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

摘要

Long-range transport of continental dust makes theseparticles a significant constituent even at locations far from theirsources. It is important to study the temporal variations in dust loadingover desert regions and the role of meteorology, in order to assess itsradiative impact. In this paper, infrared radiance (10.5-12.5 µm),acquired by the METEOSAT-5 satellite (~5-km resolution) during 1999 and2003 was used to quantify wind dependence of dust aerosols and to estimatethe radiative forcing. Our analysis shows that the frequency of occurrence ofdust events was higher during 2003 compared to 1999. Since the dustproduction function depends mainly on the surface wind speed over regions whichare dry and without vegetation, the role of surface wind on IDDI was examinedin detail. It was found that an increase of IDDI with wind speed was nearlylinear and the rate of increase in IDDI with surface wind was higher during2003 compared to 1999. It was also observed that over the Indian desert, whenwind speed was the highest during monsoon months (June to August), the dustproduction rate was lower because of higher soil moisture (due to monsoonrainfall). Over the Arabian deserts, when the wind speed is the highest during June toAugust, the dust production rate is also highest, as soil moisture is lowestduring this season. Even though nothing can be said precisely on the reasonwhy 2003 had a greater number of dust events, examination of monthly mean soilmoisture at source regions indicates that the occurrence of high windssimultaneous with high soil moisture could be the reason for the decreased dustproduction efficiency in 1999. It appears that the deserts of Northwest Indiaare more efficient dust sources compared to the deserts of Saudi Arabia andNortheast Africa (excluding Sahara). The radiative impact of dust over varioussource regions is estimated, and the regionally and annually averaged top ofthe atmosphere dust radiative forcing (short wave, clear-sky and over land)over the entire study region (0-35° N; 30°-100° E) was in therange of -0.9 to +4.5 Wm-2. The corresponding values at the surfacewere in the range of -10 to -25 Wm-2. Our studies demonstrate thatneglecting the diurnal variation of dust can cause errors in the estimation oflong wave dust forcing by as much as 50 to 100%, and nighttime retrievalof dust can significantly reduce the uncertainties. A method to retrievedust aerosols during nighttime is proposed. The regionally and annuallyaveraged long wave dust radiative forcing was +3.4±1.6 Wm-2.