ALBERT

Description: Abstract Dust devils are convective vortices with a vertical axis of rotation made visible by lifted soil particles. Currently, there is great uncertainty about the extent to which dust devils contribute to the atmospheric aerosol input and thereby influence Earth's radiation budget. Past efforts to quantify the aerosol transport and study their formation, maintenance, and statistics using large‐eddy simulation (LES) have been of limited success. Therefore, some important features of dust devil‐like vortices simulated with LES still do not compare well with those of observed ones. One major difference is the simulated value of the core pressure drop, which is almost 1 order of magnitude smaller compared to the observed range of 250 to 450 Pa. However, most of the existing numerical simulations are based on highly idealized setups and coarse grid spacings. In this study, we investigate the effects of various factors on the simulated vortex strength with high‐resolution LES. For the fist time, we are able to reproduce observed core pressures by using a high spatial resolution of 2 m, a model setup with moderate background wind and a spatially heterogeneous surface heat flux. It is found that vortices mainly appear at the lines of horizontal flow convergence above the centers of the strongly heated patches, which is in contrast to some older observations in which vortices seemed to be created along the patch edges.

Description: Abstract Location information from super‐pressure balloons flown by Project Loon provide an unprecedented opportunity to analyze wind fields in the mid‐latitude stratosphere. Horizontal velocity spectra from the balloons' quasi‐intrinsic frame of reference show clear evidence of a persistent peak in the intrinsic wind spectrum around the inertial frequency. In the Southern Hemisphere mid‐latitudes, peak‐to‐peak amplitudes of horizontal velocity perturbations (on the order of 20 ms‐1) are larger than those seen in previous super‐pressure balloon campaigns in polar regions and similar to those observed in vertical soundings in the mid‐latitudes. A rotary spectral analysis shows that near‐circular anti‐cyclonic rotation of horizontal wind perturbations around the inertial frequency dominate at most times and locations. The strongest anti‐cyclonic rotation is more common in balloon flight segments with weak zonal winds and during the austral summer. Flight segments with strong eastward zonal velocities during austral winter and spring are more likely to have mixed cyclonic and anti‐cyclonic power around the inertial frequency. These results confirm previous model and radiosonde observations of the peak in horizontal kinetic energy at the inertial frequency, and demonstrate they are associated with increased anti‐cyclonic wave power indicative of near‐inertial oscillations or inertia‐gravity waves. Flight segments with mixed cyclonic and anti‐cyclonic power around the inertial frequency display a continuum of wave power from planetary to gravity‐wave scales. These results help explain the divergence of actual and modelled balloon trajectories in previous studies and provide a baseline against which reanalysis or meteorological model realizations of the intrinsic velocity field can be assessed.

Description: Abstract From 17–22 August 2017 simultaneous enhancements of ammonia (NH3), carbon monoxide (CO), hydrogen cyanide (HCN), and ethane (C2H6) were detected from ground‐based solar absorption Fourier transform infrared (FTIR) spectroscopic measurements at two high‐Arctic sites: Eureka (80.05°N, 86.42°W) Nunavut, Canada, and Thule (76.53°N, 68.74°W), Greenland. These enhancements were attributed to wildfires in British Columbia and the Northwest Territories of Canada using FLEXPART back‐trajectories and fire locations from Moderate Resolution Imaging Spectroradiometer (MODIS) and found to be the greatest observed enhancements in more than a decade of measurements at Eureka (2006–2017) and Thule (1999–2017). Observations of gas‐phase NH3 from these wildfires illustrate that boreal wildfires may be a considerable episodic source of NH3 in the summertime high Arctic. Comparisons of GEOS‐Chem model simulations using the Global Fire Assimilation System (GFASv1.2) biomass burning emissions to FTIR measurements and Infrared Atmospheric Sounding Interferometer (IASI) measurements showed that the transport of wildfire emissions to the Arctic was underestimated in GEOS‐Chem. However, GEOS‐Chem simulations showed that these wildfires contributed to surface layer NH3 and NH enhancements of 0.01–0.11 ppbv and 0.05–1.07 ppbv, respectively, over the Canadian Archipelago from 15–23 August 2017.

Description: Abstract Accurate estimates of NOx and SO2 emissions are important for air quality modeling and management. To incorporate chemical interactions of the two species in emission estimates, we develop a joint hybrid inversion framework to estimate their emissions in China and India (2005–2012). Pseudo observation tests and posterior evaluation with surface measurements demonstrate that joint assimilation of SO2 and NO2 can provide more accurate constraints on emissions than single‐species inversions. This occurs through synergistic change of O3 and OH concentrations, particularly in conditions where satellite retrievals of the species being optimized have large uncertainties. The percentage changes of joint posterior emissions from the single‐species posterior emissions go up to 242% at grid scales, although the national average of monthly emissions, seasonality, and interannual variations are similar. In China and India, the annual budget of joint posterior SO2 emissions is lower, but joint NOx posterior emissions are higher, because NOx emissions increase to increase SO2 concentration and better match Ozone Monitoring Instrument SO2 observations in high‐NOx regions. Joint SO2 posterior emissions decrease by 16.5% from 2008 to 2012, while NOx posterior emissions increase by 24.9% from 2005 to 2011 in China—trends which are consistent with the MEIC inventory. Joint NOx and SO2 posterior emissions in India increase by 15.9% and 19.2% from 2005 to 2012, smaller than the 59.9% and 76.2% growth rate using anthropogenic emissions from EDGARv4.3.2. This work shows the benefit and limitation of joint assimilation in emission estimates and provides an efficient framework to perform the inversion.

Description: Abstract By using 90 radiosonde stations with high vertical resolution data during the period 1998–2011, the latitudinal variation of the tropopause inversion layer (TIL) in different seasons and the interactions with the inertial gravity wave (IGW) activities in the region covering the Northern Hemispheric latitudes from 5° to 75° are studied. For the midlatitudes, the TIL features show obviously seasonal variations. In the Arctic region, TIL is strong and thick. The averaged Arctic TIL intensity peaks in summer. The intense interaction between the TIL and IGW is found in the region of 5°N to 75°N. The TIL could inhibit the upward propagation of IGWs from ~2 km below the tropopause in a larger region (40–75°N). It is found that for the middle‐latitude regions, the enhanced wind shear layer just above the tropopause could lead to instability and finally result in IGW breaking and intensive turbulence, which then leads to strong wave energy dissipation and a downward heat flux. The IGW‐induced cooling around the tropopause, which resulted from the downward heat flux, then makes a colder and sharper tropopause and finally form the TIL. The IGW‐associated strong downward heat flux is also found around the Arctic tropopause. However, there is no corresponding wind shear enhancement above the tropopause. This indicates that this strong heat flux may result from some other processes and then form the strong TIL in the Arctic.

Description: Abstract Proxy system models (PSMs) are an important bridge between climate simulations and climate records prior to the period where instrumental observations are available. PSMs help to interpret what proxies show and how they record climate. Although previous studies have evaluated PSMs for individual sites, their systematic evaluation on a global scale has not yet been conducted. This study evaluated the performance of PSMs for stable water isotopes in ice cores, corals, and tree‐ring cellulose for the period 1950–2007. Spatial distributions of the mean state were well simulated for all proxy types, albeit with a bias for tree‐ring cellulose. Interannual variability was well simulated for corals and tree‐ring cellulose. These results indicate that the models represent key mechanisms for the proxies. In contrast, the reproducibility of interannual variability in ice cores was markedly lower than that for the other proxies. Although the reproducibility was limited by the atmospheric forcing used to drive the model, the results suggest that the PSM may be missing post‐depositional processes, such as sublimation for ice cores on the interannual timescale.

Description: Abstract Three years of nighttime Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observation data was used in synergy with CloudSat measurements to quantify how strongly aerosol type and aerosol load affect the cloud phase in low‐level clouds over the Arctic. Supercooled liquid layers were present in the majority of observed low‐level clouds (0.75 ≤ z ≤ 3.5 km) between −10 and −25 °C. Furthermore, based on the subset (6%) of data with high quality assurance for aerosol typing, ice formation is more common in the presence of dust or continental aerosols as opposed to marine or elevated smoke aerosols. With the first aerosol group, glaciated clouds were found at cloud top temperatures of 2 to 4 °C warmer than with the latter aerosol types. Further association of the aerosol concentration with the cloud phase showed that the aerosol concentration outweighs the aerosol type effect. Depending on the aerosol load, the temperature at which a cloud completely glaciates can vary by up to 6–10 °C. However, this behavior was most pronounced in stable atmospheric conditions and absent over open ocean with lower tropospheric stability values and probably less stratified clouds. Finally, more mixed‐phase clouds were associated with high aerosol load, suggesting that mixed‐phase clouds have an extended lifetime in the Arctic under high cloud condensation nuclei concentrations. This implies that in a pristine environment, with few or no local aerosol sources, and under the investigated conditions the amount of aerosol particles affects the cloud phase more than the aerosol type does.

Description: Abstract Aerosol pH is a useful diagnostic of aerosol chemistry for formation of secondary aerosol and has been hypothesized to be a key factor in specific chemical reaction routes producing sulfate and nitrate (Yue et al., 2009; Zhang et al., 2012; Hu et al., 2014). In this study, we measured hourly concentrations of water soluble ions (WS‐ions) in PM2.5, along with gaseous pollutants in Tianjin, China, from 4th January to 31st January 2015. The following source contributions to WS ions were estimated by PMF (Positive Matrix Factorization): secondary sulfate (13%), secondary nitrate (44%), coal (14%), vehicle (16%), and dust (13%). ISORROPIA‐II was used to investigate the complex relationships among aerosol pH, ammonia, and secondary aerosol formation. The estimated hourly aerosol pH varied from ‐0.3 to 7.7, with an average of 4.9 (±0.78); the median value was 4.89, and the interquartile range (IQR) was 0.72. During less polluted conditions, aerosol pH ranged from less than 0 to about 7; during heavily polluted conditions, pH was close to 5 (3.9‐7.9) despite large amounts of sulfate. Sufficient ammonia/ammonium were present to balance high sulfate and nitrate formation. NH4+/NH3 (g) helped stabilize pH while nonvolatile cations contributed less to decreasing aerosol acidity. High acidy (pH〈3), light pollution (Total water soluble ions (TWI)〈30 μg m‐3), and low water content (less than 5μg m‐3) were more correlated with higher rates of sulfate formation than nitrate formation in the winter.

Description: Abstract SO2 column densities from OMI provide important information on emission trends and missing sources, but there are discrepancies between different retrieval products. We employ three OMI SO2 retrieval products (NASA standard (SP), NASA prototype, and BIRA) to study the magnitude and trend of SO2 emissions. SO2 column densities from these retrievals are most consistent when viewing angles and solar zenith angles are small, suggesting more robust emission estimates in summer and at low latitudes. We then apply a hybrid 4D‐Var/mass balance emission inversion to derive monthly SO2 emissions from the NASA SP and BIRA products. Compared to HTAPv2 emissions in 2010, both posterior emission estimates are lower in US, India and Southeast China, but show different changes of emissions in North China Plain. The discrepancies between monthly NASA and BIRA posterior emissions in 2010 are less than or equal to 17% in China and 34% in India. SO2 emissions increase from 2005 to 2016 by 35% (NASA) ‐ 48% (BIRA) in India, but decrease in China by 23% (NASA) ‐ 33% (BIRA) since 2008. Compared to in‐situ measurements, the posterior GEOS‐Chem surface SO2 concentrations have reduced NMB in China, the US, and India but not in South Korea in 2010. BIRA posteriors have better consistency with the annual growth rate of surface SO2 measurement in China and spatial variability of SO2 concentration in China, South Korea and India, whereas NASA SP posteriors have better seasonality. These evaluations demonstrate the capability to recover SO2 emissions using OMI observations.

Description: Abstract Deep convective clouds similar to those arising in the TC eyewall are simulated using a parcel model and 2D slab symmetric cloud model with spectral bin microphysics (the Hebrew University Cloud Model, HUCM). The size distribution of sea spray particles (SSP) at cloud base is calculated using the Lagrangian‐Eulerian bin‐microphysics model (LEM). The model describes the SSP production, advection and formation of the size distribution of SSP in the hurricane atmospheric boundary layer at different strong wind speeds. The SSP distributions calculated by the LEM are used in the parcel model and the HUCM to investigate the microphysical and dynamical effects of SSP on clouds. The SSP ascending in cloud updrafts dramatically increase the number concentration of cloud drops within a wide range of drop sizes. As a result, sea spray creates clouds with unique property combinations of both maritime and continental types. These clouds have droplet size distributions characterized by a high drop concentration and a low effective radius, as in continental clouds. At the same time, the presence of SSP of a few hundred microns in radii triggers intense rain just above the cloud base, which is typical of extreme maritime clouds. In the presence of large sea spray drops, the smallest cloud condensational nuclei, including the smallest SSP, are activated, giving rise to the permanent in‐cloud nucleation of small droplets which produce a high concentration of small ice crystals above the level of homogeneous freezing. We showed that the SSP substantially increased the maximum vertical velocity, cloud water content and mass contents of ice particles. The results are compared with available observed data.

Description: Abstract Terrestrial gamma ray flashes (TGFs) are very short bursts of gamma radiation associated to thunderstorm activity and are the manifestation of the highest‐energy natural particle acceleration phenomena occurring on Earth. Photon energies up to several tens of megaelectronvolts are expected, but the actual upper limit and high‐energy spectral shape are still open questions. Results published in 2011 by the AGILE team proposed a high‐energy component in TGF spectra extended up to ≈100 MeV, which is difficult to reconcile with the predictions from the Relativistic Runaway Electron Avalanche (RREA) mechanism at the basis of many TGF production models. Here we present a new set of TGFs detected by the AGILE satellite and associated to lightning measurements capable to solve this controversy. Detailed end‐to‐end Monte Carlo simulations and an improved understanding of the instrument performance under high‐flux conditions show that it is possible to explain the observed high‐energy counts by a standard RREA spectrum at the source, provided that the TGF is sufficiently bright and short. We investigate the possibility that single high‐energy counts may be the signature of a fine‐pulsed time structure of TGFs on time scales ≈4 μs, but we find no clear evidence for this. The presented data set and modeling results allow also for explaining the observed TGF distribution in the (Fluence × duration) parameter space and suggest that the AGILE TGF detection rate can almost be doubled.

Description: Abstract The processes that control the isotopic composition of precipitation in the mid‐latitudes are complicated, but can provide valuable insights into precipitation‐generating processes and are critical for interpreting stable isotope‐based paleoclimate records. In this study, we investigated the controls on changes in the isotopic composition of rainwater in central Texas using a combination of existing monthly stable isotope data from the global network of isotopes in precipitation (GNIP) and 20 months of event‐based rainwater collection from Austin, TX. We find that the strongest control on the isotopic composition of precipitation is the varying proportion of convective and stratiform rainfall, with other factors such as precipitation amount, temperature, storm track playing a secondary role. Isotopic values are generally lower in the cold season than the warm season precipitation because cold season precipitation is predominantly stratiform often associated with a northerly storm track. However, the majority of the precipitation in the south‐central United States (US) occurs during the warm season in association with mesoscale convective systems (MCS) that are fed with moisture by the southerly winds. MCS are characterized by a combination of a leading edge of organized deep convection and trailing stratiform precipitation. Stronger MCS tend to contain higher proportions of stratiform rainfall and as a result, have more isotopically depleted values. Therefore, changes in the stable isotopic composition of rainfall may be interpreted as reflecting changes in the intensity of MCS.

Description: Abstract Atmospheric nitrate (NO3− = particulate NO3− + gas‐phase nitric acid [HNO3]) and sulfate (SO42−) are key molecules that play important roles in numerous atmospheric processes. Here, the seasonal cycles of NO3− and total suspended particulate sulfate (SO42−(TSP)) were evaluated at the South Pole from aerosol samples collected weekly for approximately 10 months (26 January to 25 October) in 2002 and analyzed for their concentration and isotopic compositions. Aerosol NO3− was largely affected by snowpack emissions in which [NO3−] and δ15N(NO3−) were highest (49.3 ± 21.4 ng/m3, n = 8) and lowest (−47.0 ± 11.7‰, n = 5), respectively, during periods of sunlight in the interior of Antarctica. The seasonal cycle of Δ17O(NO3−) reflected tropospheric chemistry year‐round with lower values observed during sunlight periods and higher values observed during dark periods, reflecting shifts from HOx‐ to O3‐dominated oxidation chemistry. SO42−(TSP) concentrations were highest during austral summer and fall (86.7 ± 73.7 ng/m3, n = 18) and are indicated to be derived from dimethyl sulfide (DMS) emissions, as δ34S(SO42−)(TSP) values (18.5 ± 1.0‰, n = 10) were similar to literature δ34S(DMS) values. The seasonal cycle of Δ17O(SO42−)(TSP) exhibited minima during austral summer (0.9 ± 0.1‰, n = 5) and maxima during austral fall (1.3 ± 0.3‰, n = 6) and austral spring (1.6 ± 0.1‰, n = 5), indicating a shift from HOx‐ to O3‐dominated chemistry in the atmospheric derived SO42− component. Overall, the budgets of NO3− and SO42−(TSP) at the South Pole were complex functions of transport, localized chemistry, biological activity, and meteorological conditions, and these results will be important for interpretations of oxyanions in ice core records in the interior of Antarctica.

Description: Abstract Understanding global soil moisture‐air temperature (θ‐Ta) coupling is needed to improve the representation of land‐atmosphere interactions in Earth system models. Most studies on θ‐Ta coupling have focused on hot extremes, where precipitation‐related indices and model‐derived soil moisture products are commonly used. In this study, global θ‐Ta coupling is examined based on monthly air temperature anomalies and the Gravity Recovery and Climate Experiment (GRACE) terrestrial water storage (TWS). A discrete wavelet decomposition is used to partition the TWS into different components. The results show that TWS is useful in revealing the spatial patterns of θ‐Ta coupling. Decomposed GRACE TWS shows improved skill compared to raw TWS in explaining temporal variability of monthly air temperature, which likely reflects different roles of soil moisture at different depths in the θ‐Ta coupling. The explanatory power improves further by using a combination of decomposed GRACE TWS and precipitation. Such improvement is observed particularly in places where vegetation tends to have a deeper rooting system, such as eastern region of South America, the southern tip of Africa, and north of the Tropic of Capricorn in Australia. The occurrence of θ‐Ta coupling is mainly constrained by the coupling of root zone moisture and land surface temperature. In addition to deeper rooting systems, clear wet and dry season alternation is another favorable factor for developing significant monthly θ‐Ta coupling.

Description: Abstract This study investigates heat wave variability over Korea during 1979‐2017. It is found that most of heat waves in Korea can be classified into two distinct types based on the spatial patterns of atmospheric circulation anomalies: the zonal wave (Z‐wave) type and the meridional wave (M‐wave) type. The Z‐wave type is accompanied by large‐scale atmospheric waves across the Eurasian continent, while the M‐wave type is associated with convective activities over the subtropical western North Pacific. The Z‐wave type occurs when the high‐pressure node of eastward propagating wave located around Korea and it seems that the associated wave energy could originate from North Atlantic Ocean. The M‐wave type, on the other hand, is driven by northward propagating wave train from subtropical western North Pacific to East Asia, which is triggered by anomalous convective activity over the subtropical western North Pacific. By analyzing thermodynamical as well as dynamical variables, detailed descriptions on the physical characteristics of two types of heat wave are presented in this study along with the possible implications for summer climate variability over Korea.

Description: Abstract In this study, we estimate atmospheric turbulence in the free atmosphere in terms of the Thorpe scale (LT) and eddy dissipation rate (ε) using U.S. high vertical‐resolution radiosonde data over 4 years (September 2012 to August 2016) at 68 operational stations. In addition, same calculations are conducted for 12 years (October 2005 to September 2017) at four stations among the 68 stations. These high vertical‐resolution radiosonde data have a vertical resolution of approximately 5 m and extend to an altitude of approximately 33 km, and thus, turbulence can be retrieved in the entire troposphere and lower stratosphere. There are thicker and stronger turbulent layers in the troposphere than in the stratosphere, with mean ε values of 1.84 × 10−4 and 1.37 × 10−4 m2/s3 in the troposphere and stratosphere, respectively. The vertical structure of ε exhibits strong seasonal variations, especially in the upper troposphere and lower stratosphere, with the largest ε values in summer and the smallest in winter. In the horizontal distribution of ε, large ε is seen mainly above the mountainous region in the troposphere, but this pattern is not seen in the stratosphere. Although ε is estimated by the square of LT multiplied by the cube of the Brunt‐Väisälä frequency (N), the regions of large ε are matched with large LT regions where N is relatively small. For the time series of ε near the tropopause for 12 years at four stations, an annual variation is prominent at all stations without significant interannual variations. There is, however, a slightly increasing trend of ε at two stations.

Description: Abstract The second Radiative Heating in Underexplored Bands Campaign (RHUBC‐II) was conducted in 2009 by the U.S. Department of Energy Atmospheric Radiation Measurement program to improve water vapor spectroscopy in the far‐infrared spectral region. RHUBC‐II was located in an extremely dry region of Chile to ensure very low opacities in this spectral region. Spectrally resolved measurements by a far‐infrared spectrometer and a submillimeter interferometer from RHUBC‐II are compared with line‐by‐line radiative transfer model calculations. Water vapor amounts and temperatures used in the calculations come from collocated radiosondes, with extensive adjustments to correct for issues due to the campaign's dry conditions and mountainous terrain. A reanalysis is also performed of far‐infrared measurements taken at the Atmospheric Radiation Measurement North Slope of Alaska site before and during the first RHUBC campaign. These analyses determine that differences between the measurements and model calculations using existing spectroscopic parameters are significant in the far‐infrared and submillimeter regions, leading to the derivation of improved water vapor continuum absorption coefficients and air‐broadened widths of 74 water vapor lines. The foreign continuum is increased by more than 50% in part of the far‐infrared and the widths of more than 20 lines are changed by more than 10%. The uncertainty in the foreign continuum coefficients is estimated as greater than 20% in some spectral regions, primarily a consequence of the uncertainty in the specification of water vapor. The improved far‐infrared spectroscopic parameters have a notable impact on calculated spectral radiances and a modest impact on broadband radiative fluxes and heating rates.

Description: Abstract Loess is an important dust component of airborne particles in Northwestern China. Knowledge of the chemical composition, mixing state, and processing of loess particles in urban plumes is still limited. Urban loess particles were characterized using a single‐particle aerosol mass spectrometer. To understand sources and processing of loess particles, source samples from the road, urban background, soil, construction, and biomass burning ash were collected in the urban areas and characterized. Loess particles were determined as a kind of calcium‐silicate‐rich ones, which were internally mixed with calcium, silicates, potassium, elemental carbon, organics, ammonium, sulfate, and nitrate. Road and soil were major sources of loess particles. Among the aged loess particles, the average peak areas of taken‐up nitrate and sulfate were comparable to that of (Fe+Ca+Al). Diurnal uptake profiles of chloride, sulfate, oxalate, and nitrate on loess particles were analyzed. The nocturnal elevation of chloride occurred significantly due to the uptake of HCl (g). Nighttime nitrate formation occurred prevalently under high relative humidity conditions via the heterogeneous hydrolysis of N2O5. The nighttime enrichment of oxalate, which is a marker for aqueous‐formatted secondary organic aerosol, was also found. Besides the nighttime chemistry, the daytime photochemical activities were also a drive for the elevations of sulfate, nitrate, and ammonium. Conclusively, the processing of loess particles in polluted urban plumes significantly altered their chemical composition and mixing state.

Description: Abstract As one of the typical midlatitude synoptic‐scale disturbances, extra‐tropical cyclones (ECs) can exert significant impacts on the atmospheric general circulation through its interaction with the time‐mean flow. Under the background of global warming, Eurasian continent exhibits evident non‐uniform warming, which has the potential to alter the atmospheric baroclinicity by changing the meridional temperature gradient and further affect the ECs activity. In this study, we investigated the possible connection between the land surface thermal anomaly over Eurasia and the summer ECs activity over East Asia together with relevant mechanisms. We found that the land surface warming (cooling) near 50°N of East Asia is associated with anomalous weak (strong) summer ECs activity over East Asia. Warm (cool) land surface usually reduces (increases) the meridional temperature gradient and further the atmospheric baroclinicity in the key area of cyclone activity, resulting in low (high) frequency of the extratropical cyclogenesis and weakened (intensified) ECs activity. The land surface warming (cooling) can also depress (benefit) the associated baroclinic conversion between the time‐mean effective potential energy and eddy effective potential energy, resulting in decrease (increase) in the eddy kinetic energy. As a result, the energy obtained by the synoptic‐scale eddy from the time‐mean flow has been reduced (increased), which favors (hampers) the extra‐tropical cyclogenesis, causing weak (strong) cyclone activity in the middle latitude of East Asia.

Description: Abstract This paper reports a study to understand the radio spectrum of thunderstorm narrow bipolar events (NBEs) or compact intracloud discharges, which are powerful sources of high frequency (HF) and very high frequency (VHF) electromagnetic radiation. The radio spectra from 10 kHz to about 100 MHz are obtained for three NBEs, including one caused by fast positive breakdown and two by fast negative breakdown. The results indicate that the two polarities of fast breakdown have similar spectra, with a relatively flat spectrum in the HF and VHF band. The ratio of energy spectral densities in the very low frequency and high frequency bands is (0.9‐5)× 105. We develop a statistical modeling approach to investigate if a system of streamers can explain the main features of fast breakdown. Assuming that the current moment peak and charge moment of individual streamers vary in the ranges of 5‐10 A‐m and 5‐20 μC‐m, respectively, the modeling results indicate that a system of 107‐108 streamers can reproduce the current moment, charge transfer and radio spectrum of fast breakdown. The rapid current variation on a timescale of nanoseconds required for fast breakdown to produce strong HF/VHF emissions is provided by exponentially accelerating and expanding streamers. Our study therefore supports the hypothesis that fast breakdown is a system of streamers. Finally, suggestions are given regarding future streamer simulations and NBE measurements in order to further develop our understanding of NBEs and lightning initiation.

Description: Abstract The long‐term trend in dust loading over East Asia remains under debate and is dependent on the study period chosen. In this study, the long‐term trends in springtime dust over East Asia and the North Pacific Ocean (EA&NPO) during 1980–2017 were examined based on the Modern‐Era Retrospective Analysis for Research and Applications version 2 (MERRA‐2) reanalysis. Results showed that there was a spatial gradient in dust aerosol loadings, with decreases from western China eastward towards the NPO. This pattern was corroborated by Cloud‐Aerosol Lidar with Orthogonal Polarization (CALIOP) observations. Furthermore, the empirical orthogonal function (EOF) method was used to reveal the leading modes of springtime dust aerosol optical depth (AOD) over EA&NPO. An abrupt shift occurred in the dust AOD trend in 2010 for the EOF1 mode. The dust AOD increased at a rate of approximately 2×10‐4 per year during 1999–2009, and then decreased more sharply (around 5×10‐4 per year) afterwards. This trend reversal of dust AOD was closely associated with a decrease in 10‐m wind velocity, which induces reduced dust emission. Compared with 10‐m wind, the soil moisture is less correlated with the trend reversal in dust AOD. Additionally, the trends of dry (wet) deposition were closely associated with the trends of the dust AOD, especially for the period 2010–2016. Overall, our findings add new insights to the long‐term nonlinear variability of dust.

Description: Abstract The Monin‐Obukhov similarity theory (MOST) is widely used for the surface turbulence flux‐gradient relations in modeling and data analysis. Here we quantify multi‐scale turbulence processes by applying our newly developed analysis technique to large‐eddy simulation data, and find that in the unstable surface layer, large convective eddies (with the scaling of boundary layer depth) and local free convection exist in addition to small eddies. An empirical MOST function (considering the last two processes only) is found to underestimate the surface friction velocity and heat flux both by about 30%. Much better results can be obtained using a function that explicitly considers all three processes. Generally, the non‐dimensional wind shear exhibits larger scatter and deviates more from the MOST than the temperature gradient. Based on these results, we propose the revised Sorbjan (1986) function (with coefficients determined from this study) for wind shear and MOST function for temperature gradient, for estimating surface fluxes in the unstable surface layer. The three‐dimensional multi‐scale analysis method we develop in this study is of general nature and can be of interest for problems of three‐dimensional multi‐scale process description in other disciplines.

Description: Abstract This paper investigates the relationship between long‐term trends (1980‐2017) in intensity and wind evolution for tropical cyclones (TCs) within the Western Tropical Atlantic (WTA) and Central/Eastern Tropical Atlantic (CETA) sub‐basins. Long‐term TC trends in intensity, intensification time, and wind variability for the CETA were generally more significant than, and in some cases opposite to those for the WTA. Both the TC intensity levels, as measured by the power dissipation index (PDI) normalized by storm hours and proportion of rapid intensification (RI) intervals (defined as a 12‐hour wind speed increase of 20 knots or more), exhibits no long‐term trends in either sub‐basin. A TC wind variability index (WVI) calculated over 72‐h intervals of the TC lifecycle decreases for the WTA over the decades, while the CETA has the 72‐h intervals with the greatest wind speed fluctuations. The average period of intensification before the peak in TC intensity increases ~0.97 h per year for the CETA. TC maximum intensity exhibits no trend, suggesting that TCs in the tropical North Atlantic have a trend favoring a longer intensification period to reach their lifetime maximum intensity. A correlation analysis suggests that warmer sea surface temperatures (SST) and greater moisture favors longer intensification and greater WVI. In contrast, greater 850‐200 hPa vertical wind shear is associated with shorter intensification periods and less WVI.

Description: ABSTRACT Decreases in stratospheric NOx associated with enhanced aerosol have been observed after large volcanic eruptions, e.g., after the eruption of Mt. Pinatubo in 1991. While the 1991 Mt. Pinatubo eruption was the last large explosive eruption, recent studies have shed light on the impacts of moderate‐sized eruptions since the year 2000 on the global stratospheric aerosol budget. We use an ensemble of simulations from a coupled climate‐chemistry model to quantify and analyze changes in NO and NO2 (NOx), N2O5, HNO3, ClO, and ClONO2 during periods of increased stratospheric volcanic aerosol concentrations since 2000. By using an ensemble approach, we are able to distinguish forced responses from internal variability. We also compare the model ensemble results to satellite measurements of these changes in atmospheric composition, including measurements from the Optical Spectrograph and Infrared Imaging Spectrometer on the Odin satellite and the Aura Microwave Limb Sounder. We find decreases in stratospheric NOx concentrations up to 20 hPa, consistent with increases in stratospheric HNO3 concentrations. The HNO3 perturbations also extend higher, up to 5 hPa associated with periods of increased volcanic aerosol concentrations in both model simulations and observations, though correlations with volcanic aerosol are considerably higher in the model simulations. The model simulates increases in ClO at altitudes and magnitudes similar to the NOx reductions, but this response is below the detectable limit in the available observations (100 pptv). We also demonstrate the value of accounting for transport‐related anomalies of atmospheric trace gases by regression onto N2O anomalies.

Description: Abstract Information regarding dust concentrations and size distributions is very important for determining air quality and aerosol–radiation–cloud interactions. Only by using a correct erosion database can the sectional dust emission scheme resolve detailed size distributions and determine where and how dust will be emitted. In this paper, the bias and reasons for dust emission in China Meteorological Administration Unified Atmospheric Chemistry Environment – Dust (CUACE/Dust), an operational dust forecasting model, are analyzed using a heavy sand and dust storm (SDS) episode. We used 18 years of routine SDS phenomena recorded at meteorological stations to retrieve and update the desertification details in the MBA sectional dust emission scheme adopted in CUACE/Dust. New desertification details include decreased erodibility in the area adjacent to Uzbekistan, Turkmenistan, and southern Kazakhstan, where Kyzylkum, Karakum, and Aralkum are located in Central Asia, and in the Chinese deserts of Onqin Daga, Mu Us, and Gurbantungut. New desertification also results in increased erodibility in northern Mongolia. Comparisons show that the new desertification database decreases overestimation of dust emission in Central Asia, including western Mongolia. It improves the underestimation of dust emission in northern Mongolia and the Gobi Desert in southeast Mongolia, and the Taklimakan Desert in China. Consequently, it corrects the overestimated dust cloud in the source area and in areas impacted by dust transportation. The timing, quantitative mass concentrations, and dust size distributions determined here are all more reasonable and rational than those of the original case.

Description: Abstract Groundwater (GW) and recharge as the main drivers of the water budget are challenging to quantify, due to the complexity of hydrological processes and limited observations. Understanding these processes in relation to climate is crucial for evaluating future water availability of Tibetan Plateau (TP). By computing storage changes in Gravity Recovery and Climate Experiment (GRACE) Terrestrial Water Storage (TWS) and Global Land Surface Data Assimilation System (GLDAS) land surface state variables and water balance approach, we calculated GW storage changes and recharges. In the Qaidam Basin (northern TP), TWS from the GRACE revealed a significant increasing trend of 25.5 mm/year during 2002‐2012. However, an obviously turning point was found around 2012 and TWS revealed a significant decreasing rate of 37.9 mm/year during 2013‐2016. Similarly, GW (recharge) had a significant increasing trend of 21.2 (4.5) mm/year before 2012 and a decreasing rate of 32.1 (10.9) mm/year after 2012. Domain‐averaged difference (P‐ET) between precipitation (P) and evapotranspiration (ET) also exhibited an increasing trend of 4.4 mm/year during 2002‐2012 and a decreasing rate of 9.0 mm/year during 2013‐2016. Precipitation followed dissimilar pattern with an increasing rate of 5.3 mm/year during 2002‐2012 while no significant trend during 2013‐2016. However, ET had a consistent increasing trend over the basin during the past 15 years (0.9 mm/year before 2012 and 9.0 mm/year thereafter). This study concluded that GW amount and distribution is mainly controlled by precipitation and ET. Decrease in precipitation at high elevations and increase in ET may impact future groundwater availability in this region.

Description: Abstract In this work we investigate interannual variations in lower stratospheric ozone from 1984 to 2016 based on a satellite‐derived data set and simulations from a chemical transport model. An empirical orthogonal function (EOF) analysis of ozone variations between 2000 and 2016 indicates that the first, second, and third EOF modes are related to the quasi‐biennial oscillation (QBO), canonical El Niño–Southern Oscillation (ENSO), and ENSO Modoki events, respectively; these three leading EOFs capture nearly 80% of the variance. However, for the period 1984–2000, the first, second, and third modes are related to the QBO, ENSO Modoki, and canonical ENSO events, respectively. The explained variance of the second mode in relation to ENSO Modoki is nearly twice that of the third mode for canonical ENSO. Since the frequency of ENSO Modoki events was higher from 1984 to 2000 than after 2000, the Brewer‐Dobson circulation anomalies related to ENSO Modoki were stronger during 1984–2000, which caused ENSO Modoki events to have a greater effect on lower stratospheric ozone before 2000 than after. Ozone anomalies associated with QBO, ENSO Modoki, and canonical ENSO events are largely caused by dynamic processes, and the effect of chemical processes on ozone anomalies is opposite to that of dynamic processes. Ozone anomalies related to dynamic processes are 3–4 times greater than those related to chemical processes.

Description: Abstract Oxides of nitrogen are critical trace gases in the troposphere and are precursors for nitrate aerosol and ozone, which is an important pollutant and greenhouse gas. Lightning is the major source of NOx (NO + NO2) in the mid‐ to upper troposphere. We estimate the production efficiency (PE) of lightning NOx (LNOx) using satellite data from the Ozone Monitoring Instrument (OMI) and the ground‐based World Wide Lightning Location Network (WWLLN) in three northern midlatitude, primarily continental regions that include much of North America, Europe and East Asia. Data were obtained over 5 boreal summers, 2007 – 2011 and comprise the largest number of midlatitude convective events to date for estimating the LNOx PE with satellite NO2 and ground‐based lightning measurements. In contrast to some previous studies, the algorithm assumes no minimum flash‐rate threshold and estimates freshly produced LNOx by subtracting a background of aged NOx estimated from the OMI dataset itself. We infer an average value of 180 ± 100 moles LNOx produced per lightning flash. We also show evidence of a dependence of PE on lightning flash rate and find an approximate empirical power function relating moles LNOx to flashes. PE decreases by an order of magnitude for a 2‐order of magnitude increase in flash rate. This phenomenon has not been reported in previous satellite LNOx studies but is consistent with ground‐based observations suggesting an inverse relationship between flash rate and size.

Description: Abstract We study the statistical properties of tidal weather (variability period 〈30 days) of DW1 amplitude using the extended Canadian Middle Atmospheric Model (eCMAM) and Sounding of the Atmosphere using Broadband Emission Radiometry (SABER). A hierarchy of statistical models, for example, the autoregressive (AR), vector AR, and parsimonious vector AR models, are built to predict tidal weather. The quasi 23‐day oscillation found in the tidal weather is a key parameter in the statistical models. Comparing to the more complex vector AR and parsimonious vector AR models, which consider the spatial correlations of tidal weather, the simplest AR model can predict one‐day tidal weather with an accuracy of 89% (R2: correlation coefficient squared). In the AR model, 23 coefficients at each latitude and height are obtained from seven years of eCMAM data. Tidal weather is predicted via a linear combination of 23 days of tidal weather data prior to the prediction day. Different sensitivity tests are performed to prove the robustness of these coefficients. These coefficients obtained from eCMAM are in very good agreement with those from SABER. SABER tidal weather is predicted with an accuracy of 86% and 87% at one day by the AR models with coefficients from eCMAM and SABER, respectively. The five‐day forecast accuracy is between 60 and 65%.

Description: Abstract Summertime surface‐level ozone (O3) is known to vary with temperature, but the relative roles of different processes responsible for causing the O3‐temperature relationship are not well quantified. In this study we use simulations of NASA's Global Modeling Initiative (GMI) chemical transport model (CTM) to isolate and assess the relative impact of atmospheric transport, chemistry, and emissions on large‐scale O3 variability, events, and its covariance with temperature. Using observations from CASTNet in the contiguous United States, we show that the GMI CTM reproduces the spatiotemporal variability of O3 and its relationship with temperature during the summer. We use the change in O3 given a change in temperature (dO3/dT) along with other metrics to understand differences between our simulations. In regions with moderate to strong positive correlations between temperature and O3 such as the Northeast, Great Lakes, and Great Plains, temperature's association with transport yields a majority of the total O3‐temperature relationship (∼60%) while temperature‐dependent chemistry and anthropogenic NO emissions play smaller roles (∼30% and ∼10%, respectively). There are regions, however, with insignificant correlations between temperature and O3, and our findings suggest that transport is still an important driver of O3 variability in these regions, albeit not correlated with temperature. Transport is not directly dependent on temperature but rather is linked through an indirect association, and it is therefore important to understand the exact mechanisms that link transport to O3 and how these mechanisms will change in a warming world.

Description: Abstract Snow cover in mountainous terrain plays an important role in regional and global water and energy balances, climate change, and ecosystems. Blowing snow is a frequent and important weather phenomenon over the Tibetan Plateau (TP); however, this process is neglected in most current land surface models, despite the consequential role it plays in the land surface and atmospheric water and energy budgets. In this paper, we present a blowing snow model PIEKTUK coupled with the Community Land Model (CLM4.5) to provide a better estimate of the snow dynamics for the consideration of snow redistribution induced by wind. Two simulations with a 0.065° spatial resolution were performed in 2010 over the TP, namely, a sensitivity experiment with the inclusion of blowing snow effects (CLM_BS) and a control run with the original model (CLM). A specific objective of this study was to evaluate the improvements in the simulations of snow dynamics and other key variables in surface energy partitioning provided by the coupled model, such as the surface albedo and land surface temperature (LST). Compared with a variety of remote‐sensing observations, the results show that the surface snow cover, snow depth, and surface albedo can be better reproduced in most of the TP region by CLM_BS than by the original CLM, particularly in the Kunlun Mountains, Hoh Xil area, and the southwestern TP. In areas with reduced bias, variations in the monthly mean snow cover fraction can be reflected particularly well by CLM_BS. For LST, however, a significant decrease in the nighttime LST bias was detected in CLM_BS, while the bias in the daytime LST increases. The results show considerable potential for the inclusion of the blowing snow process to promote the modeling of snow dynamics and land‐atmosphere interactions on the TP.

Description: Abstract The Lightning Cluster Filter Algorithm (LCFA) in the Geostationary Lightning Mapper (GLM) ground system identifies lightning flashes from the stream of event detections. It excels at clustering simple flashes, but experiences anomalies with complex flashes that last longer than 3 s or contain more than 100 groups, leading to flashes being artificially split. We develop a technique that corrects these anomalies and apply it to the 2018 GLM data to document all lightning across the Americas. We produce statistics describing the characteristics and frequencies of “reclustered” GLM flashes as well as thunderstorm “area” features. The average GLM Americas flash rate in 2018 was 11.7 flashes s‐1 with the greatest flash rate densities occurring over Lake Maracaibo (157 flashes km‐2 year‐1). Lloró, Chocó, Colombia had the most thunderstorm activity with 256 thunder days. The longest GLM flash spanned 673 km, the largest flash covered 114,997 km2, and the longest‐lasting flash had a 13.496 s duration. The first case occurred over Rio Grande do Sul in Brazil, while the other two cases occurred in the central United States. All three extreme flashes are located in the stratiform regions of Mesoscale Convective Systems (MCSs). The highest flash rate for a thunderstorm area feature was 17.6 flashes s‐1 while the largest thunderstorm was 216,865 km2 in size. Both storms occurred in South America. These initial results demonstrate the value that the development of a reprocessed GLM “science” product would offer and how such a product might be created at a reduced computational cost.

Description: Abstract The central and western Sahara (CWS) is the largest source of mineral aerosols during boreal summer, but observed ground‐based data are extremely scarce and typically distant from key source regions. Knowledge of dust emission mechanisms has therefore been mostly limited to short‐term observations from a point or model approximations. To address this deficiency, dust plumes from the CWS are classified according to emission mechanism for June, July and August of 2004‐2017 using an automated inference method which accurately tracks the timing, convective association and geometry of plumes observed with the Spinning Enhanced Visible and Infrared Imager (SEVIRI) aboard Meteosat Second Generation satellites. From these characteristics, plumes are classified as either low‐level jet or cold pool outflow events. The extensive data set is used to generate the largest available climatology of dust emission sources and Saharan emission mechanisms. Automated inference compares well with ground‐based measurements from the Fennec Campaign (76% accuracy) as well as with an entirely manual approach (88% accuracy). Cold pool activity accounts for 82% of total observed dust and 88% at the point of emission. Dust from cold pools evolves seasonally from hotspots around the Mali‐Niger‐Algeria border triple point towards the central Sahara to the northwest, while dust from low‐level jets is organised along the axis of the northeasterly Harmattan, and dominates emission within the Tidihelt Depression of central Algeria. The widespread importance of cold pool outflows in this research supports the findings of the Fennec Campaign, but low‐level jets remain highly significant in certain isolated hotspots.

Description: Abstract Intense forest fires in western North America during August 2017 caused smoke plumes that reached the stratosphere. While this phenomenon has often been observed, this particular event caused increases in stratospheric aerosol extinction at higher altitudes with greater magnitude than previously observed in the satellite record. Here we use multiple satellite limb sounding observations, which provide high sensitivity to thin aerosol layers and good vertical resolution, to show that enhancements in aerosol extinction from the fires reached as high as 23 km in altitude and persisted for more than 5 months. Within 1 month, the aerosol is observed to cover latitudes from 20°N to 60°N, which is essentially the northernmost limit of the observations. At midlatitudes between 15‐ and 20‐km altitudes, the sustained level of median aerosol extinction measured at 750 nm increased by almost an order of magnitude, from approximately 10−4 km−1 to nearly 10−3 km−1. Agreement between limb scatter and occultation measurements is generally within 20% despite potential bias due to modified aerosol shape and composition.

Description: Abstract Entrainment rate (λ) in convective parameterizations remains a sensitive parameter with much uncertainty in model simulations. This study estimates λ using carbon monoxide (CO) measurements jointly from the Microwave Limb Sounder (MLS) and Tropospheric Emission Spectrometer (TES) onboard the Aura satellite, associated with deep convective cases identified by CloudSat and CALIPSO observations. CO is treated as a conserved quantity over convective transport time scales and a simple entraining‐plume model is used to derive entrainment rates. The relationships of the observational estimates of λ as a function of convective height, environmental relative humidity and convective available potential energy (CAPE) derived from Atmospheric Infrared Sounder (AIRS) data are compared with those from Goddard Earth Observing System Model (GEOS‐5) simulations. Bulk statistics of λ show that the values of λ are predominately below 20 % km‐1 for deep convection and the occurrence frequency of any λ decreases with increasing λ. Composite λ values are generally lower in the tropics compared to northern mid‐latitudes in both observations and the GEOS‐5 model. A decrease of λ with increasing convective height is found in both observations and model simulations. We also find that λ tends to decrease with increasing CAPE in the observation‐based λ’s and plume‐based GEOS‐5 λ’s, although the model given λ’s have a non‐monotonic relation with CAPE. The observed λ’s have a weak relation with lower‐to‐mid tropospheric RH, while both GEOS‐5 plume‐based and given λ increases with increasing RH.

Description: Abstract Fresh volcanic eruption deposits tend to be loose, bare, and readily resuspended by wind. Major resuspension events in Patagonia, Iceland, and Alaska have lofted ash clouds with potential to impact aircraft, infrastructure, and downwind communities. However, poor constraints on this resuspension process limit our ability to model this phenomenon. Here, we present laboratory experiments measuring threshold shear velocities and emission rates of resuspended ash under different environmental conditions, including relative humidity of 25–75% and simulated rainfall with subsequent drying. Eruption deposits were replicated using ash collected from two major eruptions: the 18 May 1980 eruption of Mount St. Helens and the 1912 eruption of Novarupta, in Alaska's Valley of Ten Thousand Smokes. Samples were conditioned in a laboratory chamber and prepared with bulk deposit densities of 1,300–1,500 kg/m3. A control sample of dune sand was included for comparison. The deposits were subjected to different wind speeds using a modified PI‐SWERL® instrument. Under a constant relative humidity of 50% and shear velocities 0.4–0.8 m/s, PM10 emission by resuspension ranged from 10 to 〉100 mg·m−2·s−1. Addition of liquid water equivalent to 5 mm of rainfall had little lasting effect on Mount St. Helens wind erosion potential, while the Valley of Ten Thousand Smokes deposits exhibited lower emissions for at least 12 days. The results indicate that particle resuspension due to wind erosion from ash deposits potentially exceeds that of most desert surfaces and approaches some of the highest emissions ever measured.

Description: Abstract Clouds and aerosols play essential roles in regulating surface incident solar radiation (Rs). It has been suggested that the increased aerosol loading over China is a key factor for the decadal variability in Rs and can explain the bias in its trend from reanalyses because the reanalyses do not include the interannual variability of aerosols. In this study, we compare the Rs derived from sunshine duration at 2,400 weather stations in China and that from five reanalyses from 1980 to 2014. The determining factors for the biases in the mean values and trends of Rs from the reanalyses are examined, with the help of Rs and the cloud fraction (CF), from satellite and 2,400 weather stations. Our results show that all reanalyses overestimate the multiyear Rs by 24.10–40.00 W/m2 due to their underestimations of CF, which is more obvious in southern China. The biases in the simulated CF in the reanalyses can explain the biases in Rs by 55–41%, and the bias in clear‐sky surface solar radiation (Rc), which is primarily due to biases in aerosol loading, can explain 32–9% of the bias in Rs. The errors in the trend of the simulated CF can explain the errors in the Rs trends in the reanalyses by 73–12%, and the trend errors in the Rc can explain 43–30% of the trend error in Rs. Our study suggests that more work is needed to improve the simulation of aerosols, clouds, and aerosol‐cloud‐radiation interactions in the reanalyses.

Description: Abstract Tropopause‐penetrating convection is a frequent seasonal feature of the Central United States climate. This convection presents the potential for consistent transport of water vapor into the upper troposphere and lower stratosphere (UTLS) through the lofting of ice, which then sublimates. Water vapor enhancements associated with convective ice lofting have been observed in both in situ and satellite measurements. These water vapor enhancements can increase the probability of sulfate aerosol‐catalyzed heterogeneous reactions that convert reservoir chlorine (HCl and ClONO2) to free radical chlorine (Cl and ClO) that leads to catalytic ozone loss. In addition to water vapor transport, lofted ice may also scavenge nitric acid and further impact the chlorine activation chemistry of the UTLS. We present a photochemical model that resolves the vertical chemical structure of the UTLS to explore the effect of water vapor enhancements and potential additional nitric acid removal. The model is used to define the response of stratospheric column ozone to the range of convective water vapor transported and the temperature variability of the lower stratosphere currently observed over the Central United States in conjunction with potential nitric acid removal and to scenarios of elevated sulfate aerosol surface area density representative of possible future volcanic eruptions or solar radiation management. We find that the effect of HNO3 removal is dependent on the magnitude of nitric acid removal and has the greatest potential to increase chlorine activation and ozone loss under UTLS conditions that weakly favor the chlorine activation heterogeneous reactions by reducing NOx sources.

Description: Abstract With the highlight of environmental problems over the Tibetan Plateau (TP), aerosol pollution and the influence of this pollution on cloud properties are becoming a new area of research. Based on the aerosol index and cloud property parameters derived from satellite observations, in this study, the inconsistent effects of aerosols on ice cloud properties between daytime and nighttime over the TP are investigated. The results indicate that ice clouds are mainly distributed over the TP margin area, especially over the north slope, during both daytime and nighttime. The occurrence frequency of ice cloud is higher during the daytime than during the nighttime over the margin areas of the TP. Similarly, aerosols are mainly concentrated over the northern margin of the TP. A potential relationship may exist between the aerosol index and ice cloud properties. When the aerosol index increases from 0.05 to 0.17, the ice cloud droplet radius (ICDR) during the daytime decreases from 32.1 to 27.9 μm, while the ICDR during the nighttime remains almost constant (approximately 25 μm); furthermore, the ice water path (IWP) during the daytime decreases slightly due to the saturation effect, while the nocturnal IWP increases significantly. The changes in ice cloud optical depth (ICOD) during daytime and nighttime show significant and completely opposite trends. The removal of the influence of meteorological factors showed that aerosols have a more dominant influence than meteorological conditions on ice cloud properties (except for the nocturnal ICDR and IWP during the daytime).

Description: Abstract The Royal Netherlands Meteorological Institute (KNMI) operates a three‐dimensional microbarometer array at the Cabauw Experimental Site for Atmospheric Research observatory. The array consists of five microbarometers on a meteorological tower up to an altitude of 200 m. Ten ground‐based microbarometers surround the tower with an array aperture of 800 m. This unique setup allows for the study of infrasound propagation in three dimensions. The added value of the vertical dimension is the sensitivity to wind and temperature in the atmospheric boundary layer over multiple altitudes. In this study, we analyze infrasound generated by an accidental chemical explosion at the Moerdijk petrochemical plant on 3 June 2014. The recordings of the tower microbarometers show two sequential arrivals, whereas the recordings on the ground show one wavefront. This arrival structure is interpreted to be the upgoing and downgoing wavefronts. The observations are compared with propagation modeling results using global‐scale and mesoscale atmospheric models. Independent temperature and wind measurements, which are available at the Cabauw Experimental Site for Atmospheric Research, are used for comparison with model output. The modeling results explain the signal arrival times; however, the tower wavefront arrivals are not explained. This study is important for understanding the influence of the atmospheric boundary layer on infrasound detections and propagation.

Description: Abstract The sudden stratospheric warming (SSW) of 12 February 2018 was not forecast by any extended‐range model beyond 12 days. From early February, all forecast models that comprise the subseasonal‐to‐seasonal (S2S) database abruptly transitioned from indicating a strong stratospheric polar vortex (SPV) to a high likelihood of a major SSW. We demonstrate that this forecast evolution was associated with the track and intensity of a cyclone in the northeast Atlantic, with an associated anticyclonic Rossby wave break, which was not well forecast. The wave break played a pivotal role in building the Ural high, which existing literature has shown was a precursor of the 2018 SSW. The track of the cyclone built an anomalously strong sea level pressure dipole between Scandinavia and Greenland (termed the S‐G dipole), which we use as a diagnostic of the wave break. Forecasts that did not capture the magnitude of this event had the largest errors in the SPV strength and did not show enhanced vertical wave activity. A composite of 49 similarly strong wintertime (November–March) S‐G dipoles in reanalysis shows associated anticyclonic wave breaking leading to significantly enhanced vertical wave activity and a weakened SPV in the following days, which occurred in 35% of the 15‐day periods preceding observed major SSWs. Our results indicate a particular transient trigger for weakening the SPV, complementing existing results on the importance of tropospheric blocking for disruptions to the Northern Hemisphere extratropical stratospheric circulation.

Description: Abstract Soot particles are generally considered to be poor ice nucleating particles. Involvement of soot in clouds and their release back into the atmosphere can form residual particles with altered cloud forming potential. The impact and extent of such different cloud processing scenarios on ice nucleation is however not well understood. In this work, we present the impact of cloud processing of soot aerosols on subsequent ice nucleation cycles at T ≤ 233 K. Coupling of two continuous flow diffusion chambers allows the simulation of different cloud processing scenarios and investigation of subsequent ice nucleation activity of the processed particles. The processing scenarios presented here encompass contrail, cirrus and mixed‐phase cloud processing, mimicking typical pathways that soot particles can be exposed to in the atmosphere. For all scenarios tested, the processed particles showed an enhanced ice active fraction for T 〈 233 K. The relative humidity with respect to water for the ice nucleation onset was observed to be on average approximately 10% (relative humidity with respect to ice, RHi ≈ 16 %) lower for the cloud processed particles compared to the unprocessed soot, for which ice nucleation was observed close to or at homogeneous freezing conditions of solution droplets. We attribute the enhanced ice nucleation abilities of the cloud processed soot to a pore condensation and freezing mechanism and have identified key parameters governing these changes. Enhanced ice nucleation abilities of soot in cirrus clouds can have significant impacts, given the importance of the atmospheric ice phase for precipitation formation and global climate.

Description: No abstract is available for this article.

Description: Abstract Based on a successful cloud‐resolving simulation with the Weather Research and Forecasting Model, this study examines the evolution and the role of midtropospheric mesoscale cyclonic vortex in the formation of Super Typhoon Nepartak (2016). The midtropospheric vortex is correlated with the convective activity in pre‐Nepartak. Once the deep convection outbreaks, the midtropospheric vortex intensifies first via the vertical advection associated with the severe updrafts and then through the midlevel convergence associated with stratiform precipitation. As the stratiform precipitation dissipates, the midlevel vortex weakens slightly in the following shallow convection phase. The above‐described processes recur sequentially during the pregenesis of Nepartak, and the midtropospheric vortex demonstrates diurnal variations. Its intensification usually corresponds to the weakening of low‐level cyclonic circulation except for the deep convection phase, indicating that the development of midtropospheric vortex can inhibit the development of self‐sustained low‐level cyclonic circulation. Although the midtropospheric vortex is not always a quasi‐balanced perturbation, a cold core can be found in the lower troposphere below it during the most of the pregenesis stage. The appearance of the cold core enhances the low‐level temperature gradient around it, which favors convection burst. In addition, the closed cyclonic circulation associated with the midlevel vortex can serve as a pouch protecting the vorticity, moisture, and convection inside from the vertical wind shear and dry air intrusion when the low‐level and midlevel vortices are overlapped in the late pregenesis stage, which facilitates the sustained deep convection and the formation of Nepartak.

Description: Abstract The dynamical behavior of the mesosphere and lower thermosphere (MLT) region during strongly disturbed wintertime conditions commonly known as polar‐night jet oscillations (PJOs) is described in detail and compared to other wintertime conditions. For this purpose, wind measurements provided by two specular meteor radars located at Andenes (69°N, 16°E) and Juliusruh (54°N, 13°E) are used to estimate horizontal mean winds and tides as an observational basis. Winds and tidal main features are analyzed and compared for three different cases: major sudden stratospheric warming (SSW) with (a) strong PJO event, (b) non‐PJO event, and (c) no major SSWs. We show that the distinction into strong PJOs, non‐PJOs, and winters with no major SSWs is better suited to identify differences in the behavior of the mean winds and tides during the boreal winter. To assess the impact of the stratospheric disturbed conditions on the MLT region, we investigate the 30‐year nudged simulation by the Extended Canadian Middle Atmosphere Model. Analysis of geopotential height disturbances suggests that changes in the location of the polar vortex at mesospheric heights are responsible for the jets observed in the MLT mean winds during strong PJOs, which in turn influence the evolution of semidiurnal tides by increasing or decreasing their amplitudes depending on the tidal component.

Description: Abstract Forced by Pacific Decadal Oscillation‐related sea surface temperature (SST) anomalies with the same pattern but opposite signs in the western‐central North Pacific, nonlinear wintertime atmospheric responses are produced by a state‐of‐the‐art atmospheric general circulation model (GFDL AM2.1); that is, an obvious equivalent barotropic geopotential low appears over the cold SST forcing (“CSST”), whereas a weak baroclinic structure shows up corresponding to the warm SST forcing (“WSST”), and both of them have similar characteristics in the lower troposphere. Specifically, because of the relatively dry environment in the central North Pacific, nonlinear responses of moisture process including latent heat flux and low‐level atmosphere moisture advection induce asymmetric diabatic heating (Qd): in WSST, Qd tends to increase in the middle‐lower troposphere but decrease in the middle‐upper level, whereas it always increases in the whole troposphere in CSST. Thus, Qd has the same low‐level positive vertical gradient in both CSST and WSST, which produces similar atmospheric circulation anomalies in the lower troposphere. In turn, the asymmetric responses of low‐level temperature advection further modify air temperature meridional gradient as well as atmospheric baroclinicity in the lower troposphere, significantly shifting the transient eddy activities southward in CSST and greatly weakening their intensity in WSST, respectively. Accordingly, the transient eddy vorticity forcing primarily determines the upper‐level atmospheric responses in CSST, but it has unsystematic effects in WSST that are overtaken by Qd. Therefore, the dominance of diabatic heating in WSST and transient eddy forcing in CSST over the central North Pacific lead to the asymmetric atmospheric responses among which the asymmetry of moisture plays an essential role.

Description: Abstract Thirty southerly low‐level jet (LLJ) events were observed during the Plains Elevated Convection at Night (PECAN) field campaign in the Great Plains region of the United States during summer 2015. Here we present Doppler lidar wind data from three PECAN instrumentation sites to explore characteristics of LLJs and the boundary layer as well as some of the heterogeneities possible within the wind field of a LLJ. Southerly LLJs were observed on 66% of nights at the southwestern site (Greensburg, KS) but only 56% and 53% of nights at the eastern and northern sites, respectively (Hesston and Ellis, KS). The northernmost site had a relative abundance of weaker jets or nonjet conditions due to fronts or convective systems that only affected part of the observation domain. Plotting mean wind fields of each LLJ type reveals that the strongest LLJs tend to develop under very similar conditions but begin to show variability in wind profile evolution after several hours. A robust mixed layer height retrieval algorithm is used to investigate the interplay between the jets and the turbulent convective boundary layer, showing that stronger LLJs are preceded by deeper afternoon mixed layers and often have a later decoupling of mixing between the upper convective mixed layer and the near‐surface layer. Only the strongest LLJs generated a shallow mixing layer overnight. Comparing jet strength and direction to pristine nocturnal convection initiation shows that the strongest southerly LLJs yielded the most pristine nocturnal convection initiation events per night, and the pristine nocturnal convection initiation occurred farther north.

Description: Abstract In this study, the interannual variations in the tropical cyclone (TC) over the western North Pacific (WNP) and the influences of regional sea surface temperature (SST) anomalies are documented by separating the WNP into four quadrants considering nonuniform SST‐induced environmental changes. Our analysis shows that the TC variations in the northwest and southeast quadrants are related to both equatorial central‐eastern Pacific Ocean (EPO) and tropical Indian Ocean (TIO) SST anomalies. The TC variation in the northeast quadrant is mainly related to tropical North Atlantic Ocean SST anomalies. The main environmental variables differ for the TC variations in the four quadrants. Lower‐level (850‐hPa) vorticity is important for the TC variations in the northwest, southwest, and southeast quadrants. Midlevel (700‐hPa) humidity contributes to the TC variations in the northwest, northeast, and southeast quadrants. The vertical shear has a supplementary contribution to the TC variation in the southeast quadrant. The potential intensity (PI) negatively affects the TC variations in the southwest and southeast quadrants. The remote SST anomalies modulate different environmental variables over the WNP. The TIO SST influence is manifested in the lower‐level vorticity and vertical motion. The tropical North Atlantic SST impact occurs through the lower‐level vorticity change. The EPO SST effect occurs via changing the lower‐level vorticity and vertical motion as well as the midlevel moisture and vertical shear. The environmental variables experience more prominent changes when SST anomalies coexist in two remote regions. Numerical experiments confirm the EPO and TIO SST anomaly impacts on the environmental conditions affecting the WNP TC variations.

Description: Abstract An anomalous “north‐south” dipole mode of the snow water equivalent (SWE) persisting from winter to spring is detected over the Eurasian mid‐to‐high latitudes in this study. Using observational data sets and numerical experiments of the Community Atmospheric Model (5.0), we show that this mode contributes to prolonged winter‐springtime coldness in midlatitude Eurasia and is closely linked to the declining November Arctic sea ice concentration. The decline in the sea ice concentration over the Barents‐Laptev Seas can induce a teleconnection pattern over the mid‐to‐high latitudes in the following winter, accompanied by an anomalous ridge over the Ural Mountains and an anomalous trough over Europe and East Asia. Such changes in the large‐scale circulation lead to more cold surges and heavy snowfall in the midlatitudes and light snowfall in the high latitudes, forming an anomalous north‐south dipole mode of the SWE, which further reduces the temperature through thermodynamic feedback. Due to seasonal memory, this SWE pattern can persist into the following spring and can lead to springtime midlatitude coldness via thermodynamic and dynamic processes. For the thermodynamic process, the anomalous SWE condition can lead to anomalous wet soil, reduced incoming surface solar radiation, and cooling air in the midlatitudes. This phenomenon induces an enhanced Siberian High and a deepened East Asian trough via the snow‐Siberian high‐feedback mechanism, which favors a cold spring in northern East Asia. Further analysis suggests that an empirical seasonal prediction model based on the SWE can reasonably predict East Asian spring temperature anomalies.

Description: Abstract A complete and quantitative understanding of cumulus entrainment remains elusive, in part due to the difficulty of directly observing cloud entrainment rates. Multiple approaches to ground‐based observational retrieval of bulk fractional entrainment rates (ε) within cumuli have been developed, such as the parcel model by Jensen and Del Genio (JDG, 2006, https://doi.org/10.1175/JCLI3722.1) and Entrainment Rate In Cumulus Algorithm (ERICA) by Wagner et al. (2013, https://doi.org/10.1175/JTECH-D-12-00187.1). In this paper, a new cumulus entrainment retrieval based on a turbulent kinetic energy (TKE) similarity theory is presented. This method estimates ε based on only the environmental and subcloud conditions. By conducting large‐eddy simulations of a range of continental and maritime shallow cumulus convection cases as Observing System Simulations Experiments, the first numerical verification of the three retrieval methods is produced. These simulations consider a broad range of shallow cumulus environments along with variations of the numerical configuration. The diagnosed ε from these simulations is found to be robustly larger in cumuli over the ocean than in cumuli over land. For continental cumuli, the experiments also reveal a diurnal cycle with increasing ε in the late afternoon. These diagnosed ε serve as the “truth” against which the pseudo‐retrieved entrainment rates from several different implementations of each retrieval are verified. Overall, the simpler JDG and TKE retrievals outperform the more sophisticated ERICA method and better capture the sensitivity to continentality. Only the TKE method reproduces the diurnal variations in ε within continental cumuli. The mean error in the ε retrievals are between 20% and 30% for the TKE and JDG methods, but 50% for ERICA.

Description: Abstract The present study documents intraseasonal snow cover variations over western Eurasia and associated atmospheric processes using the latest Moderate Resolution Imaging Spectroradiometer/Terra daily snow cover product and National Centers for Environmental Prediction/National Center for Atmospheric Research atmospheric reanalysis. It is found that 9‐ to 30‐day variation dominates total intraseasonal snow cover variations over western Siberia. Composite analysis based on 69 positive snow events over western Siberia reveals that atmospheric circulation anomalies control the 9‐ to 30‐day snow variation over western Siberia. A zonal wave train associated with the North Atlantic Oscillation leads to the development of an anomalous cyclone over western Siberia. The associated anomalous ascending motion, anomalous water vapor convergence, and water vapor increase in the atmosphere provide a favorable condition for snowfall. The snowfall starts when anomalous ascending motion reaches the strongest. The maximum snow cover appears about 1 day after the peak of anomalous descending motion and water vapor flux divergence. The surface air temperature tends to vary out of phase with snow cover over western Siberia. Surface air temperature anomalies over western Siberia are contributed by horizontal advection and diabatic heating. The adiabatic heating has a damping effect in surface air temperature variation.

Description: Abstract With a six‐year (2009–2014) summer climate simulation using the Weather Research and Forecasting model at convection‐permitting resolution (4‐km grid spacing), the effects of microphysics parameterization (MP) schemes on precipitation characteristics are investigated in this study. The convection‐permitting simulations employ three popular MP schemes, namely, Lin (single‐moment bulk MP), Weather Research and Forecasting Single‐Moment 5‐class (one‐moment and mixed‐phased MP), and Thompson (two‐moment and mixed‐phase MP) scheme. By evaluating the simulations against the CMORPH, rain gauge (Station), and ERA‐Interim data, it is found that the convection‐permitting model reproduce well the summer precipitation amount and the associated large‐scale atmospheric circulations, which are insensitive to the choice of MP schemes. The simulations with three MP schemes overestimate the precipitation amount, especially over the Yangtze‐Huaihe River Valley. The overestimations may be due to the systematic biases, and cannot be significantly reduced by using different MP schemes. Moreover, all simulations capture well the major features of precipitation diurnal variations and their transition characteristics, but they significantly overestimate the precipitation frequency while underestimate the precipitation intensity. The analysis on the microphysical hydrometeors shows that the model‐simulated precipitation amount is considerably affected by the vertical profiles of solid hydrometeors, especially the snow and graupel particles. The Thompson scheme creates more snow particles and less graupel than the other schemes, while produces the least precipitation amount that best matches the CMORPH.

Description: Abstract Carbonaceous matter in the atmosphere has an important influence on climate change. Currently, the deposition of carbonaceous matter is one of the largest uncertainties in the climate system. This phenomenon is common in remote regions, such as the Himalayas and Tibetan Plateau. In this study, for the first time, we reported in situ measurements of wet and dry deposition rates of carbonaceous matter at three remote stations: Nam Co, Lulang, and Everest. The results showed that the annual wet deposition rates of water‐insoluble organic carbon (WIOC) and black carbon (BC) were 60.2 and 5.8 mg·m−2·year−1, 330 and 34.6 mg·m−2·year−1, and 47.0 and 2.6 mg·m−2·year−1 at the Nam Co, Lulang, and Everest stations, respectively. Seasonal variations in the wet deposition rates of WIOC and BC were controlled by precipitation amount and their atmospheric concentrations. In addition, the wet scavenging ratios of WIOC and BC at Nam Co Station were close to those observed in other remote areas. The total BC deposition at Nam Co Station (15.3 mg·m−2·year−1) was higher than that from chemical transport models, implying a dominant role of dry deposition of BC in the total deposition at this station and an urgent need to improve the aerosol deposition in models for the Himalayas and Tibetan Plateau. It was found that the deposition rates of carbonaceous matter in the Himalayas and Tibetan Plateau had large spatial variation; thus, high‐resolution models need to be applied in the future.

Description: Abstract It is important but difficult to measure the shortwave radiative forcing of the dust aerosols over land from satellite‐observed radiance because the inhomogeneous surface albedo varies in a large dynamic range. In this study, we proposed a satellite‐based equi‐albedo method to derive the dust aerosol shortwave direct forcing over land. In the method, an equal radiance at the top of atmosphere was assumed for the region with the similar surface albedo and the similar solar zenith angle. The aerosol optical depth (AOD) from Moderate Resolution Imaging Spectroradiometer and the shortwave radiance product from Clouds and Earth's Radiant Energy System were used to derive the dust aerosol radiative forcing. The dust storm events outbroken on 9 and 24 April 2010 in Taklimakan desert were selected as study cases. The mean dust shortwave direct forcing efficiency is −35.08 W/m2 per unit of dust AOD during the dust storm events. The results were validated with the calculated radiative forcing from Moderate Resolution Imaging Spectroradiometer AOD product by the radiative transfer model. It shows that the derived radiative forcing is well correlated with the simulated one. The mean difference is 10.57 and the standard deviation is 1.35. Moreover, uncertainty has been estimated. The regional mean‐directed radiative forcing due to dust are −28.98 ± 7.99 and −35.76 ± 10.61 W/m2 of these two cases directly from satellite observations. This research indicates that the proposed method is reliable and effective, which can be used to estimate the shortwave direct radiative forcing of the dust storm event.

Description: Abstract Multiyear droughts are a common occurrence in southwestern North America (SWNA), but it is unclear what causes these persistent dry periods. The ocean‐atmosphere conditions coinciding with droughts have traditionally been studied using correlation and composite methods, which suggest that cool conditions in the tropical Pacific are associated with SWNA droughts and warm conditions are associated with wet periods in SWNA. Nevertheless, the extent to which multiyear droughts are truly consistent with this paradigm remains unknown. This is, in part, because the temporal trajectory of ocean‐atmosphere conditions during these dry periods have not been sufficiently characterized. Here we examine the continuum of ocean‐atmosphere trajectories before, during, and after multiyear droughts in SWNA using observation‐based data and an ensemble of climate model simulations from the Community Earth System Model. An examination of sea surface temperature patterns at the beginning, middle, and end of SWNA droughts shows that an El Niño event tends to precede SWNA droughts, a cool tropical Pacific occurs during droughts, and central Pacific El Niño events end droughts. However, moderate El Niño events can occur in the middle of persistent droughts, so a warm tropical Pacific does not always end these dry periods. These findings are important for drought predictability and emphasize the need to improve simulations of the magnitude, life cycle, and frequency of occurrence of El Niño events.

Description: Abstract Atmospheric rivers (ARs) can significantly modulate surface hydrological processes through the extreme precipitation they produce. However, there is a lack of comprehensive evaluation of ARs' impact on surface hydrology. This study uses a high‐resolution regional climate simulation to quantify the impact of ARs on surface hydrological processes across the western U.S. watersheds. The model performance is evaluated through extensive comparison against observations. Our analysis indicates that ARs produce heavy precipitation but suppress evapotranspiration. Snowpack ablates more during ARs, with higher air temperature and increased longwave radiation playing the primary and secondary roles, respectively. At the 0 °C to 10 °C temperature range, ARs increase the probability of snow ablation from 0.33 to 0.57. The runoff‐to‐precipitation ratio is primarily controlled by antecedent soil moisture, but it almost doubles in the northwestern watersheds due to the intensification of snow ablation during AR events. From the analysis of the relationship between the hydrological responses and different meteorological factors, precipitation, temperature, and radiation are identified as the key drivers that distinguish the hydrologic responses between AR and non‐AR events. Lastly, analysis of ARs and total runoff at annual scale and 1 April snowpack and winter precipitation shows that ARs explain 30% to 60% of the variability of annual total runoff and sharpen the seasonality of water resources availability in the west coast mountain watersheds.

Description: Abstract As a widespread landscape, rugged terrain significantly distorts the land surface albedo. Simply neglecting the topographic effects in the land surface albedo modeling and retrievals can lead to large biases and uncertainties over rugged terrain. In spite of gradually increasing research about the topographic effects on the albedo, the albedo sensitivities over rugged terrain to different variables remain unclear. In this paper, the sensitivities of coarse‐scale snow‐free albedo to topography were quantitatively investigated using the Moderate Resolution Imaging Spectroradiometer (MODIS) land surface albedo data and the variance‐based global sensitivity analysis based on a well‐established mechanistically‐based land surface albedo parameterized model over rugged terrain. The results based on the MODIS data revealed that MODIS land surface albedo over the Tibetan Plateau was highly sensitive to the topographic distribution, and the differences of the spatially averaged snow‐free black‐sky albedos over grasslands induced by different terrain could reach up to 0.10 in winter. The topographic effects on MODIS albedo are tightly relevant to the land cover type, solar illumination geometries, and vegetation characteristics. The global sensitivity analysis results underscored that topography was an important driving factor of the snow‐free albedo, and it accounted for more than 30% of the total variance, respectively. These results highlight the necessities for the topographic consideration in the land surface albedo modeling and retrievals even though the terrain is gentle (10–20°) and advance our understanding of the albedo sensitivities to different variables over rugged terrain, which will facilitate the improvement of land surface albedo parameterization in the land surface models.

Description: Abstract An anelastic numerical model is used to study the influences of fine structure (FS) in the wind and stability profiles on gravity wave (GW) propagation in the Mesosphere and Lower Thermosphere (MLT). Large amplitude GWs interacting with FS, that is, thin regions of enhanced wind and stability, evolve very differently depending on the precise vorticity source and sink terms for small‐scale motions induced by the FS gradients. The resulting small‐scale dynamics are deterministic, promoting local instabilities, dissipation, and momentum deposition at locations and orientations determined by the initial FS. The resulting momentum depositions yield significant changes to the background wind structure, having scales and amplitudes comparable to the effects of large‐scale features in the ambient atmosphere. The deterministic nature of the large‐scale impacts further suggests that they can be estimated without fully resolving the underlying instability dynamics. Given the significant amplitudes and ubiquitous occurrence of FS throughout the atmosphere, the influences of these important and diverse flow evolutions merit inclusion in broader modeling efforts.

Description: Abstract One unique long‐range transport event with multiple layers of aerosol plumes was observed over Taiwan during 29–31 March 2006. A synergy of ground‐based observation, remote sensing, and backward trajectory simulation collectively indicated the high‐altitude (above 3 km) plume originated from biomass burning in Southeast Asia while the midaltitude (around 0.8–2 km) plume was attributed to dust from the Gobi Desert. Aerosol optical properties measured at a low‐altitude site were characterized of abundant coarse mode particles and increased single scattering albedo as a function of increased wavelength, indicating the influence from dust particles. While at a high mountain site (elevation of ~3 km), aerosol optical depth was elevated by a factor of 3–4 compared to its background value and mainly comprised of fine particles. It was diagnosed that the high‐altitude aerosols were influenced by the transported smoke plumes but exempted from dust. Simulation of the meteorological conditions against a Taiwan‐wide meteorology network showed strong near surface temperature rise of more than 2° during this long‐range transport event as well as for the vertical temperature profiles. Both dust and biomass burning aerosol plumes via long‐range transport contributed significantly to the atmospheric warming, resulting in strong instantaneous aerosol radiative forcing of 46.0 W/m2 in the atmosphere. A “double dome” warming effect mechanism was proposed that both biomass burning and dust plumes above the boundary layer could efficiently reserve the solar energy and heat the lower troposphere.

Description: Abstract Accurate representation of cloud vertical overlap in climate models is particularly significant for predicting the total cloud fraction (TCF) and calculating radiative budget. It refers to the parameterization of overlap parameter—decorrelation length‐scale L, but the potential of dynamical factors in developing parameterization of L has still received far less attention. Using ground‐based radar observation over Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) site, here long‐term seasonal‐averaged L is retrieved and shows a very high anti‐correlation with TCF from different datasets, indicating that TCF is sensitive to the way of cloud overlap. Therefore, combined with meteorological reanalysis dataset, a robust multiple regression model between L and dynamical factors is built, and exhibits smaller TCF bias compared with previous parameterization of L. Contribution calculation further verifies that atmospheric instability contributes 70% of L variation, indicating it dominates the long‐term variation of L over SGP site. This finding implies that dynamical factors should be taken into account in the parameterization of L.

Description: Abstract Previous work has shown that the Madden‐Julian Oscillation (MJO) can influence the North Atlantic Oscillation (NAO) via a Rossby wave teleconnection that propagates through the troposphere (i.e., a tropospheric pathway). In addition, recent work suggests that the MJO can influence the stratospheric polar vortex, which is also known to influence the tropospheric NAO—thus, there likely exists a stratospheric pathway for MJO influence as well. Here, we apply two methods to shed more light on the pathways linking the MJO to the NAO. First, we use a traditional approach in climate science based on analyzing conditional probabilities. Second, we use methods from causal discovery theory based on probabilistic graphical models. Together, these two analysis approaches reveal that the MJO can impact the NAO via both a tropospheric and stratospheric pathway. The stratospheric pathway is shown to come about in two ways: First, both methods show that the MJO itself influences the strength of the stratospheric polar vortex on a timescale of ∼10 days, and then 5 days later the vortex can drive changes in the NAO. Second, the state of the stratospheric polar vortex acts to condition the NAO to be conducive (or not) to MJO influence. When the vortex is in a state that opposes the expected NAO response to the MJO, we find little influence of the MJO on the NAO, however, when the vortex supports the expected NAO response, the NAO is up to 30% more likely to be in a particular state following active MJO periods.

Description: Abstract The National Centers for Environmental Prediction (NCEP) Climate Forecast System version 2 (CFSv2) has a large cold bias in the model's deep soil temperature during summer. This study explores the potential triggering effect of that bias on excessive Eurasian snow cover in early winter. Snow cover appears erroneously early in the fall, especially in western Eurasia, in long simulations with CFSv2. The seasonal transition may be too early because the model land surface temperature (LST) reaches its freezing point earlier than observed, so that new snow cannot melt. This process initiates snow‐albedo feedback too early. The early cooling of LST is partially influenced by a seasonal resurfacing of the cold bias in the deep soil layer. From winter to early spring, a cold bias prevails in LST and upper soil temperature as snow cover remains. During this season, the temperature in the deep soil is generally warmer than in the upper soil and has relatively little bias. From spring to summer, the cold bias in the upper soil becomes smaller as it warms up in response to solar heating. On the other hand, the deep soil temperature has a noticeably smaller seasonal change than observed, resulting in a severe cold bias during summer. As the solar radiation declines quickly in early fall, the cold deep soil temperature causes additional cooling in the upper soil layer and helps to bring LST to the freezing point early in the western Eurasia, which leads to enhanced bias in the snow properties.

Description: Abstract Season‐dependent interannual variability of surface winds across Antarctica and the Southern Ocean is investigated using European Centre for Medium‐Range Weather Forecasts Interim (ERA‐Interim) reanalysis for 1979–2017 and the empirical orthogonal function (EOF) technique. The first three EOF modes, which account for 18–23%, 11–15%, and 8–10% of the total variances for the four austral seasons, are related to several known modes of Southern Hemisphere large‐scale circulation including the southern annular mode, Pacific South American (PSA), and Zonal Wave 3 (ZW3) and to sea surface temperature anomalies in eastern (Niño 3) and central (Niño 4) tropical Pacific Ocean. Specifically, EOF1 and EOF2 are significantly correlated with southern annular mode in all seasons except for autumn EOF2. Neither EOF1 nor EOF2 are significantly correlated to Niño 3, but Niño 4 is significantly correlated to winter EOF1 and to EOF2 in all seasons but winter. EOF1 is significantly related to ZW3 for all seasons but summer; however, no significant connection is found between EOF2 and ZW3. Significant correlations exist between spring and winter EOF2 and PSA1 and between spring EOF1 and PSA2. EOF3 is significantly correlated with Niño 3, Niño 4, and PSA2 in all seasons but summer. The spatial patterns of all three modes can be largely explained by the anomalous mean sea level pressure fields that strengthen or weaken climatological winds in different regions of the Antarctica and the Southern Ocean, although variations in katabatic forcing also plays a role in the variations of surface winds especially in East Antarctica and during winter season where and when katabatic forcing dominates.

Description: Abstract To investigate the mechanisms for the record‐breaking rainfall in the coastal metropolitan city of Guangzhou, China during 6–7 May 2017, budget analyses of advection and source/sink terms of the water vapor, potential temperature, and vertical momentum equations were conducted using the model output of a nested very large eddy simulation with the Weather Research and Forecasting model. Results show that the warm and moist air flows from the south and east onshore in the lower troposphere provided the main moisture source for the heavy rainfall. The structure of vertical velocity and hydrometeors (low‐echo centroid structure), in which the heavy rainfall was separated from the low‐level updraft, was favorable for the formation and maintenance of a heavy precipitation rate. The removal of the heat due to the advection (cooling tendency) in the upper troposphere increased the convective available potential energy of parcels rising from the lower troposphere, maintaining the development of updrafts. Although the total buoyancy forcing was the main contribution term for maintaining the updrafts, total dynamic acceleration played an important role in the vertical acceleration below the maximum vertical velocity core. In particular, the nonlinear dynamic perturbation pressure gradient force in the lower troposphere induced by the rotations aloft maintained the strong updrafts.

Description: Abstract Global reanalysis products are extensively used for hydrologic applications in sparse data regions. ERA‐5, among the new‐era reanalysis products, has been significantly improved for horizontal and vertical resolutions and data assimilation. However, the new‐era reanalysis products (ERA‐5, CFSR, ERA‐Interim, MERRA‐2, and JRA‐55) have not been evaluated for the hydrologic applications in India specially to understand if ERA‐5 outperforms the other reanalysis products or not. Here, we use the five new‐era reanalysis products for the monsoon (June‐September) season precipitation, maximum (Tmax) and minimum (Tmin)temperatures, total runoff, evapotranspiration (ET), and soil moisture against the observations from India Meteorological Department (IMD) in India for 1980‐2018. We use a well calibrated and evaluated hydrological model [the Variable Infiltration Capacity (VIC) model] to simulate hydrologic variables using the forcing from IMD and reanalysis products. In addition, we evaluated the reanalysis products for streamflow and annual water budget for the two basins located in the diverse climatic settings in India. Our results show that ERA‐5 outperforms the other reanalysis products for the monsoon season precipitation, Tmax, ET, and soil moisture. However, CFSR performs better than ERA‐5 for the monsoon season total runoff in India. Performance for streamflow and annual water budget for ERA‐5 is either better or comparable to the other reanalysis products in the two river basins. Overall, we find that ERA‐5 performs better than the other reanalysis products and can be used for the hydrologic assessments in India.

Description: Abstract The Madden‐Julian oscillation (MJO) is the leading source of global subseasonal predictability; however, many dynamical forecasting systems struggle to predict MJO propagation through the Maritime Continent. Better understanding the biases in simulated physical processes associated with MJO propagation is the key to improve MJO prediction. In this study, MJO prediction skill, propagation processes, and mean state biases are evaluated in reforecasts from models participating in the Subseasonal Experiment (SubX) and Subseasonal to Seasonal (S2S) prediction projects. SubX and S2S reforecasts show MJO prediction skill out to 4.5 weeks based on the Real‐time Multivariate MJO index consistent with previous studies. However, a closer examination of these models' representation of MJO propagation through the Maritime Continent reveals that they fail to predict the MJO convection, associated circulations, and moisture advection processes beyond 10 days with most of models underestimating MJO amplitude. The biases in the MJO propagation can be partly associated with the following mean biases across the Indo‐Pacific: a drier low troposphere, excess surface precipitation, more frequent occurrence of light precipitation rates, and a transition to stronger precipitation rates at lower humidity than in observations. This indicates that deep convection occurs too frequently in models and is not sufficiently inhibited when tropospheric moisture is low, which is likely due to the representation of entrainment.

Description: Abstract Distributions of eight targeted current‐use pesticides (CUPs) chloroneb, simazine, atrazine, alachlor, dacthal, chlorobenzilate, methoxychlor, and permethrin were investigated in seawater and the atmosphere in a region covering the North Pacific to the Arctic Oceans during the 7th and 8th Chinese National Arctic Research Expedition (2016 and 2017) voyages of the research vessel R/V Xuelong (Snow Dragon in English). Total CUP concentrations in seawater have prominent seasonal and latitudinal trends, with higher concentrations occurring at lower latitudes in early summer. The major contributors of the CUPs (∑8CUP) did not alter over different seasons with dominant chloroneb, alachlor, and atrazine accounting for more than 90%, but their concentrations did have marked seasonal changes. However, the compositions of the eight analyzed CUPs varied indistinctly between seasons indicating a possible combined environmental impact on these compounds rather than the effects of individual chemical properties. Three‐day backward air mass trajectories indicate that atmospheric masses from northeastern China are responsible for the high concentrations of CUPs in East China and Japan Seas, whereas those from the North Atlantic Ocean contribute to the low levels in local area. Fugacity ratios indicate potential volatilization and equilibrium of chloroneb in the Canada Basin of the Arctic Ocean and Japan Sea, respectively, and deposition of other CUPs in both regions. However, atmospheric concentrations are decoupled from those in seawater, which indicates a low exchange rate.

Description: Abstract In this study, we examine the impacts of urbanization and open water surface on heavy convective rainfall based on numerical modeling experiments using the Weather Research and Forecasting model. We focus on a severe storm event over the emerging Xiong'an City in northern China. The storm event consists of two episodes and features intense moisture transport and strong large‐scale forcing. A set of Weather Research and Forecasting simulations were implemented to examine the sensitivity of spatiotemporal rainfall variability in and around the urban area to different land use scenarios. Modeling results highlight contrasting roles of open water and urban surface in dictating space‐time organizations of convective rainfall under strong large‐scale forcing. Dynamic perturbation to atmospheric forcing dominates the impacts of open water and urban surface on spatial rainfall distribution during the second storm episode, while urban surface promotes early initiation of convection during the first storm episode through enhanced buoyant energy. Open water surface contributes to convective inhibition through evaporative cooling but can enhance moist convection when the impact of urban surface is also considered. The synergistic effect of open water and urban surface leads to rainfall enhancement both over and in the downwind urban area. Changes in rainfall accumulation with different spatial extents of urban coverage highlight strong dependence of urban‐induced rainfall anomalies on urbanization stages. Our results provide improved understandings on hydrometeorological impacts due to emerging cities in complex physiographic settings and emphasize the importance of atmospheric forcing in urban rainfall modification studies.

Description: Abstract Numerical modeling is applied to elucidate the formation mechanism of the large lower positive charge centers (LPCCs) observed during thunderstorms over the Tibetan Plateau based on the simulation of a storm at the northeastern boundary of the plateau. Four sensitivity tests were carried out to explore the impacts of inductive charging, reversal temperature, and the choice of noninductive charging scheme. The results show that the unique environmental conditions of the Qinghai‐Tibet Plateau, which include weak convection and low freezing level, are fundamental to the formation of large LPCC. A weakened charge density in the upper positive charge center highlights the role of a LPCC in lightning initiation although the charge density of the LPCC has no obvious change compared to that in the LPCC of the typical tripole structure. This accounts for Tibetan Plateau thunderstorms having low frequencies of lightning flashes, which occur mainly in the lower dipole. Inductive electrification, which provided more than 50% of the positive charge on graupel and increased the positive and negative charge on cloud drops by 2 orders of magnitude, is an important complement to the lower dipole of the tripole charge structure originally established by noninductive electrification. The inductive electrification also evidently enhances the LPCC and the middle negative charge center while slightly reducing the upper positive charge center. Subsequently, the lightning activity is strengthened, and lightning flashes are more likely to be initiated at the lower dipole. Varying the reversal temperatures and noninductive charging scheme does not fundamentally affect the formation of the LPCC.

Description: Abstract The melting layer of precipitation has a major impact on remote sensing and telecommunications. However, there is a shortage of observational studies to validate and constrain the melting layer models especially for high‐frequency radar bands. In this paper, we report how multifrequency radar Doppler spectra can be used to retrieve the melting layer attenuation at Ka‐ and W‐bands. The presented analysis is based on identifying Rayleigh scattering regions in radar Doppler spectra measurements where dual‐wavelength spectral ratios can be related to differential attenuation. We show that the estimated attenuation at Ka‐ and W‐bands agrees reasonably well with previously reported studies, but there are indications of differences at higher rain rates. We advocate that this technique can be applied to long‐term observations to advance our knowledge of the melting process. The parameterizations of melting layer attenuation as a function of rain rate and radar reflectivity are also presented.

Description: Abstract Lightning channels are made of plasma. As a consequence, the driving electrical current changes the channel's resistance in a nonlinear fashion. The resistance has an intricate dependence on the history of Joule heating and various cooling processes, as well as on the various kinetic processes that dictate the population balance of electrons within the channel. Such dependence cannot be captured by an analytic function, as often attempted. In this paper, we introduce a minimal numerical model that can qualitatively capture the temporal dynamics of the key plasma properties of a lightning channel, including its electric field, temperature, plasma density, radius, and the resulting nonlinear resistance. Through a series of novel parameterizations, we introduce six zero‐dimensional equations that can capture both nonequilibrium/low‐temperature and local thermodynamic equilibrium/high‐temperature plasma regimes. In this manuscript, we go to great lengths to validate the model, showing that it can reproduce the finite time scale of streamer‐to‐leader transition, replicate the negative differential resistance behavior of steady‐state plasma arcs, and properly describe the temporal evolution of temperature in a return stroke channel. Finally, the model is applied to the simulation of optical emissions from rocket‐triggered lightning strikes, explaining the measured delay between the rise of current and visible light, as well as reproducing the direct relationship between peak current and peak radiated power and between charge transferred to ground and total radiated energy.

Description: Abstract This study analyzes the summertime precipitation bias over the Central United States and its relationship to the simulated large‐scale environment and the convection scheme in the Energy Exascale Earth System Model Atmosphere Model version 1. This relationship is mainly examined in a set of short‐term hindcasts initialized with realistic large‐scale conditions for the summer of 2011. Besides the uniform 1° model resolution, we adopt Regionally Refined Meshes to increase the model resolution to 0.25° over the contiguous United States. Additional five‐year Atmospheric Model Intercomparison Project simulations are conducted to confirm that the results from the hindcasts are consistent with the climate runs. We find that the summertime dry precipitation bias over the Great Plains and the wet bias over the Rockies cannot be reduced simultaneously by changing resolution or tuning parameters. As for the diurnal cycle, Energy Exascale Earth System Model Atmosphere Model version 1 captures the general diurnal variation of the large‐scale moisture transport and the large‐scale upward motion over the Central United States. However, the diurnal cycle of precipitation over the Great Plains is out of phase with the diurnal variation of the large‐scale environment because the convective precipitation dominates the total precipitation and its diurnal cycle, and it does not directly respond to the local moisture convergence and the large‐scale upward motion. These results reemphasize the importance of improving the coupling of the convection to the large‐scale environment in reducing the summer precipitation bias over the Central United States in climate models with the resolution of ~0.25°.

Description: Abstract The rarity of reports in the literature of brief and spatially limited observations of drizzle at temperatures below ‐20°C suggest that riming and other temperature‐dependent cloud microphysical processes such as heterogeneous ice nucleation and ice crystal depositional growth prevent drizzle persistence in cold environments. In this study, we report on a persistent drizzle event observed by ground‐based remote‐sensing measurements at McMurdo Station, Antarctica. The temperatures in the drizzle‐producing cloud were below ‐25°C and the drizzle persisted for a period exceeding 7.5 hours. Using ground‐based, satellite, and reanalysis data we conclude that drizzle was likely present in parts of a widespread cloud field, which stretched more than ~1000 km along the Ross Ice Shelf coast. Parameter space sensitivity tests using two‐moment bulk microphysics in large‐eddy simulations constrained by the observations suggest that activated ice freezing nuclei (IFN) and accumulation‐mode aerosol number concentrations aloft during this persistent drizzle period were likely on the order of 0.2 L‐1 and 20 cm‐3, respectively. In such constrained simulations, the drizzle moisture flux through cloud base exceeds that of ice. The simulations also indicate that drizzle can lead to the formation of multiple peaks in cloud water content profiles. This study suggests that persistent drizzle at these low temperatures may be common at the low aerosol concentrations typical of the Antarctic and Southern Ocean atmospheres.

Description: ABSTRACT 36Cl production in the atmosphere is modulated by the magnetic field intensity of both the Sun and the Earth. The record of 36Cl concentration along with that of 10Be in ice cores may therefore provide information as to their variability. To better understand the 36Cl signal in glaciological archives, we measured its concentration in Talos Dome snow samples (mean accumulation rate of 8 g.cm‐2.yr‐1 water equivalent) spanning the 1955 to 1980 C.E. period with a resolution of one sample every 3 years, and in Vostok snow samples (mean accumulation rate of 1.96 g.cm‐2.yr‐1 water equivalent) spanning the 1949 to 2007 C.E. period with a six‐month resolution that had never before been obtained. Marine nuclear bomb tests in the late 1950s produced anthropogenic 36Cl which was injected into the stratosphere and spread around the globe. In the late 1950s this anthropogenic pulse led to an increase of 36Cl concentration at Talos Dome that was more than 100 times higher than the pre‐ and post‐ bomb values. It is noteworthy that the atmosphere of Vostok remains polluted by anthropogenic 36Cl today. This pollution results from gaseous H36Cl mobility at low accumulation sites and implies re‐emission of 36Cl from the snowpack that is not observed at Talos Dome. The 36Cl/Cl‐ ratio may be used to discriminate the stratospheric anthropogenic 36Cl source from the tropospheric natural 36Cl source, which allows us to discuss the immobile vs. mobile 36Cl in the Vostok snowpack.

Description: Abstract We use World Wide Lightning Location Network (WWLLN) data on the radiated radio frequency electromagnetic energy per stroke to identify the upper tip of the global lightning stroke energy distribution. The mean stroke energy is about 1,000 J per stroke in the very low frequency band between 5 and 18 kHz, while the distribution used in this paper is limited to strokes in that band above 1 MJ, about 3 orders of magnitude above the mean. It is shown that these energies are representative of the tip of the optical distribution, first identified by Turman (1977) above 10 GW per stroke, which he termed “superbolts.” The distribution peaks globally in the Northern Hemisphere winter (November–February) with most superbolts being found in the North Atlantic west of Europe, the winter Mediterranean Sea, and a strong local maximum over the Andes in South America. We identify regions with somewhat fewer superbolts in the North Pacific east of Japan in winter, along the equator of the Atlantic and Indian Oceans and south of South Africa. We find very few superbolts during April–October each year. While superbolts are scattered around the globe, the local occurrence peaks do not coincide with the usual three main lightning “chimneys.” Unlike the distribution of all normal global lightning, we find superbolts predominantly over the oceans and seas, with fewer over the continents, just the opposite of all global lightning.

Description: Abstract Atmospheric rivers (ARs) manifest as transient filaments of intense water vapor transport that contribute to synoptic‐scale extremes and interannual variability of precipitation. Despite these influences, the synoptic to planetary scale processes that lead to ARs remain inadequately understood. In this study, North Pacific ARs within the November‐April season are objectively identified in both reanalysis data and the Community Earth System Model version 2 (CESM2), and atmospheric patterns preceding AR landfalls beyond one week in advance are examined. Latitudinal dependence of the AR processes is investigated by sampling events near the Oregon (45°N, 230°E) and southern California (35°N, 230°E) coasts. Oregon ARs exhibit a pronounced anticyclone emerging over Alaska 1‐2 weeks before AR landfall that migrates westward into Siberia; dual midlatitude cyclones developing over southeast coastal Asia and the northeast Pacific; and a zonally elongated band of enhanced water vapor transport spanning the entire North Pacific basin that guides anomalous moisture toward the North American west coast. The precursor high‐latitude anticyclone corresponds to a significant increase in atmospheric blocking probability, suppressed synoptic eddy activity, and an equatorward‐shifted storm track. Southern California ARs also exhibit high‐latitude blocking but have an earlier‐developing and more intense northeast Pacific cyclone. Compared to reanalysis, CESM2 underestimates Northeast Pacific AR frequencies by 5‐20% but generally captures AR precursor patterns well, particularly for Oregon ARs. Collectively, these results indicate that the identified precursor patterns represent physical processes that are central to ARs and are not simply an artifact of statistical analysis.

Description: Abstract The spatial non‐uniformity of the electric field in air discharges, such as streamers, can influence the accuracy of spectroscopic diagnostic methods and hence the estimation of the peak electric field. In this work, we use a self‐consistent streamer discharge model to investigate the spatial non‐uniformity in streamer heads and streamer glows. We focus our analysis on air discharges at atmospheric pressure and at the low pressure of the mesosphere. This approach is useful to investigate the spatial non‐uniformity of laboratory discharges as well as sprite streamers and blue jet streamers, two types of Transient Luminous Event (TLE) taking place above thunderclouds. This characterization of the spatial non‐uniformity of the electric field in air discharges allows us to develop two different spectroscopic diagnostic methods to estimate the peak electric field in cold plasmas. The commonly employed method to derive the peak electric field in streamer heads underestimates the electric field by about 40‐50 % as a consequence of the high spatial non‐uniformity of the electric field. Our diagnostic methods reduce this underestimation to about 10‐20%. However, our methods are less accurate than previous methods for streamer glows, where the electric field is uniformly distributed in space. Finally, we apply our diagnostic methods to the measured optical signals in the Second Positive System of N2 and the First Negative System of of sprites recorded by Armstrong et al. (1998) during the SPRITE's 95 and 96 campaigns.

Description: Abstract It is increasingly acknowledged that cold pools can influence the initiation of new convective cells. Yet, the full complexity of convective organization through cold pool interaction is poorly understood. This lack of understanding may partially be due to the intricacy of the dynamical pattern formed by precipitation cells and their cold pools. Additionally, how exactly cold pools interact is insufficiently known. To better understand this dynamics, we develop a tracking algorithm for cold pool gust fronts. Rather than tracking thermodynamic anomalies, which do not generally coincide with the gust front boundaries, our approach tracks the dynamical cold pool outflow. Our algorithm first determines the locus of the precipitation event. Second, relative to this origin and for each azimuthal bin, the steepest gradient in the near‐surface horizontal radial velocity vr is employed to determine the respective locus of the cold pool gust front edge. Steepest vr‐gradients imply largest updraft velocities, hence strongest dynamical triggering. Results are compared to a previous algorithm based on the steepest gradient in temperature — highlighting the benefit of the method described here in determining dynamically active gust front regions. Applying the method to a range of numerical experiments, the algorithm successfully tracks an ensemble of cold pools. A linear relation emerges between the peak rain intensity of a given event and maximal vr for its associated cold pool gust front — a relation found to be nearly independent of the specific sensitivity experiment.

Description: Abstract Subseasonal‐to‐seasonal (S2S) forecasts of Northern Hemisphere (NH) extratropical winter weather patterns continue to be a challenging venture. Past studies have considered the individual influence of two modes of climate variability – the Madden‐Julian Oscillation (MJO) and the state of the stratospheric polar vortex (SPV) – on the NH polar jet stream and associated weather regimes. This study takes a different approach and quantifies the joint influence of the SPV and the MJO on NH S2S winter weather patterns. Using ERA‐Interim, we illustrate that variability associated with the MJO primarily influences tropospheric patterns across the North Pacific and western North America 10 to 14 days later, while SPV variability has a stronger influence on tropospheric patterns over the North Atlantic and Europe for the same lags. Over the rest of North America and into the Arctic, however, constructive and destructive interference between the MJO and the SPV teleconnections yield unique jet stream and temperature patterns that differ from the single‐mode composites. As such, S2S forecasts of temperature and jet stream patterns across much of North America may improve in accuracy by considering both modes. The study also shows that MJO Phase 2,3 events influence the resulting tropospheric circulation via primarily a tropospheric pathway. By contrast, MJO Phase 7,8 events modify both the tropospheric and stratospheric circulation with potential feedbacks on the tropospheric circulation at longer lags. Hence, the study suggests another benchmark by which to test S2S dynamical prediction systems, including the importance for modeling stratosphere‐troposphere coupling dynamics correctly.

Description: Abstract Four years (201404‐201803) of Global Precipitation Measurement (GPM) Precipitation Features (PF) data along with co‐located the Modern Era Retrospective‐Analysis for Research and Applications (MERRA‐2) model data are used to identify Freezing Rain Features (FRFs). A PF with presence of both melting layer (maximum temperature of the vertical column 〉 4 0C), and a layer of subfreezing air (2 m temperature 〈 0 0C) adjacent to the surface is considered an FRF. During four years of observations, GPM and MERRA‐2 identify approximately 3096 FRFs globally (650S‐650N). Most of them are observed over northern hemispheric land in the winter season. The majority of FRFs originate through the “melting process” whereas only 35 features are associated with “warm rain” process. The locations, seasonal and diurnal distribution patterns of the FRFs over the United Stated are well matched with the ground‐based observations. The ground‐based observations verify approximately 70% of the FRFs over the United States. Ku band radar properties show that FRFs are found relatively shallower (2‐5 km) and less intense (〈 27 dBZ) than precipitation features in general, but deeper and more intense than Snow Features (SFs). Passive microwave properties show that FRFs Tbs and Polarization Corrected Temperature (PCT) are warmer than SFs at all GMI channels with the largest differences in 166 GHz. The enhancement in Tbs are more distinct with “warm rain” FRFs. FRF Tb tends to decrease as echo top height increases at all GMI channels except for 183 GHz, where Tbs have lack of dependence on echo top height.

Description: Abstract Global warming is expected to produce modifications in the intensity, as well as in the seasonality and spatiotemporal structure of extreme precipitation. In the present study, the temporal evolution of simulated daily and sub‐daily precipitation extremes was analyzed to assess how they respond to climate warming over different time horizons. Pooling series from the recent 50‐member Canadian Regional Climate Model v5 Large Ensemble (CRCM5‐LE), the probability distributions, date and time of occurrences, and spatiotemporal structure of simulated Annual Maxima (AM) precipitation were analyzed at various spatial scales and for durations between 1h and 3 days. In agreement with previous studies, the results underline the large increases in AM precipitation quantiles, especially for the shortest durations and for the more extreme events (i.e. longest return periods), and modifications in their spatiotemporal scaling properties and annual and diurnal cycles. For instance, sub‐daily AM extremes are expected to occur later in the evening, while, no matter the duration, the extremes are expected to occur over a wider period of the year in future climate. Finally, the analysis of projected AM probability distributions showed that heavy‐tail Generalized Extreme Value (GEV) distributions will most likely be observed in the future climate, with some model grid boxes experiencing a significant increase of GEV shape parameters. These results may have major consequences in terms of the occurrence and possible impacts of the most extreme precipitation events.

Description: Abstract Interactions between convection and the Saharan Air Layer in the tropical Atlantic Ocean are quantified using a novel compositing technique that leverages geostationary cloud observations to add temporal context to the polar orbiting CloudSat and CALIPSO satellites allowing aerosol optical depth (AOD) changes to be tracked throughout a typical convective storm lifecycle. Four years of CALIPSO observations suggest that approximately 20% of the dust mass in every 10° longitude band between 10°W and 80°W is deposited into the ocean. Combining a new convective identification algorithm based on hourly geostationary cloud products with AODdust profiles along the CALIPSO track reveals that wet scavenging by convection is responsible for a significant fraction of this deposition across the Atlantic. Composites of four years of convective systems reveal that, on average, convection accounts for 15% ± 7% of the dust deposition in each longitude band relative to pre‐convective amounts, implying that dry deposition and scavenging by non convective events are responsible for the remaining 85% of dust removal. In addition, dust layers are detrained at upper levels of the atmosphere between 8 km and 12 km by convective storms across the Atlantic. The dust budget analysis presented here indicates that convection lofts 1.5% ± 0.6% of dust aerosol mass to altitudes greater than 6 km. This may have significant implications for cloud formation downstream of convection since lofted dust particles can act as effective ice nucleating particles (INP), altering cloud microphysical and radiative properties, latent heating, and precipitation rates.