Diurnal cycle characteristics of precipitation in the east Pacific

This study examines the diurnal cycle of precipitation features in two regions of the tropical east Pacific where recent field campaigns have been conducted. Precipitation features are defined as contiguous regions of radar echo exceeding a threshold of 10 dBZ and are sub-divided into MCS and sub-MCS categories, based on an area threshold of 1000 km^2 and a requirement that at least one grid element be classified as convective. Features of any size that do not contain convective elements are classified as non-convective (NC). EPIC (10N, 95W) was conducted in September 2001 and TEPPS (8N, 125W) was carried out in August 1997. Both studies employed a C-band radar on board the NOAA ship Ronald H. Brown (RHB) as well as periodic upper air sounding launches (6 times per day) to observe convection and variability in the surrounding environment. TRMM-PR and TAO buoy data are also used to place the RHB data in a climatological context and to compare the diurnal cycle of radar observed convection with changes in boundary layer fluxes.

Results show that MCSs in EPIC and TEPPS regions have distinct diurnal signatures. In the TEPPS region, MCSs peak in terms of echo coverage and rain volume in the early morning hours, similar to radar observations from the west Pacific. In contrast, MCSs in the EPIC region peak near local sunrise. These observations are in agreement previous satellite studies over the Americas and surrounding coastal regions, which show a phase lag response (in the adjacent oceanic regions) to late evening convection over the central American land mass. Thus, it appears that the diurnal cycle of MCSs in the EPIC region is a mixture of oceanic and continental signatures. The sub-MCS features in both regions show a continuous buildup of echo area and rain volume from the late afternoon through the late evening hours. These features range from trade wind cumulus to cumulus congestus clouds. Because these features predominate in relatively undisturbed periods when large scale forcing is minimized, their evolution is probably more closely linked to diurnal changes in SST and resulting boundary layer flux variability. NC features in both regions displayed little diurnal variability. These features tended to be transient, sometimes occurring as isolated elements and sometimes in association with decaying convection.