Rainfall and Radiative Heating Rates from TOGA COARE Atmospheric Budgets

Abstract

Atmospheric heat and moisture budgets are used to determine rainfall and radiative heating rates over the western Pacific warm pool during the Tropical Ocean Global Atmosphere Coupled Ocean?Atmosphere Response Experiment (TOGA COARE). Results are compared to independent estimates of these quantities from the other sources. Using the COARE bulk flux algorithm to estimate surface evaporation over the intensive flux array (IFA), the IFA moisture budget-derived average rainfall for the 120-day intensive observing period (IOP) is 8.2 mm day?1. This value agrees closely with recent estimates from satellites and the ocean salinity budget. For a smaller area within the IFA containing the rain-mapping domain of the TOGA and Massachusetts Institute of Technology 5-cm radars, the atmospheric budget for the 101-day radar deployment yields 6.8 mm day?1, slightly greater than the independent radar rain rate estimate of 5.4 mm day?1. Comparison of budget-derived rainfall with National Centers for Environmental Prediction and European Centre for Medium-Range Weather Forecasts reanalyses indicates that the reanalyses produce excessive precipitation in the northern ITCZ (around 10°N) in association with anomalously moist low-level conditions at those latitudes. These anomalous conditions arise from moist-biased VIZ humidity sensors on rawinsondes launched at operational sites there, while outside those latitudes dry-biased Vaisala sensors were almost exclusively used. Computation of the vertically integrated net radiative heating rate ?QR? as a residual from the heat and moisture budgets reveals a ?1.5 K day?1 variation on the timescale of the Madden?Julian oscillation. The implied horizontal variation of ?QR? is large enough to have significant impacts on the tropical Walker and Hadley circulations. The IFA?IOP mean ?QR? is ?0.41 K day?1. This net cooling rate is less than many previous estimates for the Tropics but is within the range of independent estimates for COARE based on radiation models and observations. This small value may arise from decreased longwave emission to space due to abundant cirrus over the warm pool and, in addition, may reflect some shortwave absorption by cirrus, but not necessarily ?anomalous absorption? as has been recently proposed.