Aerosols have significant and complex impacts on regional climate in East Asia. Cloud‐aerosol‐precipitation interactions (CAPI) remain most challenging in climate studies. The quantitative understanding of CAPI requires good knowledge of aerosols, ranging from their formation, composition, transport, and their radiative, hygroscopic, and microphysical properties. A comprehensive review is presented here centered on the CAPI based chiefly, but not limited to, publications in the special section named EAST‐AIRcpc concerning (1) observations of aerosol loading and properties, (2) relationships between aerosols and meteorological variables affecting CAPI, (3) mechanisms behind CAPI, and (4) quantification of CAPI and their impact on climate. Heavy aerosol loading in East Asia has significant radiative effects by reducing surface radiation, increasing the air temperature, and lowering the boundary layer height. A key factor is aerosol absorption, which is particularly strong in central China. This absorption can have a wide range of impacts such as creating an imbalance of aerosol radiative forcing at the top and bottom of the atmosphere, leading to inconsistent retrievals of cloud variables from space‐borne and ground‐based instruments. Aerosol radiative forcing can delay or suppress the initiation and development of convective clouds whose microphysics can be further altered by the microphysical effect of aerosols. For the same cloud thickness, the likelihood of precipitation is influenced by aerosols: suppressing light rain and enhancing heavy rain, delaying but intensifying thunderstorms, and reducing the onset of isolated showers in most parts of China. Rainfall has become more inhomogeneous and more extreme in the heavily polluted urban regions.

East Asian Study of Tropospheric Aerosols and their Impact on Regional Clouds, Precipitation, and Climate (EAST-AIRCPC)

[1] We explore the effects of increases in aerosol concentration on cloud lifetime for warm convective clouds using a two-dimensional single cloud model and three-dimensional large eddy simulations (LES). The models include size-resolved treatment of drop size distributions and warm microphysical processes. It is shown using a variety of soundings representing marine trade cumulus, and continental convective clouds that contrary to expectation, an increase in aerosol concentration from very clean to very polluted does not increase cloud lifetime, even though precipitation is suppressed. Cloud lifetimes are statistically similar although individual clouds may experience decreases in lifetime of 10–40%. An evaporation-entrainment feedback that tends to dilute polluted clouds more than clean clouds is identified. It is proposed that the small changes in cloud lifetime are due to competing effects of precipitation suppression and enhanced evaporation, with the latter tending to dominate in these shallow clouds.

Aerosol effects on the lifetime of shallow cumulus

. Our knowledge is still poor regarding the response of the precipitation vertical
structure to aerosols, partly due to the ignorance of precipitation occurring
at different spatial scales. A total of 6 years of collocated ground-based PM 10 
and satellite-based (Tropical Rainfall Measuring Mission, TRMM) radar data, along with ERA-Interim reanalysis, are
used in this study to investigate the aerosol effects on three localized rain
regimes (shallow, stratiform, and convective rain) over the Pearl River Delta
region of China. A subjective analysis method is proposed to discriminate
between the localized and synoptic-scale precipitations based on weather
composite charts where daily averaged wind field at 850 hPa is overlaid with
the geopotential height at 500 hPa. In general, average rain rate tends to be
greater under polluted conditions than under clean conditions. But such
potential aerosol effects are regime dependent: as the atmosphere becomes
slightly polluted (PM 10≤38 µ g m −3 ), the top 1 % radar reflectivity ( Z ) for all regimes
initially increases, followed by continued increases and weak decreases for
convective and stratiform/shallow rain regimes, respectively. As the
atmosphere becomes much more polluted, such regime dependences of aerosol
effects are more significant. From a perspective of the vertical Z structure,
comparisons between polluted conditions (days with the highest third of
PM 10 concentration) and clean conditions (days with the lowest third of
PM 10 concentration) show that the convective rain regime exhibits a deeper
and stronger Z pattern, whereas a much shallower and weaker Z pattern is
observed for stratiform and shallow precipitation regimes. In particular, the
top height of the 30 dBZ rain echo increases by ∼29  %
( ∼1.27  km) for the convective regime, but decreases by
 ∼10.8  % ( ∼0.47  km) for the stratiform regime.
However, no noticeable changes are observed for the shallow precipitation regime.
Impacts of meteorological factors are further studied on both rain top height
(RTH) and the center of gravity of Z , including vertical velocity, vertical wind
shear, convection available potential energy, and vertically integrated
moisture flux divergence (MFD). The possible invigoration effect on convective
precipitation seems dependent on wind shear, in good agreement with previous
findings. Overall, the observed dependence of the precipitation vertical
structure on ground-based PM 10 supports the notion of aerosol
invigoration or suppression effect on cold or warm rain and adds new insights into
the nature of the complex interactions between aerosol and various localized
precipitation regimes.

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Aerosol-induced changes in the vertical structure of precipitation: a perspective of TRMM precipitation radar

Weather prediction is essential to the daily life of human beings. Current numerical weather prediction models such as the Global Forecast System (GFS) are still subject to substantial forecast biases and rarely consider the impact of atmospheric aerosol, despite the consensus that aerosol is one of the most important sources of uncertainty in the climate system. Here we demonstrate that atmospheric aerosol is one of the important drivers biasing daily temperature prediction. By comparing observations and the GFS prediction, we find that the monthly-averaged bias in the 24-h temperature forecast varies between ± 1.5 °C in regions influenced by atmospheric aerosol. The biases depend on the properties of aerosol, the underlying land surface, and aerosol–cloud interactions over oceans. It is also revealed that forecast errors are rapidly magnified over time in regions featuring high aerosol loadings. Our study provides direct “observational” evidence of aerosol’s impacts on daily weather forecast, and bridges the gaps between the weather forecast and climate science regarding the understanding of the impact of atmospheric aerosol.

Aerosol as a critical factor causing forecast biases of air temperature in global numerical weather prediction models

Aerosols can act as cloud condensation nuclei and ice nuclei, resulting in changes in cloud droplet/particle number/size, and hence altering the radiation budget. This study investigates the interactions between aerosols and ice clouds by incorporating the latest ice clouds parameterization in an atmospheric general circulation model. The simulation shows a decrease in effective ice cloud crystal size corresponding to aerosol increase, referred to as the aerosol first indirect effect, which has not been comprehensively studied. Ice clouds with smaller particles reflect more shortwave radiation and absorb more infrared radiation, resulting in radiation change by 0.5-1.0 W/m2 at the top of the atmosphere (TOA). The TOA radiation field is also influenced by cloud cover change due to aerosol-induced circulation change. Such aerosol effects on precipitation highly depend on the existence of a deep convection system: interactions between aerosols and ice clouds create dipole precipitation anomalies in the Asian monsoon regions; while in West Africa, enhanced convections are constrained by anticyclone effects at high levels and little precipitation increase is found. We also conduct an experiment to assess interactions between aerosols and liquid clouds and compare the climatic effects with that due to ice clouds. Radiation and temperature changes generated by liquid clouds are normally 1-2 times larger than those generated by ice clouds. The radiation change has a closer relationship to liquid cloud droplet size than liquid cloud cover, in contrast with what we find for ice clouds.

/pdf/assessing-aerosol-indirect-effect-on-clouds-and-regional-26zz2l1aqo.pdf

Assessing aerosol indirect effect on clouds and regional climate of East/South Asia and West Africa using NCEP GFS

. Aerosol–cloud interactions (ACIs) have been widely recognized as a factor affecting precipitation. However, they have not been considered in the operational National Centers for Environmental Predictions Global Forecast System model. We evaluated the potential impact of neglecting ACI on the operational rainfall forecast using ground-based and satellite observations and model reanalysis. The Climate Prediction Center unified gauge-based precipitation analysis and the Modern-Era Retrospective analysis for Research and Applications Version 2 aerosol reanalysis were used to evaluate the forecast in three countries for the year 2015. The overestimation of light rain (47.84 %) and underestimation of heavier rain (31.83, 52.94, and 65.74 % for moderate rain, heavy rain, and very heavy rain, respectively) from the model are qualitatively consistent with the potential errors arising from not accounting for ACI, although other factors cannot be totally ruled out. The standard deviation of the forecast bias was significantly correlated with aerosol optical depth in Australia, the US, and China. To gain further insight, we chose the province of Fujian in China to pursue a more insightful investigation using a suite of variables from gauge-based observations of precipitation, visibility, water vapor, convective available potential energy (CAPE), and satellite datasets. Similar forecast biases were found: over-forecasted light rain and under-forecasted heavy rain. Long-term analyses revealed an increasing trend in heavy rain in summer and a decreasing trend in light rain in other seasons, accompanied by a decreasing trend in visibility, no trend in water vapor, and a slight increasing trend in summertime CAPE. More aerosols decreased cloud effective radii for cases where the liquid water path was greater than 100 g m−2. All findings are consistent with the effects of ACI, i.e., where aerosols inhibit the development of shallow liquid clouds and invigorate warm-base mixed-phase clouds (especially in summertime), which in turn affects precipitation. While we cannot establish rigorous causal relations based on the analyses presented in this study, the significant rainfall forecast bias seen in operational weather forecast model simulations warrants consideration in future model improvements.

/pdf/potential-influences-of-neglecting-aerosol-effects-on-the-1el07wx8wd.pdf

Yuejian Zhu

Papers

Potential influences of neglecting aerosol effects on the NCEP GFS precipitation forecast