JGR–Atmospheres

Process modeling of Aerosol-cloud interaction (ACI) is essential to bridging gaps between observational analysis and climate modeling of aerosol effects in the Earth system and eventually reducing climate projection uncertainties. In this study, we examine ACI in summertime precipitating shallow cumuli observed during the Aerosol Cloud meTeorology Interactions oVer the western ATlantic Experiment (ACTIVATE). Aerosols and precipitating shallow cumuli were extensively observed with in-situ and remote-sensing instruments during two research flight cases on 02 June and 07 June, respectively, during the ACTIVATE summer 2021 deployment phase. We perform observational analysis and large-eddy simulation (LES) of aerosol effect on precipitating cumulus in these two cases. Given the measured aerosol size distributions and meteorological conditions, LES is able to reproduce the observed cloud properties by aircraft such as liquid water content (LWC), cloud droplet number concentration (N c ) and effective radius r eff. However, it produces smaller liquid water path (LWP) and larger N c compared to the satellite retrievals. Both 02 and 07 June cases are over warm waters of the Gulf Stream and have a cloud top height over 3 km, but the 07 June case is more polluted and has larger LWC. We find that the N a -induced LWP adjustment is dominated by precipitation feedback for the 2 June precipitating case and there is no clear entrainment feedback in both cases. An increase of cloud fraction due to a decrease of aerosol number concentration is also shown in the simulations for the 02 June case.

No abstract is available for this article.

Elevated moisture transport over the Antarctic ice sheet can increase snowfall and ice mass. Previous studies used ground-based observations, reanalysis products, and atmospheric models to evaluate the relationship between extreme moisture transport and snowfall properties. Here, we build on previous studies by combining reanalysis and CloudSat radar snowfall retrievals to examine impacts of extreme moisture intrusions on changes in snowfall frequency and intensity over glacier basins on the Antarctic ice sheet. We examine the impacts of enhanced moisture transport events on snowfall frequency and intensity over two different regions on the Antarctic ice sheet: the Amery Ice Shelf of East Antarctica and the Thwaites and Pine Island glacier basins of West Antarctica. We determine when the median integrated water vapor transport from reanalysis exceeds the 95th percentile within the glacier basins of interest to define enhanced moisture transport events. We then use CloudSat radar snowfall retrievals to evaluate differences between snowfall frequency and intensity during enhanced water vapor transport events compared to the seasonal means for 2007–2010. We find that enhanced moisture transport events over the Amery Ice Shelf and Thwaites and Pine Island glacier basins coincide with higher snowfall frequency and intensity. These enhanced moisture transport events have the potential to alter surface mass balance within glacier basins, with implications for future rates of sea level rise.

We analyze the performance of the European Centre for Medium-Range Weather Forecasts (ECMWF) and UK Met Office (UKMO) meteorological models in predicting offshore blowing wind in coastal areas. Our attention is mainly on the Mediterranean coast, up to 200 km distance from the shore. We compare forecast neutral winds with Advanced Scatterometer measurements. The results indicate that the ECMWF forecasts systematically underestimate wind speed with respect to scatterometer data, while the UKMO model tends to overestimate. A cross-analysis suggests that, better than fetch, model biases are a function of the model horizontal discretization, hence of the number of grid steps the wind runs over the sea. The steepness and roughness of the land orography before entering the sea, together with the related drag parameters, appear to have a strong role in determining the coastal offshore wind values. Surface drag over land tends to reduce the related wind speed, and it takes a few grid points to adjust to the smooth sea surface conditions. This is supported by a detailed study with a high resolution grid, but different orography resolutions. In these simulations, practically identical coastal wind speed distributions are found when the subgrid orography schemes are switched off. Vertical cross-sections of potential temperature and wind in bora and mistral conditions illustrate the strong role of gravity waves, wind channeling and turbulent diffusion in coastal numerical weather prediction. Besides local wave modeling, this may be relevant for offshore wind farms, typically situated within 5–50 km from the coast.

Shifts in Southern Ocean (SO, 40–85°S) shortwave cloud feedback (SW FB ) toward more positive values are the dominant contributor to higher effective climate sensitivity (ECS) in Coupled Model Intercomparison Project Phase 6 (CMIP6) models. To provide an observational constraint on the SO SW FB , we use a simplified physical model to connect SO SW FB with the response of column-integrated liquid water mass (LWP) to warming and the susceptibility of albedo to LWP in 50 CMIP5 and CMIP6 GCMs. In turn, we predict the responses of SO LWP using a cloud-controlling factor (CCF) model. The combination of the CCF model and radiative susceptibility explains about 50% of the variance in the GCM-simulated SW FB in the SO. Observations of SW radiation fluxes, LWP, and CCFs from reanalysis are used to constrain the SO SW FB . Observations suggest a SO LWP increase in response to warming and albedo susceptibility to LWP that is on the lower end relative to GCMs. The overall constraint on the contribution of SO to global mean SW FB is −0.168 to +0.051 W m−2 K−1, relative to −0.277 to +0.270 Wm−2 K−1. In summary, observations suggest SO SW FB is less likely to be as extremely positive as predicted by some CMIP6 GCMs, but more likely to range from moderately negative to weakly positive.

Disentangling the response of tropical convective updrafts to enhanced aerosol concentrations has been challenging. Leading theories for explaining the influence of aerosol concentrations on tropical convection are based on the dynamical response of convection to changes in cloud microphysics, neglecting possible changes in the environment. In recent years, global convection-permitting models (GCPM) have been developed to circumvent problems arising from imposing artificial scale separation on physical processes associated with deep convection. Here, we use a global model in the convective gray zone that partially simulates deep convection to investigate how enhanced concentrations of aerosols that act as cloud condensate nuclei (CCN) impact tropical convection features by modulating the convection-circulation interaction. Results from a pair of idealized non-rotating radiative-convective equilibrium simulations show that the enhanced CCN concentration leads to weaker large-scale circulation, the closeness of deep convective systems to the moist cluster edges, and more mid-level cloud water at an equilibrium state in which convective self-aggregation occurred. Correspondingly, the enhanced CCN concentration modulates how the physical processes that support or oppose convective aggregation maintain the aggregated state at equilibrium. Overall, the enhanced CCN concentration facilitates the development of deep convection in a drier environment but reduces mean precipitation. Our results emphasize the importance of allowing atmospheric phenomena to evolve continuously across spatial and temporal scales in simulations when investigating the response of tropical convection to changes in cloud microphysics.

The topography plays an essential role in the initiation and development of precipitating clouds, therefore has a profound effect on the ultimate spatial distribution of precipitation. This study investigates the fine-scale characteristics of a synoptic-induced precipitation event in Southwest China, a region characterized by a sequence of steep mountains aligned roughly north-south. The convection-permitting simulation successfully reproduces the observed rainband induced by a synoptic-scale shear line. The spatial distribution of accumulated precipitation over three small-scale mountains (named M1, M2, and M3 from east to west) exhibits distinct inhomogeneity. The accumulated precipitation is significantly enhanced on the western slope of M1, the high-altitude area of M2, and the eastern slope of M3. The low-level vortex generated on the western slope of M1, as well as the convergence established over M2 and the eastern slope of M3, dynamically contributes to the enhanced precipitation over the various mountain locations. As the highest of the three mountains, M2 exhibits pronounced blocking effects on the mesoscale circulations. Additional sensitivity experiment demonstrates that the mesoscale circulations and corresponding precipitation are sensitive to the highest elevation in the continuous mountains. The rainfall accumulation over M2 (M3) could decrease (increase) by 54.3% (63.4%), when the terrain of M2 is reduced to the comparable height of surrounding mountains. With more upslope wind flowing over M2, the convergence over M3 is strengthened. The more intense upward motion and stronger potential instability both contribute to the notable increase of precipitation over M3. This study implies the possible mechanisms of the inhomogeneous precipitation over Southwest China and could deepen the current understanding of the topographic effects on precipitation over complex terrains.

High temporal resolution geostationary hyperspectral infrared sounders can simultaneously profile atmospheric temperature and moisture, and track moisture features to provide 3D horizontal winds in clear and partially cloudy scenarios. The thermodynamic information obtained has been integral to enhancing tropical cyclone (TC) forecasts. However, the potential benefits derived from the dynamic information provided by geostationary hyperspectral infrared sounders (GeoHIS) are yet to be fully comprehended for numerical weather prediction (NWP). With the re-estimated observation error and multivariate hydrometeor background error covariance, the 3D horizontal winds from the Geostationary Interferometric Infrared Sounder (GIIRS) are synergistically assimilated with the hydrometeor information. The impact of the GIIRS-derived 3D horizontal wind assimilation on Typhoon Maria (2018) and Lekima (2019) analysis and forecast are evaluated. The results show that the assimilating GIIRS-derived 3D horizontal wind notably reduces the RMSEs of analysis and forecast, particularly in the U and V components. The improvement in accuracy of large-scale fields substantially enhances the forecasts of typhoons' track and maximum wind speed, as well as the central sea-level pressure. Moreover, the prediction of the spatial distribution and intensity for landfall precipitation is also improved. The detailed diagnoses of the Maria show that a more southerly subtropical high generated by the additional horizontal wind assimilation improves the rainfall spatial distribution, and the reasonable water vapor transportation caused by the improved dynamic conditions corrects the precipitation intensity. This work highlights the importance of dynamic information in TC forecasting and emphasizes the need for efficient and high-precision 3D wind measurements to improve NWP.

Hydroxyl dicarboxylic acids (OHDCA) are ubiquitous in the atmosphere as an important constituent of secondary organic aerosol, yet the formation mechanisms remain unclear. At an urban background site on the coast of South China, we observed notable levels of OHDCA, with the highest concentration of malic acid (a typical OHDCA species) reaching 533 ng m−3. In the coastal air, the correlation between OHDCA and sulfate was better (R 2 = 0.48) in the period when the relative humidity was higher and the sulfate size distribution was in a droplet mode, fitting the features of aqueous formation. In the short-range continental air, a significant rise in OHDCA levels from morning through early afternoon (588 ng m−3) was observed under marked daytime increment of ozone that was corrected for titration loss (O3_corr, sum of ozone and nitrogen dioxide). In addition, good correlation between OHDCA and O3_corr was identified in this period, illuminating the role for gas-phase photochemistry in regulating OHDCA formation. Therefore, the elevated OHDCA was likely attributed to aqueous photooxidation, and the dominant factors varied under different atmospheric conditions. The precursors of OHDCA could be derived from biogenic emissions, as indicated by the correlations of OHDCA with 2-methylglyceric acid (bihourly data) and isoprene and monoterpenes (daily average data). However, anthropogenic aromatics might also be involved in OHDCA formation, especially in the short-range continental air. The formation mechanisms probed through observational evidence will be an important reference for rectifying simulations of OHDCA and its impact on air quality and climate.

One prominent mode of the variability in the summer wet-bulb temperature (WBT) over eastern China exhibits a distinct north‒south dipole pattern. This pattern demonstrates a positive center in northern China (NC), while a negative center is observed in southern China (SC). Our results indicate that the dipole mode of WBT could be partially ascribed to the impact of spring snow anomalies in eastern Europe–western Siberia (EEWS). The reduction in the snow depth, coupled with dry soil conditions, enhances the surface heat flux and consequently leads to an increase in the near-surface air temperature. The signal of soil moisture could persist from spring to summer, stimulating the generation of zonal Rossby waves. Consequently, the significant wave flux anomalies propagate from EEWS downstream and influence the atmospheric circulation over eastern China. These patterns play a role in the increase in the surface air temperature and moisture accumulation over NC, ultimately leading to the establishment of the dipole mode of WBT over eastern China in summer. Further analysis indicates that the atypical low sea surface temperature in the tropical Ocean, induced by the El Niño-Southern Oscillation, establishes a climatic context favorable for the persistence of an anomalous cyclone in the Northwest Pacific (NWP) throughout the summer season. The strengthened convection over the NWP and SC induces a dipole pattern of atmospheric circulation by stimulating a meridional wave train. This pattern creates favorable temperature and moisture conditions, contributing to the development of the dipole mode of the summer WBT across eastern China.

Renewable energy is recognized in Africa as a means for climate change mitigation, but also to provide access to electricity in sub-Saharan Africa, where three-quarters of the global population without electricity resides. Reliable and highly resolved renewable energy potential (REP) information is indispensable to support power plants expansion. Existing atmospheric data sets over Africa that are used for REP estimates are often characterized by data gaps, or coarse resolution. With the aim to overcome these challenges, the ICOsahedral Nonhydrostatic (ICON) Numerical Weather Prediction (ICON-NWP) model in its Limited Area Mode (ICON-LAM) is implemented and run over southern Africa in a hindcast dynamical downscaling setup at a convection-permitting 3.3 km horizontal resolution. The simulation time span covers contrasting solar and wind weather years from 2017 to 2019. To assess the suitability of the novel simulations for REP estimates, the simulated hourly 10 m wind speed (sfcWind) and hourly surface solar irradiance (rsds) are extensively evaluated against a large compilation of in situ observations, satellite, and composite data products. ICON-LAM reproduces the spatial patterns, temporal evolution, the variability, and absolute values of sfcWind sufficiently well, albeit with a slight overestimation and a mean bias (mean error (ME)) of 1.12 m s−1 over land. Likewise the simulated rsds with an ME of 50 W m−2 well resembles the observations. This new ICON simulation data product will be the basis for ensuing REP estimates that will be compared with existing lower resolution data sets.

The downward surface longwave flux (DSLF) plays a relevant role in the Earth’s surface radiative budget, which is crucial to monitor, understand and model the impact of changes at local and global scales on surface temperature and surface conditions. This study focuses on the evaluation and intercomparison of four DSLF products: (a) a recently developed all-weather DSLF product based on the multivariate adaptive regression splines (MARS) algorithm driven by satellite cloud information from the Meteosat Second Generation (MSG) and ERA5 reanalysis screen variables; (b) the Satellite Application Facility on Land Surface Analysis (LSA SAF); (c) CERES Synoptic top-of-atmosphere and surface fluxes and clouds (CERES-SYN1deg) and (d) ERA5 reanalysis. The study covers the period 2005–2021 and the MSG region focusing on monthly means. The evaluation performed against 48 ground stations from the Baseline Surface Radiation Network (BSRN) and FLUXNET2015 networks showed that the MARS product outperforms the remaining products, particularly the LSA SAF, while ERA5 and CERES show similar performance metrics. The fours products are intercompared in terms of their mean spatial variability and temporal mean annual cycles and inter-annual variability in four selected regions, showing a high level of agreement, particularly between MARS, ERA5 and CERES. Our results highlight the clear added value of MARS with respect to LSA SAF, while providing higher spatial resolution (0.05°), constrained by satellite cloud information, when compared with ERA5 (0.25°) or CERES (1°).

Spring thermal forcing over the Tibetan Plateau (TP) is largely determined by surface sensible heating (SSH), which is an important fuel for atmospheric circulations over subtropics and even the globe. However, insufficient efforts were devoted on exploring the potential influencing factors of spring TP's SSH. In the current study, observational analysis and model results confirm that the sea ice concentration in the north of Greenland can act as a precursor. Specifically, the winter shrinkage of the sea ice concentration can stimulate a meridional Rossby wave train, with a remarkable positive-negative-positive pattern of geopotential height anomalies controlling the Arctic, western Eurasia, and the southeastern Mediterranean, respectively. This teleconnection is equivalent barotropic in the whole troposphere, which shifts southeastward from winter to spring, with a significant anomalous anticyclone and increased geopotential height dominating the western TP in spring. The surface northeasterly anomalies in the southeastern flank of the anticyclone decrease the climatological westerlies and weaken the SSH over the central-southern TP. Existing literature mainly focused on the Arctic-TP relationship in autumn and winter. Our finding can not only provide a new insight onto the variation of spring thermal condition over the TP, but also deepen our knowledge of cross-seasonal relationships between the Arctic and the TP with similar snow-ice albedo feedback.

The prevalence of the maximum radius of maximum wind (RMW) contraction rate for the tropical cyclone (TC) before the maximum TC intensification rate has been observed in several previous studies. However, it remains unclear whether and how the maximum RMW contraction rate preceding the maximum TC intensification rate affects the TC lifetime maximum intensity (LMI). In this study, tropical cyclones are grouped into three types depending on whether the peak RMW contraction rate precedes, occurs simultaneously with, or lags the time of the peak intensification rate in the North Atlantic and western North Pacific. Results indicate that when the maximum RMW contraction rate occurs before the maximum intensification rate, TCs are more likely to gain a greater LMI. TCs with the time of maximum RMW contraction rate preceding the time of the maximum intensification rate are more likely to achieve a small RMW and intermediate intensity after the maximum RMW contraction rate period, which is conducive to higher intensification rates. An intermediate initial intensity and higher maximum intensification rate both lead to a greater LMI. Environmental factors are also examined but are found to have little impact on the LMI for the three types of TCs. This work suggests that TCs with a higher LMI are often characterized by a notable maximum rate of RMW contraction before the peak intensification rate and helps to a better understanding of TC structure and intensity changes.

Aeolus is the first satellite mission focusing on wind profile detection from near the surface to about 30 km in height on a global scale. This study evaluates the contribution of Aeolus winds to sea surface wind forecasts geographically by further analyzing the Observing System Experiments from the European Centre for Medium-Range Weather Forecasts (ECMWF) with scatterometer winds from the meteorological operational satellites (assimilated into the model) and the Haiyang-2B satellite (not assimilated into the model). The findings indicate that Aeolus has the ability to reduce the root-mean-square difference between scatterometer winds and background forecasts (short-range) by about 0.05%–0.16% on average for climatic regions, except for the meridional wind component in the tropics. Also, Aeolus can generally reduce zonal biases of the background forecasts, while its beneficial impact on meridional biases mainly occurs in the Northern Hemisphere extratropics and tropics. For medium-range forecast assessments, as the forecast step extends up to day 5, the positive impact of Aeolus on sea surface wind forecasts becomes more evident and is even greater than 3%, especially for extratropical ocean regions in the Southern Hemisphere. Furthermore, the impact of Aeolus shows seasonal variation, with a substantial positive impact from September 2019 to February 2020 and a negative impact mainly in March, April, and May 2020.

We used observational data and a long-term piControl simulation from the Community Earth System Model Version 2 to investigate the influence of the Pacific Decadal Oscillation (PDO) on the winter climate over the Tibetan Plateau. The results showed that changes in the phase of the PDO have a significant effect on winter temperatures and precipitation over the southern Tibetan Plateau. Changes in the sea surface temperature (SST) during the positive PDO can weaken the Walker circulation and increase the SST in the Indian Ocean. Our analyses of the moist static energy showed that warming of the tropical troposphere over the Indian Ocean caused by the increased SST has resulted in the horizontal advection of anomalous moist enthalpy by the climatological zonal winds, which was responsible for anomalous ascending motion over the Tibetan Plateau. The additional moisture budget suggests that enhanced vertical motion contributes to the increase in winter precipitation and related total cloud cover over the Tibetan Plateau, leading to the increase of snow depth. The increased total cloud cover and snow depth, in turn, reduces net surface shortwave radiation. The surface air temperature of the Tibetan Plateau is then decreased as a result of the reduction in the net surface shortwave radiation. The PDO therefore has an important modulating role in the interdecadal variability of the winter climate over the Tibetan Plateau. We therefore need to focus on changes in the PDO in research related to the decadal prediction of the climate over the Tibetan Plateau.

Coupled fire-atmosphere models often struggle to simulate important fire processes like fire generated flows, deep flaming fronts, extreme updrafts, and stratospheric smoke injection during large wildfires. This study uses the coupled fire-atmosphere model, WRF-Fire, to examine the sensitivities of some of these phenomena to the modeled total fuel load and its consumption. Specifically, the 2020 Bear Fire and 2021 Caldor Fire in California's Sierra Nevada are simulated using three fuel loading scenarios (1X, 4X, and 8X LANDFIRE derived surface fuel), while controlling the fire rate of spread using observations. This approach helps isolate the fuel loading and consumption needed to produce fire-generated winds and plume rise comparable to radar observations of these events. Increasing fuel loads and corresponding fire residence time in WRF-Fire leads to deep plumes in excess of 10 km, strong vertical velocities of 40–45 m s−1, and combustion fronts several kilometers in width (in the along wind direction). These results indicate that LANDFIRE-based surface fuel loads in WRF-Fire likely under-represent fuel loading, having significant implications for simulating landscape-scale wildfire processes, associated impacts on spread, and fire-atmosphere feedbacks.

The Hengduan Mountains (HM) are located on the southeastern edge of the Tibetan Plateau and feature high mountain ridges (>6,000 m MSL) separated by deep valleys. The HM region also features an exceptionally high biodiversity, believed to have emerged from the topography interacting with the climate. To investigate the role of the HM topography on regional climate, we conduct simulations with the regional climate model COSMO at high horizontal resolutions (at ∼12 km and a convection-permitting scale of ∼4.4 km) for the period 2001–2005. We conduct one control simulation with modern topography and two idealized experiments with modified topography, inspired by past geological processes that shaped the mountain range. In the first experiment, we reduce the HM's elevation by applying a spatially non-uniform scaling to the topography. The results show that, following the uplift of the HM, the local rainy season precipitation increases by ∼25%. Precipitation in Indochina and the Bay of Bengal (BoB) also intensifies. Additionally, the cyclonic circulation in the BoB extends eastward, indicating an intensification of the East Asian summer monsoon. In the second experiment, we remove deep valleys by applying an envelope topography to quantify the effects of terrain undulation with high amplitude and frequency on climate. On the western flanks of the HM, precipitation slightly increases, while the remaining fraction of the mountain range experiences ∼20% less precipitation. Simulations suggest an overall positive feedback between precipitation, erosion, and valley deepening for this region, which could have influenced the diversification of local organisms.

The fine particulate matter (PM2.5) in Chinese megacities has declined significantly after the implementation of strict mitigation strategies since 2013. However, the concentration of wintertime nitrate in PM2.5 (p-NO3 -) has only changed slightly and became the dominant inorganic component despite considerable precursor emission reductions. Discerning chemical mechanisms leading to nitrate growth during haze events is critical to implementing effective pollution mitigation policies. The oxygen isotope anomaly of nitrate (Δ17O(NO3 −)) is a powerful means to distinguish nitrate formation mechanisms. Nevertheless, the observed high Δ17O(NO3 −) values were significantly underestimated by chemical transport models during extreme haze events (PM2.5 > 225 μg m−3) in Beijing, indicating an incomplete understanding of nitrate chemistry. To reconcile this model-observation discrepancy, we compiled reported Δ17O(NO3 −) data in Beijing haze along with relevant observational parameters (e.g., hydroxyl (OH) reactivity, peroxyl radical concentrations), then tested assumptions on Δ17O of key precursors (e.g., OH and nitrogen dioxide (NO2)), recalculated Δ17O(NO3 −) and compared them with observations. Our results indicate that considering heterogeneous dinitrogen pentoxide reactions on chlorine-containing aerosols (N2O5 + Cl− chemistry) with a nitryl chloride (ClNO2) yield of ∼0.75 can explain the observed high Δ17O(NO3 −) during extreme haze events. According to the Δ17O(NO3 −) data, on average this heterogeneous N2O5 + Cl− chemistry can explain ∼60% of nighttime nitrate production and make daytime and nocturnal pathways equally important in winter Beijing haze (PM2.5 > 75 μg m−3). Our results highlight the critical role of reactive chlorine chemistry in air pollution/chemistry in inland cities.

Although air quality in China has improved substantially over recent years, haze pollution events still occur frequently, especially over the North China Plain (NCP). Previous studies showed that typhoons are conducive to regional pollution events in eastern China; however, the underlying mechanism and quantitative understanding of the typhoons' impact on haze pollution remain unclear there. Here, based on ground-based and satellite observations, reanalysis data, and model simulations, we show that northward typhoons approaching China are essential for autumn haze pollution over NCP. Elevated relative humidity levels and enhanced pollution accumulation, caused by northward typhoons and the corresponding high-pressure systems, are responsible for the pollution enhancements over NCP. Compared with episodes without typhoon influence, cities near Taihang and Yan Mountain suffer from heavier haze pollution when typhoons approach, with PM2.5 concentrations increasing from 87.1 to 106.4 μg m−3. More water vapor from the Yellow and Bohai Seas and pollutants from eastern China are transported to these cities by typhoon-induced southeasterly wind anomalies, facilitating the chemical formation of aerosols there. In addition, by the block of mountains, these southeasterly wind anomalies also lead to stronger local accumulation over cities and an elevation of pollutants along the mountains. What is more, with the implementation of emission reduction, the relative changes of PM2.5 concentrations between typhoon-induced episodes and no-typhoon episodes increase. This work highlights the importance of understanding the impact of synoptical weather on PM2.5 transport, accumulation, and formation processes in haze pollution mitigation in eastern China.