Aerosol lofting from sea breeze during the Indian Ocean Experiment

Abstract

This work was carried out to understand the mechanisms leading to lofting and large‐scale advection of aerosols over the Indian Ocean region due to interaction of the sea breeze with the northeast monsoon winds along the west coast of India. European Centre for Medium‐Range Weather Forecasts (ECMWF) wind fields for the months of February and March 1999 were analyzed at various times of day. Intense sea breeze activity was observed at 1200 UT (1730 local time) along the west coast of India with average intensity larger in March than in February. The sea breeze was seen to extend inland deeper in March than in February. Lofting of air observed as high as 800 hPa (approximately 2 km above sea level) could lead to entrainment of aerosols into the free troposphere and long‐range transport. Upward motion of air was observed everywhere along the west coast of India (from 8° to 20°N), on average higher in March than in February, because of convergence between the sea breeze and the synoptic‐scale flow. A region of intense lofting of air and well‐defined convergence was observed along the coast of the Karnataka region (12°–16°N). A simulation with a general circulation model nudged with ECMWF data indicated that the intrusion of marine air masses with low concentrations of organic matter is seen as deep as 64 km inland in the evening (1500 UT). Intrusion of the sea‐salt plume is seen to a maximum distance of around 200 km from 1500 until 2300 UT. A well‐developed lofted layer of aerosols as high as 3 km was also simulated during sea breeze activity along the west coast of India. The general circulation model simulation shows a clear diurnal evolution of the vertical profile of the aerosol extinction coefficient at Goa but fails to reproduce several features of the lidar observations, for example, the marked diurnal variability of the upper layers between 1 and 3 km. However, the model simulates a diurnal cycle at the surface (0–0.7 km) that is not apparent in lidar measurements. The model simulates long‐range transport and captures the lofted plume downwind of the west coast of India. However, there was a 1–2 day delay in the model transport of lofted aerosols at higher layers to Hulule, 700 km downwind of India, when compared to lidar observations.