Variability of Radiative Cooling during the Asian Summer Monsoon and Its Influence on Intraseasonal Waves

Abstract

Infrared radiative cooling rates are calculated over the Asian summer monsoon between 5°S?20°N and 40°?135°E at a spatial resolution of 5° ? 5° for the summer seasons of 1984 and 1987. A medium spectral resolution infrared radiative transfer model with specified temperature, moisture, clouds, and trace gas distributions is used to obtain the cooling rate profiles. Cloud distributions for the two summers are obtained from Indian National Satellite measurements. Seasonal mean and intraseasonal variations of clouds and radiative cooling rates over a 21?76-day range of periods are examined. The analysis identifies centers over the central and eastern Indian Ocean, and western Pacific Ocean, along the equator, and along 15°N, where seasonal mean cloud amounts range from 40% to 80% with cloud tops mostly in the middle and upper troposphere. Intraseasonal variability of clouds is also large over these centers (% variances >25%). Consistently, seasonal mean cooling rates are at a maximum (3°?5°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° and 3°C day?1. The cooling rates exhibit intraseasonal amplitudes of 1.0°?1.5°C day?1. The largest amplitudes are found between 300 and 500 mb, indicating that cooling rate variability is directly related to intraseasonal variability of convective clouds. Spatial distributions of clouds and cooling rates 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 and lower troposphere. Based on a one-dimensional radiative?convective equilibrium model, it is estimated that the radiative damping timescale over the Indian Ocean region is ?3 days. Based on this damping timescale and in conjunction with a model of equatorial Kelvin waves with first baroclinic mode, it is hypothesized that the variable cloud-radiative cooling rates can alter phase speeds of Kelvin waves by up to 60%. This helps explain why the frequency range of intraseasonal oscillations is so broad.