Dust aerosols over India and adjacent continents retrieved using METEOSAT infrared radiance Part II: quantification of wind dependence and estimation of radiative forcing

Abstract

Abstract. Long-range transport of continental dust makes these particles a significant constituent even at locations far from their sources. It is important to study the temporal variations in dust loading over desert regions and the role of meteorology, in order to assess its radiative impact. In this paper, infrared radiance (10.5-12.5 µm), acquired by the METEOSAT-5 satellite (∼5-km resolution) during 1999 and 2003 was used to quantify wind dependence of dust aerosols and to estimate the radiative forcing. Our analysis shows that the frequency of occurrence of dust events was higher during 2003 compared to 1999. Since the dust production function depends mainly on the surface wind speed over regions which are dry and without vegetation, the role of surface wind on IDDI was examined in detail. It was found that an increase of IDDI with wind speed was nearly linear and the rate of increase in IDDI with surface wind was higher during 2003 compared to 1999. It was also observed that over the Indian desert, when wind speed was the highest during monsoon months (June to August), the dust production rate was lower because of higher soil moisture (due to monsoon rainfall). Over the Arabian deserts, when the wind speed is the highest during June to August, the dust production rate is also highest, as soil moisture is lowest during this season. Even though nothing can be said precisely on the reason why 2003 had a greater number of dust events, examination of monthly mean soil moisture at source regions indicates that the occurrence of high winds simultaneous with high soil moisture could be the reason for the decreased dust production efficiency in 1999. It appears that the deserts of Northwest India are more efficient dust sources compared to the deserts of Saudi Arabia and Northeast Africa (excluding Sahara). The radiative impact of dust over various source regions is estimated, and the regionally and annually averaged top of the atmosphere dust radiative forcing (short wave, clear-sky and over land) over the entire study region (0-35 • N; 30 • -100 • E) was in the range of -0.9 to +4.5 W m −2 . The correspondingCorrespondence to: S. K. Satheesh (satheesh@caos.iisc.ernet.in) values at the surface were in the range of -10 to -25 W m −2 . Our studies demonstrate that neglecting the diurnal variation of dust can cause errors in the estimation of long wave dust forcing by as much as 50 to 100%, and nighttime retrieval of dust can significantly reduce the uncertainties. A method to retrieve dust aerosols during nighttime is proposed. The regionally and annually averaged long wave dust radiative forcing was +3.4±1.6 W m −2 .

Retrieval of dust aerosols during night: improved assessment of long wave dust radiative forcing over Afro-Asian regions

“…The regionally (0-35 N; 30 E-100 E) and annually averaged longwave dust radiative forcing was + 2.6 ± 1.8 W m −2 . This is lower than the value of + 3.4 ± 1.6 W m −2 reported by Deepshikha et al (2006b) without considering the diurnal variation of dust loading. Liao and Seinfeld (1998) estimated global mean clear-sky, long-wave radiative forcing at TOA in the range of + 0.2 to + 1.0 W m −2 .…”

“…However, we note that the IDDI algorithm, valid for daytime, will no longer hold during the night because dust is then hotter than the theoretical dust-free reference, while clouds are colder as also acknowledged in Deepshikha et al (2006b). Hence for nighttime retrieval of dust, we have followed a modified approach by re-ordering the steps as follows (a) Follow the daytime IDDI algorithm: create "reference image" representing the hottest pixels; (b) create "difference image" and then use it for cloud identification and masking; (c) apply the cloud mask to the original IRBT images; (d) apply the IDDI algorithm again, but with the "minimum reference".…”

Spectral surface albedo over Morocco and its impact on radiative forcing of Saharan dust

“…Lyamani et al (2006) obtained solar forcings of a dust event in Spain between −2 Authors TOA TOA TOA Surface Surface Surface Remarks SW LW net SW LW net Annual and global mean 0.14 Miller and Tegen (1998) −0. Costa et al (2006) −40 < − 400 Yellow Sea, single event, ω 0 = 0.76, AOT=0.4-2.0 Deepshikha et al (2006) +4.5 −22.8 Regional and annual mean, ω 0 = 0.82 −0.9 −11.8 Regional and annual mean, ω 0 = 0.97 Wang et al (2006 and −12 W m −2 in dependence on the solar zenith angle, with strongest forcing occurring at 70 • .…”

Absorbing aerosol variability over the Indian subcontinent and its increasing dependence on ENSO

“…Over IGP, a rapid increase in population and industrialisation has resulted in a sustained increase in the anthropogenic aerosol burden (Guttikunda et al, 2003;Dey et al, 2004;Deepshikha et al, 2006;Nair et al, 2007;Kaskaoutis et al, 2011;Abish and Mohanakumar, 2013). An excessive dependence on fossil fuels in heavy industries, that includes thermal power plants and vehicular transportations, has significantly increased the emission of carbonaceous aerosols (soot).…”