted FDFTLE ridges, the authors differentiate LCS extracted from lidar data. It is found that these LCS derived from horizontal lidar scans compare well to convergence and divergence suggested by vertical slice scans. At HKIA, horizontal scans are predominant: they cover much bigger azimuthal ranges as compared with only two azimuthal angles from the vertical scans. LCS extracted from horizontal scans are thus advantageous in providing organizing turbulence structures over the entire observational domain as compared with a single line along the vertical scan direction. In Part II of this study, the authors will analyze the evolution of LCS and their impacts on landing aircraft based on recorded flight data.

Description: During the rainy season over the East China Sea, convective rainfalls often show melting layer (ML) characteristics in polarimetric radar variables. In this research, the appearance ratio of the ML (the ratio of rainfall area accompanied by polarimetric ML signatures) and the variation in height of the level of the ML signature maximum (MLSM level; defined by the level of the ρhv minimum in the ML) in a convective rainfall region in a rainfall system over the East China Sea observed on 2 June 2006 were studied using C-band polarimetric radar (COBRA). For this analysis, a method of rainfall type classification that evaluates the presence of an ML in addition to providing conventional convective–stratiform classification using range–height indicator (RHI) observation data was developed. This rainfall type classification includes two steps: conventional convective–stratiform separation using the horizontal distribution of Zh at 2-km altitude, and ML detection using the vertical profile of ρhv at each horizontal grid point. Using a combination of these two classifications, the following four rainfall types were identified: 1) convective rainfall with an ML, 2) convective rainfall with no ML, 3) stratiform rainfall with an ML, and 4) stratiform rainfall with no ML. An ML was detected in 53.9% of the convective region in the rainfall system. Using the same definition, an ML was detected in 83.1% of the stratiform region. The ML in the convective region showed a marked decrease in ρhv coincident with an increase in ZDR around the ambient 0°C level, as did that in the stratiform region. Melting aggregated snow was the likely cause of the ML signature in the convective region. The average height of the MLSM level in the convective region was 4.64 km, which is 0.46 km higher than that in the stratiform region (4.18 km) and 0.27 km higher than the ambient 0°C level (4.37 km).

Description: Numerical simulations for severe downslope winds as well as trapped lee waves in Nevada’s Las Vegas Valley were performed in this study. The goal of this study was to improve model forecasts of downslope-wind-event intensities. This was measured by assessing different planetary boundary layer (PBL) schemes in the mountain–valley region. The Weather Research and Forecasting Model was adopted for this research. The numerical experiments were constructed using two nested domains, with 4- and 1-km grid resolution. The working hypothesis was that the occurrence of low-level wind shear and surface gustiness in the Las Vegas Valley was guided by the inversion layer in the valley. The choice of boundary layer scheme and model vertical resolution will influence inversion-layer height and consequently result in significant differences in surface wind and temperature forecast error below some near-surface height. Simulations of severe downslope wind events on 15 April 2008 and on 4 October 2009 were conducted. Statistical analyses of model results from three different PBL schemes and different vertical resolutions were performed. The results from the domain with 1-km grid spacing demonstrated remarkable detail of the severe downslope winds associated with low-level wind shear and surface gustiness in the Las Vegas Valley. The simulation results demonstrated that model vertical resolution was primarily responsible for the detail of the lower-level wind and temperature structures. The inverse Froude number and Froude number are two indices that may be included as the forecasting guidance for downslope winds, lee waves, and rotors for the Las Vegas Valley.

Description: The community of Tuktoyaktuk (Northwest Territories, Canada) along the Beaufort Sea experiences dramatic shoreline erosion during storm surge events that tend to occur during persistent northwesterly wind events in the late summer months (July–September) when the sea ice coverage of the Beaufort Sea reaches its annual minimum. This study compiles the climatology of hourly surface wind, low-level geostrophic wind, and static stability to investigate the physical mechanisms responsible for the high frequency of northwesterly winds observed at Tuktoyaktuk during the late summer. The results link the prevalence of westerly to northwesterly winds at the surface to the high frequency of northwesterly geostrophic winds and a tendency for low static stability. With an environment that favors strong northwesterly geostrophic wind and suggests lower static stability, the high frequency of strong northwesterlies observed at the surface appears to be associated with momentum mixing by turbulent eddies. A composite analysis indicates that persistently strong northwesterly winds are associated with anomalously low pressure northeast of Tuktoyaktuk and high pressure over the Bering Sea and eastern Siberia. The high pressure anomalies over the Bering Sea also extend well to the east along the northern edge of the Brooks Range. An apparent topographic modification of the sea level pressure (SLP) field by cold air trapped to the north of mountains produces the pressure gradient favorable for strong westerly to northwesterly geostrophic winds at Tuktoyaktuk. The results suggest that cold-air damming contributes to the wind regime at Tuktoyaktuk by altering the pressure gradient along the Beaufort coast.

Description: In this study, unusual radar anomalous propagation (AP) phenomena associated with foehn winds induced by Typhoon Krosa (2007) were documented by using observations from radar, surface stations, and soundings. The AP echoes embedded within rainband areas and exhibited inward motions toward the radar site within 2–3 h prior to the occurrences of foehn winds at the radar site, which would interfere with the interpretation of radar data and associated downstream applications. As Typhoon Krosa appeared in the vicinity of the northeastern coast of Taiwan, foehn winds with significant subsidence warming and drying generated by downslope winds were observed in southeastern Taiwan. The foehn winds continuously moved northward within confined areas from the southeastern to eastern–central parts of Taiwan. Before the foehn winds penetrated to the surface, the subsidence warming introduced a temperature inversion layer above the surface and caused the ducting of radar beams. Analyses of refractive index and ray tracing suggested that the occurrence and evolution of the AP echoes during Typhoon Krosa were closely related to the varying inversion heights induced by downslope winds.

Description: The Tropical Rainfall Measuring Mission (TRMM) satellite has provided invaluable data for tropical cyclone (TC) research since December 1997. The challenge, however, is how to analyze and efficiently utilize all of the information from several instruments on TRMM that observe the same target. In this study, a tropical cyclone precipitation, cloud, and convective cell feature (TCPF) database has been developed by using observations of the TRMM precipitation radar (PR), Microwave Imager (TMI), Visible and Infrared Scanner (VIRS), Lightning Imaging System (LIS), and the TRMM 3B42 rainfall product. The database is based on an event-based method that analyzes the measurements from multiple sensors. This method condenses the original information of pixel-level measurements into the properties of events, which can significantly increase the efficiency of searching and sorting the observed historical TCs. With both convective and rainfall properties included, the database offers the potential to aid the research aiming to improve both TC intensification and rainfall forecasts. Using the TRMM TCPF database, regional variations of TC convection and diurnal variations of TC rainfall are examined. In terms of absolute number, the northwest Pacific Ocean basin has the deepest and most intense TCPFs according to IR, radar, and 85-GHz microwave measurements. However, the North Atlantic TCPFs appear to have the highest lightning production. Globally, TC rainfall has a maximum at 0430–0730 local solar time (LST) and a minimum around 1930–2230 LST. However, after separating ocean from land, a distinct difference is seen. Over land, the diurnal variation of TC rainfall shows double peaks: one around 0130–0730 LST and the other at 1630–1930 LST. The minimum is at 1030–1330 LST.

Description: A new homogeneous climate division monthly precipitation dataset [based on full network estimated precipitation (FNEP)] was created as an alternative to the National Climatic Data Center (NCDC) climate division dataset. These alternative climate division monthly precipitation values were estimated using an equal-weighted average of Cooperative Observer Program stations that contained serially complete time series. Missing station observations were estimated by a procedure that was optimized through testing on U.S. Historical Climate Network stations. Inhomogeneities in the NCDC dataset arise from two principal causes. The pre-1931 estimation of NCDC climate division monthly precipitation from statewide averages led to a significant time series discontinuity in several climate divisions. From 1931 to the present, NCDC climate division averages have been calculated from a subset of available station data within each climate division, and temporal changes in the location of available stations have caused artificial changes in the time series. The FNEP climate division dataset is recommended over the NCDC dataset for studies involving climate trends or long-term climate variability. According to the FNEP data, the 1895–2009 linear precipitation trend is positive across most of the United States, and trends exceed 10% per century across the southern plains and the Corn Belt. Remaining inhomogeneities from changes in gauge technology and station location may be responsible for an artificial trend of 1%–3% per century.

Description: A prototype generic, unified land surface classification and screening methodology for Global Precipitation Measurement (GPM)-era microwave land precipitation retrieval algorithms by using ancillary datasets is developed. As an alternative to the current radiometer-determined approach, the new methodology is shown to be promising in improving rain detection by providing better surface-cover-type information. The early prototype new surface screening scheme was applied to the current version of the Goddard profiling algorithm that is used for the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (GPROFV6). It has shown improvements in surface-cover-type classification and hence better precipitation retrieval comparisons with TRMM precipitation radar level-2 (L2) (2A25) data and the Global Precipitation Climatology Project (GPCP) version-2.1 (GPCPV2.1) datasets. The new ancillary data approach removes the current dependency of the screening step on relatively different satellite-specific channels and ensures the comparability and continuity of satellite-based precipitation products from different platforms. This is particularly important for advancing the current state of precipitation retrieval over land and for use in merged rainfall products.

Description: To understand agro-climatic suitability for spring soybean growth in north China, an integrated crop-response-function method was developed. This method includes crop-response functions for temperature, precipitation, and sunshine and is assessed by a weighting method based on the coefficient of determination. The results show that the most suitable area (S1) for spring soybean growth occupied approximately 21.35% of the total area of north China. Among three types of spring soybeans of early maturity, middle maturity, and late maturity, middle maturity was the most suitable variety to grow in the study area, covering nearly 1.133 × 106 km2 or about 99.75% of the total area of S1. As a result of this study, the authors suggest that breeders pay more attention to middle-maturity cultivars in north China. The findings from this study may provide useful information for policy makers issuing guidelines for agricultural production.

Description: Conventional methods of viewing and combining retrieved geophysical fields from polar-orbiting satellites often complicate the work of end users because of the erratic time differences between overpasses, the significant time gaps between elements of a composite image, or simply the different requirements for interpretation between contributing instruments. However, it is possible to mitigate these issues for any number of retrieved quantities in which the tracer lifetime exceeds the sampling time. This paper presents a method that uses “advective blending” to create high-fidelity composites of data from polar-orbiting satellites at high temporal resolution, including a characterization of error as a function of time gap between satellite overpasses. The method is especially effective for tracers with lifetimes of longer than 7 h. Examples are presented using microwave-based retrievals of total precipitable water (TPW) over the ocean, from the Cooperative Institute for Meteorological Satellite Studies (CIMSS) Morphed Integrated Microwave Imagery at CIMSS TPW product (MIMIC-TPW). The mean average error of a global 0.25° × 0.25° product at 1-h resolution is 0.5–2 mm, which is very reasonable for most applications.

Description: The measurement of the physical characteristics of hailstones reaching the ground is usually carried out by means of hailpads, on which the impact of hailstones leaves dents. Hailstone dents provide information about parameters, such as the number N of hailstones, their size M, and their kinetic energy E. In the case of intense hailfalls, however, the dents often overlap and the final measurement may not be totally reliable. This paper presents a computerized simulation with the aim of assessing measurement errors caused by dent overlap. The simulated dents represent several random hailfalls with both exponential size distributions and monodispersed size distributions. The simulated hailpads were measured following the procedure employed in the case of hailpads exposed to authentic hailfalls, and it was thus possible to assess the error due to dent overlap. The results show that dent overlap makes it impossible to measure all the dents, which means that in a real hailfall the number of hailstones registered will often be lower than the number of hailstones that actually hit the ground (up to 25% may go undetected). Consequently, the energy and mass of the hailstones are also underestimated (they may be up to 50% higher than the values registered on a hailpad). The maximum size registered, however, does not depend on the degree of overlapping and neither does the slope parameter λ of the exponential distribution, except when λ takes higher values. Finally, the authors suggest a heuristic correction of the data obtained by real hailpads based on the results of the simulations. An example is provided that applies these corrections to the 228 hailfalls registered by the Italian hailpad network over a period of 10 yr. The results show that, on average, the correction applied because of overlapping increases the number of hailstones in 3.2%, the mass in 1.9%, and the energy in 5.4%. However, there are cases in which these corrections reached much higher values of up to 6.9% in N and M, and up to 25.2% in E. It is therefore advisable to correct dent overlap before carrying out a regional climatic study of hail, since this study would certainly be affected by the errors accumulated by all the hailpads.

Description: Hygroscopic pollution aerosols have the potential to alter winter orographic snowfall totals and spatial distributions by modification of high-elevation supercooled orographic clouds and the riming process. The authors investigate the cumulative effect of varying the concentrations of hygroscopic aerosols during January–February for four recent winter snowfall seasons over the high terrain of Colorado. Version 6.0 of the Regional Atmospheric Modeling System (RAMS) is used to determine the particular mountain ranges and seasonal conditions that are most susceptible. Multiple winter seasonal simulations are run at both 3- and 1-km horizontal grid spacing with varying aerosol vertical profiles. Model-predicted snowfall accumulation trends are compared with automated snow water equivalent observations at high-elevation sites. An increase in aerosol concentration leads to reduced riming of cloud water by ice particles within supercooled, liquid orographic clouds, thus leading to lighter rimed hydrometers with slower fall speeds and longer horizontal trajectories. This effect results in a spillover of snowfall from the windward slope to the leeward slope. A snowfall spillover effect is most evident in the southern and western regions of the San Juan Range where high-moisture-laden storms are more prevalent. The effect over the Park Range is also present in each simulated season, but with lower amplitudes and slightly varying magnitudes among seasons. Seasons with greater overall snowfall exhibit a greater response in magnitude and percentage change. The smallest spillover effect occurred downwind of the primary western slope mountain barriers. Although the aerosol effect on snowfall can be locally significant in particularly wet winter seasons, the interseasonal variability in synoptic conditions can impose much larger widespread changes in snowfall accumulation.

Description: Since 2007 the advancement of the National Weather Radar Testbed Phased-Array Radar (NWRT PAR) hardware and software capabilities has been supporting the implementation of high-temporal-resolution (∼1 min) sampling. To achieve the increase in computational power and data archiving needs required for high-temporal-resolution sampling, the signal processor was upgraded to a scalable, Linux-based cluster with a distributed computing architecture. The development of electronic adaptive scanning, which can reduce update times by focusing data collection on significant weather, became possible through functionality added to the radar control interface and real-time controller. Signal processing techniques were implemented to address data quality issues, such as artifact removal and range-and-velocity ambiguity mitigation, absent from the NWRT PAR at its installation. The hardware and software advancements described above have made possible the development of conventional and electronic scanning capabilities that achieve high-temporal-resolution sampling. Those scanning capabilities are sector- and elevation-prioritized scanning, beam multiplexing, and electronic adaptive scanning. Each of these capabilities and related sampling trade-offs are explained and demonstrated through short case studies.

Description: When fitting a raindrop size distribution using a gamma model from data collected by a disdrometer, some consideration needs to be given to the small drops that fail to be recorded (typical disdrometer minimum size thresholds being in the 0.3–0.5-mm range). To this end, a gamma estimation procedure using maximum likelihood estimation has recently been published. The current work adds another procedure that accounts for the left-truncation problem in the data; in particular, an L-moments procedure is developed. These two estimation procedures, along with a traditional method-of-moments procedure that also accounts for data truncation, are then compared via simulation of volume samples from known gamma drop size distributions. For the range of gamma distributions considered, the maximum likelihood and L-moments procedures—which perform comparably—are found to outperform the procedure of method-of-moments. As these three procedures do not yield simple estimates in closed form, salient details of the R statistical code used in the simulations are included.

Description: The characteristics of aviation turbulence over South Korea during the recent five years (2003–08, excluding 2005) are investigated using pilot reports (PIREPs) accumulated by the Korea Aviation Meteorological Agency (KAMA). Among the total of 8449 PIREPs, 4607 (54.53%), 1646 (19.48%), 248 (2.94%), 7 (0.08%), and 1941 (22.97%) correspond to the turbulence categories of null, light, moderate, severe, and missing, respectively. In terms of temporal variations, the annual total number of turbulence events increased from 2003 to 2008, and the seasonal frequency is the highest in the spring. With regard to spatial distributions, reported turbulence encounters are dominant along the prevailing flight routes, but are locally higher over the west coast, Jeju Island, and the Sobaek and Taebaek mountains. The turbulence events in these regions vary by season. To examine the regional differences and possible sources of the observed turbulence, lightning flash data, Regional Data Assimilation and Prediction System (RDAPS) analysis data with a 30-km horizontal grid spacing provided by the Korean Meteorological Administration (KMA), and a digital elevation model (DEM) dataset with a 30-s resolution, are additionally used. Convectively induced turbulence (CIT) and clear-air turbulence (CAT) events comprised 11% and 89% of the total 255 moderate or greater (MOG)-level turbulence events, respectively. CAT events are classified as tropopause/jet stream–induced CAT (TJCAT) and mountain-wave-induced CAT (MWCAT) events. The MOG-level TJCAT and MWCAT events are responsible for 41.2% and 19.6% of the total MOG-level turbulence events, respectively. The CIT events in summer and the TRCAT and MWCAT events in spring occur most frequently over the previously mentioned regions of South Korea, associated with specific generation mechanisms.

Description: This study presents a 2-yr-long comparison of Weather Surveillance Radar-1988 Doppler (WSR-88D) refractivity retrievals with Oklahoma Mesonetwork (“Mesonet”) and sounding measurements and discusses some challenges to implementing radar refractivity operationally. Temporal and spatial analyses of radar refractivity exhibit high correlation with Mesonet data; however, periods of large refractivity differences between the radar and Mesonet are observed. Several sources of refractivity differences are examined to determine the cause of large refractivity differences. One source for nonklystron radars includes magnetron frequency drift, which can introduce errors up to 10 N-units if the frequency drift is not corrected. Different reference maps made at different times can “shift” refractivity values. A semiautomated method for producing reference maps is presented, including trade-offs for making reference maps under different conditions. Refractivity from six Mesonet stations within the clutter domain of the Oklahoma City, Oklahoma, WSR-88D (KTLX) is compared with radar refractivity retrievals. The analysis revealed that the six Mesonet stations exhibited a prominent diurnal trend in differences between radar and Mesonet refractivity measurements. The diurnal range of the refractivity differences sometimes exceeded 20 or 30 N-units in the warm season, which translated to a potential dewpoint temperature difference of several degrees Celsius. A seasonal analysis revealed that large refractivity differences primarily occurred during the warm season when refractivity is most sensitive to moisture. Ultimately, the main factor in determining the magnitude of the differences between the two refractivity platforms is the vertical gradient of refractivity because of the difference in observation height between the radar and a surface station.

Description: Doppler radar measurements at different frequencies (50 and 2835 MHz) are used to characterize the terminal fall speed of hydrometeors and the vertical air motion in tropical ice clouds and to evaluate statistical methods for retrieving these two parameters using a single vertically pointing cloud radar. For the observed vertical air motions, it is found that the mean vertical air velocity in ice clouds is small on average, as is assumed in terminal fall speed retrieval methods. The mean vertical air motions are slightly negative (downdraft) between the melting layer (5-km height) and 6.3-km height, and positive (updraft) above this altitude, with two peaks of 6 and 7 cm s−1 at 7.7- and 9.7-km height. For the retrieved hydrometeor terminal fall speeds, it is found that the variability of terminal fall speeds within narrow reflectivity ranges is typically within the acceptable uncertainties for using terminal fall speeds in ice cloud microphysical retrievals. This study also evaluates the performance of previously published statistical methods of separating terminal fall speed and vertical air velocity from vertically pointing Doppler radar measurements using the 50-/2835-MHz radar retrievals as a reference. It is found that the variability of the terminal fall speed–radar reflectivity relationship (Vt–Ze) is large in ice clouds and cannot be parameterized accurately with a single relationship. A well-defined linear relationship is found between the two coefficients of a power-law Vt–Ze relationship, but a more accurate microphysical retrieval is obtained using Doppler velocity measurements to better constrain the Vt–Ze relationship for each cloud. When comparing the different statistical methods to the reference, the distribution of terminal fall speed residual is wide, with most residuals being in the ±30–40 cm s−1 range about the mean. The typical mean residual ranged from 15 to 20 cm s−1, with different methods having mean residuals of

Description: An investigation on vertical variations of the mixing lengths for momentum and heat under neutral and stable conditions was conducted using the data collected from the Cooperative Atmosphere–Surface Exchange Study in 1999 (CASES-99). By comparing κz with the mixing lengths under neutral conditions calculated using the observations from CASES-99, the vertical layer where the Monin–Obukhov similarity theory (MOST) is valid was identified. Here κ is the von Kármán constant and z is the height above the ground. On average, MOST is approximately valid between 0.5 and 10 m. Above the layer, the observed mixing lengths under neutral conditions are smaller than the MOST κz and can be approximately described by Blackadar’s mixing length, κz/[1 + (κz/l∞)], with l∞ = 15 m for up to z ~ 20 m for the mixing length for momentum and up to the highest observation height for the mixing length for heat. Above ~20 m, the mixing length for momentum approaches a constant. Both MOST κz and Blackadar’s formula systematically overestimate the mixing length for momentum above ~20 m, leading to overestimates of turbulence.

Description: A method to estimate objectively the surface wind fields associated with tropical cyclones using only data from multiple satellite platforms and satellite-based wind retrieval techniques is described. The analyses are computed on a polar grid using a variational data-fitting method that allows for the application of variable data weights to input data. The combination of gross quality control and the weighted variational analysis also produces wind estimates that have generally smaller errors than do the raw input data. The resulting surface winds compare well to the NOAA Hurricane Research Division H*Wind aircraft reconnaissance–based surface wind analyses, and operationally important wind radii estimated from these wind fields are shown to be generally more accurate than those based on climatological data. Most important, the analysis system produces global tropical cyclone surface wind analyses and related products every 6 h—without aircraft reconnaissance data. Also, the analysis and products are available in time for consideration by forecasters at the Joint Typhoon Warning Center, the Central Pacific Hurricane Center, and the National Hurricane Center in preparing their forecasts and advisories. This Multiplatform Tropical Cyclone Surface Wind Analysis (MTCSWA) product is slated to become an operationally supported product at the National Environmental Satellite Data and Information Service (NESDIS). The input data, analysis method, products, and verification statistics associated with the MTCSWA are discussed within.

Description: The algorithms used to estimate rainfall from polarimetric radar variables show significant variance in error characteristics over the range of naturally occurring rain rates. As a consequence, to improve rainfall estimation accuracy using polarimetric radar, it is necessary to optimally combine a number of different algorithms. In this study, a new composite method is proposed that weights the algorithms by the inverse of their theoretical error. A number of approaches are discussed and are investigated using simulated radar data calculated from disdrometer measurements. The resultant algorithms show modest improvement over composite methods based on decision-tree logic—in particular, at rain rates above 20 mm h−1.

Description: In-cloud icing on aircraft and ground structures can be observed every winter in many countries. In extreme cases ice can cause accidents and damage to infrastructure such as power transmission lines, telecommunication towers, wind turbines, ski lifts, and so on. This study investigates the potential for predicting episodes of in-cloud icing at ground level using a state-of-the-art numerical weather prediction model. The Weather Research and Forecasting (WRF) model is applied, with attention paid to the model’s skill to explicitly predict the amount of supercooled cloud liquid water content (SLWC) at the ground level at different horizontal resolutions and with different cloud microphysics schemes. The paper also discusses how well the median volume droplet diameter (MVD) can be diagnosed from the model output. A unique dataset of direct measurements of SLWC and MVD at ground level on a hilltop in northern Finland is used for validation. A mean absolute error of predicted SLWC as low as 0.08 g m−3 is obtained when the highest model resolution is applied (grid spacing equal to 0.333 km), together with the Thompson microphysics scheme. The quality of the SLWC predictions decreases dramatically with decreasing model resolution, and a systematic difference in predictive skill is found between the cloud microphysics schemes applied. A comparison between measured and predicted MVD shows that when prescribing the droplet concentration equal to 250 cm−3 the model predicts MVDs ranging from 12 to 20 μm, which corresponds well to the measured range. However, the variation from case to case is not captured by the current cloud microphysics schemes.

Description: This paper presents a convenient method for diagnosing the sources of infrasound in a numerical simulation of a convective storm. The method is based on an exact acoustic wave equation for the perturbation Exner function Π′. One notable source term (Suu) in the Π′ equation is commonly associated with adiabatic vortex fluctuations, whereas another (Sm) is directly connected to the heat and mass generated or removed during phase transitions of moisture. Scale estimates suggest that other potential sources are usually unimportant. Simple numerical simulations of a disturbed vortex and evaporating cloud droplets are carried out to illustrate the infrasound of Suu and Sm. Moreover, the diagnostic method is applied to a towering cumulonimbus simulation that incorporates multiple categories of ice, liquid, and mixed-phase hydrometeors. The sensitivity of Sm to the modeling of the hail-to-rain category conversion is briefly addressed.

Description: Polarimetric X-band radar measurements of differential reflectivity ZDR in stratiform rainfall were used for retrieving mean mass-weighted raindrop diameters Dm and estimating their spatial variability δDm at different scales. The ZDR data were calibrated and corrected for differential attenuation. The results revealed greater variability in Dm for larger spatial scales. Mean values of δDm were respectively around 0.32–0.34, 0.28–0.30, and 0.24–0.26 mm at scales of 20, 10, and 4.5 km, which are representative of footprints of various spaceborne sensors. For a given spatial scale, δDm decreases when the mean value of Dm increases. At the 20-km scale the decreasing trend exhibits a factor-of-1.7 decrease of δDm when the average Dm changes from 1 to 2 mm. Estimation data suggest that this trend diminishes as the spatial scale decreases. Measurement noise and other uncertainties preclude accurate estimations of Dm variability at smaller spatial scales because for many data points estimated variability values are equal to or less than the expected retrieval errors. Even though they are important for retrievals of absolute values of Dm, the details of the drop shape–size relation did not significantly affect estimates of size spatial variability. The polarization cross coupling in simultaneous transmission–simultaneous receiving measurement mode presents another limiting factor for accurate estimations of Dm. This factor, however, was not too severe in estimations of the size variability. There are indications that tuning the differential attenuation correction scheme might balance off some possible cross-coupling ZDR bias if differential phase accumulation is less than approximately 40°.

Description: The severity of drought has many implications for society. Its impacts on rain-fed agriculture are especially direct, however. The southeastern United States, with substantial rain-fed agriculture and large variability in growing-season precipitation, is especially vulnerable to drought. As commodity markets, drought assistance programs, and crop insurance have matured, more advanced information is needed on the evolution and impacts of drought. So far many new drought products and indices have been developed. These products generally do not include spatial details needed in the Southeast or do not include the physiological state of the crop, however. Here, a new type of drought measure is described that incorporates high-resolution physical inputs into a crop model (corn) that evolves based on the physical–biophysical conditions. The inputs include relatively high resolution (as compared with standard surface or NOAA Cooperative Observer Program data) (5 km) radar-derived precipitation, satellite-derived insolation, and temperature analyses. The system (referred to as CropRT for gridded crop real time) is run in real time under script control to provide daily maps of crop evolution and stress. Examples of the results from the system are provided for the 2008–10 growing seasons. Plots of daily crop water stress show small subcounty-scale variations in stress and the rapid change in stress over time. Depictions of final crop yield in comparison with seasonal average stress are provided.

Description: The Wallops-2000 experiment took place in April and May 2000 in the vicinity of Wallops Island, Virginia, to collect high-resolution measurements of microwave propagation and coincident meteorological parameters in a complex coastal environment. These data are used in conjunction with a mesoscale numerical weather prediction model to examine the impact of sea surface temperature (SST) on microwave ducting. Analysis of time series of meteorological fields at the location of an instrumented buoy indicates reliable forecast skill. Statistics from vertical profiles and of derived ducting characteristics (duct frequency, duct strength, duct-base height, and duct thickness) show that the model reproduced observed duct characteristics with modest accuracy, allowing for a 3–6-h error in synoptic airmass transitions. In addition to the control run, two experiments are conducted to examine the impact of SST on ducting. In one experiment a climatological SST field is used, and in the other a diurnal variation in SST is imposed. The higher SST in the diurnally varying simulations promotes stronger turbulent mixing, deep boundary layers, and small vertical gradients in mixing ratio in comparison with the control, which lead to reduced duct frequency and strength in many cases. The study further reveals that, while advection of large-scale air masses (vertical and horizontal) plays a crucial role in determining whether an environment is favorable for microwave ducting, diurnal variations in SST can be influential in determining the onset of ducting and the frequency of surface-based ducting in coastal regions.

Description: This paper describes a time-sensitive approach to climate change projections that was developed as part of New York City’s climate change adaptation process and that has provided decision support to stakeholders from 40 agencies, regional planning associations, and private companies. The approach optimizes production of projections given constraints faced by decision makers as they incorporate climate change into long-term planning and policy. New York City stakeholders, who are well versed in risk management, helped to preselect the climate variables most likely to impact urban infrastructure and requested a projection range rather than a single “most likely” outcome. The climate projections approach is transferable to other regions and is consistent with broader efforts to provide climate services, including impact, vulnerability, and adaptation information. The approach uses 16 GCMs and three emissions scenarios to calculate monthly change factors based on 30-yr average future time slices relative to a 30-yr model baseline. Projecting these model mean changes onto observed station data for New York City yields dramatic changes in the frequency of extreme events such as coastal flooding and dangerous heat events. On the basis of these methods, the current 1-in-10-year coastal flood is projected to occur more than once every 3 years by the end of the century and heat events are projected to approximately triple in frequency. These frequency changes are of sufficient magnitude to merit consideration in long-term adaptation planning, even though the precise changes in extreme-event frequency are highly uncertain.

Description: Lidar measurements obtained during the Surface Heat Budget of the Arctic Ocean (SHEBA) experiment under a mixed-phase stratus cloud that was lightly precipitating ice show a range of surprisingly low depolarization ratios (4%–23%), despite an absence of cloud droplets there. These depolarization ratios are much lower than the range of theoretical values obtained for various ice habits. The depolarization ratios correlate well with radar reflectivity, suggesting that the variation in depolarization ratios results from variations in ice water content, rather than variation in ice habits or orientation. By calculating lidar depolarization based on (i) large-eddy simulations and (ii) in situ ice size distribution measurements, it is shown that the presence of humidified aerosol particles in addition to the ice precipitation can explain the distribution and vertical profile of the observed depolarization ratios, although uncertainties related to the aerosol size distributions are substantial. These calculations show that humidified aerosol must be taken into account when interpreting lidar depolarization measurements for cloud and precipitation phase discrimination or ice habit classification, at least under conditions similar to those observed during SHEBA.

Description: Representative patterns from multichannel microwave brightness temperature Tb in the midlatitude oceanic region, observed by the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI), are studied during precipitation events detected by the TRMM precipitation radar (PR) for three summer and winter seasons using empirical orthogonal function (EOF) analysis. The first three patterns are interpreted as rain liquid water, solid particles, and rain type based on the frequency distributions of vertical profiles of the radar reflectivity factor and the heights of the storm top, cloud top, and freezing level. The first EOF (EOF1) correlates with the near-surface rain rate. While the eigenvector for the 85.5-GHz channel is less significant for EOF1 variability in summer, those in all channels contribute equally to the variability in winter. This difference suggests that summer precipitation is caused by additional solid particles formed in developing precipitation systems. The second EOF (EOF2) represents the number of solid particles and also corresponds to the near-surface rain rate. This result suggests an increase of solid particles with the development of precipitation systems. EOF2 varies largely by echo-top height in summer and by echo-top height and freezing height in winter. The positive component score has double Tb peaks. Dividing the score into two patterns according to these peaks reveals highly developed precipitation systems, such as convective rainbands and frontal systems, and weak precipitation with shallow systems caused by cold outbreaks in the winter case. The negative component score also shows shallow and weak precipitation systems with warm rain. The third EOF (EOF3) is related to rain type. Vertical profiles show a significant bright band with a small height difference between the echo top and freezing level for negative EOF3, while positive EOF3 has no bright band with a high echo top relative to the freezing height. The results indicate that stratiform and convective precipitation systems can be characterized by EOF3.

Description: The multiple-single-column approach is proposed as a new concept to study the boundary layer parameterization scheme in the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5). The results are compared with the Dutch Atmospheric Large-Eddy Simulation Model (DALES). Numerical experiments were performed over homogeneous and heterogeneous surfaces under clear convective boundary layer conditions. Identical simulations using MM5 and DALES were performed, which enabled an evaluation of the MM5 boundary layer scheme with DALES results. From the experiment with a homogeneous surface, MM5 shows a slightly shallower, colder, and moister boundary layer than DALES. This result is produced by an underestimation of turbulent mixing near the surface and less-vigorous entrainment of heat and dry air in MM5. In the heterogeneous surface experiment, the domain is divided into dry and wet patches, with the result that both models produce a mesoscale circulation. However, relative to the homogeneous case, larger differences were found between the models in the representation of the boundary layer dynamics. In DALES, the surface heterogeneity influenced the turbulent motions, making the mesoscale circulation much stronger (wmax is 6 times as large) than in MM5. Because of this stronger circulation, the boundary layer height, bulk temperature, and humidity also displayed differences in time and spatial patterns. Because of the land–atmosphere coupling in MM5, the mesoscale circulation strengthened the surface flux heterogeneity. Cold and moist air advection close to the surface from the wet patch to the dry patch increased the sensible heat flux above the dry patch and thus the induced mesoscale flow.

Description: Continental temperature inversions significantly influence air quality, yet little is known about their variability in frequency and intensity with time or sensitivity to dynamical changes with climate. Inversion statistics for six upper-air stations in the American Southwest are derived for the period 1994–2008 from radiosonde data reported by the Global Telecommunication System (GTS) and National Climatic Data Center (NCDC), which use different significant level standards. GTS data indicate that low-level elevated inversions have increased in frequency at four of six sites, consistent with enhanced regional stagnation projected by models. NCDC data, in contrast, show remarkable declines in weak, near-surface inversions through 2001, indicating local surface conditions may counteract atmospheric dynamics in regulating inversion activity and air quality. To further test the sensitivity of inversion activity to climate, associations between wintertime inversion frequency and large-scale circulation are quantified using the self-organizing map technique. Twenty-four representative circulation patterns are derived from North American Regional Reanalysis (NARR) 500-hPa geopotential height fields, and these patterns are correlated with inversion frequency at each site. Inversion activity in Salt Lake City, Utah, and Albuquerque and Santa Teresa, New Mexico, is found to correspond well with large-scale anticyclonic ridging; however, sensitivities to large-scale circulation in Denver, Colorado, and Flagstaff and Tucson, Arizona, are weak. Denver stands out in exhibiting a higher percentage of near-surface inversions in winter than the other southwestern sites. These findings indicate that dynamical changes with climate will not uniformly influence inversions and hence urban air quality conditions in the American Southwest.

Description: Comparisons are described for infrared-derived cloud products retrieved from the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Atmospheric Infrared Sounder (AIRS) using measured spatial response functions obtained from prelaunch AIRS calibration. One full day (1 January 2005) of global collection-5 MODIS and version-5 AIRS retrievals of cloud-top temperature Tc, effective cloud fraction f, and derived effective brightness temperature Tb,e is investigated. Comparisons of Tb,e demonstrate that MODIS and AIRS are essentially radiatively consistent and that MODIS Tb,e is 0.62 K higher than AIRS Tb,e for all scenes, increasing to 1.43 K for cloud described by AIRS as single layer and decreasing to 0.50 K for two-layer clouds. Somewhat larger differences in Tc and f are observed between the two instruments. The magnitudes of differences depend partly on whether MODIS uses a CO2-slicing or 11-μm brightness temperature window retrieval method. Some cloud- and regime-type differences and similarities between AIRS and MODIS cloud products are traceable to the assumptions made about the number of cloud layers in AIRS and also to the MODIS retrieval method. This (partially) holistic comparison approach should be useful for ongoing algorithm refinements, rigorous assessments of climate applicability, and establishment of the capability of synergistic MODIS and AIRS retrievals for improved cloud quantities and also should be useful for future observations to be made by the National Polar-Orbiting Operational Environmental Satellite System (NPOESS) Preparatory Project (NPP).

Description: This study summarizes recent improvements in the development of bulk scattering/absorption models at solar wavelengths. The approach combines microphysical measurements from various field campaigns with single-scattering properties for nine habits including droxtals, plates, solid/hollow columns, solid/hollow bullet rosettes, and several types of aggregates. Microphysical measurements are incorporated from a number of recent field campaigns in both the Northern and Southern Hemisphere. A set of 12 815 particle size distributions is used for which Tcld ≤ −40°C. The ice water content in the microphysical data spans six orders of magnitude. For evaluation, a library of ice-particle single-scattering properties is employed for 101 wavelengths between 0.4 and 2.24 μm. The library includes the full phase matrix as well as properties for smooth, moderately roughened, and severely roughened particles. Habit mixtures are developed for generalized cirrus, midlatitude cirrus, and deep tropical convection. The single-scattering properties are integrated over particle size and wavelength using an assumed habit mixture to develop bulk scattering and absorption properties. In comparison with global Cloud Aerosol Lidar with Orthogonal Polarization (CALIOP) data, models built with severely roughened particles compare best for all habit mixtures. The assumption of smooth particles provided the largest departure from CALIOP measurements. The use of roughened rather than smooth particles to infer optical thickness and effective diameter from satellite imagery such as the Moderate Resolution Imaging Spectroradiometer (MODIS) will result in a decrease in optical thickness and an increase in particle size.

Description: An equation is presented for wet-bulb temperature as a function of air temperature and relative humidity at standard sea level pressure. It was found as an empirical fit using gene-expression programming. This equation is valid for relative humidities between 5% and 99% and for air temperatures between −20° and 50°C, except for situations having both low humidity and cold temperature. Over the valid range, errors in wet-bulb temperature range from −1° to +0.65°C, with mean absolute error of less than 0.3°C.

Description: Single-layer physically based urban canopy models (UCM) have gained popularity for modeling urban–atmosphere interactions, especially the energy transport component. For a UCM to capture the physics of conductive, radiative, and turbulent advective transport of energy, it is important to provide it with an accurate parameter space, including both mesoscale meteorological forcing and microscale surface inputs. While field measurement of all input parameters to a UCM is rarely possible, understanding the model sensitivity to individual parameters is essential to determine the relative importance of parameter uncertainty for model performance. In this paper, an advanced Monte Carlo approach—namely, subset simulation—is used to quantify the impact of the uncertainty of surface input parameters on the output of an offline modified version of the Weather Research and Forecasting (WRF)-UCM. On the basis of the conditional sampling technique, the importance of surface parameters is determined in terms of their impact on critical model responses. It is found that model outputs (both critical energy fluxes and surface temperatures) are highly sensitive to uncertainties in urban geometry, whereas variations in emissivities and building interior temperatures are relatively insignificant. In addition, the sensitivity of the model to input surface parameters is also shown to be very weakly dependent on meteorological parameters. The statistical quantification of the model’s sensitivity to input parameters has practical implications, such as surface parameter calibrations in UCM and guidance for urban heat island mitigation strategies.

Description: During the Queensland Cloud Seeding Research Program, the “CP2” polarimetric radar parameter differential radar reflectivity Zdr was used to examine the raindrop size evolution in both maritime and continental clouds. The focus of this paper is to examine the natural variability of the drop size distribution. The primary finding is that there are two basic raindrop size evolutions, one associated with continental air masses characterized by relatively high aerosol concentrations and long air trajectories over land and the other associated with maritime air masses with lower aerosol concentrations. The size evolution difference is during the growth stage of the radar echoes. The differential radar reflectivity in the growing continental clouds is dominated by large raindrops, whereas in the maritime clouds differential reflectivity is dominated by small raindrops and drizzle. The drop size evolution in many of the maritime air masses was very similar to those observed in the maritime air of the Caribbean Sea observed with the NCAR S-band polarimetric radar (S-Pol) during the Rain in Cumulus over the Ocean (RICO) experiment. Because the tops of the Queensland continental clouds ascended almost 2 times as fast as the maritime ones in their growth stage, both dynamical and aerosol factors may be important for the systematic difference in drop size evolution. Recommendations are advanced for future field programs to understand better the causes for the observed variability in drop size evolution. Also, considering the natural variability in drop size evolution, comments are provided on conducting and evaluating cloud seeding experiments.

Description: Shelter temperature and wind forecasts from numerical weather prediction models are subject to large systematic errors. Kalman filtering and model output statistics (MOS) are commonly used postprocessing methods, but how effective are they in comparison with steadily increasing resolution of the forecast model? Observations from over 1100 stations in central Europe are used to compare the different postprocessing methods and the influence of model resolution in complex and simple terrain, respectively. A 1-yr period with hourly, or at least 3-hourly, data is used to achieve statistically meaningful results. Furthermore, the importance of real-time observations as MOS predictors and the effects of daily training of the MOS equations are studied.

Description: The study of precipitation in different phases is important to understanding the physical processes that occur in storms, as well as to improving their representation in numerical weather prediction models. A 2D video disdrometer was deployed about 30 km from a polarimetric weather radar in Norman, Oklahoma, (KOUN) to observe winter precipitation events during the 2006/07 winter season. These events contained periods of rain, snow, and mixed-phase precipitation. Five-minute particle size distributions were generated from the disdrometer data and fitted to a gamma distribution; polarimetric radar variables were also calculated for comparison with KOUN data. It is found that snow density adjustment improves the comparison substantially, indicating the importance of accounting for the density variability in representing model microphysics.

Description: The strongest hurricanes over the North Atlantic Ocean are getting stronger, with the increase related to rising ocean temperature. Here, the authors develop a procedure for estimating future wind losses from hurricanes and apply it to Eglin Air Force Base along the northern coast of Florida. The method combines models of the statistical distributions for extreme wind speed and average sea surface temperature over the Gulf of Mexico with dynamical models for tropical cyclone wind fields and damage losses. Results show that the 1-in-100-yr hurricane from the twentieth century picked at random to occur in the year 2100 would result in wind damage that is 36% [(13%, 76%) = 90% confidence interval] greater solely as a consequence of the projected warmer waters in the Gulf of Mexico. The method can be applied elsewhere along the coast with modeling assumptions modified for regional conditions.

Description: The Snowy Precipitation Enhancement Research Project (SPERP) was undertaken in winters from May 2005 to June 2009 in the Snowy Mountains region of southeastern Australia. Part I of this paper describes the design and implementation of the project, as well as the characteristics of the key datasets collected during the field phase. The primary analysis in this paper (Part II) shows an unequivocal impact on the targeting of seeding material, with the maximum level of silver in snow samples collected from the primary target area found to be significantly greater in seeded than unseeded experimental units (EUs). A positive but not statistically significant impact on precipitation was found. Further analysis shows that a substantial source of uncertainty in the estimation of the impacts of seeding on precipitation is associated with EUs where the seeding generators operated for relatively few hours. When the analysis is repeated using only EUs with more than 45 generator hours, the increase in precipitation in the primary target area is 14% at the 8% significance level. When applying that analysis to the overall target area, the precipitation increase is 14% at the 3% significance level. A secondary analysis of the ratio of silver to indium in snow supports the hypothesis that seeding material affected the cloud microphysics. Other secondary analyses reveal that seeding had an impact on virtually all of the physical variables examined in a manner consistent with the seeding hypothesis.

Description: Model investigations of aerosol–cloud interactions across spatial scales are necessary to advance basic understanding of aerosol impacts on climate and the hydrological cycle. Yet these interactions are complex, involving numerous physical and chemical processes. Models capable of combining aerosol dynamics and chemistry with detailed cloud microphysics are recent developments. In this study, predictions of aerosol characteristics from the Weather Research and Forecasting Model with Chemistry (WRF/Chem) are integrated into the Regional Atmospheric Modeling System microphysics package to form the basis of a coupled model that is capable of predicting the evolution of atmospheric aerosols from gas-phase emissions to droplet activation. The new integrated system is evaluated against measurements of cloud condensation nuclei (CCN) from a land-based field campaign and an aircraft-based field campaign in Colorado. The model results show the ability to capture vertical variations in CCN number concentration within an anthropogenic pollution plume. In a remote continental location the model-forecast CCN number concentration exhibits a positive bias that is attributable in part to an overprediction of the aerosol hygroscopicity that results from an underprediction in the organic aerosol mass fraction. In general, the new system for predicting CCN from forecast aerosol fields improves on the existing scheme in which aerosol quantities were user prescribed.

Description: An artificial seeding experiment was carried out over the Genkai Sea, Japan, using liquid carbon dioxide. The seeded cloud was followed by an aircraft and radar at Kyushu University. A radar-echo intensity of 19 dBZ was formed in the seeded cloud where no radar echo was observed previously. The results of the experiment are analyzed using a numerical model [Weather Research and Forecasting Model (WRF), version 3.1] to evaluate the effects of the artificial seeding. The numerically simulated radar echo has a slightly stronger intensity and wider area than that observed. The results of the experiments were similar to those produced by the numerical model, however. Evaporation of cloud liquid water; growth of snow particles that is due to deposition, autoconversion, aggregation, and collection; and convection caused by released latent heat are shown in the numerical results as effects of the seeding experiment. The falling snow particles cause downward flow, which is compensated for by upward flow that causes the formation of new cloud liquid water as a secondary effect of seeding, although this secondary effect is confirmed only in the numerical results.

Description: From the 344 state climate divisions in the conterminous United States, nine distinct regions of warm-season drought variability are identified using principal component analysis. The drought metric used is the Palmer hydrological drought index for the period 1895–2008. The focus of this paper is multidecadal drought variability in the Southeast (SEUS) and eastern Gulf South (EGS) regions of the United States, areas in which the low-frequency forcing mechanisms of warm-season drought are still poorly understood. Low-frequency drought variability in the SEUS and EGS is associated with smoothed indexed time series of major ocean–atmosphere circulation features, including two indices of spatiotemporal variability in the North Atlantic subtropical anticyclone (Bermuda high). Long-term warm-season drought conditions are significantly out of phase between the two regions. Multidecadal regimes of above- and below-average moisture in the SEUS and EGS are closely associated with slow variability in sea surface temperatures in the North Atlantic Ocean and with the summer mean position and mean strength of the Bermuda high. Multivariate linear regression indicates that 82%–92% of the low-frequency variability in warm-season moisture is explained by two of the three leading principal components of low-frequency variability in the climate indices. The findings are important for water resource managers and water-intensive industries in the SEUS and EGS. The associations identified in the paper are valuable for enhanced drought preparedness and forecasting in the study area and potentially for global models of coupled ocean–atmosphere variability.

Description: Urban heat islands (UHIs) are one of the best-recorded incidences of anthropogenic climate change. Studies from across the globe have examined this phenomenon, but relatively few have focused on cold-winter cities and the effects of snow cover and snowfall. This study uses hourly temperature measurements from 1 December 2008 through 28 February 2009 at 22 urban sites in Minneapolis, Minnesota, to investigate the effect of snow cover and snowfall on the UHI. Snow effects on temperature are estimated for calm conditions using a linear mixed-effect (LME) model. For the winter of 2008/09, the average wintertime UHI was approximately 1.0°C, with a peak near midday rather than at night. The LME model results suggest that a snow cover of 5 cm or more increased the magnitude of the UHI by about 1.0°C during the day and by about 0.5°C at night. The model also indicates that the presence of moderate snowfall decreased the magnitude of the UHI by up to 2.0°C, although this result is based on a comparatively small number of events. The high albedo of snow is posited to contribute to the enhanced UHI during the day, and snow’s insulating properties are likely contributors to the characteristics of the nighttime UHI.

Description: Three closely related issues that affect drought estimation in regions with limited precipitation data are addressed by investigating methods for filling missing daily precipitation data, handling short-term records, and deriving drought information for unsampled locations. The analysis yields three general conclusions: 1) it is better to conduct spatial interpolation prior to calculating drought index values, 2) using weather stations with moderate lengths of records (usually at least 10 years) improves the spatial–temporal characterization of drought, and 3) alternative precipitation sources of the National Weather Service multisensor precipitation rainfall estimates and the Tropical Rainfall Measuring Mission (TRMM) satellite monthly rainfall product (3B43) do not outperform spatially interpolated daily precipitation data in most regions, except in the western United States where the TRMM-based precipitation data work better than the spatially interpolated values for drought monitoring.

Description: Sounding of nonprecipitating clouds with the 10-cm wavelength Weather Surveillance Radar-1988 Doppler (WSR-88D) is discussed. Readily available enhancements to signal processing and volume coverage patterns of the WSR-88D allow observations of a variety of clouds with reflectivities as low as −25 dBZ (at a range of 10 km). The high sensitivity of the WSR-88D, its wide velocity and unambiguous range intervals, and the absence of attenuation allow accurate measurements of the reflectivity factor, Doppler velocity, and spectrum width fields in clouds to ranges of about 50 km. Fields of polarimetric variables in clouds, observed with a research polarimetric WSR-88D, demonstrate an abundance of information and help to resolve Bragg and particulate scatter. The scanning, Doppler, and polarimetric capabilities of the WSR-88D allow real-time, three-dimensional mapping of cloud processes, such as transformations of hydrometeors between liquid and ice phases. The presence of ice particles is revealed by high differential reflectivities and the lack of correlation between reflectivity and differential reflectivity in clouds in contrast to that found for rain. Pockets of high differential reflectivities are frequently observed in clouds; maximal values of differential reflectivity exceed 8 dB, far above the level observed in rain. The establishment of the WSR-88D network consisting of 157 polarimetric radars can be used to collect cloud data at any radar site, making the network a potentially powerful tool for climatic studies.

Description: The links between daily ozone levels in Southern California and atmospheric circulation at regional and large scales are examined for July–September 1994–2001. The monitoring station in Pasadena is used as the primary basis for ozone analysis; comparison with other stations validates its representativeness for Southern California. Comparing the 10% of highest-ozone days with the 10% of lowest-ozone days for Pasadena reveals a large regional difference in 700-hPa vertical velocity over Southern California, consistent with changes to the ventilation and depth of the boundary layer. Analysis of the associated changes in midlevel (500 hPa) circulation reveals near-continental-scale differences, with very large modifications in the strength and position of the North American anticyclone. These links between daily ozone levels and regional and large-scale atmospheric circulation features suggest the potential for using currently available medium-range weather forecasts in ozone prediction.

Description: In the last two decades, mesoscale models (MMs) with urban canopy parameterizations have been widely used to study urban boundary layer processes. Different studies show that such parameterizations are sensitive to the urban canopy parameters (UCPs) that define the urban morphology. At the same time, high-resolution UCP databases are becoming available for several cities. Studies are then needed to determine, for a specific application of an MM, the optimum degree of complexity of the urban canopy parameterizations and the resolution and details necessary in the UCP datasets. In this work, and in an attempt to answer the previous issues, four urban canopy schemes, with different degrees of complexity, have been used with the Weather Research and Forecasting (WRF) model to simulate the planetary boundary layer over the city of Houston, Texas, for two days in August 2000. For the UCP two approaches have been considered: one based on three urban classes derived from the National Land Cover Data of the U.S. Geological Survey and one based on the highly detailed National Urban Database and Access Portal Tool (NUDAPT) dataset with a spatial resolution of 1 km2. Two-meter air temperature and surface wind speed have been used in the evaluation. The statistical analysis shows a tendency to overestimate the air temperatures by the simple bulk scheme and underestimate the air temperatures by the more detailed urban canopy parameterizations. Similarly, the bulk and single-layer schemes tend to overestimate the wind speed while the multilayer schemes underestimate it. The three-dimensional analysis of the meteorological fields revealed a possible impact (to be verified against measurements) of both the urban schemes and the UCP on cloud prediction. Moreover, the impact of air conditioning systems on the air temperature and their energy consumption has been evaluated with the most developed urban scheme for the two simulated days. During the night, this anthropogenic heat was responsible for an increase in the air temperature of up to 2°C in the densest urban areas, and the estimated energy consumption was of the same magnitude as energy consumption obtained with different methods when the most detailed UCP database was used. On the basis of the results for the present case study, one can conclude that if the purpose of the simulation requires only an estimate of the 2-m temperature a simple bulk scheme is sufficient but if the purpose of the simulation is an evaluation of an urban heat island mitigation strategy or the evaluation of the energy consumption due to air conditioning at city scale, it is necessary to use a complex urban canopy scheme and a detailed UCP.

Description: Snow cover and amount are important components in the interaction between the land surface and atmosphere, but until recently there has been no observational snow information incorporated into the Met Office global numerical weather prediction (NWP) model. This paper presents a Northern Hemisphere (NH) snow analysis, based on satellite-derived observations of snow cover, that has recently been implemented in the operational global Unified Model (UM). The analysis uses daily snow maps from the NOAA National Environmental Satellite Data and Information Service Interactive Multisensor Snow and Ice Mapping System (IMS) to modify the UM background snow amount. Assimilation experiments carried out during the NH snow accumulation and ablation periods show improvements in analyzed snow cover, both qualitatively and quantitatively, when compared with unmodified control runs. Although the effect on forecast accuracy of introducing the snow analysis is largely neutral, there is some evidence of small improvements in screen-level temperature and humidity forecasts. Retention of information introduced by the snow analysis is hard, and so the limited impact on forecast accuracy is not unexpected. Several methods of validation and verification are presented, and their results and implications are discussed. The NH snow analysis delivers a basic snow assimilation system that can be expanded and developed further in future iterations to improve the representation of snow cover and amount in global and regional forecasting models.

Description: The movement of convective rainbands embedded in a tropical cyclone (TC) is usually derived from satellite images via the atmospheric motion vector (AMV) method or through the calculation of a radar’s echo track. In estimating the rotation speed of a TC rainband, however, the land-based radar can only detect approaching tropical cyclones within the vicinity. The AMV method is unable to fully account for the TC eyewall movement, thus making it difficult to estimate the TC intensity. The widely used method in estimating the TC maximum wind speed is the Dvorak technique in which the cloud pattern is extracted from only one image. In this study, the rainband rotation speeds are computed via satellite imagery and further applied in estimating the TC maximum wind speed. In contrast to previous research, this study adopts an innovative method by using two subsequent geostationary satellite images. The TC spin rates observed by weather satellites could often be seen to be positively related to the TC intensity. Analyses of the relationship between the typhoon wind intensity and estimated rotation speed at the 130–260-km ring via infrared channels are conducted for major typhoon cases that occurred during 2000–05 in the northwestern Pacific Ocean. Results show that the correlation between the wind intensity and estimated rotation speeds is strong for most of the cases. The highest R2 value from the individual cases could reach 0.93, and on an annual basis it could attain a value of 0.67. The mean R2 value for the 2000–05 dataset was roughly 0.53. The correlation between the wind intensity and estimated rotation speeds is further improved by factoring in the previous 6-h average rotation speeds. A regression equation is derived from the chosen typhoon cases between 2000 and 2005, which is utilized in verifying the major typhoon occurrences during 2006–08. The mean absolute error (MAE) of the hourly and 6-h average intensity estimates during 2000–08 was 20 and 18.7 kt, respectively (1 kt ≃ 0.5 m s−1). The best verification result occurred during 2008, for which the R2 value and MAE could reach 0.7 and 15.6, respectively. These research results demonstrate the suitability of using geostationary satellite image data in estimating the maximum wind speed. Nevertheless, the drawback of this study is that sometimes the rotation speeds will become slower when tropical cyclones mature because of the strong outflow of the secondary circulation. It is assumed that the relationship between the estimated rotation speeds and wind intensity can be further improved if the outflow speed of the tropical cyclones is also considered.

Description: Following the discovery of anomalously high values of lidar integrated attenuated backscatter near the top center layers of mesoscale convective systems (MCSs) observed by the NASA Lidar In-Space Technology Experiment (LITE), a search of Cloud Aerosol Lidar with Orthogonal Polarization (CALIOP) data on board the Cloud Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) platform revealed the same phenomena in a sample of eight MCSs investigated. The backscatter depolarization ratio also showed changes concurrent with the high integrated backscatter and either increased or decreased concurrently with the anomalous backscatter. Simultaneous CloudSat data in the A-Train formation showed a cloud-top altitude similar to that measured by CALIOP, indicating fairly large ice crystals were reaching cloud top. Based on previous work, the CALIOP and CloudSat returns were likely due to a mix of small ice droxtals or frozen drops extending in a continuous spectrum to large crystals composed of well-formed hexagonal columns, thick hexagonal plates, spheroids, and irregular particles. The CALIOP lidar would detect the whole spectrum whereas CloudSat would detect ice crystals greater than ∼30 μm in effective radius; there were apparently enough of such crystals to allow CloudSat to detect a cloud-top height similar to that found by CALIOP. Using such a model, it was estimated that the measured backscatter phase function in the most active part of the cloud could be reconciled approximately with theoretical values of the various crystal habits. However, it was harder to reconcile the changes in depolarization ratio given the absence of values of this parameter for small droxtal crystals.

Description: The ability of C-band polarimetric radar to account for strong attenuation/differential attenuation is demonstrated in two cases of heavy rain that occurred in the Chicago, Illinois, metropolitan area on 5 August 2008 and in central Oklahoma on 10 March 2009. The performance of the polarimetric attenuation correction scheme that separates relative contributions of “hot spots” (i.e., strong convective cells) and the rest of the storm to the path-integrated total and differential attenuation has been explored. It is shown that reliable attenuation correction is possible if the radar signal is attenuated by as much as 40 dB. Examination of the experimentally derived statistics of the ratios of specific attenuation Ah and differential attenuation ADP to specific differential phase KDP in hot spots is included in this study. It is shown that these ratios at C band are highly variable within the hot spots. Validation of the attenuation correction algorithm at C band has been performed through cross-checking with S-band radar measurements that were much less affected by attenuation. In the case of the Oklahoma storm, a comparison was made between the data collected by closely located C-band and S-band polarimetric radars.

Description: A better understanding of the interaction between the built environment and the atmosphere is required to more effectively manage urban airsheds. This paper reports an analysis of data from an atmospheric measurement campaign in Oklahoma City, Oklahoma, during the summer of 2003 that shows wind flow patterns, turbulence, and thermal effects in the downtown area. Experimental measurements within a street canyon yielded airflow patterns, stability conditions, and turbulence properties as a function of the incoming wind direction and time of the day. Air and surface temperatures at two different sites, one within the downtown urban canyon and the other in a nearby park, were measured. A study of the stability conditions within the urban canyon during the campaign indicates that dynamically stable conditions did not occur within the canyon. This provides evidence that the built environment can strongly influence the thermal characteristics in cities. Mean flow patterns close to the street level are analyzed for two different ranges of incoming wind directions and are compared with those obtained from a previous field experiment featuring idealized building configurations. This paper presents an approach allowing the estimation of wind direction in an urban canyon, given inflow conditions, that shows good agreement with wind patterns in the Oklahoma City street canyon. Turbulence statistics were calculated and normalized using different velocity scales to investigate the efficacy of the latter in specifying turbulence levels in urban canopies. The dependence of turbulence quantities on incoming wind direction and time of the day was investigated.

Description: Short-term (0–1 h) convective storm nowcasting remains a problem for operational weather forecasting, and convective storms pose a significant monetary sink for the aviation industry. Numerical weather prediction models, traditional meteorological observations, and radar are all useful for short-term convective forecasting, but all have shortcomings. Geostationary imagers, while having their own shortcomings, are valuable assets for addressing the convective initiation nowcast problem. The University of Wisconsin Convective Initiation (UWCI) nowcasting algorithm is introduced for use as an objective, satellite-based decision support tool. The UWCI algorithm computes Geostationary Operational Environmental Satellite (GOES) Imager infrared window channel box-averaged cloud-top cooling rates and creates convective initiation nowcasts based on a combination of cloud-top cooling rates and satellite-derived cloud-top type–phase trends. The UWCI approach offers advantages over existing techniques, such as increased computational efficiency (decreased runtime) and day–night independence. A validation of the UWCI algorithm relative to cloud-to-ground lightning initiation events is also presented for 23 convective afternoons and 11 convective nights over the central United States during April–June and 1 night of July during 2008 and 2009. The mean probability of detection and false-alarm ratio are 56.3% (47.0%) and 25.5% (34.8%), respectively, for regions within a Storm Prediction Center severe storm risk area (entire validation domain). The UWCI algorithm is shown to perform 1) better in regimes with storms developing in previously clear to partly cloudy skies and along sharp boundaries and 2) poorer in other regimes such as scenes covered with cirrus shields, existing convective anvils, and fast cloud motion.

Description: A new method of deriving statistical moments related to the distribution of liquid water path over partially cloudy scenes is tested using a satellite cloud climatology. The method improves the ability to reconstruct total-scene visible reflectance when compared with an approach that relies on valid liquid water path retrievals, and thus it maintains physical consistency with the primary satellite observations when deriving cloud climatologies. A global application of the new method finds a mean bias of −0.008 ± 0.017 when reconstructing total-scene reflectance from liquid water path distributions, as compared with a bias of 0.05 ± 0.047 when using a conventional approach. Application of the method to a multidecadal cloud climatology suggests that this may provide a means of identifying data artifacts that could affect long-term cloud property trends. The conservation of reflectance plus the ease of applicability to various satellite datasets makes this method a valuable tool for model validation and comparison of satellite climatologies. Gaussian and gamma functions are used to approximate the distribution of horizontal subgrid-scale liquid water path for 1° × 1° scenes, and while both functions perform well for the majority of atmospheric conditions, it is found that the Gaussian distribution generates a negative bias for cases in which visible reflectance is very high and that neither function is able to represent liquid water path well in the few cases in which the observed distribution is bi- or multimodal.

Description: Recent developments to the Local-scale Urban Meteorological Parameterization Scheme (LUMPS), a simple model able to simulate the urban energy balance, are presented. The major development is the coupling of LUMPS to the Net All-Wave Radiation Parameterization (NARP). Other enhancements include that the model now accounts for the changing availability of water at the surface, seasonal variations of active vegetation, and the anthropogenic heat flux, while maintaining the need for only commonly available meteorological observations and basic surface characteristics. The incoming component of the longwave radiation (L↓) in NARP is improved through a simple relation derived using cloud cover observations from a ceilometer collected in central London, England. The new L↓ formulation is evaluated with two independent multiyear datasets (Łódź, Poland, and Baltimore, Maryland) and compared with alternatives that include the original NARP and a simpler one using the National Climatic Data Center cloud observation database as input. The performance for the surface energy balance fluxes is assessed using a 2-yr dataset (Łódź). Results have an overall RMSE 〈 34 W m−2 for all surface energy balance fluxes over the 2-yr period when using L↓ as forcing, and RMSE 〈 43 W m−2 for all seasons in 2002 with all other options implemented to model L↓.

Description: Fine dust particles emitted from Owens (dry) Lake in California documented during the Terrain-Induced Rotor Experiment (T-REX) of 2006 have been examined using surface observations and a mesoscale aerosol model. Air quality stations around Owens (dry) Lake observed dramatic temporal and spatial variations of surface winds and dust particulate concentration. The hourly particulate concentration averaged over a 2-month period exhibits a strong diurnal variation with a primary maximum in the afternoon, coincident with a wind speed maximum. The strongest dust event documented during the 2-month-long period, with maximum hourly and daily average particulate concentrations of 7000 and 1000 μg m−3, respectively, is further examined using output from a high-resolution mesoscale aerosol model simulation. In the morning, with the valley air decoupled from the prevailing westerlies (i.e., cross valley) above the mountaintop, fine particulates are blown off the dry lake bed by moderate up-valley winds and transported along the valley toward northwest. The simulated strong westerlies reach the western part of the valley in the afternoon and more fine dust is scoured off Owens (dry) Lake than in the morning. Assisted by strong turbulence and wave-induced vertical motion in the valley, the westerlies can transport a substantial fraction of the particulate mass across the Inyo Mountains into Death Valley National Park.

Description: This study quantifies the potential impacts on ship-defense high-energy-laser (HEL) performance due to atmospheric effects in the marine boundary layer driven by recent observations and analysis of worldwide sea surface temperatures (SSTs). The atmospheric effects are defined using the worldwide probabilistic climatic database available in the High Energy Laser End-to-End Operational Simulation (HELEEOS) model, which includes an SST database for the period 1854–1997. A more recent worldwide sea surface temperature database was provided by the Naval Postgraduate School for the period 1990–2008. Mean differences and trends between the two SST databases are used to deduce possible climate change impacts on simulated maritime HEL engagements. The anticipated effects on HEL propagation performance are assessed at an operating wavelength of 1.0642 μm across the world’s oceans and mapped onto a 1° × 1° grid. The scenario evaluated is near surface and nearly horizontal over a range of 5000 m in which anticipated clear-air maritime aerosols occur. Summer and winter scenarios are considered. In addition to realistic vertical profiles of molecular and aerosol absorption and scattering, correlated optical turbulence profiles in probabilistic (percentile) format are used.

Description: Presented are quantitative estimates of specific attenuation and specific differential attenuation of 5-cm-wavelength radiation (C band) obtained by comparison with measurements at 10-cm wavelength (S band), which are much less affected by attenuation. The data originated from two almost-collocated radars in central Oklahoma. To avoid biases in estimates, the slopes with respect to range of differences in reflectivities and differential reflectivities are assumed to represent the specific attenuations. Observations on a day with no reports of hail on the ground and on a day with large hail are contrasted. A simple one-dimensional model of melting hail is used to qualify these observations. Examples of volumetric fields of the polarimetric variables obtained at the two wavelengths are presented to illustrate that much can be learned about size, orientation, and phase of hydrometeors over volumes that play a role in precipitation formation.