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Aerosol indirect effect during successive contrasting monsoon seasons over Indian subcontinent using MODIS data

https://doi.org/10.1016/j.atmosenv.2010.02.015Get rights and content

Abstract

Aerosol indirect effect (AIE) was estimated over six Indian regions, which have been identified as main source regions of absorbing aerosol emissions, for four successive contrasting monsoon years, 2001 (normal monsoon rainfall year), 2002 (drought year), 2003 (excess monsoon rainfall year) and 2004 (below normal rainfall year). The AIE has been estimated both for fixed cloud liquid water path (CLWP) and for fixed cloud ice path (CIP) bins, ranging from 1 to 350 gm−2 at 25 gm−2 intervals obtained from Moderate resolution imaging spectroradiometer (MODIS). In 2002 and 2004, AIE found to be of positive (Twomey effect) in majority of the fixed CLWP and CIP bins, while in 2001 and 2003 majority of the bins were found to be showing negative indirect effect (Anti-Twomey effect). Changes in circulation patterns during contrasting monsoon seasons, bringing up air mass containing aerosols of different source origins may be the main reason for this positive and negative AIE. The study suggests that AIE could be one of the factors in modulating Indian summer monsoon. However, further research on this topic is to be carried out to establish the relationship between AIE and Indian monsoon rainfall and also AIE values may be parameterized in climate models for better prediction of monsoon.

Introduction

Atmospheric aerosols influence the climate indirectly by modifying the cloud microphysical properties, known as aerosol indirect effect. The intergovernmental panel on climate change fourth assessment report (IPCC, 2007) estimate for the global annual radiative forcing of the first indirect effect is −0.7 Wm−2 with an uncertainty range of −1.8 to −0.3 Wm−2. Very few observations report quantification of aerosol indirect effect across the world (Pandithurai et al., 2009, Feingold et al., 2003, Kim et al., 2003). While different types of AIE have been proposed (Twomey, 1977, Albrecht, 1989, Kaufman and Fraser, 1997), the most widely accepted one is the first indirect effect or Twomey effect, which describes the reduction in cloud effective radius with increase in aerosol loading for fixed liquid water path. Opposite to this effect i.e. increase in cloud droplet size with increase in aerosol load also were reported over some parts of the world at certain environmental conditions (Yuan et al., 2008). It is also reported that precipitation has strong effect in aerosol scavenging by clouds (Kawamoto, 2008). Even though many studies report aerosol direct radiative effects; studies on AIE are very sparse over Indian region. Indian summer monsoon season (June–September) and associated rainfall, which has a strong connection to agricultural food production, has been less predictable by conventional models in recent years (Patra et al., 2005). Many studies qualitatively suggest that aerosols and AIE has a key role in modulating and in determining breaks and floods of Indian Summer Monsoon Rainfall (ISMR) (Ravi Kiran et al., 2009, Patra et al., 2005). Hence it is necessary to have a quantitative estimate of AIE to find the modulation of ISMR by aerosol-cloud interactions. In this study we quantitatively estimate the AIE values in four successive contrasting monsoon years viz. 2001 (normal monsoon year), 2002 (drought year), 2003 (excess monsoon rainfall year), 2004 (below normal rainfall year) and show that AIE could be one of the factors modulating ISMR. This is believed to be the first study to report a quantitative estimate of first AIE with fixed CLWP and CIP constraints over different aerosol source regions in India.

Section snippets

Data and methodology

Since rainfall during mid-monsoon month (July) determines the main features of ISMR, we used July data sets obtained from Moderate resolution imaging spectroradiometer (MODIS) for this study. The analysis also was extended for September (i.e. last month of monsoon season) to study that whether there is any change in primary features of AIE observed during July. MODIS is a 36 band scanning radiometer onboard NASA's Terra and Aqua satellites. We have used Terra MODIS Level-3 daily data products

Variation of aerosol properties and cloud effective radii over different regions

Fig. 2 shows the variation of aerosol optical depth (AOD), Fine mode fraction (FMF), Aerosol Index (AI), cloud droplet effective radius for ice phase (CIR) and droplet effective radius for water phase (CWR) during July and September of 2001–2004 over all six regions. The AOD values indicating the aerosol loading over all regions found to be highest during July 2002 (Except over R6) as compared to all other months. Lower AOD values were found over all regions (Except over R4) during July 2003.

Summary and conclusions

In this study we have quantitatively estimated the aerosol indirect effect during four successive contrasting monsoon seasons, 2001 (normal monsoon rainfall year), 2002 (drought year), 2003 (excess monsoon rainfall year) and 2004 (below normal rainfall year). The AIE has been estimated both for fixed cloud liquid water path (CLWP) and for fixed cloud ice path (CIP) bins. It is found that in 2002 and 2004, majority of the fixed CLWP and CIP bins were found to be showing positive indirect effect,

Acknowledgements

The Authors would like to thank B. N. Goswami, Director, IITM and P.C.S. Devara, Head, PM& A Division Dr. R. Krishnan, Director In-charge, CCCR for their encouragement. Data from MODIS and TOMS are also acknowledged with thanks. A. S. Panicker and S. Dipu gratefully acknowledge CSIR for research fellowship.

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