Dilip Ganguly

Physical and optical properties of aerosols over an urban location in western India: Implications for shortwave radiative forcing

[1] We discuss results on implications of seasonal and interannual variabilities in aerosol parameters measured over Ahmedabad, an urban location in western India, for the regional-scale shortwave aerosol direct radiative forcing. Results on physical and optical properties of aerosols are discussed in a companion paper. A discrete ordinate radiative transfer model has been used to carry out the radiative transfer computations. Two different approaches are followed to generate spectral values of aerosol parameters required as input for the radiative transfer computations, and the estimated values are found comparable for both methods. Magnitudes of surface forcing are found to be highest during postmonsoon (-63 ± 10 W/m 2 ), followed by dry (-54 ± 6 W/m 2 ) and lower values during premonsoon (-41.4 ± 5 W/m 2 ) and monsoon (-41 ± 11 W/m 2 ) seasons. In case of TOA, radiative forcing are found to be negative during dry (-26 ± 3 W/m 2 ) and postmonsoon (-22), while positive values are obtained during monsoon (14) and premonsoon (8). Large differences between TOA and surface forcing during monsoon and premonsoon indicate large absorption of radiant energy (∼50 W/m 2 ) within the atmosphere during these seasons. Different properties of aerosols and differences in their vertical distribution give rise to different heating rates within the atmosphere for different seasons. Heating rates at the surface are found to be highest during postmonsoon (5.6°K/day) but decreases sharply with increase in height. Atmosphere is heated strongly at higher levels between 1 and 2 km during monsoon. Results from several sensitivity studies have emphasized the importance of solar zenith angle and other related factors in modulating the values of aerosol radiative forcing.

On the dust load and rainfall relationship in South Asia: an analysis from CMIP5

This study is aimed at examining the consistency of the relationship between load of dust and rainfall simulated by different climate models and its implication for the Indian summer monsoon system. Monthly mean outputs of 12 climate models, obtained from the archive of the Coupled Model Intercomparison Project phase 5 (CMIP5) for the period 1951–2004, are analyzed to investigate the relationship between dust and rainfall. Comparative analysis of the model simulated precipitation with the India Meteorological Department (IMD) gridded rainfall, CRU TS3.21 and GPCP version 2.2 data sets show significant differences between the spatial patterns of JJAS rainfall as well as annual cycle of rainfall simulated by various models and observations. Similarly, significant inter-model differences are also noted in the simulation of load of dust, nevertheless it is further noted that most of the CMIP5 models are able to capture the major dust sources across the study region. Although the scatter plot analysis and the lead–lag pattern correlation between the dust load and the rainfall show strong relationship between the dust load over distant sources and the rainfall in the South Asian region in individual models, the temporal scale of this association indicates large differences amongst the models. Our results caution that it would be pre-mature to draw any robust conclusions on the time scale of the relationship between dust and the rainfall in the South Asian region based on either CMIP5 results or limited number of previous studies. Hence, we would like to emphasize upon the fact that any conclusions drawn on the relationship between the dust load and the South Asian rainfall using model simulation is highly dependent on the degree of complexity incorporated in those models such as the representation of aerosol life cycle, their interaction with clouds, precipitation and other components of the climate system.

Investigation of the Relationship Between Natural Aerosols and Indian Summer Monsoon Rainfall Using a Climate Model

Using all forcing simulations of fully coupled climate model GFDL-CM3, an attempt has been made to represent the role of natural aerosols (such as dust) in modulating the summer monsoon rainfall over the Central Indian region. For this purpose, long-term data set of dust, winds and rainfall have been obtained from CMIP5 data portal for the past 54 years of time period. The spatial pattern of dust load from GFDL-CM3 is able to capture the potential dust sources such as Sahara Desert, Arabian Peninsula region. Further to this, it has been observed that the load of dust over the Arabian Sea and Arabian Peninsula is significantly correlated (significant at 1% significance level) with the rainfall over the central Indian region, suggesting an in-phase relationship between the two parameters.

Aerosols and contrasting monsoon conditions over the Himalayan region

Impact of aerosols on the Indian summer monsoon (ISM) variability is well documented; however there are limited studies which have quantified the role of aerosols in modifying the amount of rainfall. To address this research problem, we make use of the remotely sensed data set of precipitation and aerosols from different observations. In the present study remotely sensed precipitation data set has been utilised to define contrasting monsoon conditions over the Himalayan region. As per the classical definition, active and break spells are defined over the central part of the Indian land region, and during the break spells over the central Indian region, the Himalayan region receives substantial amount of rainfall. It is found that accumulation of more dust over the Uttarakhand region significantly (negative correlation with rainfall; significant at 5% significance level) suppresses the rainfall during break spells. We propose that the substantial aerosol loading and its associated dynamical feedback over the Himalayan foothills may have considerable impact on the amount of rainfall over the mountainous regions of the Indian subcontinent. Results presented in this paper are supported by the statistically robust significance test and would be useful to develop the understanding of the role of aerosols in modulating the rainfall intensity during the summer monsoon season.

Accuracy of MODIS-derived AOD over an urban location in western India

Correct assessment of aerosol properties is a pre-requisite for climate change study. On account of large heterogeneity in their properties both on spatial and temporal scales, satellite remote sensing is an ideal tool to study them. But the advantage offered by satellites is inhibited by contamination from surface reflectance and cloud interference. In the past, satellite remote sensing of aerosols was limited to over oceans, which offered a dark background. With the launch of MODIS instrument onboard Terra and Aqua satellites observations have been extended to over land. MODIS derives aerosol properties by making an assessment of surface reflectance at visible wavelengths based on mid-IR reflectance. This process has an empirical basis but the validity can only be verified by comparing the results against ground truth data. In this study MODIS derived AOD is validated against the ground based sunphotometer observations made at Ahmedabad (23.03° N, 72.53°E), an urban location in Western India, from 2002 to 2005. The local meteorology is summer from March till July, monsoon during July to September and winter from October to February. MODIS AOD data at 470 nm and 660 nm from both Terra and Aqua averaged over a 0.5×0.5 degree box centered at Ahmedabad are compared with the ground truth data. An overestimation up to 150% by MODIS during April- June and an underestimation up to 50% during October to March is found. An attempt to explain these differences in terms of seasonal variation in surface reflectivity and cloud contamination is presented and discussed.