Abstract
Abstract. Aerosol–cloud interaction (ACI) is examined using 10 years of data from the MODIS/Terra (morning orbit) and MODIS/Aqua (afternoon orbit) satellites. Aerosol optical depth (AOD) and cloud properties retrieved from both sensors are used to explore in a statistical sense the morning-to-afternoon variation of cloud properties in conditions with low and high AOD, over both land and ocean. The results show that the interaction between aerosol particles and clouds is more complex and of greater uncertainty over land than over ocean. The variation in d(Cloud_X), defined as the mean change in cloud property Cloud_X between the morning and afternoon overpasses in high-AOD conditions minus that in low-AOD conditions, is different over land and ocean. This applies to cloud droplet effective radius (CDR), cloud fraction (CF) and cloud top pressure (CTP), but not to cloud optical thickness (COT) and cloud liquid water path (CWP). Both COT and CWP increase over land and ocean after the time step, irrespective of the AOD. However, the initial AOD conditions can affect the amplitude of variation of COT and CWP. The effects of initial cloud fraction and meteorological conditions on the change in CF under low- and high-AOD conditions after the 3 h time step over land are also explored. Two cases are considered: (1) when the cloud cover increases and (2) when the cloud cover decreases. For both cases, we find that almost all values of d(CF) are positive, indicating that the variations of CF are larger in high AOD than that in low AOD after the 3 h time step. The results also show that a large increase in cloud fraction occurs when scenes experience large AOD and stronger upward motion of air parcels. Furthermore, the increase rate of cloud cover is larger for high AOD with increasing relative humidity (RH) when RH is larger than 20 %. We also find that a smaller increase in cloud fraction occurs when scenes experience larger AOD and larger initial cloud cover. Overall, the analysis of the diurnal variation of cloud properties provides a better understanding of aerosol–cloud interaction over land and ocean.
Highlights
Clouds and cloud systems are crucial elements in the energy cycle of our planet (Hartmann et al, 1992; Webb et al, 2006)
For cloud optical thickness (COT) (Fig. 3b1–b2) the values are significantly higher at high Aerosol optical depth (AOD) over the ECS and the Beijing– Tianjin–Hebei (BTH); COT does not show a significant difference between the situations at low and high AOD over the Yangtze River Delta (YRD) and Pearl River Delta (PRD)
These results indicate that there is no clear dependence of COT on aerosol load and the aerosol type may influence the aerosol effect on COT
Summary
Clouds and cloud systems are crucial elements in the energy cycle of our planet (Hartmann et al, 1992; Webb et al, 2006). In addition to the radiative effects, clouds influence the hydrological cycle of the Earth through precipitation (Stephens et al, 2002). The increase in CCN, while the liquid water path remains constant, usually results in more numerous cloud droplets with smaller cloud droplet radius (CDR) due to the competition for the same amount of water vapor. Cloud albedo increases and the smaller cloud droplet effective radius in most cases results in the suppression of precipitation, which in turn results in a longer cloud lifetime, and the maintenance of a larger liquid water path (Albrecht, 1989; Feingold et al, 2001). It is important to understand the interaction between aerosols and clouds and the effect of different processes on cloud development
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