VARIABILITY OF RADIATIVE COOLING DURING THE ASIAN SUMMER MONSOON AND ITS INFLUENCE ON INTRASEASONAL WAVES

Infrared radiative cooling rates are calculated over the Asian summer monsoon between 5 degrees S-20 degrees N and 40 degrees-135 degrees E at a spatial resolution of 5 degrees X 5 degrees for the summer season s of 1984 and 1987. A medium spectral resolution infrared radiative tr ansfer model with specified temperature, moisture, clouds, and trace g as distributions is used to obtain the cooling rate profiles. Cloud di stributions for the two summers are obtained from Indian National Sate llite measurements. Seasonal mean and intraseasonal variations of clou ds and radiative cooling rates over a 21-76-day range of periods are e xamined. The analysis identifies centers over the central and eastern Indian Ocean, and western Pacific Ocean, along the equator, and along 15 degrees N, where seasonal mean cloud amounts range from 40% to 80% with cloud tops mostly in the middle and upper troposphere. Intraseaso nal variability of clouds is also large over these centers (% variance s >25%). Consistently, seasonal mean cooling rates are at a maximum (3 degrees-5 degrees C day(-1)) in the upper troposphere between 300 and 400 mb, related to cloud-top cooling. The cooling rates below 400 mb are between 1 degrees and 3 degrees C day(-1). The cooling rates exhib it intraseasonal amplitudes of 1.0 degrees-1.5 degrees C day(-1). The largest amplitudes are found between 300 and 500 mb, indicating that c ooling rate variability is directly related to intraseasonal variabili ty of convective clouds. Spatial distributions of clouds and cooling r ates remain similar during the 1984 and 1987 summer seasons. However, during 1987, intraseasonal amplitudes of deep convective cloud amount and cooling rate over the Indian Ocean are 10%-15% larger than in 1984 . It is shown that intraseasonal variability of cooling rates over the Indian Ocean can perturb convective heating by 10%-30% in the upper a nd lower troposphere. Based on a one-dimensional radiative-convective equilibrium model, it is estimated that the radiative damping timescal e over the Indian Ocean region is similar to 3 days. Based on this dam ping timescale and in conjunction with a model of equatorial Kelvin wa ves with first baroclinic mode, it is hypothesized that the variable c loud-radiative cooling rates can alter phase speeds of Kelvin waves by up to 60%. This helps explain why the frequency range of intraseasona l oscillations is so broad.