Daniel Kirshbaum's webpage
Small-scale mechanisms of orographic precipitation enhancement
Main collaborators:
Dirk Cannon, Phd student, University of Reading
Dr Suzanne Gray, Senior Lecturer, University of Reading
Mountain (aka orographic) regions are well known to receive heavier precipitation than surrounding areas. This enhancement arises from a number of physical mechanisms, some of which involve stratiform (stable) processes and others that involve convective (statically unstable) processes. Stable orographic clouds frequently form within the warm-frontal and warm-sector regions of mid-latitude cyclones. Because of the generally weak vertical motions within these clouds (which scale with the product of the cross-barrier wind speed and terrain slope), they require some time to produce precipitation-sized particles that reach the ground. Because the flow only ascends the mountain for a brief time, these clouds may not produce precipitation on their own. However, they may still enhance precipitation when the impinging airflow is already precipitating. As the falling precipitation passes through the orographic cloud, it sweeps out cloud particles and grows, which enhances the precipitation rate over the high terrain. This mechanism is often called the seeder-feeder process, where the "seeder" cloud is the impinging precipitating cloud and the "feeder" cloud is the low-level orographic cloud.
Convective orographic clouds, on the other hand, regularly create precipitation even in the absence of pre-existing frontal clouds. The strong updrafts within convective cells or bands form precipitation readily, which falls out as intermittent showers or thunderstorms. Convective orographic clouds may form in a range of background environments: warm-sector flow, potentially unstable flow following the passage of a surface cold front, and in conditionally unstable flows during the warm season. In the latter case, mountains are particularly effective in acting as elevated heat sources that drive horizontal convergence and initiate deep convection.
The distinction between stratiform and convective orographic precipitation is blurred when the orographic cloud is nominally stratiform but develops embedded convective cells. This occurs when the orographic flow is only marginally unstable.
Please see my publications page for a full list of my publications.
Last updated 6th March 2010