Stratiform Rain in the Tropics as Seen by the TRMM Precipitation Radar*

Abstract

Across the Tropics (20°N–20°S), the Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) indicates that for reflectivities ≥17 dBZ, 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 4.1 on average at the horizontal resolution of the PR data. Convective rain rates remain constant or decrease as the stratiform contribution to total rain increases, implying that stratiform rain 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 efficient in the production of stratiform precipitation through either the sustainability of convection by a warm, moist boundary layer with only a weak diurnal variation and/or by the near–moist adiabatic stratification 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 (i.e., mesoscale convective complexes). 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 Pacific, with a minimum ∼25% over the Maritime Continent and a maximum ∼60% in the intertropical convergence zone (ITCZ) of the eastern-central Pacific. This near-equatorial trans-Pacific gradient becomes exaggerated during El Niño. A higher stratiform rain fraction concentrates latent heating at upper levels, which implies a stronger upper-level circulation response to the heating. Thus, the variations in stratiform rain fraction that occur before the monsoon and during the monsoon, across the Pacific basin, and between La Niña and El Niño imply vertical variations in the large-scale circulation response to tropical precipitating systems that would not occur if the stratiform rain fraction was temporally and spatially uniform across the Tropics.