Stratiform Rain in the Tropics as Seen by the TRMM ....pdf

1J UNE 2003 1739 SCHUMACHERAND HOUZE q 2003 American Meteorological Society Stratiform Rain in the Tropics as Seen by the TRMM Precipitation Radar * COURTNEY SCHUMACHER AND ROBERT A. H OUZE J R . Department of Atmospheric Sciences, University of Washington, Seattle, Washington (Manuscript received 6 September 2002, in ?nal form 9 December 2002) ABSTRACT Across the Tropics (20 8 N–20 8S), the Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) indicates that for re?ectivities $17 dB Z, stratiform precipitation accounts for 73% of the area covered by rain and 40% of the total rain amount over a 3-yr period (1998–2000). The ratio of the convective rain rate to the stratiform rain rate is on average at the horizontal resolution of the PR data. Convective rain rates remain constant or decrease as the stratiformcontribution to total rain increases, implying that stratiformrain production is not very dependent on the strength of convection. This relationship is especially evident over the ocean, where there are weaker convective rain rates than over land but relatively larger stratiform rain amounts. The ocean environment appears more ef?cient in the production of stratiform precipitation through either the sus- tainability of convection by a warm, moist boundary layer with only a weak diurnal variation and/or by the near–moist adiabatic strati?cation of the free atmosphere. Factors such as wind shear and the relative humidity of the large-scale environment can also affect the production of stratiform rain. Over land, higher stratiform rain fractions often occur during the season of maximum insolation and with the occurrence of very large, organized precipitation systems (., mesoscale plexes). Monsoon regions show the largest seasonal variations in stratiform rain fraction, with very low values in the season before the monsoon and higher values during the monsoon. A strong gradient in stratiform rain fraction exists across the Paci?c, with a minimum