The radiative budgets of a tropical mesoscale convective system during the EMEX-STEP-AMEX experiment: 2 model results

This paper describes calculations of the spatial and temporal variation of the radiation budget of a tropical mesoscaleconvectivesystem (MCS). A combination of cloud model simulations, radiation model simulations, and analysesof observationsobtained during the Equatorial MesoscaleExperiment (EMEX), the Stratosphere-TroposphereExchange Program (STEP), and the Australian Monsoon Experiment (AMEX) are used to obtain these heating rates. The twodimensional version of the Colorado State University regional atmospheric modeling system is used to simulate a tropical MCS that occurred during EMEX mission 9 on February 2, 1987. The simulation is shown to broadly agree with the observations reported in a related paper. The spatial radiative heating distributions derived from a two-stream radiative transfer model corresponding to the mature stage of the simulated cloud system indicate that significant horizontal inhomogeneitiesexist. According to the model results the effects of the MCS are to (1) increase in the infrared emissionto the surface and to decreasein the net infrared energy lossfrom the atmosphere relative to the clear sky emissionand (2) change the transmissionof solar flux to the surface, the shortwave albedo of the atmosphere, and the solar absorption in the atmosphere. The results show how the MCS significantly reducesthe solar flux to the surface relative to the clear sky valuesand that the largest reduction occursunder the convectiveportions of the mature MCS. (3) The MCS createsa total (solar plus infrared) radiative warming in the atmosphererelative to the surrounding clear sky. The value of this total heating is governed by both infrared and solar absorption. Vertical profiles of this heating show the dominance of infrared cooling near cloud top and infrared heating inside and near cloud base. The shortwave heating rate can also be as large as the infrared cooling near the cloud top region of the tropical MCS, especially at a local noon. (4) The temporal changesin radiation profiles also demonstrate how the MCS modulates the radiation budget of the atmosphere. Specifically, the total radiation energy loss of the entire