Diurnal Variations of Tropical Convection Observed during the TOGA-COARE.

Abstract

Diurnal variations of tropical convection in the western Pacific are examined by using the data observed during the Tropical Ocean and Global Atmosphere-Coupled Ocean Atmosphere Response Experiment (TOGA-COARE) Intensive Observing Period (IOP) (November 1992 to February 1993). High resolution data of satellite infrared histograms, MIT radar from the TOGA project at NASA/GSFC, upper-air soundings, and improved meteorological surface mooring (IMET) buoy data by the WHOI are utilized for analyses. Over tropical western Pacific, maximum convective activity occurs during the evening hours to midnight on large islands. On the other hand, in the vicinity of large islands, the Intertropical Convergence Zone (ITCZ) and the South Pacific Convergence Zone (SPCZ), the peak activity occurs during the morning hours to noon. The local time of peak activity varies depending on the focused cloud top height. Comparing the results in different regions, local time of peak convective events of different cloud top heights change little over large islands, but it varies among the surrounding ocean, ITCZ and SPCZ. Diurnal variations of precipitation tend to be more prominent in the case of heavy rainfall with a nocturnal maximum from the TOGA-COARE special data. Therefore, we focused on the days with nocturnal precipitation maxima and examined the diurnal variations of atmospheric vertical structures over the TOGA-COARE region. At the onset of convection in the evening, water vapor is increased in lower troposphere. During this time low-level clouds appear, and upward motion is observed in the lower layers. Convective activity reaches its peak around 00-03 LT, which coincides with the maximum precipitation. Large-scale upward motion, apparent heat source, and moisture sink are also observed at the peak of convective activity. While precipitation decreases gradually in the morning to noon, the activity of high-level clouds decay. It is suggested that the water vapor increase at low levels in the evening plays a role in the development of nocturnal convection. The maximum activity of convection over the TOGA-COARE region however, is observed about 6 hours prior to the peak of other convergence regions over the tropical western Pacific.