Short-period modulations in aerosol optical depths over the central Himalayas: Role of mesoscale processes

Multiyear measurements of spectral aerosol optical depths (AODs) were made at Manora Peak in the central Himalaya Range (29 degrees 22'N, 79 degrees 27'E, similar to 1950 m above mean sea level), using a 10-channel multi-wavelength solar radiometer for 605 days during January 2002-December 2004. The AODs at 0.5 mu m were very low (<= 0.1) in winter and increased steeply to reach high values (similar to 0.5) in summer. It was observed that monthly mean AODs vary significantly (by more than a factor of 6) from January to June. Strong short-period fluctuations (within a daytime) were observed in the AODs. Further investigations of this aspect have revealed that boundary layer dynamics plays a key role in transporting aerosols from the polluted valley region to higher altitudes, causing large contrast in AODs between forenoon and afternoon. The seasonal variations in AODs, while examined in conjunction with synoptic-scale wind fields, have revealed that the transport of dust aerosols from arid regions to the valley regions adjacent to the observational site and their subsequent transport upward by boundary layer dynamics are responsible for the summer increases.