Estimation of Latent Heating of Rainfall during the Onset of the Indian Monsoon Using TRMM PR and Radiosonde Data

Abstract

The objective of this study is to estimate the vertical structure of the latent heating of precipitation in the vicinity of the Himalayas. Based on a cloud physics parameterization and the thermodynamic equilibrium equation, a simple algorithm is proposed to estimate latent heating from a combination of radiosonde and Tropical Rainfall Measuring Mission (TRMM) precipitation radar (PR) data, specifically, the radar reflectivity and the rain-rate estimates. An evaluation of the algorithm against 6-hourly areal averages from diagnostic budget studies during the South China Sea Monsoon Experiment (SCMEX) suggests that the algorithm captures well the vertical structure of latent heating between the top of the moist layer and the cloud-top detrainment layer. The retrieval algorithm was applied systematically over the Indian subcontinent and Tibetan plateau within a region comprising 15°?32°N and 70°?95°E during June, the month of monsoon onset, for three different years (1999, 2000, and 2001). The estimated latent heating profiles exhibit large spatial and temporal variability in the magnitude and position of maximum latent heating within the same TRMM overpass, and from one year to the next. This reflects the presence of convective activity with varying degrees of organization during the monsoon, and also the interannual variability of large-scale conditions. Along the Himalayan range, the diurnal cycle of latent heating profiles suggests more intense convective activity in the early morning and during nighttime (1-km difference in the height of maximum latent heating), consistent with the diurnal cycle of rainfall observations and cloudiness. The height of maximum latent heating at stations in the Indian subcontinent varies over a wide range, reflecting a mix of stratiform and convective precipitation systems, respectively, 5.7 ± 2, 3.8 ± 1.5, and 4.8 ± 1.7 km MSL, for 1999, 2000, and 2001. Overall, the peak production of latent heating is roughly at the effective terrain elevation of the Himalayan range with regard to synoptic circulation and orographic enhancement effects. The Tibetan plateau behaves as an elevated heat source with maximum heating produced at 7?8 km MSL. Average values of the maximum latent heating ranged between 1.3 and 1.6 K day?1 per unit rainfall (1 cm day?1), with maximum values of up to 10 K day?1.