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 Wm-2. The corresponding values at the surface were in the range of -10 to -25 Wm-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 Wm-2.
Highlights
Soil derived particles are among the largest aerosols, with radii ranging from less than 0.1 μm to ∼100 μm (Kalu, 1979; Arimoto et al, 1995, 1997; Tanreet al., 2001; Prospero et al, 2002)
In this paper (Part II), we examine in detail the role of surface wind speed on Infrared Difference Dust Index (IDDI) over regions which are dry and without vegetation
While monthly mean IDDI values are representative of the average dust load, it does not provide any information on the number of dust events
Summary
Soil derived particles are among the largest aerosols, with radii ranging from less than 0.1 μm to ∼100 μm (Kalu, 1979; Arimoto et al, 1995, 1997; Tanreet al., 2001; Prospero et al, 2002). There have been a number of investigations in the literature regarding the transport of aerosols from the continents to the ocean (Bergametti et al, 1989; Arimoto et al, 1995; Gong et al, 2003; Zender et al, 2003) Some of these authors found the existence of Saharan dust even over the remote areas of the Atlantic and Pacific Oceans (d’Almeida et al, 1991). Radiometric Counts were converted into radiance and radiance data were converted into brightness temperature (IRBT) using an inverse Planck Function This IRBT formed the “original image”, which contains all the radiative information about the surface and the atmosphere. The “difference image” represents features of clouds and dust patterns separated from the permanent surface features This was obtained by subtracting the individual day “original image” (or IRBT) from the “reference image” for that period. We present a method to retrieve dust aerosol properties during the nighttime and discuss its implications to long wave radiative forcing
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