OROGRAPHIC PRECIPITATION
Precipitation that has been generated or modified by topography, typically through the forcing of
vertical atmospheric motions.
Introduction
The influence of mountains upon rain and snowfall is often profound, creating some of the Earth's
wettest places (e.g. Cherrapunji in India, where monsoon flow encounters the southern Himalayas, has
received 26.5 m in one year) and driest places (e.g. The central valleys of the Atacama desert, shielded
by surrounding mountains, can go for decades without rainfall). Orographic effects on precipitation are
also responsible for some of the planet's sharpest climatic transitions. The classic example is the so-
called 'rain shadow'; for a mountain range oriented perpendicular to the prevailing winds, precipitation
is greatly enhanced on the windward side and suppressed in the lee. However, the full gamut of
orographic influences is much broader than this: precipitation can be enhanced in the lee, over the crest,
or well upwind of a mountain.
The mass balance of Earth's snow and ice is strongly affected by orographic precipitation.
Accumulation of mountain snowpack and on alpine glaciers is typically dominated by orographic
snowfall. The Greenland and Antarctic Ice sheets are themselves substantial topographic features
responsible for orographic effects on precipitation. Avalanches are sensitive to the detailed stratigraphy
of the snowpack, which is partly determined by the sequence of orographic graupel, rain, ice, and
various snow crystals that fall during storms.
This article will focus on the important physical processes controlling orographic precipitation, and the
observational and modeling techniques that have been used to characterize and understand it. More
complete reviews may be found in: Smith (1979); Roe (2005); and Smith (2006).
Fundamentals
Orographic precipitation is shaped by myriad non-linear processes operating on scales ranging from the
1000 km size of storms and major mountains to the sub-micron size of cloud droplets. Still, the most
fundamental of these processes are thermodynamic in nature and are well understood. Almost all
orographic influences on precipitation occur due to rising and descending atmospheric motions forced
by topography. These motions can be forced mechanically, as air impinging on a mountain is lifted
over it, or thermally, as heated mountain slopes trigger buoyancy-driven circulations. Rising motion
causes the air to expand and cool, which is important since the amount of water that may exist as vapor
in air is an approximately exponential function of temperature (described by the Clausius Clayperon
equation). Thus if cooling is sufficient, air saturates and the water vapor condenses into cloud droplets
or forms cloud ice crystals. These droplets and crystals grow by various processes until they become
large enough to fall as rain and snow. It is important to emphasize that moist ascent over topography
alone is typically insufficient to generate precipitation: these orographic effects mainly modify
precipitation during preexisting storms (e.g. Browning et al., 1974; Smith, 2006). Conversely, when air
descends it warms and dries, and both cloud and precipitation evaporate.
A useful tool for understanding some of the basic controls on orographic precipitation is the āupslopeā
model (e.g. Smith, 1979; Smith, 2006). This idealized and physically-based model predicts the water
condensed when flow with given surface specific humidity (qv, expressed as a mixing ratio), density
(
Ļ
), and uniform wind velocity ( ), impinges upon topography (with height: h(x,y)). The model
