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Spencer's Mesoscale Meteorology Wiki Page

"Sawtooth National Forest near Stanley Idaho, by Spencer Daines - Second place at 2006 AMS Photo Contest"

University of Utah Department of Meteorology Homepage

Orography and Orographic Precipitation
Orography As defined in the | American Meteorological Society’s Glossary of Meteorology Orography is the nature of a region with respect to its elevated terrain. Orography can also be thought of as the study and nature of hills and mountains.



Orographic Precipitation

Orographic Precipitation is, therefore, precipitation caused by the forced lifting of moist air by a mountain barrier. This lifting can be caused by two mechanisms: The upward deflection of large scale horizontal flow by the Orography. 2) The daytime heating of the mountain barrier surface causes anabatic or upward vertical propagation of moist air up an orographic slope. Upon ascent, the air that is being lifted will expand and cool. The expansion and cooling of a rising moist air parcel will allow for evaporation of the surrounding water vapor forming clouds and eventually precipitation.

Sources of Orographic Precipitation along the Wasatch Front

In Utah, terrain induced precipitation is a major factor for meteorologists as they forecast the local weather. The Wasatch Front mountain range plays a major role in the type, amount, intensity and duration of precipitation events. Studies by local researchers are beginning to understand the sources of orographic precipitation along the Wasatch Front as studied from in situ observations, aircraft observations and atmospheric model output such as the MM5 (Mesoscale Model).

A study entitled High-Resolution Simulations and Microphysical Validation of an Orographic Precipitation event over the Wasatch Mountains during IPEX IOP3 describes the evolution of and Mesoscale dynamics involved for the creation of optimal orographic precipitation along the Wasatch Front. In effect, the study shows that barrier width, slope steepness and updraft speed are major contributors for the amount and intensity of orographic precipitation. Model simulations for these factors showed that narrow barriers and steeper slopes produced stronger updraft speeds which, in turn, enhanced orographic precipitation.

In sutu observations showed that during the time of the orographic precipitation event, strong convergence of synoptic scale south-westerly flow and along barrier southerly flow some 20-40 km upstream of the barrier resulted in enhanced precipitation at the barrier.