Dust aerosols over India and adjacent continents retrieved using METEOSAT infrared radiance Part I: sources and regional distribution

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 (∼5-km 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 beenCorrespondence to: S. K. Satheesh (satheesh@caos.iisc.ernet.in) 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.

Accumulation of aerosols over the Indo-Gangetic plains and southern slopes of the Himalayas: distribution, properties and radiative effects during the 2009 pre-monsoon season

“…On the other hand, Northwestern India and adjacent arid regions largely remained dry and received little rainfall during the entire measurement period further reinforcing the persistence of regional high AOD. The enhanced influx of dust over northern India is not only due to dust emissions over the Thar Desert region but also due to its long-range transport from the Afghanistan/Iran and Middle-Eastern peninsular regions moving eastward over the Northern Arabian Sea (Prospero et al, 2002;Deepshikha et al, 2006;Satheesh et al, 2006, Prasad and. Figure 5a shows simulated 5-day airmass trajectories (1500 m a.g.l.)…”

McClear: a new model estimating downwelling solar radiation at ground level in clear-sky conditions

“…While the all-sky SSI changes mainly with clouds, the SSI under clear sky is strongly dependent on the composition of the atmosphere, namely the variable parameters aerosol and water vapour. Aerosols play a major role due to the diversity in scattering and absorbing properties according to their type, and the spatial and temporal heterogeneity of their number, size, chemical composition, and shape (Deepshikha et al, 2006;Deneke et al, 2008;Elias and Roujean, 2008;Gueymard, 2005;Xu et al, 2011).…”

On the role of dust storms in triggering atmospheric gravity waves observed in the middle atmosphere

“…The perturbation brought into the atmosphere due to dust storms can sustain from a few hours to a few days. These storms are common in western India during premonsoon season (March-May), and the Thar Desert centered in Western India and eastern Pakistan is the primary potential source of dust storms observed over the Indian subcontinent (Deepshikha et al, 2006;Pandithurai et al, 2008).…”

“…The satellite observations suggest that this type of absorbing dust loading into the atmosphere is high over the Thar Desert due to frequently occurring dust storms; therefore, the Thar Desert becomes a hot spot of warming over Indian subcontinental region (Deepshikha et al, 2006). Such hot-spots produced in the lower atmosphere are sudden and forced changes that have potential to trigger gravity waves.…”