Precipitation Rates in the Tropics Based on the Q1-Budget Method: 1 June 1984–31 May 1987

Abstract

The ?apparent? heat source method (Q1 budget) is used to compute the total derivative of dry static energy (s = cpT + gz) from 30°N to 30°S for the period 1 June 1984?31 May 1987. The dataset is produced from the ECMWF global analyses and consists of twice-daily values of temperature, geopotential height, horizontal wind components, and vertical velocity at increments of 2.5° ? 2.5° lat/long at seven pressure levels. Vertically integrated values of ds/dt, which are equal to total diabatic heating, Q1, are combined with estimates of net columnar radiation and surface sensible heat exchange to compute mean monthly precipitation rates, P0, as the residual in the Q1 budget. The accuracy of these P0 values is thoroughly examined, and it is suggested that the technique produces reliable estimates of precipitation over tropical oceanic areas on a monthly basis. Time series of mean monthly P0 for several geographic regions of the Southern Hemisphere tropics and the equatorial western Pacific (TOGA-COARE region) reveal that 1) the South Pacific convergence zone (SPCZ) has the highest precipitation rates in the Southern Hemisphere; 2) a clear and distinct seasonal cycle is prominent in all regions; and 3) the 1986?87 ENSO event is easily identified, particularly in the TOGA-COARE region. In addition, Hovmoeller diagrams of 5-day running means of Q1 show a gradual eastward shift of diabatic heating over the Pacific Ocean, which corresponds to the ENSO event of 1986?87. They also show some evidence of an intraseasonal (40?50 day) oscillation over the western and central Pacific.