Diurnal variations in tropical diabatic heating profiles

The diurnal cycle of precipitation largely dictates diurnal variations in the magnitude of diabatic heating (Q1). However, the diurnal cycle of cloud type (both precipitating and non-precipitating) can significantly affect diurnal variations in the vertical structure of Q1. This study uses sounding network data from three Tropical Rainfall Measuring Mission (TRMM) field campaigns (KWAJEX, TRMM-LBA and SCSMEX) to compare diurnal Q1 variations in different tropical environments (ocean, continental and monsoon). In general, afternoon and evening cloud processes contribute more to the bottom-heavy aspect (i.e., heating below 600 mb) of the average Q1 profiles and early morning cloud processes contribute more to the top-heavy aspect (i.e., maxima from 600-400 mb).

Over the Kwajalein Atoll, the diurnal cycle of precipitation is weak but Q1 profiles show significant diurnal variation. Convective clouds of small to moderate vertical extent appear to be prevalent during the afternoon and evening hours, while more organized precipitating systems tend to occur in the later morning hours. These different cloud types factor strongly into the lower and upper level heating maxima in the average KWAJEX Q1 profile, respectively. Over the southwestern Amazon, deep convection with a broad mid-level heating maximum occurs in the afternoon and contributes to the large afternoon maximum in rainfall. Mesoscale convective systems (MCSs) and remnant stratiform regions from convective systems originating near the Brazilian coast predominate in the early morning hours. These large, early-morning cloud systems don't produce as much rain as the afternoon convection, but they contribute significantly to the top heaviness of the TRMM-LBA Q1 profile. The north South China Sea has a moderate diurnal cycle of rain during the monsoon, but little variability in vertical Q1 structure. Convective systems appear to form along the China coast and propagate south over the sounding network during the day as mature and decaying MCSs. The systems exit the sounding network in the late evening and early morning hours. Thus, the SCSMEX Q1 profile is top heavy throughout the diurnal cycle, with an afternoon maximum in magnitude.

The above variations in Q1 provide a challenge to models and satellite retrievals in determining the vertical structure of heating associated with tropical convection. The results of this study point to the importance of accurately capturing the diurnal cycle of tropical cloud systems in order to better describe vertical variations in heating to which the general circulation responds.