Possible impacts of upper-tropospheric clouds and pollution on precipitation over Asia

The subtropical anticyclone during Asian summer monsoon has great influences on distributions and variations of upper-tropospheric clouds, water vapor, pollution, and precipitation in the region. Observations from EOS MLS (Earth Observing System Microwave Limb Sounder) on NASA Aura satellite show that the anticyclone traps large amount of polluted air, cloud ice and water vapor above the South Asia High (SAH) during August and September 2004. The amount of cloud ice and water over SAH was built up slowly (over 20 days) in August but removed rapidly (within 10 days) in early September. This slow buildup is possible if polluted air tends to delay precipitation by holding more cloud ice and water vapor in the upper troposphere than clear air. The removal of a large amount of cloud ice in the upper troposphere, according to MLS observations, is associated with the breakdown of the subtropical anticyclone and the SAH. GPCP data reveal heavy rainfalls in south central China during the period when the upper-tropospheric clouds were reducing. Winds from the UK Met Office analysis show strong updrafts in the heavy rainfall regions (e.g., south central China) while the anticyclone was weakening. As the upper-tropospheric anticyclone moved eastward, the associated rainband drifted to the southwest, producing heavy rainfalls and flooding in southern China and South Asia during 3-8 September 2004. Together, these observations suggest that the removal of upper-tropospheric clouds and humidity is mostly carried out by the breakdown of the subtropical anticyclone. Slowly accumulating and rapidly removing high clouds is likely to intensify the rainfalls in these polluted regions.