Land-ocean Contrast in Aerosol-cloud Interaction over India: the Role of Moisture

Abstract:

Warm stratiform clouds contribute to large uncertainty in the climate system through aerosol indirect effect. Earlier studies focused mostly on deep convective mixed-phased monsoonal clouds or over a limited region in the Indian subcontinent. Here we study the aerosol-cloud interaction in warm stratiform clouds, through the relationship between the cloud droplet size and aerosol abundance (aci), separately over the land and ocean of Indian region, and check its sensitivity to the moisture availability. MODIS Aqua collection 6.1 (Level 3, 1°*1°, MYD08_D3) daily observational gridded data are used for total precipitable water (TPW), aerosol, and cloud variables between 2002-2018. Satellite retrieval artifacts are avoided through appropriate data screening, with maintaining the homogeneity in cloud dynamics by binning clouds in the interval of 25 hPa of cloud top pressure. Interestingly, a land-ocean contrast in aci is observed, consistent with the conventional positive aci (Twomey effect) over the ocean; while an opposing negative aci (“anti-Twomey” effect) is observed over the land. The land-ocean contrast in aci is conserved across different spatio-temporal resolution. Such contrast is also consistently observed across warm stratiform clouds of different vertical extent. Both positive and negative aci, observed over ocean and land respectively, are enhanced under TPW, revealing a strong influence of moisture. The droplets attain larger size under increasing aerosol concentration over the land, by possible suppression of smaller droplet activation under intense competition for moisture, contrasting the Twomey effect observed under relatively pristine atmosphere over the ocean. The cloud droplets over the ocean, under higher moisture abundance, grow to a larger size than over land in moderately polluted condition, but converge to a common droplet size (14µm) in polluted condition, affecting the aci. Importantly, negative aci indicates warming from cloud radiative forcing with a future regional aerosol level increase over land.