Spatio-temporal distribution of aerosol direct radiative forcing over mid-latitude regions in north hemisphere estimated from satellite observations

Abstract

An empirical method is used to estimate the aerosol direct radiative forcing (ADRF) over 20oN − 40oN regions from March 2000 to March 2019. The ADRF is calculated as the difference between the cloud-free sky and clean-sky (non-aerosol) radiative fluxes, which are fitted to an exponential function of the aerosol optical depth (AOD). The regional averaged ADRFs are negative (cooling effect) at the surface (SUR) and the top of atmosphere (TOA) and positive (warming effect) in the atmosphere (ATM). The spatial and temporal distributions of ADRF are closely linked to the spatio-temporal distributions of AOD. Higher AOD and stronger ADRF are found in spring (March to May) and summer (June to August). ADRFs are larger in regions with frequent sandstorm outbreaks and rapid economic growth since 2000 than other regions. The uncertainty of ADRF due to data source is 1.12 W/m2 at the surface and 0.91 W/m2 at the TOA according to the stochastic error propagation function. The ADRFs in our study regions show statistically significant but different changes of −0.074 W/m2 /year and 0.1 W/m2 /year at the surface during 2000 to 2009 and 2010 to 2019, respectively. Recent trend analysis also shows that the reduced aerosol contributes to the increasing short-wave flux about 0.32 W/m2 under the background of the global warming during the period from 2000 to 2019 in our study area, which indicates that it may alter the pattern of atmospheric circulation or enhance the global warming effect.