Aerosol-cloud interaction in water clouds observed using ground-based, in-situ, and satellite-based observations over an Indian continental region

The impact of atmospheric aerosols on climate by acting as cloud condensation nuclei is very complex and unable to capture completely by climate models. In this study, an observational approach to estimate the aerosol-cloud interaction (ACI) in water clouds is presented using a combination of ground-based, in-situ balloon-borne, and satellite-based measurements of nearly 7 years over an Indian region. Aerosol and cloud optical depths (AOD and COD retrieved from lidar) are the aerosol and cloud proxies, respectively, used for estimating ACI by constraining liquid water path (LWP retrieved from radiosonde) and background meteorological conditions. Seasonal variations of the occurrence of water clouds detected from backscattered lidar signals using the wavelet covariance transform method are identical to those obtained from radiosonde observations. The ACI analysis shows that the correlation between AOD and COD is highest near the cloud base and it decreases as the height above the cloud base increases for all the LWP values ranging from 30 to 150 g/m2. The calculated ACI tau parameter is within the theoretical limit. The highest correlation value near the cloud base are observed at the LWP bin of 60 to 90 g/m2. ACI at the cloud base shows a decreasing pattern as the cloud base temperature decreases for LWP values of 30 to 60 g/m2 whereas different patterns for all other LWPs. High ACI is observed when the wind direction at the cloud base is northeasterly and low ACI when it is southeasterly and southwesterly. This result supports the observation of the high occurrence of water clouds during winter followed by the post-monsoon season and low occurrence during monsoon and pre-monsoon seasons over the observational site. Thus, the estimated ACI is in accordance with the theoretical concept of aerosol indirect effect i.e. the polluted atmospheric condition leads to clouds with more cloud fraction and cloud optical depth.

Automated summary

In this study, an observational approach is used to estimate the aerosol-cloud interaction (ACI) in water clouds over an Indian region. The analysis shows a decrease in correlation between aerosol and cloud optical depths as the height above cloud base increases. The observed patterns of high and low ACI support the seasonal variations of water cloud occurrence.