ALBERT

Description: The diurnal variations from a high-resolution regional climate model (Regional Spectral Model; RSM) are analyzed from 6 independent decade long integrations using lateral boundary forcing data separately from the National Centers for Environmental Prediction Reanalysis 2 (NCEPR2), and European Center for Medium-Range Weather Forecasts (ECMWF) 40-year Reanalysis (ERA40) and the 20 th Century Reanalysis (20CR). With each of these lateral boundary forcing data, the RSM is integrated separately using two convection schemes: the Relaxed Arakawa-Schubert (RAS) and Kain-Fritsch (KF) schemes. The results show that RSM integrations forced with 20CR have the least fidelity in depicting the seasonal cycle and diurnal variability of precipitation and surface temperature over the Southeastern United States (SEUS). The remaining four model simulations show comparable skills. The differences in the diurnal amplitude of rainfall during the summer months of the 20CR forced integration from the corresponding NCEPR2 forced integration, for example, is found to be largely from the transient component of the moisture flux convergence. The root mean square error (RMSE) of the seasonal cycle of precipitation and surface temperature of the other four simulations (not forced by 20CR) were comparable to each other and highest in the summer months. But the RMSE of the diurnal amplitude of precipitation and the timing of its diurnal zenith were largest during winter months and least during summer and fall months in the four model simulations (not forced by 20CR). The diurnal amplitude of surface temperature in comparison showed far less fidelity in all models. The phase of the diurnal maximum of surface temperature however showed significantly better validation with corresponding observations in all of the 6 model simulations

Description: A high-resolution transect of atmospheric soundings across the Kuroshio Current in the East China Sea was conducted onboard a ship in June 2012 with the objective of analyzing the influence of the complex sea surface temperature (SST) distribution on the Baiu frontal zone (BFZ). Expendable bathythermograph castings and continuous surface meteorological observations were also examined. Two distinct mesoscale atmospheric fronts, characterized by changes of wind direction in the lower troposphere and surface air temperature (SAT), were found in the BFZ. One (northern) atmospheric front was observed around the SST front in relation to a warm water tongue extending from the Kuroshio. A high SST region around the northern atmospheric front enhances unstable near surface stratification and intensifies turbulent heat flux. They help modify the marine atmospheric boundary layer in the BFZ. The other (southern) atmospheric front was at the southern end of the BFZ. Intense evaporation over the Kuroshio and moisture transport by southerly winds were important in forming the conditionally unstable air masses in the lower troposphere of the BFZ.

Description: We analyze the variability of mean age of air (AoA) and of the local effects of the stratospheric residual circulation and eddy mixing on AoA within the framework of the isentropic zonal mean continuity equation. AoA for the period 1988–2013 has been simulated with the Lagrangian chemistry transport model CLaMS driven by ERA-Interim winds and diabatic heating rates. Model simulated AoA in the lower stratosphere shows good agreement with both in-situ observations and satellite observations from MIPAS (Michelson Interferometer for Passive Atmospheric Sounding), even regarding interannual variability and changes during the last decade. The interannual variability throughout the lower stratosphere is largely affected by the QBO-induced circulation and mixing anomalies, with year-to-year AoA changes of about 0.5 years. The decadal 2002–2012 change shows decreasing AoA in the lowest stratosphere, below about 450 K. Above, AoA increases in the NH and decreases in the SH. Mixing appears to be crucial for understanding AoA variability, with local AoA changes resulting from a close balance between residual circulation and mixing effects. Locally, mixing increases AoA at low latitudes (40S-40N) and decreases AoA at higher latitudes. Strongest mixing occurs below about 500 K, consistent with the separation between shallow and deep circulation branches. The effect of mixing integrated along the air parcel path, however, significantly increases AoA globally, except in the polar lower stratosphere. Changes of local effects of residual circulation and mixing during the last decade are supportive of a strengthening shallow circulation branch in the lowest stratosphere and a southward shifting circulation pattern above.

Description: The interaction between sea ice and atmosphere depends strongly on the near-surface transfer coefficients for momentum and heat. A parametrization of these coefficients is developed on the basis of an existing parametrization of drag coefficients for neutral stratification that accounts for form drag caused by the edges of ice floes and melt ponds. This scheme is extended to better account for the dependence of surface wind on limiting cases of high and low ice concentration and to include near-surface stability effects over open water and ice on form drag. The stability correction is formulated on the basis of stability functions from Monin-Obukhov similarity theory and also using the Louis concept with stability functions depending on thebulk Richardson numbers. Furthermore, a parametrization is proposed that includes the effect of edge related turbulence also on heat transfer coefficients. The parametrizations are available in different levels of complexity. The lowest level only needs sea ice concentration and surface temperature as input while the more complex level needs additional sea ice characteristics. An important property of our parametrization is that form drag caused by ice edges depends on the stability over both ice and water which is in contrast to the skin drag over ice. Results of the parametrization show that stability has a large impact on form drag and, thereby, determines the value of sea ice concentration for which the transfer coefficients reach their maxima. Depending on the stratification, these maxima can occur anywhere between ice concentrations of 20 and 80%.

Description: The eruption of Mount Pinatubo in 1991 injected a large amount of SO 2 into the stratosphere, which formed sulfate aerosols. Increased scattering and absorption of UV radiation by the enhanced stratospheric SO 2 and aerosols decreased the amount of UV radiation reaching the troposphere, causing changes in tropospheric photochemistry. These changes affected the oxidizing capacity of the atmosphere and the removal rate of CH 4 in the years following the eruption. We use the three-dimensional chemistry transport model TM5 coupled to the aerosol microphysics module M7 to simulate the evolution of SO 2 and sulfate aerosols from the Pinatubo eruption. Their effect on tropospheric photolysis frequencies and concentrations of OH and CH 4 are quantified for the first time. We find that UV attenuation by stratospheric sulfur decreased the photolysis frequencies of both ozone and NO 2 by about 2% globally, decreasing global OH concentrations by a similar amount in the first two years after the eruption. SO 2 absorption mainly affects OH primary production by ozone photolysis, while aerosol scattering also alters OH recycling. The effect of stratospheric sulfur on global OH and CH 4 is dominated by the effect of aerosol extinction, while SO 2 absorption contributes by 12.5% to the overall effect in the first year after the eruption. The reduction in OH concentrations causes an increase in the CH 4 growth rate of 4 and 2 ppb/yr in the first and second year after the eruption, respectively, contributing 11 Tg to the 27 Tg observed CH 4 burden change in late 1991 and early 1992.

Description: Emission source contributions of tropospheric ozone (O 3 ) were comprehensively investigated by using the higher-order decoupled direct method (HDDM) for sensitivity analysis and the ozone source apportionment technology (OSAT) for mass balance analysis in the comprehensive air-quality model with extensions (CAMx). The response of O 3 to emissions reductions at various levels in mainland China, Korea, and Japan were estimated and compared with results calculated by the brute force method (BFM) where one model parameter is varied at a time. Emissions were assessed at three receptor sites in Japan that experienced severe pollution events in May 2009. For emissions from China, HDDM assessed O 3 response with a bias of only up to 3 ppbv (a relative error of 4.5%) even for a 50% reduction, but failed to assess a more extreme reduction. OSAT was reasonably accurate at 100% reduction, with a −4 ppbv (−7%) bias, but was less accurate at moderate ranges of reduction (⊔50-70%). For emissions from Korea and Japan, HDDM captured the nonlinear response at all receptor sites and at all reduction levels to within 1% in all but one case; however, the bias of OSAT increased with the increasing reduction of emissions. One possible reason for this is that OSAT does not account for NO titration. To address this, a term for potential ozone (PO; O 3 and NO 2 together) was introduced. Using of PO instead of O 3 improved the performance of OSAT, especially for emissions reductions from Korea and Japan. The proposed approach with PO refined the OSAT results and did not degrade HDDM performance.

Description: Using observed precipitation and the NCEP-NCAR reanalysis, the changes in the metrics of the summer precipitation in China, the dominance of frequency and intensity of daily extreme precipitation, and the linkage with changes in moisture and air temperature are explored. Results show that over the recent 50 years, total summer rainfall increased over the southeast and the west and decreased over the northeast. The changes in the frequency, identified with the 95% threshold and Poisson regression, and rainfall extremes show similar spatial patterns. The relative importance of the changes in frequency and intensity in the variability and changes in extreme precipitation is estimated. It is shown that, while the interannual variability of the rainfall amount is dominated by the frequency change in almost all stations, the long-term change of rainfall amount can be dominated by both frequency and intensity, depending on the station. The change in the rainfall total is linked to changes in atmospheric moisture and temperature. Results show that the variability and change of the rainfall total can be dominated by changes in both moisture and air temperature, and the relative importance depends on the region.

Description: We examine variations in water vapor in air entering the stratosphere through the tropical tropopause layer (TTL) over the past three decades in satellite data and in a trajectory model. Most of the variance can be explained by three processes that affect the TTL: the quasi-biennial oscillation, the strength of the Brewer-Dobson circulation, and the temperature of the tropical troposphere. When these factors act in phase, significant variations in water entering the stratosphere are possible. We also find that volcanic eruptions, which inject aerosol into the TTL, affect the amount of water entering the stratosphere. While there is clear decadal variability in the data and models, we find little evidence for a long-term trend in water entering the stratosphere through the TTL over the past 3 decades.

Description: Eight months (June 2011-January 2012) of aerosol property data were obtained at the remote site of Alborán Island (35.95 ° N, 3.03 ° W) in the western Mediterranean basin. The aim of this work is to assess the aerosol properties according to air-mass origin and transport over this remote station with a special focus on air mass transport from North Africa. For air masses coming from North Africa, different aerosol properties showed strong contributions from mineral dust lifted from desert areas. Nevertheless, during these desert dust intrusions, some atmospheric aerosol properties are clearly different from pure mineral dust particles. Thus, Angström exponent α(440–870) presents larger values than those reported for pure desert dust measured close to dust source regions. These results combine with α(440,670)-α(670,870) ≥ 0.1 and low single scattering albedo (ω(λ)) values, especially at the largest wavelengths. Most of the desert dust intrusions over Alborán can be described as a mixture of dust and anthropogenic particles. The analyses support that our results apply to North Africa desert dust air masses transported from different source areas. Therefore, our results indicate a significant contribution of fine absorbing particles during desert dust intrusions over Alborán arriving from different source regions. The aerosol optical depth (AOD) data retrieved from sun-photometer measurements have been used to check MODIS retrievals, and they show reasonable agreement, especially for North African air masses.

Description: The impact of aircraft type on contrail evolution is assessed using a large-eddy simulation (LES) model with Lagrangian ice microphysics. Six different aircraft ranging from the small regional airliner Bombardier CRJ to the largest aircraft Airbus A380 are taken into account. Differences in wake vortex properties and fuel flow lead to considerable variations in the early contrail geometric depth and ice crystal number. Larger aircraft produce contrails with more ice crystals (assuming that the number of initially generated ice crystals per kg fuel is constant). These initial differences are reduced in the first minutes, as the ice crystal loss during the vortex phase is stronger for larger aircraft. In supersaturated air, contrails of large aircraft are much deeper after 5 minutes than those of small aircraft. A parametrization for the final vertical displacement of the wake vortex system is provided, depending only on the initial vortex circulation and stratification. Cloud resolving simulations are used to examine whether the aircraft-induced initial differences have a long-lasting mark. These simulations suggest that the synoptic scenario controls the contrail-cirrus evolution qualitatively. However, quantitative differences between the contrail-cirrus properties of the various aircraft remain over the total simulation period of six hours. The total extinctions of A380-produced contrails are about 1.5 to 2.5 times higher than those from contrails of a Bombardier CRJ.