Vertical profiling of optical and physical particle properties over the tropical Indian Ocean with six-wavelength lidar 1. Seasonal cycle

For the first time the seasonal cycle of optical and physical particle prop erties over the Indian Ocean was observed with a six-wavelength aerosol lid ar. The measurements were performed in the framework of the Indian Ocean Ex periment at the Maldives International Airport in Hulule island (4.1 degree sN, 73.3 degreesE) from February 1999 until March 2000. The advection of ai r masses from India and Southeast Asia during the northeast monsoon season showed multiple particle layers with strong backscatter and extinction coef ficients to heights of 4 km. Approximately 30-45% of the monthly-mean optic al depth was contributed by elevated particle layers above 1000 m height. M ean optical depths of similar to0.32 at 530 nm and mean extinction-to-backs catter (lidar) ratios from 45 to 75 sr at 532 nm indicated heavily polluted aerosols. Observations during March 2000 showed that 56% of the mean optic al depth of 0.29 was contributed by elevated particle layers. Lidar ratios between 45 and 60 sr were in the same range as in the previous year. The so uthwest monsoon season was characterized by the advection of particles in m aximum heights of 5000 m from Africa and Arabia. Backscatter coefficients w ere considerably less compared to the findings during the winter monsoon. L ow mean optical depths of 0.15, with a contribution of 48% from layers abov e 1000 m, and lidar ratios from 35-55 sr indicated a mixture of clean-marin e and clean-continental aerosol conditions. During the intermonsoon season, clean-marine conditions prevailed with particles to maximum heights of 2.5 km, mean optical depths of 0.13, and lidar ratios from 20 to 30 sr.