Heating, Moistening, and Rainfall over the Western Pacific Warm Pool during TOGA COARE

Abstract

Rawinsonde, surface, and satellite data collected from the Tropical Ocean Global Atmosphere (TOGA) Coupled Ocean Atmosphere Response Experiment (COARE) are used to investigate the distributions of beating, moistening, precipitation, and evaporation over the western Pacific warm pool. The behavior of the atmosphere and the response of the ocean surface before, during, and after the passages of westerly wind bursts are examined. The tropospheric vertical wind shear associated with tropical low-frequency oscillations strongly modulated convective beating and moistening. Heavy precipitation usually fell 1 to 3 weeks prior to the peak westerly wind bursts. SSTs reached their maximum during the undisturbed phase of the intraseasonal oscillations (ISOs), gradually decreased as convection intensified, and reached their minimum during the periods of peak westerly winds when deep convection was generally suppressed over the intensive flux array (IFA). Surface latent heat fluxes were positively correlated with surface wind speed and varied between 50 and 100 W m?2 during light winds to more than 200 W m?2 during strong westerly wind bursts. Surface sensible heat fluxes, however, did not follow the pattern of surface wind speed and usually peaked during organized deep convection over the IFA. Intensive observing period (IOP)-mean evaporation and sensible heat flux over the large-scale array (LSA) were obtained by adjusting the European Centre for Medium-Range Weather Forecasts (ECMWF) fields toward buoy estimates over the IFA. These values were then used to determine IOP-mean rainfall distribution over the LSA from the moisture budget. The results generally compare well with two satellite remote-sensing estimations, SSM/I retrievals and GPI, and the ECMWF model forecast. All four methods indicate an east-west zone of minimal rainfall along the equator. Two heavy rainfall bands coinciding with the double-ITCZ structure were located north and south of the equator. The IFA was mainly located within the minimum rainfall band. Budget-diagnosed rainfall rates over the IFA agree with SSM/I retrievals, but the GPI values are excessive, particularly at times of extensive cirrus. Comparison of the apparent heat source Q1 profiles suggests that the long-term mean heating rates over the warm pool have large positive values at all levels of the atmosphere and peak between 400 and 450 hPa. In contrast, the apparent moisture sink Q2 profiles show distinctly different features over the warm pool. Positive values (indicating drying) exist in the ITCZ bands north and south of the IFA. However, low-level moistening is evident in the profiles over the IFA, probably due to strong evaporation and upward transport of moisture by shallow cumuli during high winds.