Aerosol-induced perturbation of surface fluxes over different landscapes in a tropical region

ABSTRACT

Global picture of modification of land surface fluxes due to aerosol loading, especially over different landscapes, is not yet clear. The impact of aerosols on surface fluxes over Indo-Gangetic Basin, a highly polluted region, is studied by selecting four stations in the north Indian plains that are agriculturally heterogeneous in nature, viz., short crop area, industrial area, seasonal agricultural area, and permanent dense canopy area. Data from satellite (Moderate Resolution Imaging Spectroradiometer (MODIS)), model  (Global Land Data Assimilation (GLDAS-NOAH), National Centre for Environmental Prediction/Oregon State University/Airforce/Hydrology Research Laboratory, and reanalysis (MERRA-Modern-Era Retrospective analysis for Research and Applications) as well as ground observations (AERONET (AErosol RObotic NETwork) and Dibrugarh University measurements) on daily and monthly scales are used to validate and assess the aerosol-induced modification of surface fluxes. To ensure that satellite products are reasonably accurate, aerosol optical depth (AOD) and evapotranspiration (ET) from MODIS are validated against ground truth of AOD and ET from GLDAS-NOAH and found to have correlations of 0.90 and 0.76, respectively. Aerosol radiative forcing (ARF) was calculated as the difference between ‘clear sky’ and ‘clean clear sky’ of net radiation (R) at surface obtained from MERRA. Since surface fluxes are perturbed directly by ARF on a daily scale with maximum effect during ‘sunlit’ hours, change in fluxes is calculated on the daily scale. The impact of aerosols alone on latent heat flux (LE) is quantified by using the linear relation between R and LE along with the aerosol-induced reduction of net radiation. Latent heat flux and R from GLDAS-NOAH showed an excellent correlation between 0.75 and 0.97. The impact of aerosols on surface fluxes, represented as perturbed latent and sensible heat fluxes normalized by ARF, is found to be followed an asymptotic curve against Bowen ratio (B) which indicates aridity of the land surface. This study shows that the aerosols reduce LE by 60% of ARF over the permanent dense canopy with considerable soil moisture (B = 0.5), while the same amount of ARF reduces LE by 25% only over the semi-arid region (B = 2) over the north Indian plains for the period from October 2008 to May 2009.

Acknowledgements

The authors are thankful to the Director, IITM, for the infrastructure and logistic support.

The data used in this effort were acquired as part of the activities of NASA’s Science Mission Directorate and are archived and distributed by the Goddard Earth Sciences (GES) Data and Information Services Centre (DISC). The authors are thankful to them.

The authors sincerely acknowledge the Global Modelling and Assimilation Office (GMAO) and the GES DISC for the dissemination of MERRA.

Disclosure statement

No potential conflict of interest was reported by the authors.