Solar dimming over the tropical urban region of Hyderabad, India: Effect of increased cloudiness and increased anthropogenic aerosols

Abstract

The present work examines the multidecadal changes in the net downward shortwave radiation (NDSWR) over the urban region of Hyderabad, India, during the period 1979-2005. The trends in NDSWR and in cloud optical depth (COD) are analyzed over the region using the Modern Era Retrospective-Analysis for Research and Applications (MERRA 2D), which is a newly available data set. The NDSWR exhibits a significant reduction of ∼-20.25 W m -2 for the period 1993-2005; however, it declined at a slower rate (0.468 W m -2) during the period 1979-1992. This reduction translates to -1.56 W m -2 yr -1 in the first period and to -0.04 W m -2 yr -1 in the second. The large difference in the NDSWR trends between the two periods is mainly attributed to the dramatic increase in high COD and in aerosol optical depth (AOD) during the last decade. The total COD over the region suggests an increase of 6.85% during the study period, with the larger increase to be depicted in the high-level clouds (70.86%). Furthermore, retrievals of AOD 550 from Microtops-II Sun photometer and Terra-Moderate Resolution Imaging Spectroradiometer (MODIS) show a significant increase of ∼34% and ∼20% during the period 2002-2008 over Hyderabad, respectively. It is demonstrated that aerosols and clouds contribute to the remarkable reduction in NDSWR over Hyderabad in the recent years, thus resulting in a continuous solar dimming over the area. The main role in the decline of NDSWR has been the increase in COD of the high clouds, mainly in monsoon and premonsoon. On the other hand, the dramatic increase in the AOD 550 plays an important role in the decrease of NDSWR, mainly in winter. Furthermore, the indirect aerosol effect can modify the cloud formation, albedo, and lifetime, also playing a key role in the aerosol-cloud-radiation interactions. Copyright 2010 by the American Geophysical Union.