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

摘要

Mineral dust constitutes the single largest contributorto continental aerosols. To accurately assess the impact of dust aerosols onclimate, the spatial and temporal distribution of dust radiative propertiesis essential. Regional characteristics of dust radiative properties,however, are poorly understood. The magnitude and even sign of dustradiative forcing is uncertain, as it depends on a number of parameters, suchas vertical distribution of dust, cloud cover and albedo of the underlyingsurface. In this paper, infrared radiance (10.5-12.5 µm), acquired fromthe METEOSAT-5 satellite ( resolution), was used to retrieve regionalcharacteristics of dust aerosols for all of 1999. The infrared radiancedepression, due to the presence of dust in the atmosphere, has beenused 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 Saharausing IDDI as a measure of dust load. Over the Indian region, however,studies on dust aerosols are sparse. Spatial and temporal variability indust loading and its regional distribution over various arid and semiaridregions of India and adjacent continents (0-35° N;30° E-100° E)(excluding Sahara) have been studied and the results are examined along withsurface soil conditions (such as vegetation cover and soil moisture).The advantage of the IDDI method is that information on aerosol properties, such aschemical composition or microphysical properties, is not needed. A largeday-to-day variation in IDDI was observed over the entire study region, withvalues ranging from 4 to 22 K. It was observed that dust activity starts byMarch over the Indian deserts, as well as over deserts of the Africa and Arabianregions. The IDDI reaches maximum during the period of May to August. Regionalmaps of IDDI, in conjunction with biomass burning episodes(using TERRA satellite fire pixel counts), suggest that large IDDI valuesobserved during the winter months over Northern India could be due to a possibledeposition of black carbon on larger dust aerosols. The IDDI values havebeen compared with another year (i.e. 2003), with a large number of duststorms reported by meteorological departments based on visibility data.During the dry season, the magnitude of the monthly average IDDI during 2003 wasslightly higher than that of 1999. The monthly mean IDDI was in the range from 4to 9 K over the Indian deserts, as well as over the deserts of Africa and Arabia. Themaximum IDDI during a month was in the range from 6 to 18 K.Large IDDI values were observed even over vegetated regions (such asthe vegetated part of Africa and central India), attributed to the presence oftransported dust from nearby deserts.