Tropospheric Stability Research Articles

A segmented grid model for vertical adjustment of precipitable water vapor in China

Precipitable Water Vapor (PWV) is vital in climate research, monitoring in troposphere stability and weather disasters. This study investigates the detailed vertical variation of PWV in China, utilizing data from the 5th generation European Centre for Medium-Range Weather Forecasts (ECMWF). A PWV segmented vertical adjustment grid model (CPWV-H) was developed based on a sliding window algorithm, with a spatial resolution of 0.5°×0.5° and a temporal resolution of one day. The performance of the CPWV-H model was evaluated using multi-source PWV data and compared with the conventional empirical model (EPWV-H) and the recently released C-PWVC1 model. Results show that the CPWV-H model has superior accuracy and stability. Using ERA5 PWV profiles as a reference, the CPWV-H model has a mean Bias of -0.45 mm and a mean RMSE of 1.24 mm, improving by 21.52% over the EPWV-H model and 31.11% over the C-PWVC1 model. With radiosonde profiles as a reference, it has a mean Bias of -0.10 mm and a mean RMSE of 2.86 mm, improving by 7.14% and 13.86% over the EPWV-H and C-PWVC1 models, respectively. Consequently, the CPWV-H model exhibits enhanced stability and vertical adjustment accuracy in China, offering significant utility for the fusion of multi-source water vapor data, comparative analysis, and climate research.

Diurnally asymmetric cloud cover trends amplify greenhouse warming.

Surface air temperature (SAT) is a key indicator of climate change. Variations in cloud cover affect SAT by interacting with radiation. During daytime, clouds tend to cool the surface by blocking sunlight, while nighttime clouds warm the surface by trapping longwave radiation. Here, we show that, on the global scale, cloud cover, particularly low-level cloudiness, exhibits diurnally asymmetric trends in a warming climate. Cloud fraction on average decreases more during the day than at night. Climate models indicate that the diurnally asymmetric cloud cover variation is mainly driven by trends in the lower tropospheric stability and is largely attributed to the increasing greenhouse gases rather than natural variability. This asymmetry, therefore, turns out to be an amplifier of surface warming, by both decreasing the daytime cloud shortwave albedo effect and increasing the nighttime cloud longwave greenhouse effect.

Interactions between trade wind clouds and local forcings over the Great Barrier Reef: a case study using convection-permitting simulations

Abstract. Trade wind clouds are ubiquitous across the subtropical oceans, including the Great Barrier Reef (GBR), playing an important role in modulating the regional energy budget. These shallow clouds, however, are by their nature sensitive to perturbations in both their thermodynamic environment and microphysical background. In this study, we employ the Weather Research and Forecasting (WRF) model with a convection-permitting configuration at 1 km resolution to examine the sensitivity of the trade wind clouds to different local forcings over the GBR. A range of local forcings including coastal topography, sea surface temperature (SST), and local aerosol loading is examined. This study shows a strong response of cloud fraction and accumulated precipitation to orographic forcing both over the mountains and upwind over the GBR. Orographic lifting, low-level convergence, and lower troposphere stability are found to be crucial in explaining the cloud and precipitation features over the coastal mountains downwind of the GBR. However, clouds over the upwind ocean are more strongly constrained by the trade wind inversion, whose properties are, in part, regulated by the coastal topography. On the scales considered in this study, the warm-cloud fraction and the ensuant precipitation over the GBR show only a small response to the local SST forcing, with this response being tied to the surface flux and lower troposphere stability. Cloud microphysical properties, including cloud droplet number concentration, liquid water path, and precipitation, are sensitive to the changes in atmospheric aerosol population over the GBR. While cloud fraction shows little responses, a slight deepening of the simulated clouds is evident over the upwind region in correspondence to the increased aerosol number concentration. A downwind effect of aerosol loading on simulated cloud and precipitation properties is further noted.

Opposite effects of aerosols and meteorological parameters on warm clouds in two contrasting regions over eastern China

Abstract. The sensitivity (S) of cloud parameters to the influence of different aerosol and meteorological parameters has in most previous aerosol–cloud interaction (aci) studies been addressed using traditional statistical methods. In the current study, relationships between cloud droplet effective radius (CER) and aerosol optical depth (AOD, used as a proxy for cloud condensation nuclei, CCN), i.e., the sensitivity (S) of CER to AOD, are investigated with different constraints of AOD and cloud liquid water path (LWP). In addition to traditional statistical methods, the geographical detector method (GDM) is applied in this study to quantify the relative importance of the effects of aerosol and meteorological parameters, as well as their interaction, on S. Moderate Resolution Imaging Spectroradiometer (MODIS) C6 L3 data and European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-5 reanalysis data, for the period from 2008 to 2022, were used to investigate aci over eastern China. Two contrasting areas were selected: the heavily polluted Yangtze River Delta (YRD) and a relatively clean area over the East China Sea (ECS). Linear regression analysis shows that CER decreases with the increase in AOD (negative S) in the moderately polluted atmosphere (0.1<AOD<0.3) over the ECS, whereas, in contrast, CER increases with increasing AOD (positive S) in the polluted atmosphere (AOD>0.3) over the YRD. Evaluation of S as function of the LWP shows that in the moderately polluted atmosphere over the ECS, S is negative in the LWP interval [40 g m−2, 200 g m−2], and the sensitivity of CER to AOD is substantially stronger as LWP is larger. In contrast, in the polluted atmosphere over the YRD, S is positive in the LWP interval [0 g m−2, 120 g m−2] and does not change notably as function of LWP in this interval. The study further shows that over the ECS, the CER is larger for higher low tropospheric stability (LTS) and relative humidity (RH) but lower for higher pressure vertical velocity (PVV). Over the YRD, there is no significant influence of LTS on the relationship between CER and AOD. The GDM has been used as an independent method to analyze the sensitivity of cloud parameters to AOD and meteorological parameters (RH, LTS and PVV). The GDM has also been used to analyze the effects of interactions between two parameters and thus obtain information on confounding meteorological effects on the aci. Over the ECS, cloud parameters are sensitive to almost all parameters considered except for cloud top pressure (CTP), and the sensitivity to AOD is larger than that to any of the meteorological factors. Among the meteorological factors, the cloud parameters are most sensitive to PVV and least sensitive to RH. Over the YRD, the explanatory power of the sensitivity of cloud parameters to AOD and meteorological parameters is much smaller than over the ECS, except for RH, which has a statistically significant influence on CTP and can explain 74 % of the variation of CTP. The results from the GDM analysis show that cloud parameters are more sensitive to the combination of aerosol and a meteorological parameter than to each parameter alone, but confounding effects due to co-variation of both parameters cannot be excluded.

Assessing Radiative Impacts of African Smoke Aerosols Over the Southeastern Atlantic Ocean

AbstractBiomass burning smoke aerosols are efficient at attenuating incoming solar radiation. The Layered Atlantic Smoke Interactions with Clouds campaign was conducted from June 2016 to October 2017. The U. S. Department of Energy mobile Atmospheric Radiation Measurement site located on Ascension Island (AMF‐ASI) identified several instances of smoke plume intrusions. Increases in surface and column measurements of aerosol loading were directly related to increases in fine mode fraction, number concentrations of aerosols (Na), and cloud condensation nuclei (NCCN). During periods of weak lower tropospheric stability, smoke particles were more likely to be advected downward either by boundary layer turbulence or cloud top entrainment under non‐overcast sky conditions. Backward trajectory analysis illustrated that smoke aerosols reaching the AMF‐ASI site were fine mode, less aged, strongly absorbing, and had shorter boundary layer trajectories while longer boundary layer trajectories denoted mixtures of weakly absorbing smoke and coarse mode marine aerosols. The most polluted smoke cases of August 2016 and 2017 revealed a notable contrast in radiative forcing per unit aerosol optical depth or radiative forcing efficiency (ΔFeff) at the top of the atmosphere (TOA) and near‐surface (BOA). The weakly (strongly) absorbing 2016 cases exhibited weaker (stronger) ΔFeff at the TOA and BOA suggesting a warming (cooling) effect within the boundary layer. The 2017 cases featured the strongest ΔFeff suggesting more of a cooling effect at the TOA and BOA due to mixing of fresh smoke with marine aerosols during transport.

Longer dust events over Northwest China from 2015 to 2022

Dust events show volatile characteristics in the Northwest warming and wetting trend under global warming. Previous studies yet pay little attention to the specific characteristic of the duration of dust events. We present herein our findings on the changes of dust event duration (DED) and its influencing factors over Northwest China based on hourly resolution air quality observations. We find that the DED over Northwest China increased by ∼25% from 2015 to 2022. This increase is mainly attributed to moderate and heavy dust events, especially for regions close to dust sources, where moderate dust event frequency even decreases significantly. Correlation analyses with meteorological factors reveal that increased lower tropospheric stability is significantly associated with increased DED, implying that greater stability hinders dust dry deposition and transport processes, thereby promoting a longer residence time. Additionally, the DED in dust source regions negatively correlates with precipitation frequency (less wet deposition), while in non-dust source regions, it positively correlates with dust transport. It suggests that the factors enhancing DED vary with distance from dust sources. These findings will help elucidate dust event characteristics (especially the duration) and their changes over Northwest China, and will inform local dust management.

The impact of WRF vertical resolution on the simulated thermal-dynamic structures and intensity of Typhoon Lekima

This study utilizes the Weather Research and Forecasting (WRF) to comparatively analyze the impact of three vertical grid resolution (VGR) enhancement schemes on the simulation of super typhoon Lekima under two different horizontal resolutions. The relationship between structural changes and typhoon intensity is explored from the perspective of the simulated three-dimensional thermodynamic and dynamic structure of the typhoon. The main conclusions obtained are as follows: Typhoon track simulation is not sensitive to the setting of VGR, while intensity simulation is greatly affected by it. Increasing VGR in the lower layer can enhance the simulated intensity of the typhoon, but when VGR enhancement is made in the middle layer, the simulated typhoon intensity decreases. For VGR enhancement in the upper levels, it is only when coupled with a simultaneous increase in horizontal resolution that the simulated typhoon intensity is enhanced. Different VGR enhancement schemes result in significant differences in the simulated thermodynamic and dynamic structures of the typhoon, which is a crucial factor causing variations in simulated typhoon intensity. In terms of dynamics, increasing VGR in the lower layer reduces the hydrostatic stability of the lower troposphere, which enhances convection and improves its symmetry. This leads to strengthened inflow and outflow activities of the typhoon, resulting in a stronger simulated typhoon with a tighter and straighter eyewall. In terms of thermal structure, increasing VGR in both the lower and upper levels enhances the warm core of the typhoon, thereby increasing its simulated intensity. However, the warm core simulated with upper layer enhancement extends to a higher altitude. This may be related to two different heating mechanisms, where lower layer VGR enhancement has a more pronounced effect on changes in boundary layer latent heat flux, while upper layer VGR enhancement promotes more sinking of high-entropy air from higher levels.

Diurnal variation in the near-global planetary boundary layer height from satellite-based CATS lidar: Retrieval, evaluation, and influencing factors

Diurnal variations in planetary boundary layer height (PBLH) are essential for regulating the diffusion of pollutants and controlling heat and moisture exchange in the lower atmosphere. However, owing to the lack of continuous large-scale observations, the global behavior of diurnal variation in the PBLH remains poorly understood. This study seeks to bridge this knowledge gap by examining 33 months of profiles from Cloud Aerosol Transport System (CATS) lidar to retrieve the PBLH on a near-global scale. The retrievals were compared with other datasets, including radiosonde, CALIPSO, reanalysis, and CMIP6. The results suggest that the CATS retrievals were significantly consistent with the radiosonde and CALIPSO retrievals, with correlations of 0.6 (clear-sky) and 0.74, respectively. Spatially, the CATS PBLH exhibits correlations of 0.11–0.72 with several CMIP6 and reanalysis outputs. For diurnal variations, the CATS PBLH showed a higher correlation (reaching 0.7) with reanalysis outputs during midday and early afternoon. However, there were considerable differences between them in the morning and late afternoon, owing to the influences of the residual layer and algorithm differences. Notably, deserts, plateaus and tropical areas exhibited more discrepancies between the CATS and reanalysis PBLH, which may be attributed to multilayer aerosol structures, heterogeneous surfaces, and deep convection. Based on the CATS derived PBLH dataset, we further investigated the meteorological impacts on interannual variability in the near-global PBLH using multiple stepwise regression and contribution calculations. The results indicate that meteorological factors explain 39–58% of the interannual variability in the CATS PBLH, and their contributions are land cover dependent and exhibit diurnal variations. Lower tropospheric stability dominated the interannual variability over land, with a diurnal pattern of its contribution aligned with the PBLH itself, whereas humidity factors contributed the most over oceans and wetlands. These findings have important implications for understanding the diurnal and temporal variability in the near-global PBLH, as well as the PBL parameterization.

Time‐Evolving Radiative Feedbacks in the Historical Period

AbstractWe investigate the time‐dependence of radiative feedback in the historical period (since the late 19th century), by analyzing experiments using coupled atmosphere–ocean climate models with historical greenhouse gas, anthropogenic aerosol, and natural forcings, each separately. We find that radiative feedback depends on forcing agent, primarily through the effect of cloud on shortwave radiation, because the various forcings cause different changes in global‐mean tropospheric stability per degree of global‐mean temperature change. The large time‐variation of historical feedback driven by observed sea surface temperature change alone, with no forcing agents, is also consistent with tropospheric stability change, and differs from the similarly large and significant historical time‐variation of feedback that is simulated in response to all forcing agents together. We show that the latter results from the varying relative sizes of individual forcings. We highlight that volcanic forcing is especially important for understanding the time‐variation, because it stimulates particularly strong feedbacks that tend to reduce effective climate sensitivity. We relate this to stability changes due to enhanced surface temperature response in the Indo‐Pacific warm pool.

What Are the Similarities and Differences in Marine Boundary Layer Cloud and Drizzle Microphysical Properties During the ACE‐ENA and MARCUS Field Campaigns?

AbstractThis study compares macrophysical and microphysical properties of single‐layered, liquid‐dominant MBL clouds from the Measurements of Aerosols, Radiation, and Clouds over the Southern Ocean (MARCUS) (above 60°S) and the ARM East North Atlantic (ENA) site during the Aerosol and Cloud Experiments in Eastern North Atlantic (ACE‐ENA) field campaign. A total of 1,136 (16.5% of clouds) and 6,034 5‐min cloud samples are selected from MARCUS and ARM ENA in this study. MARCUS clouds have higher cloud‐top heights, thicker cloud layers, larger liquid water path, and colder cloud temperatures than ENA. Thinner, warmer MBL clouds at ENA can contain higher layer‐mean liquid water content due to higher cloud and ocean surface temperatures along with greater precipitable water vapor (PWV). MARCUS has a higher drizzle frequency rate (71.8%) than ENA (45.1%). Retrieved cloud and drizzle microphysical properties from each field campaign show key differences. MARCUS clouds feature smaller cloud droplets, whereas ENA clouds have larger cloud droplets, especially at the upper region of the cloud. From cloud top to cloud base, drizzle drop sizes increase while number concentrations decrease. Drizzle drop radius and number concentration decrease from cloud base to drizzle base due to net evaporation, and MARCUS' lower specific humidity leads to a higher drizzle base than ENA. The broader surface pressure and lower tropospheric stability (LTS) distributions during MARCUS have demonstrated that there are different synoptic patterns for selected cases during MARCUS with less PWV, while ENA is dominated by high pressure systems with nearly doubled PWV.

Climatology of Planetary Boundary Layer Height over Jiangsu, China, Based on ERA5 Reanalysis Data

Based on the hourly ERA5 reanalysis dataset of the European Centre for Medium-Range Weather Forecasts (ECMWF) from 1 January 1979 to 31 December 2019, the climatology of the planetary boundary layer height (PBLH) in Jiangsu, China, is studied. The PBLH based on ERA5 is verified by using radiosonde data, and the results show that the PBLH based on ERA5 fits very well with the PBLH diagnosed by the radiosonde data. Overall, the daytime average PBLH is between 700 and 1200 m, which is higher in the north and lower in the south. It is between 100 and 400 m at night, and it is lower in the north and higher in the south. The PBLH exhibits complex spatiotemporal variation. In the daytime, the PBLH in inland areas is highest in spring, followed by fall and summer, and lowest in winter. At night, the seasonal variation in the PBLH is less obvious. The seasonal variation in the PBLH in coastal areas is higher in fall and winter and lower in spring and summer. The PBLH shows an obvious diurnal cycle, usually reaching its peak at 14:00 (LST) or 15:00 (LST). The diurnal cycle of the PBLH is significantly positively correlated with the near-surface temperature and wind speed and significantly negatively correlated with the relative humidity and lower tropospheric stability. Over these 41 years, the daytime PBLH has increased significantly in most areas. The increase in the PBLH can be attributed to the increase in near-surface temperature and the decrease in near-surface relative humidity and lower tropospheric stability.

Effects of ozone levels on climate through Earth history

Abstract. Molecular oxygen in our atmosphere has increased from less than a part per million in the Archean Eon to a fraction of a percent in the Proterozoic and finally to modern levels during the Phanerozoic. The ozone layer formed with the early Proterozoic oxygenation. While oxygen itself has only minor radiative and climatic effects, the accompanying ozone has important consequences for Earth climate. Using the Community Earth System Model (CESM), a 3-D general circulation model (GCM), we test the effects of various levels of ozone on Earth's climate. When CO2 is held constant, the global-mean surface temperature decreases with decreasing ozone, with a maximum drop of ∼3.5 K at near total ozone removal. By supplementing our GCM results with 1-D radiative flux calculations, we are able to test which changes to the atmosphere are responsible for this temperature change. We find that the surface temperature change is caused mostly by the stratosphere being much colder when ozone is absent; this makes it drier, substantially weakening the greenhouse effect. We also examine the effect of the structure of the upper troposphere and lower stratosphere on the formation of clouds and on the global circulation. At low ozone, both high and low clouds become more abundant due to changes in the tropospheric stability. These generate opposing shortwave and longwave radiative forcings that are nearly equal. The Hadley circulation and tropospheric jet streams are strengthened, while the stratospheric polar jets are weakened, the latter being a direct consequence of the change in stratospheric temperatures. This work identifies the major climatic impacts of ozone, an important piece of the evolution of Earth's atmosphere.

Extreme Smog Challenge of India Intensified by Increasing Lower Tropospheric Stability

AbstractExtreme smog in India widely impacts air quality in late autumn and winter months. While the links between emissions, air quality and health impacts are well‐recognized, the association of smog and its intensification with climatic trends in the lower troposphere, where aerosol pollution and its radiative effects manifest, are not understood well. Here we use long‐term satellite data to show a significant increase in aerosol exceedances over northern India, resulting in sustained atmospheric warming and surface cooling trends over the last two decades. We find several lines of evidence suggesting these aerosol radiative effects have induced a multidecadal (1980–2019) strengthening of lower tropospheric stability and increase in relative humidity, leading to over fivefold increase in poor visibility days. Given this crucial aerosol‐radiation‐meteorological feedback driving the smog intensification, results from this study would help inform mitigation strategies supporting stronger region‐wide measures, which are critical for solving the smog challenge in India.

Diurnal MCSs Precede the Genesis of Tropical Cyclone Mora (2017): The Role of Convectively Forced Gravity Waves

Abstract A novel high-resolution regional reanalysis is used to investigate the mesoscale processes that preceded the formation of Tropical Cyclone (TC) Mora (2017). Both satellite observations and the regional reanalysis show early morning mesoscale convective systems (MCSs) persistently initiated and organized in the downshear quadrant of the preexisting tropical disturbance a few days prior to the genesis of TC Mora. The diurnal MCSs gradually enhanced the meso-α-scale vortex near the center of the preexisting tropical disturbance through vortex stretching, providing a vorticity-rich and moist environment for the following burst of deep convection and enhancement of the meso-β-scale vortex. The regional reanalysis shows that the gravity waves that radiated from afternoon convection over the northern coast of the Bay of Bengal might play an important role in modulating the diurnal cycle of pregenesis MCSs. The diurnal convectively forced gravity waves increased the tropospheric stability, reduced the column saturation fraction, and suppressed deep convection within the preexisting tropical disturbance from noon to evening. A similar quasi-diurnal cycle of organized deep convection prior to TC genesis has also been observed over other basins. However, modeling studies are needed to conclusively demonstrate the relationships between the gravity waves and pregenesis diurnal MCSs. Also, whether diurnal gravity waves play a similar role in modulating the pregenesis deep convection in other TCs is worth future investigations. Significance Statement Tropical cyclogenesis is a process by which a less organized weather system in the tropics develops into a tropical cyclone (TC). Observations indicate that thunderstorms occurring prior to the tropical cyclogenesis often show a distinct quasi-diurnal cycle, while the related physical mechanisms are still unclear. In this study, we used a novel high-resolution dataset to investigate the diurnal thunderstorms occurring prior to the genesis of TC Mora (2017). We find that the pregenesis diurnal thunderstorms played a crucial role in spinning up the circulation of the atmosphere and provided a favorable environment for the rapid formation of Mora. It is likely that gravity waves emitted by afternoon thunderstorms over the inland region were responsible for regulating the diurnal variation of pregenesis thunderstorms over the ocean.

Disentangling the mechanisms of equatorial Pacific climate change.

Most state-of-art models project a reduced equatorial Pacific east-west temperature gradient and a weakened Walker circulation under global warming. However, the causes of this robust projection remain elusive. Here, we devise a series of slab ocean model experiments to diagnostically decompose the global warming response into the contributions from the direct carbon dioxide (CO2) forcing, sea ice changes, and regional ocean heat uptake. The CO2 forcing dominates the Walker circulation slowdown through enhancing the tropical tropospheric stability. Antarctic sea ice changes and local ocean heat release are the dominant drivers for reduced zonal temperature gradient over the equatorial Pacific, while the Southern Ocean heat uptake opposes this change. Corroborating our model experiments, multimodel analysis shows that the models with greater Southern Ocean heat uptake exhibit less reduction in the temperature gradient and less weakening of the Walker circulation. Therefore, constraining the tropical Pacific projection requires a better insight into Southern Ocean processes.

On three types of sea breeze in Qingdao of East China: an observational analysis

Our knowledge of sea breeze remains poor in the coastal area of East China, due largely to the high terrain heterogeneity. Five–year (2016–2020) consecutive wind observations from a 10-m wind tower and radar wind profiler are used to characterize the sea–land breeze at Qingdao, a coastal city in East China. The sea surface temperatures at the nearby buoy station in the Yellow Sea are further used to elucidate the land–sea thermal contrast. First of all, three types of sea breeze are determined according to the temporal evolution of wind direction, including pure sea breeze (PSB), corkscrew sea breeze (CSB) and backdoor sea breeze (BSB). Statistically, there are 522 days experiencing sea breezes, of which 61 days belong to pure sea breeze, 40 days witness corkscrew sea breeze, and only 2 days see the backdoor sea breeze. The occurrence of sea breeze is found to peak from April to September and the average wind speed lies at 3–5 m s−1. This suggests a salient seasonality of sea breeze at Qingdao, which is likely caused by the seasonal dependence of land–sea thermal contrast. In terms of the diurnal variability, the sea breezes tend to occur more frequently and have more intensity in the afternoon compared with in the morning, irrespective of sea breeze types. Interestingly, the backdoor sea breeze is merely observed in autumn, whereas corkscrew sea breeze and pure sea breeze can be found year round. Among all types of sea breeze, the pure sea breeze has the highest intensity and most frequency throughout the daytime, same in the seasons of spring, summer and autumn. Further analyses are conduced of the atmospheric circulation, lower troposphere stability and bulk Richard number (Ri) for three types of sea breeze. Both pure sea breeze and corkscrew sea breeze in Qingdao are characterized by prevailing westerlies at 500 hPa. In contrast, the backdoor sea breeze is generally accompanied with easterlies at 850 hPa. Meanwhile, the backdoor sea breeze has the lowest lower troposphere stability, in sharp contrast to the highest lower troposphere stability for the pure sea breeze. This indicates that the backdoor sea breeze (pure sea breeze) tends to occur in an unstable (stable) lower troposphere. The findings obtained here highlight the importance of typing sea breeze.

Evaluation of aerosol–cloud interactions in E3SM using a Lagrangian framework

Abstract. A Lagrangian framework is used to evaluate aerosol–cloud interactions in the U.S. Department of Energy's Energy Exascale Earth System Model (E3SM) version 1 (E3SMv1) for measurements taken at Graciosa Island in the Azores where a U.S. Department of Energy Atmosphere Radiation Measurement (ARM) site is located. This framework uses direct measurements of cloud condensation nuclei (CCN) concentration (instead of relying on satellite retrievals of aerosol optical depth) and incorporates a suite of ground-based ARM measurements, satellite retrievals, and meteorological reanalysis products that when applied to over a 1500 trajectories provides key insights into the evolution of low-level clouds and aerosol radiative forcing that is not feasible from a traditional Eulerian analysis framework. Significantly lower concentrations (40 %) of surface CCN concentration are measured when precipitation rates in 48 h back trajectories average above 1.2 mm d−1 in the Integrated Multi-satellitE Retrievals for Global Precipitation Measurement (IMERG) product. The depletion of CCN concentration when precipitation rates are elevated is nearly twice as large in the ARM observations compared to E3SMv1 simulations. The model CCN concentration bias remains significant despite modifying the autoconversion and accretion rates in warm clouds. As the clouds in trajectories associated with larger surface-based CCN concentration advect away from Graciosa Island, they maintain higher values of droplet number concentrations (Nd) over multiple days in observations and E3SM simulations compared to trajectories that start with lower CCN concentrations. The response remains robust even after controlling for meteorological factors such as lower troposphere stability, the degree of cloud coupling with the surface, and island wake effects. E3SMv1 simulates a multi-day aerosol effect on clouds and a Twomey radiative effect that is within 30 % of the ARM and satellite observations. However, the mean cloud droplet concentration is more than 2–3 times larger than in the observations. While Twomey radiative effects are similar amongst autoconversion and accretion sensitivity experiments, the liquid water path and cloud fraction adjustments are positive when using a regression model as opposed to negative when using the present-day minus pre-industrial aerosol emissions approach. This result suggests that tuning the autoconversion and accretion alone is unlikely to produce the desired aerosol susceptibilities in E3SMv1.

Model spread in tropical low cloud feedback tied to overturning circulation response to warming

Among models participating in the Coupled Model Intercomparison Project phase 6 (CMIP6), here we show that the magnitude of the tropical low cloud feedback, which contributes considerably to uncertainty in estimates of climate sensitivity, is intimately linked to tropical deep convection and its effects on the tropical atmospheric overturning circulation. First, a reduction in tropical ascent area and an increased frequency of heavy precipitation result in high cloud reduction and upper-tropospheric drying, which increases longwave cooling and reduces subsidence weakening, favoring low cloud reduction (Radiation-Subsidence Pathway). Second, increased longwave cooling decreases tropospheric stability, which also reduces subsidence weakening and low cloudiness (Stability-Subsidence Pathway). In summary, greater high cloud reduction and upper-tropospheric drying (negative longwave feedback) lead to a more positive cloud feedback among CMIP6 models by contributing to a greater reduction in low cloudiness (positive shortwave feedback). Varying strengths of the two pathways contribute considerably to the intermodel spread in climate sensitivity.

A meteorological overview of the ORACLES (ObseRvations of Aerosols above CLouds and their intEractionS) campaign over the southeastern Atlantic during 2016–2018: Part 2 – Daily and synoptic characteristics

Abstract. Part 1 (Ryoo et al., 2021) provided a climatological overview of the ObsErvation of Aerosols above CLouds and their intEractionS (ORACLES) period and assessed the representativeness of the deployment years. In part 2, more detailed meteorological analyses support the interpretation of the airborne measurements for aerosol transport and its interaction with low clouds over the southeastern (SE) Atlantic Ocean during the September 2016, August 2017, and October 2018 deployments at a daily and synoptic scale. The key meteorological characteristics during the September 2016 deployment are (1) the southern African easterly jet (AEJ-S), centered at around 600 hPa (∼4 km), which strengthens throughout the month in concert with a warming continental heat low, with the strongest winds occurring around 23 September. These advect both aerosol and moisture in the free troposphere. (2) Mid-tropospheric black carbon (BC) is entrained at times into the boundary layer, and (3) convection over land is dry south of about 10∘ S and moist north of 10∘ S. The daily mean low-cloud fraction (low CF) is well correlated with the daily mean high low tropospheric stability (LTS, r=0.44–0.73 over the flight domain; 0–10∘ E, 5–25∘ S) and moderately correlated with the daily mean boundary layer height (BLH, r=0.37–0.52), defined as the altitude of the maximum vertical gradient of moisture. For the August 2017 deployment, the primary meteorological characteristics are that (1) the AEJ-S is at a lower altitude (∼3 km; ∼700 hPa) and further north (5–7∘ S) than in September 2016 and only becomes established by 20 August, with a separate easterly jet present aloft above 500 hPa (∼5.5 km) before that, (2) the mid-tropospheric BC–RH coupling strengthens after the AEJ-S develops, at around 3 km, (3) the daily mean low CF is less closely correlated with the daily mean LTS (r=0.16–0.57) and BLH (r=0.11–0.25) than in September 2016, and (4) dry convection reaches 700 hPa over the Namibian–Kalahari dryland (∼15–25∘ S, ∼18–24∘ E), generating the moist plume subsequently advected over the southeastern Atlantic by the AEJ-S. For the October 2018 deployment, the key meteorological characteristics are that (1) the AEJ-S develops around 600 hPa, driven by the Kalahari heat low (∼10–25∘ S, ∼12–24∘ E), but it diminishes over time as moist continental convection moves southward, (2) the offshore advection of mid-tropospheric BC (∼4 km, 600 hPa) and water vapor are strongly modulated by the AEJ-S around 8–10∘ S in early October, but this decreases as mid-latitude frontal systems develop and the AEJ-S weakens around mid-to-late October, (3) the AEJ-S–low-level jet (LLJ) over the coastal Namibian region relationship is the strongest among all deployment months, and the daily mean low CF is largely reduced by the strong daily mean LLJ, especially to the cloud deck's south, and (4) the relationship of the daily mean low CF with the daily mean BLH and LTS are insignificant compared to the other 2 deployment months, partially due to the variability introduced by the passage of the mid-latitude disturbance.

Dust Aerosol Impacts on the Time of Cloud Formation in the Badain Jaran Desert Area

AbstractDust, as absorbing aerosols, can change the stability of the atmosphere through radiative effects, thereby affecting the formation and characteristics of clouds. In order to explore the influence of dust aerosols on cloud formation, this study takes the Badain Jaran Desert area as the research area where cloud formation is less disturbed by human activities compared to other densely populated areas such as the North China Plain, and uses the high temporal resolution Himawari‐8 satellite data to identify 469 cases with cloud formation after 03:00 UTC (11:00 Beijing time). Through the analysis of individual cases, we find that the cloud formation time is dominated by the low tropospheric stability (LTS) when the total precipitable water (TPW) is less than about 18.5 kg/m2, and greatly affected by the TPW when it is larger than 18.5 kg/m2. Dust aerosols near the ground absorb shortwave solar radiation, making the atmospheric structure unstable (decreased LTS) and conducive to cloud formation. Statistically, for every 0.1 increase in aerosol optical depth, the cloud formation time is advanced about 138 min compared to low aerosol cases. Further analysis shows that dust aerosols actually inhibit the formation of convective clouds through microphysical processes when the TPW is small, while promote the formation of convective clouds through radiative processes when the TPW is large. This study reveals the effects of dust aerosols on the time of cloud formation along with the underlying mechanisms, which provides observational support for improving cloud parameterization in numerical models.