Satellite-Based Estimates of Aerosol Washout and Recovery over India during Monsoon

Abstract

Large aerosol optical depth (AOD) observed over the Indian subcontinent during the monsoon season in the satellite data challenges the common notion of aerosol washout by monsoon rain. Here, we examined recovery of aerosol field after washout by monsoon rain over various rainfall homogeneous zones of India in view of the duration of rainfall, recovery time and source strength. Mean (± 1 standard deviation) seasonal aerosol optical depth, AOD is highest over the central northeast 1 (0.74 ± 0.22) followed by central northeast 2 (0.60 ± 0.11), northwest (0.61 ± 0.15), west-central (0.54 ± 0.13), northeast (0.29 ± 0.08), peninsular India (0.39 ± 0.07) and hilly region (0.33 ± 0.08) in the monsoon season. Post-washout aerosol recovery in India is not a linear function to the recovery period relative to the two successive satellite overpasses. Fastest recovery is observed in the central northeast region dominated by anthropogenic emission. In general, washout is more for 9-hour spell than 3-hour spell, but not spatially uniform over the various rainfall homogeneous zones. In central northeast region it is observed that updraft plays an important role in post precipitation aerosol build up whereas in dust-dominated northwest India, monsoon rainfall (whenever occurs) suppresses dust emission because of the increased soil moisture and therefore inhibits the recovery. The number of grids where washout outweighs recovery during the monsoon season for a 3-hour rainfall increases by 5.6% with an increase in rain rate from < 2 mm day–1 to > 4 mm day–1, while the corresponding increase for a 9-hour rainfall event is 2.8%. AOD reduces in ‘cloudy-sky’ condition relative to ‘clear-sky’ condition because aerosols are scavenged by cloud drops as the clouds grow vertically during the monsoon. Quantitatively, AOD decreases by 16% per 100 hPa increase in cloud base height.