Diurnal Characteristics of Precipitation Features over the Tropical East Pacific: A Comparison of the EPIC and TEPPS Regions

Abstract

This study examines the diurnal cycle of precipitation features in two regions of the tropical east Pacific where field campaigns [the East Pacific Investigation of Climate Processes in the Coupled Ocean?Atmosphere System (EPIC) and the Tropical Eastern Pacific Process Study (TEPPS)] were recently conducted. EPIC (10°N, 95°W) was undertaken in September 2001 and TEPPS (8°N, 125°W) was carried out in August 1997. Both studies employed C-band radar observations on board the NOAA ship Ronald H. Brown (RHB) and periodic upper-air sounding launches to observe conditions in the surrounding environment. Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) and Geostationary Operational Environmental Satellite (GOES) IR data are used to place the RHB data in a climatological context and Tropical Atmosphere Ocean (TAO) buoy data are used to evaluate changes in boundary layer fluxes in context with the observed diurnal cycle of radar observations of precipitation features. Precipitation features are defined as contiguous regions of radar echo and are subdivided into mesoscale convective system (MCS) and sub-MCS categories. Results show that MCSs observed in EPIC and TEPPS have distinct diurnal signatures. Both regions show an increase in intensity starting in the afternoon hours, with the timing of maximum rain intensity preceding maxima in rain area and accumulation. In the TEPPS region, MCS rain rates peak in the evening and rain area and accumulation in the late night?early morning hours. In contrast, EPIC MCS rain rates peak in the late night?early morning, and rain area and accumulation are at a maximum near local sunrise. The EPIC observations are in agreement with previous satellite studies over the Americas, which show a phase lag response in the adjacent oceanic regions to afternoon?evening convection over the Central American landmass. Sub-MCS features in both regions have a broad peak extending through the evening to late night?early morning hours, similar to that for MCSs. During sub-MCS-only periods, the rainfall patterns of these features are closely linked to diurnal changes in SST and the resulting boundary layer flux variability.