Intraseasonal variability of infrared cooling rates during the Asian summer monsoon and their influence on intraseasonal waves

Infrared cooling rates over the Asian monsoon domain between 5{dollar}spcirc{dollar}S to 20{dollar}spcirc{dollar}N and 40{dollar}spcirc{dollar}E to 135{dollar}spcirc{dollar}E have been calculated for summer seasons of 1984 and 1987, using a medium band resolution infrared radiative transfer model. Cloudiness is specified using INSAT infrared measurements. Intraseasonal variability of cloudiness and cooling rates are examined. INSAT-derived cloud amounts have 20 to 50% of total variance in intraseasonal frequency band and the centers of maximum variance are found over the Arabian Sea, equatorial Indian Ocean, Bay of Bengal, South China Sea, and western Pacific Ocean. Intraseasonal variability of cloudiness is primarily due to variations in deep convective clouds on intraseasonal time scales. Infrared cooling rates are between 1 to 5{dollar}spcirc{dollar}C{dollar}cdot{dollar}day{dollar}sp{lcub}-1{rcub}{dollar}, and intraseasonal cooling rate anomalies have magnitudes between 0.5 to 1.5{dollar}spcirc{dollar}C{dollar}cdot{dollar}day{dollar}sp{lcub}-1{rcub}{dollar}. The centers of large intraseasonal variability of cooling rates are co-located with the centers of cloudiness variability. The cooling rate anomalies are directly associated with variability of deep convection such that cooling rates are maximum between 200 and 400 mb when deep convective clouds are present. In the absence of deep convective clouds, cooling rates are maximum lower in the troposphere. The dominant vertical mode of cooling rate anomalies has the structure in which lower and upper tropospheres have opposite phases. The difference between 1984 and 1987 monsoon seasons are primarily in the amplitudes and time periods associated with intraseasonal cloudiness and cooling rate anomalies. A conceptual model of equatorial Kelvin waves, with first baroclinic mode structure is used to show that cooling rate anomalies can modify phase speeds of planetary scale Kelvin waves by up to 12%. The change in phase speed is maximum for wave number 1 and for the faster radiative time scales.