Spaceborne Observations of the Diurnal Variation of Shortwave Aerosol Direct Radiative Effect at Top of Atmosphere Over the Dust-Dominated Arabian Sea and the Atlantic Ocean

The ScaRaB payload onboard the low-inclination Megha-Tropiques (MT) satellite has been making observations of radiative fluxes at the top of the atmosphere (TOA) for different local times (LTs) of the day over the tropics since October 2011. This provides a unique opportunity to investigate the diurnal variation of the regional instantaneous aerosol direct radiative effect efficiency (IADREE) at TOA, which is otherwise not possible using the available long-term satellite observations carried out using similar sensors onboard polar sun-synchronous satellites. In this paper, the diurnal variations of the IADREE over the Arabian Sea and the Atlantic Ocean during June–September, when both these regions are engulfed by large-scale mineral dust plumes transported from the adjoining deserts, are investigated using collocated multiyear (2012–2014) observations of the MT-ScaRaB measured shortwave fluxes and MODIS-derived aerosol optical depth. The estimates of the IADREE made using the MT-ScaRaB data at 13:30 LT are found to be in agreement with those derived from the Cloud and the Earth’s Radiant Energy System data at this LT, at which the latter observations are carried out. The IADREE derived from the MT-ScaRaB shows diurnal peak value of −53 ± 10 Wm<inline-formula> <tex-math notation="LaTeX">$^{{{-2}}}\tau _{{{550}}}^{{{-1}}}$ </tex-math></inline-formula> and −40 ± 3 Wm<inline-formula> <tex-math notation="LaTeX">$^{{{-2}}}\tau _{{{550}}}^{{{-1}}}$ </tex-math></inline-formula> at solar zenith angle of ~40° over the Arabian Sea and the Atlantic Ocean, respectively. Diurnal mean aerosol direct radiative effect efficiency at TOA during June–September is −22 ± 4.5 Wm<inline-formula> <tex-math notation="LaTeX">$^{{{-2}}}\tau _{{{550}}}^{{{-1}}}$ </tex-math></inline-formula> over the Arabian Sea and −18 ± 3.6 Wm<inline-formula> <tex-math notation="LaTeX">$^{{{-2}}}\tau _{{{550}}}^{{{-1}}}$ </tex-math></inline-formula> over the Atlantic Ocean.