Abstract
Sea-salt aerosols, prominent natural aerosols over the ocean, play a vital role in direct and indirect radiative forcing. Since surface winds are the prime cause of sea-salt generation, we have developed an empirical relationship between aerosol optical depth (AOD) and sea-surface winds over the study region (60-70 degrees E; 40 degrees S-20 degrees N). The latitudinal variation of background aerosol optical depth (tau(0)) and the wind index (b) are estimated as they are essential inputs for the estimation of the spatial variation of sea-salt aerosols and are used over the Arabian Sea (AS) to generate spatial heterogeneity of sea-salt aerosols. The latitudinal variation of tau(0) and b show a nearly exponential and linear increase, respectively, as we moved towards the north. We used an empirical-cum-model approach to construct an aerosol system to reproduce the observed AOD and aerosol optical properties. Utilizing this information as input to a radiative transfer model, we worked out direct radiative forcing (DRF) over the study region in the short wave (0.2-4 mu m) and long wave (8-14 mu m) region at the surface, top of the atmosphere (TOA) and in the atmosphere. Short wave cooling at the surface, TOA and heating in the atmosphere are estimated as 40 W m(-2), 32 W m(-2) and 8 W m(-2), respectively. Long wave heating due to the sea-salt aerosols estimated at the surface, TOA and in the atmosphere is about 9 W m-2, 6 W m-2 and 15 W m-2 respectively. Long wave forcing (LWF) partly counterbalances the effect of short wave forcing (SWF) and the cooling at the surface and at TOA. The highest value of such an offset at the surface was observed over the AS (similar to 23%) and that at TOA was similar to 19%, obviously at high wind conditions. This implies that over the AS sea-salt aerosols (in coarse mode) contribute significantly to LWF compared to other adjacent oceanic regions.
Highlights
The most omnipresent natural aerosols over the oceanic region are sea-salt aerosols (Blanchard and Woodcock, 1980; Winter and Chylek, 1997; Haywood et al, 1999; Randles et al, 2004; Satheesh and Moorthy, 2005)
Since surface winds are the prime cause of sea-salt generation, we have developed an empirical relationship between aerosol optical depth (AOD) and sea-surface winds over the study region (60–70°E; 40°S–20°N)
We estimated the values of τ0 and b for each region, which were subsequently used to derive the value of τ at different wind speeds (0–15 m s–1) for the respective regions (Fig. 2(a))
Summary
The most omnipresent natural aerosols over the oceanic region are sea-salt aerosols (Blanchard and Woodcock, 1980; Winter and Chylek, 1997; Haywood et al, 1999; Randles et al, 2004; Satheesh and Moorthy, 2005). Several campaigns and satellite data analyses have been performed to study the concentration and radiative effects of aerosols (Moorthy and Satheesh, 2000; Vinoj and Satheesh, 2003; Wai and Tanner, 2004; Satheesh and Moorthy, 2005; Satheesh et al, 2006) Various relationships between these parameters have been reported as the outcome of these studies, which include exponential, linear and power-law type relations (Woodcock, 1953, 1957; Monahan, 1968; Lovett, 1978; Kulkarni et al, 1982; Gras and Ayers, 1983; Exton et al, 1985; Marks, 1990; O’Dowd et al, 1997; Moorthy and Satheesh, 2000; Vinoj and Satheesh, 2003; Wai and Tanner, 2004; Satheesh et al, 2006). Over oceanic regions adjacent to the Indian subcontinent the dependence of sea-salt aerosols mass/number concentration on wind speed has been scrutinized by several researchers
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
