Initiation of Convective Precipitation in Complex Terrain Under Reduced Moisture Conditions
Abstract
Orographic features can encourage the initiation of precipitating thunderstorms with a number of physical processes. These processes can be due to either the mechanical or thermal effects of the mountains on the atmosphere. We wish to discuss a mechanism which predominates when the amount of moisture available in the atmosphere is relatively low and the solar heating is strong. Although we have studied this process over the Rocky Mountains, we believe it is applicable to many parts of the world experiencing similar conditions. Mountains act as an elevated heat source and the atmosphere above them gains heat from them. This process, however, does not appear to strongly influence where the precipitation forms under low moisture conditions. In this situation, convection often initiates on the lee side of the mountain. Furthermore, when the wind blows along the ridge line, thunderstorms are more likely to be generated if the wind direction is along the ridge line than if it is across the ridge line. The development of precipitating convection is also favored for slower wind speeds than faster wind speeds. Moisture convergence along and on the lee side of the ridge reduces atmospheric stability sufficiently to allow for thunderstorm development. In addition, when the wind blows along the ridge the mechanically induced subsidence on its lee side is considerably less. We illustrate these principles with a numerical model over Rocky Mountain orography. Obviously, the development of thunderstorms is heavily dependent on atmospheric instability and that is controlled by a number of processes. The orientation of the wind with respect to the topography and the speed of the wind can be applied to reveal the most favorable areas for the initiating of precipitating convection when synoptic and other mesoscale forcing are weak. We are currently examining their potential to explain the wide spatial variations in precipitation on monthly or seasonal time scales in mountainous regions.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2005
- Bibcode:
- 2005AGUFM.H53J..03T
- Keywords:
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- 1840 Hydrometeorology;
- 1843 Land/atmosphere interactions (1218;
- 1631;
- 3322);
- 1847 Modeling;
- 1854 Precipitation (3354);
- 3314 Convective processes