Auromeet Saha

Aerosol optical depths over peninsular India and adjoining oceans during the INDOEX campaigns: Spatial, temporal, and spectral characteristics

The spatial, temporal, and spectral characteristics of aerosol optical depths tau (p lambda) for the Indian Ocean Experiment (INDOEX) study period (January to April) are examined using data collected through a ground-based network of multiwavelength solar radiometers (MWR) over coastal regions of peninsular India; two island locations, one in the Arabian Sea and another in the southern Indian Ocean at 20 degreesS; in conjunction with estimates made over various locations over the Arabian Sea and Indian Ocean during the INDOEX cruises of 1996, 1998, and 1999. Spatial variations show extremely low values of tau (p) at the shorter (visible) wavelengths (lambda < 750 nm) to the south of the Intertropical Convergence Zone (ITCZ), but increases substantially at locations due north of the ITCZ due to increased source impact and advection. by favorable winds. An enhancement in tau (p) is seen in the central Arabian Sea, which is attributed to air trajectory effects. Angstrom parameters, deduced from optical depth spectra, reveal a high value of alpha (similar to0.9) for north of the ITCZ, while for the south alpha is negative, indicating a change in the aerosol size distribution. Accumulation aerosols dominate in the north, while concentration of coarse aerosols remain nearly about the same, except very close to the coast. A north-south gradient in aerosol optical depth, with scaling distance of similar to 1000 to 2000 kin at shorter wavelengths and much higher at longer wavelengths, is observed. The gradient becomes shallower at high wind speeds. The large-scale dynamics associated with the movement of the ITCZ and its interannual variation appears to significantly influence the aerosol characteristics. As the southwest monsoon sets in over India, considerable wet removal and change in air mass characteristics cause a significant depletion in optical depths, which then became comparable to those prevailing in the southern hemisphere.

Wintertime spatial characteristics of boundary layer aerosols over peninsular India

During an intense field campaign for generating a spatial composite of aerosol characteristics over peninsular India, collocated measurements of the mass concentration and size distribution of near-surface aerosols were made onboard instrumented vehicles along the road network during the dry, winter season (February-March) of 2004. The study regions covered coastal, industrial, urban, village, remote, semiarid, and vegetated forestlands. The results showed (1) comparatively high aerosol (mass) concentrations (exceeding 50 μ g m(-3)), in general, along the coastal regions (east and west) and adjacent to urban locations, and (2) reduced mass concentration ( 50% of the total) of coarse-mode aerosols (>1 μ m). The spatial composite of accumulation-mode share to the total aerosol mass concentration agreed very well with the monthly mean spatial composite of aerosol fine-mode fraction for February 2004, deduced from Moderate-Resolution Imaging Spectroradiometer data for the study region, while a point by point comparison yielded a linear association with a slope of 1.09 and correlation coefficient of 0.79 for 76 independent data pairs. Pockets of enhanced aerosol concentration were observed around the industrialized and urban centers along the coast as well as inland. Aerosol size distributions were parameterized using a power law. Spatial variation of the retrieved aerosol size index shows relatively high values (>4) along the coast compared to interior continental regions except at a few locations. Urban locations showed steeper size spectra than the remote locations.

Impact of Precipitation on Aerosol Spectral Optical Depth and Retrieved Size Distributions: A Case Study

Abstract A case study is presented on the impact of two isolated, strong thundershowers during a prevailing dry, sunny season on the spectral optical depths and inferred columnar size characteristics of atmospheric aerosols at a tropical station. Results show a remarkable decrease in the aerosol optical depth and change in the spectral slope after the rain. The scavenging was found to be dependent on the particle size distribution; the larger, supermicron particles were found to be removed faster during the first shower itself, even though it was of only moderate intensity, resulting in about a 64% decrease in the columnar mass loading. In the second shower, which was stronger and more widespread than the former, more of the submicron particles in the optically active submicron size range were removed, but the reduction in mass loading was very small. The effective radius decreased continuously and so too did the columnar mass loading (total aerosol volume). The data are used to estimate the apparent colu...

Interannual Variations of Aerosol Optical Depth over Coastal India: Relation to Synoptic Meteorology

Abstract Interannual variations in spectral aerosol optical depths (AOD) were examined using the data obtained from a chain of ground-based multiwavelength solar radiometers from various locations of the Indian peninsula during the dry winter season (January–March) of 1996–2001. All of the stations revealed significant interannual variations, even though the spatial pattern of the variations differed over the years. These interannual variations were found to be significantly influenced by the extent of the southward excursion of the intertropical convergence zone (ITCZ). The years in which the southward excursion of the ITCZ was less (i.e., the years when the wintertime ITCZ was closer to the equator) showed higher AODs than the years in which the ITCZ moved far southward. The spatial variation was found to be influenced by large-scale vertical descent of an air mass over peninsular India, the Arabian Sea, the Indian Ocean, and the Bay of Bengal.

Aerosol study during INDOEX: observation of enhanced aerosol activity over the Mid Arabian Sea during the northern winter

The spatial distribution of the aerosol optical depth during the northern winter, along the West coast of India and over the oceanic environments of the Arabian Sea and the South-West Indian Ocean (between 60°E–78°E and 15°N–20°S), has been investigated using co-ordinated ground-based and ship-borne measurements carried out during January–March of 1998 and 1999 under the Indian Ocean Experiment (INDOEX). In this paper apart from the expected results, an interesting observation is presented of enhanced aerosol activity in the mid Arabian Sea, far removed from the continent. Its implications are discussed.