Radiative Heating Profile Anomalies Associated with Boreal Summer Intraseasonal Oscillation

Intraseasonal oscillations of tropical rainfall during boreal summers (BSISO) are closely related with the active/break phases of the Asian summer monsoon rainfall. BSISO has been investigated in a number of earlier studies as it provides a primary source for the predictability of the Asian monsoon on sub-seasonal time scale. Jiang et al. (2011) quantified the anomalous cloud types, frequencies and ice and liquid water contents associated with northward propagating BSISO events.  Such information provides important observation constraints for evaluating climate models’ capability in simulating the Indian summer monsoon and its intraseasonal variations, as well as providing valuable constraints for theoretical considerations of northward propagation mechanisms of the BSISO.

This study examines the space-time structures of anomalous cloud water content and radiative heating profiles associated with northward propagating BSISO for the period 2006-10.  The observation data used in this study includes satellite-observed daily precipitation from TRMM7, cloud ice and liquid water contents from CloudSat Radar-only Cloud Water Content Product (2B-CWC-RO), broadband heating rate profiles calculated consistently with the cloud liquid- and ice-water content estimates from the CloudSat Profiling Radar (2B-FLXHR-LIDAT_P2_R04: L’Ecuyer et al. 2008), and the height-pressure relationship from the ERA-Interim reanalysis. Based on the daily precipitation data, 17 Strong northward propagating BSISV events are identified for the 5-year period 2006-10, with nearly the same northward propagating features as in Jiang et al. (2011). The latitude-height structure of the composite liquid and ice cloud water content anomalies for the 2008-10 period used in this study is nearly the same as that for the 2006-08 period in the Jiang et al. (2011) study. The latitude-height structure of the composite radiative heating anomalies are characterized by: (1) Positive (negative) SW heating anomaly maximum in the upper troposphere, approximately in the location of the positive IWC concentration maxima (low troposphere), (2) Shallow LW cooling anomaly in the upper troposphere at 200hPa, and (3) LW heating anomaly maxima in the lower troposphere. The positive SW heating anomaly in the upper troposphere indicate enhanced absorption of insolation by increased ice clouds in the upper troposphere. The negative SW heating anomaly in the lower troposphere may result from shading of enhanced upper tropospheric ice clouds and lower tropospheric liquid clouds. The positive LW heating in the lower troposphere may result from the enhanced downward LW fluxes from the positive anomalies of the upper tropospheric IWC and the LWC anomalies in the mid- and low troposphere. The net radiative heating anomalies (i.e., LW + SW heating anomalies) show a positive heating anomaly of 0.3-0.4 K day^-1 in the upper troposphere and negligible radiative heating in the low troposphere.