A Statistical Analysis of Surface Turbulent Heat Flux Enhancements Due to Precipitating Clouds Observed in the Tropical Western Pacific

Abstract

Surface heat fluxes around 2°N, 138°E in the tropical western Pacific have been acquired by Research Vessel (R/V) Mirai for a total of 74 days during four observational periods. Significant fractions of the sensible and latent heat fluxes are found to occur during occasional passages of precipitating cloud systems which cause large surface fluxes. Based on the radar reflectivity image and in situ precipitation observation at the R/V Mirai, the contributions of the surface heat flux enhancements due to precipitating clouds to the total accumulated surface heat fluxes throughout the observational periods are estimated to be 41 and 10 percent for the sensible and latent heat fluxes, respectively (12 percent for their sum). Furthermore, each 20 percent increase in the radar echo area ratio leads to increases in sensible and latent heat fluxes by about 11 and 30 W m -2 , respectively. The significant enhancements of the sensible heat flux are due to the combined effects of increased wind speeds and large air-sea temperature differences resulting from distinct drops in air temperature at the surface. On the other hand, the enhancements of the latent heat flux are primarily due to the increased wind speeds. This difference in the way the sensible and latent heat fluxes are enhanced by precipitating clouds leads to the differences in the contributions of these enhanced fluxes to the total accumulated fluxes as well as their frequency distributions. These results suggest that precipitating convective systems play an important role in determining the surface heat fluxes over the tropical western Pacific on temporal scales that range from the life cycle of an isolated cumulus to Madden-Julian oscillation.