Precipitation, Elevation and Relief in the Tropics

TRMM (Tropical Rainfall Measuring Mission) satellite precipitation radar data indicate that near-surface precipitation rates vary as a function of elevation in distinct ways in different mountain ranges across the tropics. Regional maximum precipitation rates are found at very low elevation in India's Western Ghats, Hawaii, and Eastern Australia. In New Guinea, the Northern Andes, and Taiwan, the locally maximum precipitation rates occur at moderate elevations (~1000 m). Regional maximum precipitation rates occur at very high elevation (>2000 m) in the African Rift Valley and Mexico's Sierra Madre Occidental. We present a simple model to explain the occurrence of these different relationships between precipitation and elevation as a function of large-scale atmospheric conditions, specifically mid-level moisture and its control on convective cloud depth. Additionally, we note that spatial variability in precipitation corresponds with spatial variability in ridge-valley relief in several tropical mountain ranges. We examine topography derived from SRTM (Shuttle Radar Topography Mission) data and precipitation patterns in swaths cutting perpendicular to the strike of these mountain ranges. Ridge-valley relief is defined as the standard deviation of topographic elevation parallel to the strike of the range. Ridge-valley relief varies systematically across several mountain ranges in concert with annual average precipitation. Where precipitation rates are high, ridge-valley relief is diminished and where precipitation rates are low, ridge-valley relief is maximized. The correspondence of precipitation and relief suggest a dynamic interaction between orographic precipitation and topographic development and confirms the predictions of an idealized numerical model of the co-evolution of precipitation and topography.