Radiative effects of elevated aerosol layer in Central Himalayas

Abstract

Systematic observations of light detection and ranging (LIDAR) to detect elevated aerosol layer were carried out at Manora Peak (29.4° N, 79.5° E, ∼1960 m a.s.l), Nainital, in the Central Himalayas during January–May 2008. In spite of being a remote, high-altitude site, an elevated aerosol layer is observed quite frequently in the altitude range of 2460–4460 m a.s.l with a width of ∼2 km during the observation period. We compare these profiles with the vertical profiles observed over Gadanki (13.5° N, 79.2° E, ∼370 m a.s.l), a tropical station, where no such elevated aerosol layer was found. Further, there is a steady increase in aerosol optical depth (AOD) from January (winter) to May (summer) from 0.043 to 0.742, respectively, at Manora Peak, indicating aerosol loading in the atmosphere. Our observations show north-westerly winds indicating the convective lifting of aerosols from far-off regions followed by horizontal long-range transport. The presence of strongly absorbing and scattering aerosols in the elevated layer resulted in a relatively large diurnal mean aerosol surface radiative forcing efficiency (forcing per unit optical depth) of about −65 and −63 W m⁻² and the corresponding mean reduction in the observed net solar flux at the surface (cooling effect) is as high as −22 and −30 W m⁻². The reduction of radiation will heat the lower atmosphere by redistributing the radiation with heating rate of 1.13 and 1.31 K day⁻¹ for April and May 2008, respectively, in the lower atmosphere.

Acknowledgements

This work was carried out as a part of the ARFI and ABLN&C project under Indian Space Research Organization – Geosphere Biosphere Program. We thank the director, ARIES, Nainital, for the constant support provided to undertake this work. We also thank the director, NARL, Gadanki, for providing the LIDAR data and Dr. Harish Gadhvi for help during the analysis and fruitful discussions. We thank Dr. Hegde, Mr. Arjun Reddy and Mr. Manoj Kumar for their help with the LIDAR observations. We also thank the anonymous reviewers for their useful suggestions to improve the quality of this article.