Abstract
Abstract. Mineral dust constitutes the single largest contributor to continental aerosols. To accurately assess the impact of dust aerosols on climate, the spatial and temporal distribution of dust radiative properties is essential. Regional characteristics of dust radiative properties, however, are poorly understood. The magnitude and even sign of dust radiative forcing is uncertain, as it depends on a number of parameters, such as vertical distribution of dust, cloud cover and albedo of the underlying surface. In this paper, infrared radiance (10.5-12.5 µm), acquired from the METEOSAT-5 satellite ( resolution), was used to retrieve regional characteristics of dust aerosols for all of 1999. The infrared radiance depression, due to the presence of dust in the atmosphere, has been used as an index of dust load, known as the Infrared Difference Dust Index (IDDI). There have been several studies in the past carried out over the Sahara using IDDI as a measure of dust load. Over the Indian region, however, studies on dust aerosols are sparse. Spatial and temporal variability in dust loading and its regional distribution over various arid and semiarid regions of India and adjacent continents (0-35° N; 30° E-100° E) (excluding Sahara) have been studied and the results are examined along with surface soil conditions (such as vegetation cover and soil moisture). The advantage of the IDDI method is that information on aerosol properties, such as chemical composition or microphysical properties, is not needed. A large day-to-day variation in IDDI was observed over the entire study region, with values ranging from 4 to 22 K. It was observed that dust activity starts by March over the Indian deserts, as well as over deserts of the Africa and Arabian regions. The IDDI reaches maximum during the period of May to August. Regional maps of IDDI, in conjunction with biomass burning episodes (using TERRA satellite fire pixel counts), suggest that large IDDI values observed during the winter months over Northern India could be due to a possible deposition of black carbon on larger dust aerosols. The IDDI values have been compared with another year (i.e. 2003), with a large number of dust storms reported by meteorological departments based on visibility data. During the dry season, the magnitude of the monthly average IDDI during 2003 was slightly higher than that of 1999. The monthly mean IDDI was in the range from 4 to 9 K over the Indian deserts, as well as over the deserts of Africa and Arabia. The maximum IDDI during a month was in the range from 6 to 18 K. Large IDDI values were observed even over vegetated regions (such as the vegetated part of Africa and central India), attributed to the presence of transported dust from nearby deserts.
Highlights
Soil derived particles are among the largest aerosols with radii ranging from less than 0.1 μm to ∼100 μm (Prospero et al, 1983, 2002; Kaufman et al, 1995; Haywood and Boucher, 2000; Kaufman et al, 2002; Seinfeld et al, 2004)
The monthly mean Infrared Difference Dust Index (IDDI) was in the range from 4 to 9 K over the Indian deserts, as well as over the deserts of Africa and Arabia
It has been postulated that large dust loading over widespread areas in arid and semiarid zones might reduce surface heating sufficiently to affect the dynamics of the regional circulation pattern (Brooks and Legrand, 1999; Brooks, 2000)
Summary
Soil derived particles are among the largest aerosols with radii ranging from less than 0.1 μm to ∼100 μm (Prospero et al, 1983, 2002; Kaufman et al, 1995; Haywood and Boucher, 2000; Kaufman et al, 2002; Seinfeld et al, 2004). Dust aerosols are significant contributors to radiative warming below 500 mb due to short wave absorption (Chen et al, 1995; Mohalfi et al, 1998; Alpert et al, 1998; Miller and Tegen, 1999; Prospero, 2003; Prospero and Lamb, 2003). Tegen and Fung (1994) have shown that dust from disturbed soil causes a net cooling at the surface, accompanied by an increase in atmospheric heating. Such radiative effects are found to be most pronounced over the desert regions (Mohalfi et al, 1998; Haywood et al, 2001, 2003; Tanreet al., 2001; Zhang and Christopher, 2003)
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