Journal of Geophysical Research: Atmospheres最新文献

Northwestern South America (NWSA) is the rainiest region on Earth, with diurnal precipitation exhibiting extensive westward offshore propagation of up to about 1,200 km in boreal spring (March-May). The diurnal offshore precipitation propagation begins slowly (3–10 m s⁻¹) near the coast of NWSA (<200 km) but accelerates significantly (∼20 m s⁻¹) and shows an afternoon enhancement far from the coast (>400 km). However, the driving mechanisms behind this long-distance precipitation propagation remain unclear. Using a new cloud tracking and classification data set, we found that mesoscale convective systems (MCSs) are the dominant precipitation contributors in the offshore region of NWSA. Cloud tracking shows that the long-distance propagation and the afternoon enhancement of diurnal precipitation primarily originate from MCSs initiated in the early morning, either over open oceans or from the coast of Central America. Composite tendency analysis shows that MCSs initiated near the coast of Central America have significant upward cooling and moistening signals starting from the surface before initiation. Further analysis of surface diurnal perturbation fields indicates that the land breeze is the primary driving mechanism for MCS initiation. Conversely, for MCSs initiated over open oceans, a significant downward cooling signal from 400 hPa is observed ∼7 hr before initiation, corresponding to the passage of diurnal gravity waves emitted from the Andes. Additionally, our findings highlight the critical role of lower and mid-level moisture conditions in MCS initiation, alongside the influence of gravity waves.

The associated dynamics of mid-level and low-level vortices and their relative importance in tropical cyclogenesis (TCG) are less studied and understood to date. The issues are studied using the sensitivity of TCG to radiation as a tool. Three groups of idealized full-physics ensemble experiments with distinct solar insolation conditions are conducted: the control group with real diurnal radiation, the day-only, and the night-only groups for comparison. New findings are that the daytime radiative condition is more favorable for mid-level vorticity development than the nighttime one. This is because stratiform clouds cover a large area and dominate radiative heating, inducing a greater vertical gradient of diabatic heating at altitudes of 4–6 km through increased evaporation cooling most at about 4 km and decrease of latent heat release above about 6 km. However, the vorticity budgets show that the stronger mid-level vortex cannot extend downward to enhance the low-level vortex. Moreover, daytime solar radiation increases the low-level stability and suppresses convective development at the edge of cold pools, weakening low-level convergence of advective vorticity flux, and limiting low-level vorticity enhancement. Therefore, TCG is delayed in day-only experiments compared to others. These results reveal that the acceleration of TCG by longwave radiation is primarily caused by the enhancement of low-level convergence of advective vorticity flux, which tends to occur at nighttime without solar insolation. The study suggests that the development of low-level vortex is more important to TCG relative to mid-level vortex with radiative effects.

Brown carbon (BrC) is known to have a great impact on atmospheric radiative forcing, but its absorption characteristics at the molecular level is not well understood. This study investigated the seasonal variations of light absorption characteristics and molecular composition of BrC in Xi'an, China. Results showed that BrC exhibited higher light absorption capacity in cold (autumn and winter) than warm seasons (spring and summer). Nitrogen-containing organic compounds were identified as important BrC chromophores. Oxidized-N compounds originated from biomass burning emissions and NO_x/NO₃⁻ mediated oxidation reactions were predominant in cold seasons, whereas reduced-N compounds mainly formed from NH₃/NH₄⁺ mediated reactions were abundant in warm seasons. These results contribute to a better understanding of formation mechanisms and light absorption characteristics of nitrogen-containing BrC chromophores in PM_2.5.

The impact of Antarctic sea ice reduction during early austral winter on the austral winter Antarctic stratospheric polar vortex is investigated using reanalysis data set and model simulations. Both reanalysis data set and model simulations show that the reduction of Antarctic sea ice during early austral winter leads to a northward displacement of the tropospheric mid-latitude jet, resembling the negative phase of the Southern Annular Mode. Meanwhile, the reduction of sea ice induces a weaker Antarctic stratospheric polar vortex during winter, which is accompanied by a weaker polar night jet. Further analysis indicates that the Antarctic sea ice reduction could lead to a greater excitation of Rossby waves and significant positive geopotential height anomalies over the Antarctic continent. The zonal wave 1 and 2 components of geopotential height anomalies are in phase with the climatology, corresponding to enhanced upward propagation of wave activity flux in early austral winter. Meanwhile, the reduction of sea ice in early austral winter could result in a more favorable atmospheric environment for the propagation of planetary waves into the stratosphere. These processes ultimately weaken the Antarctic stratospheric polar vortex and the polar night jet in winter. The reduction of sea ice in the Amundsen Sea sector enhances the upward propagation of planetary wave, while the reduction of sea ice in the Indian Ocean sector has the opposite effect.

Size-resolved organic functional group (OFG) concentrations in aerosol particles were measured during January−February 2019 in Hefei, a major urban city in eastern China. Alkane, carboxylic acid, and hydroxyl groups were the main components of the organic aerosol (OA), contributing 35.0%, 22.5%, and 21.8%, respectively. The mass concentrations and size distributions of the OFGs strongly depended on ambient relative humidity (RH). Specifically, the OFG mass concentrations were 50% higher during the high-RH periods than the low-RH periods. The peak size of the OFGs size distribution was 0.56–1 μm during the high-RH periods, while a peak size of 0.32–0.56 μm was observed during the low-RH periods. In addition, the concentrations of the total OA, carboxylic acid, and hydroxyl groups were positively correlated with the photochemical age (PCA), with the high-RH samples increasing faster (greater slope) with PCA and being associated with a higher oxygen-to-carbon ratio. These results suggest that aqueous-phase processing likely enhanced the production of the OFGs, especially oxygenated functional groups. Further analysis of size distribution of the oxygenated OFGs and the viscosity of aerosols indicates that the secondary OFGs were formed via surface-limited mechanisms under low-RH conditions and volume-limited mechanisms under high-RH conditions. These findings reinforce the important role of multiphase chemistry in the formation and evolution of OAs.

The primary objective of the GRACE Follow-On satellite mission is to measure temporal changes in the Earth's gravitational field. Distance variations between the two GRACE-FO satellites, recorded by a K-Band Ranging system and a new Laser Ranging Interferometer (LRI), are significantly influenced by atmospheric mass redistribution. We investigate whether the sub-monthly variations in atmospheric water mass, precipitation, and changes in total water storage during the extreme flood event in western Europe in 2021 were sufficiently large to influence the satellite gravity field measurements, if the GRACE-FO satellites would have passed directly over the region. We use several data sets such as weather forecasts (ICON-D2 model), hydrological simulations (ParFlow/CLM), observations as well as reanalyses, showing the high uncertainty between different estimations of the considered variables: total precipitable water, total precipitation, and total water storage. Our estimates suggest a potentially noticeable impact of the 2021 flood event on the GRACE-FO satellites. Although it was globally seen a rather small event, even the atmospheric water mass beyond water vapor, which is not considered within the de-aliasing process, is close to the LRI detection accuracy. This is particularly relevant for future gravity missions, which will use the LRI with potentially higher sensitivity as their main instrument. Sub-monthly variations in the total atmospheric water mass, that is, beyond water vapor of huge extreme precipitation events should be investigated further to reduce potential future aliasing errors.

Unprecedented heatwaves accompanying severe droughts hit South Europe in May‒July 2022. From the interdisciplinary perspective, this study revealed that the extreme climate events can intensify European energy crisis through pushing up electricity demand and limiting renewable energy supply that makes up more than one-third of gross electricity consumption in the European Union (EU). The record-high electricity demand over South Europe in May‒July was closely associated with the long-lasting extreme heatwaves. On the other hand, the anomalous high-pressure system over Europe contributed to the severe heatwaves and the shortage of renewable power generation, for instance, through affecting the wind speed. The wind power generation over South Europe was reduced by the anomalous anticyclone which weakened low-tropospheric prevailing northwesterly wind. The results indicate a potential intensive conflict between the increasing power demand and the decreasing renewable power generation in Europe that are induced simultaneously by the extreme heatwaves. Therefore, it is suggested that the energy sectors' resilience to extreme climate events should be well built as the EU sets to significantly increase the renewable energy target to meet the goal of climate-neutral by 2050.

Atmospheric blocking is closely linked to the occurrence of extreme weather events. However, low-resolution Earth system models often underestimate the frequency of blocking, undermining confidence in future projections. In this study, we use the high-resolution Community Earth System Model (CESM-HR; 25 km atm and 10 km ocean) to show that CESM-HR reduces biases in atmospheric blocking for both winter and summer, particularly for events lasting longer than 10 days. This improvement is partly due to reduced sea surface temperature biases at higher resolution. Additionally, applying a bias correction to the 500 hPa geopotential height further enhances blocking frequency simulations, highlighting the crucial role of the mean state. Under the Representative Concentration Pathway 8.5 scenario, CESM-HR projects a decrease in wintertime blocking over regions such as the Euro-Atlantic and Chukchi-Alaska, consistent with previous studies. In contrast, summer blocking is expected to become more frequent and persistent, driven by weakened zonal winds. The blocking center shifts from historical locations over Scandinavia and eastern Russia to central Eurasia, significantly increasing blocking over the Ural region. Summer blocking frequency over the Scandinavia-Ural region may eventually surpass historical winter blocking over the Euro-Atlantic. This increase in summer blocking could exacerbate summer heatwaves in a warming climate, making severe heatwaves, like those observed recently, more common in the future.