[1] We examine the spatio-temporal characteristics of aerosols in the recent years (2000)(2001)(2002)(2003) over the Indian region with special emphasis on the Indo-Gangetic basin (northern India) using data from Moderate Resolution Imaging Spectroradiometer (MODIS), Aerosol Robotic Network (AERONET) and Total Ozone Mapping Spectroradiometer (TOMS). First, we have compared the MODIS-derived aerosol optical depth (AOD) and fine-mode aerosol fraction (FMAF ratio of the fine-mode AOD to the total mode AOD) with those of AERONET at Kanpur (26.45°N, 80.346°E). It has been found that the MODIS captures the major part of the seasonal variation of aerosols in terms of abundance as well as aerosol type. The absolute errors in AOD were within the predicted uncertainty of Dt = ±0.05 ± 0.2t. The monthly mean regional maps of MODIS show high aerosol optical depth (AOD) over the Indo-Gangetic basin in the range 0.6-1.2 at 550 nm wavelength with significant spatial and temporal variation during the summer (April to June). The associated FMAF was found to be low (<0.4). This indicates that the coarse-mode particles are dominant in the summer. The spatial distribution of absorbing aerosol index (AAI) derived from TOMS, Å ngström exponent (a) and aerosol volume size distribution measured at Kanpur also indicated the presence of absorbing coarse-mode aerosols during summer. On the other hand, the entire Indo-Gangetic basin was dominated by the fine-mode particles during the winter (November to January) with AOD in the range 0.4-0.6. Their spatial and temporal variations, however, were quite low compared to the summer. Results reported in this paper indicate that the Indo-Gangetic basin has the largest aerosol optical depth in India during both the seasons. The region is dominated by the large absorbing coarse-mode particles (possibly transported dust from the northwest of India) in the summer and by the probable widespread emission sources of fine-mode aerosols (primarily of anthropogenic origin) in the winter. The unique topography and weather condition of the region have impact on the observed spatial and temporal distribution of aerosols.Citation: Jethva, H., S. K. Satheesh, and J. Srinivasan (2005), Seasonal variability of aerosols over the Indo-Gangetic basin,

We estimate the distribution of ice thickness for a Himalayan glacier using surface velocities, slope and the ice flow law. Surface velocities over Gangotri Glacier were estimated using sub-pixel correlation of Landsat TM and ETM+ imagery. Velocities range from ˜14–85 m a–1 in the accumulation region to ˜20–30 m a–1 near the snout. Depth profiles were calculated using the equation of laminar flow. Thickness varies from ˜540 m in the upper reaches to ˜50–60 m near the snout. The volume of the glacier is estimated to be 23.2 ± 4.2 km3.

Mineral dust is the single largest contributor of natural aerosols over land. Dust aerosols exhibit high variability in their radiative effects because their composition varies locally. This arises because of the regional distinctiveness of the soil characteristics as well as the accumulation of other aerosol species, such as black carbon, on dust while airborne. To accurately estimate the climate impact of dust, spatial and temporal distribution of its radiative properties are essential. However, this is poorly understood over many regions of the world, including the Indian region. In this paper, infrared (IR) radiance (10.5–12.5 μm) acquired from METEOSAT‐5 satellite (∼5‐km resolution) is used to retrieve dust aerosol characteristics over the “Great Indian Desert” and adjacent regions. The infrared radiance depression on account of the presence of dust in the atmosphere has been used as an index of dust load, called the Infrared Difference Dust Index (IDDI). Simultaneous, ground‐based spectral optical depths estimated at visible and near‐infrared wavelengths (using a multiwavelength solar radiometer) are used along with the IDDI to infer the dust absorption. The inferred single scattering albedo of dust was in the range of 0.88–0.94. We infer that dust over the Indian desert is of more absorbing nature (compared with African dust). Seasonally, the absorption is least in summer and most in winter. The large dust absorption leads to lower atmospheric warming of 0.7–1.2 K day−1.

Abstract.A theory is proposed to determine the onset of the Indian Summer Monsoon (ISM) in an Atmospheric General Circulation Model (AGCM). The onset of ISM is delayed substantially in the absence of global orography. The impact of orography over different parts of the Earth on the onset of ISM has also been investigated using five additional perturbed simulations. The large difference in the date of onset of ISM in these simulations has been explained by a new theory based on the Surface Moist Static Energy (SMSE) and vertical velocity at the mid-troposphere. It is found that onset occurs only after SMSE crosses a threshold value and the large-scale vertical motion in the middle troposphere becomes upward. This study shows that both dynamics and thermodynamics play profound roles in the onset of the monsoon.

Recent experiments conducted over the oceanic regions adjacent to the Indian sub continent have revealed the presence of anthropogenic aerosol haze during January to March. It has been suggested that the major source of this aerosol is South and Southeast Asia. Here we show from long term, multi‐station and ship borne observations that aerosols transported from regions northwest of Indian subcontinent especially Arabian and Saharan regions (mostly natural dust) along with the locally produced sea‐salt aerosols by sea‐surface winds constitute a more significant source of aerosols during April–May period. The radiative forcing due to Arabian/Saharan aerosols (mostly natural) during April–May period is comparable and often exceed (as much as 1.5 times) the forcing due to anthropogenic aerosols during January to March period. The presence of dust load over the Arabian Sea can influence the temperature profile and radiative balance in this region.

In order to understand the response of the Earth‐atmosphere system to anthropogenic aerosol forcing, it is essential to know the relative impact of various aerosol species on the radiative budget. Most of the models used for estimating the direct radiative forcing have assumed that the various aerosol species are mixed externally or internally. However, it is possible that one aerosol species may be coated over another species to form core‐shell structure and resulting radiative impact can be significantly different than those of the externally‐mixed or internally‐mixed aerosols. Our study suggests that changes in the state of mixing of black carbon aerosols may be one of the possible causes for ‘excess’ atmospheric absorption reported by many investigators. We show that past estimates of climate forcing due to anthropogenic black carbon aerosols represent the lower bound and that the actual values may be larger than the current estimates.

[1] Making use of aerosol optical depths (AOD) derived from MODIS (onboard TERRA satellite) and winds from NCEP, and the fact that sea-salt optical depth over ocean is determined primarily by sea-surface wind speed, we examine the contribution of sea-salt to the composite aerosol optical depth (AOD) over Arabian Sea (AS), by developing empirical models for characterizing wind-speed dependence of sea-salt optical depth. We show that at high wind speeds, sea-salt contributes 81% to the coarse mode and 42% to the composite AOD in the southern AS. In contrast to this, over the northern AS, share of sea-salt to coarse mode and composite optical depth is only 35% and 16% respectively. Comparison of the sea-salt optical depth and coarse mode optical depth (MODIS) showed excellent agreement. The sea-salt optical depth over AS at moderate to high wind speed is comparable to the anthropogenic AOD reported for this region during winter. Citation: Satheesh, S. K., J. Srinivasan, and K. K. Moorthy (2006), Contribution of sea-salt to aerosol optical depth over the Arabian Sea derived from MODIS observations, Geophys. Res. Lett., 33, L03809,

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