A New Theory for Rainfall in Tropical Mountains
Spatial patterns of tropical rainfall are strongly influenced by mountains. Although theories for precipitation induced by convectively stable upslope winds exist for the midlatitudes, these do not represent the interaction of moist convection with orographic forcing, thus being of questionable relevance in the tropics. UC Berkeley researchers who are part of the RGMA Monsoon Extremes project developed a new theory describing convective precipitation produced by the mechanical interaction of a mountain ridge with a basic-state horizontal wind. Lower tropospheric temperature and moisture anomalies created by stationary mountain waves are used with a closure for convective precipitation. Analytical solutions provide estimates of the rain distribution, including its peak, upstream extent, and rain shadow length. The theory makes good predictions when compared to idealized simulations of tropical orographic rainfall.
While existing theories for orographic precipitation essentially set rainfall proportional to forced vertical motion, this new theory highlights for the first time the role of mountain-forced thermodynamic anomalies in setting seasonal-mean rainfall in the tropics. This explains why a mountain can enhance rainfall as far as 1000 km upstream and deplete it equivalently far downstream. These extensive length scales are apparent on satellite-based rainfall retrievals in regions such as the western coasts of India and Myanmar. The theory applies to any tropical mountainous region in the presence of significant seasonal-mean wind. These new results present a new, simple framework in which to study the seasonal and interannual variability of precipitation in and around tropical mountains. This could allow a better understanding of when and why dry spells and wet spells occur in a given region. It also provides a tool to understand future changes in rainfall under anthropogenic warming, and to evaluate the ability of global climate models to simulate this change accurately.
UC Berkeley researchers derived a new theory for rainfall in and around tropical mountains, using ingredients from classic mountain wave theory and a recent convective closure suited for seasonal-mean tropical rainfall. Spatial modulation of temperature and moisture in the lower free troposphere is shown to be a dominant control on the precipitation distribution. The new theory provides quantitative estimates of rainfall strength, as well as the extent of the upstream rainfall enhancement and the rain shadow. This mechanistic understanding, a first in the context of tropical orographic rainfall, will guide future work in explaining spatiotemporal variability and projecting long-term changes in tropical orographic precipitation, with application to many regions, including South Asia in the summer monsoon.