Cloud microphysical processes occur at the smallest end of scales among cloud-related processes and thus must be parameterized not only in large-scale global circulation models (GCMs) but also in ...various higher-resolution limited-area models such as cloud-resolving models (CRMs) and large-eddy simulation (LES) models. Instead of giving a comprehensive review of existing microphysical parameterizations that have been developed over the years, this study concentrates purposely on several topics that we believe are understudied but hold great potential for further advancing bulk microphysics parameterizations: multi-moment bulk microphysics parameterizations and the role of the spectral shape of hydrometeor size distributions; discrete vs “continuous” representation of hydrometeor types; turbulence-microphysics interactions including turbulent entrainment-mixing processes and stochastic condensation; theoretical foundations for the mathematical expressions used to describe hydrometeor size distributions and hydrometeor morphology; and approaches for developing bulk microphysics parameterizations. Also presented are the spectral bin scheme and particle-based scheme (especially, super-droplet method) for representing explicit microphysics. Their advantages and disadvantages are elucidated for constructing cloud models with detailed microphysics that are essential to developing processes understanding and bulk microphysics parameterizations. Particle-resolved direct numerical simulation (DNS) models are described as an emerging technique to investigate turbulence-microphysics interactions at the most fundamental level by tracking individual particles and resolving the smallest turbulent eddies in turbulent clouds. Outstanding challenges and future research directions are explored as well.
4258 instantaneous 550 nm Aerosol Optical Depth (AOD) values were compared between the second Modern-Era Retrospective analysis for Research and Applications (MERRA-2) and the Aerosol Robotic Network ...(AERONET) in four seasons at 12 AERONET sites across China. The correlation coefficients (R) in spring, summer, autumn and winter were 0.88, 0.92, 0.91 and 0.87, respectively. The MERRA-2 AOD was compared with the Moderate resolution Imaging Spectroradiometer (MODIS/Aqua) AOD over China from 2003 to 2017, and good agreement was obtained. 4501 daily AOD values were compared between MERRA-2 and MODIS at 16 sites over China. Spatial distribution and temporal variation of MERRA-2 AOD over China were analyzed from 1980 to 2017. Mean values of MERRA-2 AOD indicated that high AOD mainly appeared in the eastern, southeastern and central China, while low AOD mostly occurred in the western and northeastern China. Mean AOD values over China during this study period in each month was also discussed, and similar spatial distribution (high AOD in developed areas, low AOD in rural and less developed areas) was found in each month. Slight AOD increase could be observed in the 1980s and 1990s, and a rapid increase happened from 2001 to 2010, followed by an AOD decrease between 2011 and 2017. Annual variation of mean AOD of the whole China (the Yangtze River Delta) showed a slight increase of 0.0010 (0.0045) per year from 1980 to 1999, a rapid increase of 0.0096 (0.0271) per year between 2000 and 2009, and a decrease of −0.0089 (−0.0206) per year from 2010 to 2017. Annual AOD Variation in Jing-jin-ji and Pearl River Delta is similar to that in the Yangtze River Delta. In four seasons, AOD change was similar to the annual AOD variation except winter with a slight decrease of −0.0012 per year between 1980 and 1999. AOD variation over China before and after the Pinatubo volcanic eruption in the Philippines in June 1991 was analyzed to get a better understanding of the transport and the impact on the environment from volcanic pollutants. AOD variation trend over China was studied in two periods. The increasing trend appeared in most part of China from 1980 to 2009, while a decreasing trend could be found in almost the entire China between 2010 and 2017. Spatial and temporal variation of MERRA-2 Absorption Aerosol Optical Depth (AAOD), Black Carbon Absorption Aerosol Optical Depth (BCAAOD) and Dust Absorption Aerosol Optical Depth (DUAAOD) were analyzed during the period between 1980 and 2017. BCAAOD (66.30%) and DUAAOD (30.56%) offered the major contribution to total AAOD in China. 38 years of variation of AAOD over China is mainly due to the variation of BCAAOD. BCAAOD experienced an increase (1980–2007) and a decrease (2008–2017) during the 38 years in China.
•An AOD increasing trend appeared in most part of China from 1980 to 2009.•An AOD decreasing trend occurred in almost the entire China between 2010 and 2017.•Variation of AAOD over China is mainly due to the variation of BCAAOD from 1980 to 2017.
The process of entrainment-mixing between cumulus clouds and the ambient air is important for the development of cumulus clouds. Accurately obtaining the entrainment rate (
λ
) is particularly ...important for its parameterization within the overall cumulus parameterization scheme. In this study, an improved bulk-plume method is proposed by solving the equations of two conserved variables simultaneously to calculate
λ
of cumulus clouds in a large-eddy simulation. The results demonstrate that the improved bulk-plume method is more reliable than the traditional bulk-plume method, because
λ
, as calculated from the improved method, falls within the range of
λ
values obtained from the traditional method using different conserved variables. The probability density functions of
λ
for all data, different times, and different heights can be well-fitted by a log-normal distribution, which supports the assumed stochastic entrainment process in previous studies. Further analysis demonstrate that the relationship between
λ
and the vertical velocity is better than other thermodynamic/dynamical properties; thus, the vertical velocity is recommended as the primary influencing factor for the parameterization of
λ
in the future. The results of this study enhance the theoretical understanding of
λ
and its influencing factors and shed new light on the development of
λ
parameterization.
This work empirically examines the dependence of entrainment‐mixing mechanisms on the averaging scale in cumulus clouds using in situ aircraft observations during the Routine Atmospheric Radiation ...Measurement Aerial Facility Clouds with Low Optical Water Depths Optical Radiative Observations (RACORO) field campaign. A new measure of homogeneous mixing degree is defined that can encompass all types of mixing mechanisms. Analysis of the dependence of the homogenous mixing degree on the averaging scale shows that, on average, the homogenous mixing degree decreases with increasing averaging scales, suggesting that apparent mixing mechanisms gradually approach from homogeneous mixing to extreme inhomogeneous mixing with increasing scales. The scale dependence can be well quantified by an exponential function, providing first attempt at developing a scale‐dependent parameterization for the entrainment‐mixing mechanism. The influences of three factors on the scale dependence are further examined: droplet‐free filament properties (size and fraction), microphysical properties (mean volume radius and liquid water content of cloud droplet size distributions adjacent to droplet‐free filaments), and relative humidity of entrained dry air. It is found that the decreasing rate of homogeneous mixing degree with increasing averaging scales becomes larger with larger droplet‐free filament size and fraction, larger mean volume radius and liquid water content, or higher relative humidity. The results underscore the necessity and possibility of considering averaging scale in representation of entrainment‐mixing processes in atmospheric models.
Key Points
Entrainment‐mixing tends to be more inhomogeneous over a longer averaging scaleThe scale dependence is related to three factors, e.g., filament propertiesThe results could be used in scale‐dependent parameterizations for mixing
The seasonal and diurnal variations of cloud systems are profoundly affected by the large-scale and local environments. In this study, a one-year-long simulation was conducted using a two-dimensional ...cloud-resolving model over the Eastern Tibetan Plateau (ETP) and two subregions of Eastern China: Southern East China and Central East China. Deep convective clouds (DCCs) rarely occur in the cold season over ETP, whereas DCCs appear in Eastern China throughout the year, and the ETP DCCs are approximately 20%–30% shallower than those over Eastern China. Most strong rainfall events (precipitation intensity, PI> 2.5 mm h
−1
) in Eastern China are related to warm-season DCCs with ice cloud processes. Because of the high elevation of the ETP, the warm-season freezing level is lower than in Eastern China, providing favorable conditions for ice cloud processes. DCCs are responsible for the diurnal variations of warm-season rainfall in all three regions. Warm-season DCCs over the ETP have the greatest total cloud water content and frequency in the afternoon, resulting in an afternoon rainfall peak. In addition, rainfall events in the ETP also exhibit a nocturnal peak in spring, summer, and autumn due to DCCs. Strong surface heat fluxes around noon can trigger or promote DCCs in spring, summer, and autumn over the ETP but produce only cumulus clouds in winter due to the cold and dry environment.
One of the major factors controlling the phase partitioning in mixed‐phase cloud is entrainment mixing, but it is still poorly understood. In this study, the liquid‐ice mass partitioning across the ...edge of shallow to moderately deep cumulus clouds are analyzed using airborne measurements. The results show the concentration and water content of both liquid and ice decrease toward the cloud edge. However, the liquid mass fraction remains similar across the cloud. The mechanism responsible for the phase partitioning is that extreme inhomogeneous entrainment‐mixing dominates. This is evident as both the droplet and ice sizes remain similar with changing concentrations. The comparison between the time scale of turbulent mixing and the phase relaxation time of water also suggests the turbulence strength is too low to homogenize the cloud. The findings from this study improve our understanding on the role of entrainment in phase partitioning, and are useful in evaluating model simulations.
Plain Language Summary
In mixed‐phase clouds, the coexistence of liquid and ice has significant impacts on the cloud lifecycle and radiative properties, but the phase partitioning in mixed‐phase cloud is still not fully understood. In cumulus clouds, one of the major factors controlling the microphysics across cloud edges is the entrainment of dry air into cloud. During the mixing process, the liquid‐ice partitioning is determined by the mixing type (homogeneous or inhomogeneous), the evaporation (growth) of liquid, and sublimation (growth) of ice. Currently, there is a lack of observational evidence for the main mixing mechanism that controls the phase partitioning. In this study, the liquid‐ice mass partitioning across the edge of shallow to moderately deep cumulus clouds are analyzed using airborne in situ measurements. The results show the liquid mass fraction remains similar across the cloud, and inhomogeneous mixing dominates the phase partitioning. The results improve our understanding on the role of entrainment, and are useful in evaluating model simulations.
Key Points
Liquid‐ice mass partitioning across the edges of cumulus clouds is characterized using airborne in situ measurement
In most of the clouds, the liquid and ice water content decrease toward the edge, while liquid mass fraction remains similar
The inhomogeneous mixing dominates the phase partitioning across the cloud edges
Mixed‐phase stratiform clouds contain numerous liquid, mixed‐phase, and ice clusters, quantifying the cluster scales is potentially helpful to improve the parameterizations of microphysics and ...radiation models. However, the scales of hydrometeor clusters at different levels of stratiform clouds are not well understood. In this study, using airborne measurements and a large eddy simulation, we show that turbulence plays an important role in controlling the clusters with length of a few hundred meters, while the scales of larger clusters have stronger vertical variations from cloud base to top. The liquid clusters are the largest near the cloud top, while the lengths of ice clusters decrease from cloud base to top. The lengths of mixed‐phase clusters depend on the glaciation process, a faster glaciation results in smaller mixed‐phase clusters. The results improve our understanding on how the liquid and ice are mixed at different levels in stratiform clouds.
Plain Language Summary
Mixed‐phase clouds are vital to the global radiative balance and water cycle. The coexistence of liquid and ice, and the way they mix have significant impacts on the cloud microphysical properties. Observations suggest hydrometeors organize in clusters in mixed‐phase stratiform clouds, while such inhomogeneity is still poorly represented in models. In this study, using airborne measurements and model simulation, we analyzed the scales of liquid, mixed‐phase, and ice clusters at different levels in a stratiform cloud observed in Northeast China. The results suggest turbulence controls the scale of clusters with length of a few hundred meters, while larger clusters have stronger vertical variations. The results improve our understanding on the cluster scales of hydrometeors at different levels in stratiform clouds, and are potentially helpful to improve the parameterizations of microphysics and radiation.
Key Points
Turbulence plays an important role in controlling the scale of hydrometeor clusters
The scales of liquid, mixed‐phase, and ice clusters have different height dependencies from cloud top to base
A faster glaciation process results in smaller mixed‐phase clusters
In mixed‐phase cumulus clouds, droplets and ice crystals are inhomogeneously distributed, such spatial inhomogeneity can be enhanced by inhomogeneous entrainment as it can strengthen the particle ...clustering, which may further influence the phase partitioning and interactions among hydrometeors. However, the scale of particle clustering induced by inhomogeneous entrainment is not well known. Utilizing high‐resolution in‐situ aircraft measurements in mixed‐phase cumulus clouds, this study shows due to inhomogeneous entrainment‐mixing, the cluster scales of droplets and ice crystals decrease by approximately 10 m from the cloud center to the edge. Changes in the clustering are correlated with the intensity of entrainment‐mixing. Clouds that are significantly affected by entrainment exhibit stronger enhancement of particle clustering and a more noticeable reduction in cluster scale. The findings from this study improve our understanding of the scale of particle clustering induced by entrainment‐mixing, and are potentially helpful in evaluating models.
Plain Language Summary
Entrainment‐mixing is one of the key processes controlling cloud life cycle and the spatial distribution of hydrometeor. It is known that inhomogeneous entrainment mixing would enhance the clustering of particle distribution in cumulus clouds, but the scales of droplet and ice clusters induced by inhomogeneous entrainment have not been quantified. Utilizing high‐resolution aircraft data sampled in mixed‐phase cumulus clouds, we showed that extreme inhomogeneous entrainment‐mixing fosters pronounced clustering of both droplet and ice particles. Opposite gradients in cluster index and cluster scale from the cloud center to the edge under varying temperature conditions are discovered. On average, the droplet and ice cluster scales decrease by 10 m due to the entrainment. In clouds that undergo intense entrainment, the amplified particle clustering is more discernible and the decrease of cluster scale is markedly greater from the cloud center to the edge. The results enhance the comprehension by quantification of extreme inhomogeneous entrainment‐mixing and are potentially helpful for assessing the performance of model simulations.
Key Points
The scales of droplet and ice crystal clustering in mixed‐phase cumulus clouds are quantified using airborne in‐situ measurements
The inhomogeneous entrainment‐mixing enhances particle clustering near cloud edge, with cluster scales decreasing by 10 m on average
Clustering variation is related to the intensity of entrainment, stronger mixing leads to a more substantial decrease in cluster scales
Surface solar radiation (SSR) can affect climate, the hydrological cycle, plant photosynthesis, and solar power. The values of solar radiation at the surface reflect the influence of human activity ...on radiative climate and environmental effects, so it is a key parameter in the evaluation of climate change and air pollution due to anthropogenic disturbances. This study presents the characteristics of the SSR variation in Nanjing, China, from March 2016 to June 2017, using a combined set of pyranometer and pyrheliometer observations. The SSR seasonal variation and statistical properties are investigated and characterized under different air pollution levels and visibilities. We discuss seasonal variations in visibility, air quality index (AQI), particulate matter (PM
10
and PM
2.5
), and their correlations with SSR. The scattering of solar radiation by particulate matter varies significantly with particle size. Compared with the particulate matter with aerodynamic diameter between 2.5 µm and 10 µm (PM
2.5–10
), we found that the PM
2.5
dominates the variation of scattered radiation due to the differences of single-scattering albedo and phase function. Because of the correlation between PM
2.5
and SSR, it is an effective and direct method to estimate PM
2.5
by the value of SSR, or vice versa to obtain the SSR by the value of PM
2.5
. Under clear-sky conditions (clearness index ➞0.5), the visibility is negatively correlated with the diffuse fraction, AQI, PM
10
, and PM
2.5
, and their correlation coefficients are −0.50, −0.60, −0.76, and −0.92, respectively. The results indicate the linkage between scattered radiation and air quality through the value of visibility.
Abstract Terrestrial evapotranspiration plays a critical role in drought monitoring and water resource management. Changes in evapotranspiration are significantly influenced by cloud-related ...precipitation and radiation effects. However, the impact of cloud amount (CA) on evapotranspiration through its influence on precipitation remains uncertain, especially in the transition zone affected by the East Asian summer monsoon (EASM), which limits the understanding of the water cycle. Therefore, this study deeply explores the impact of CA on evapotranspiration and its potential physical mechanisms in Northwest China. The results show that the correlation between 31-year average evapotranspiration and CA is negative only in the semi-arid region and is positive in other climatic regions of Northwest China. This unique negative correlation is related to the change of precipitation pattern in the semi-arid region caused by the weak EASM. Smaller CA in weak monsoons results in more short-wave radiation reaching the surface, larger sensible heat, and weaker convective inhibition. Consequently, the proportion of convective clouds (CCs) increases and precipitation from these CCs enhances evapotranspiration. Less CA increases evapotranspiration and potentially exacerbates aridity in the semi-arid region of Northwest China. These results emphasize the role of cloud type in evapotranspiration. It is well known that global warming can change cloud type with more CCs. Therefore, this study sheds new light on evapotranspiration change under global warming.
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