Evaluation of an Upslope Precipitation Model
Abstract
A linear orographic precipitation model applicable on complex terrain for an arbitrary wind direction has been developed. The model includes mountain wave dynamics as well as condensed water advection and two micro-physical time delay mechanisms. Atmospheric input variables in the model are wind speed and direction, specific humidity, wet static stability and two conversion factors for the micro-physics. In addition, the underlying terrain is needed. Various closed-form solutions for the precipitation behavior over ideal mountains have been derived and verified with numerical mesoscale models. The model is tested in real terrain against observations. Several locations are used to evaluate the model performance (southern Norway, the Alps and the Wasatch mountains in Utah). The model results are of same magnitude as the observations, which indicate that the fundamental physics is included in the model. The ratio of condensate that is carried over the mountain crest to the amount that is left as precipitation is crucial, and the model seem to reproduce this well. When the model results are evaluated against observations with statistical measure such as correlation coefficient, it performs well overall. This requires that detailed input information such as wind direction and stability are provided and that the observations are taken frequently. Traditional observation samplings are normally unevenly distributed between valleys and mountain tops which cause a bias in objective analysis. Such an analysis can, in this case, not be held directly against model results. For the same reason, if a model for instance perform well on mountain tops, but poorly in valleys, observations will give a wrong impressions of the model performance. From our tests, the model perform well in smaller region where the input variables are representative for the whole area. Some model deficiencies are also discovered. The model performance seems to improve with slightly smoothed terrain which partly compensates for non-linear effects of flow in complex terrain.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2002
- Bibcode:
- 2002AGUFM.H62C0872B
- Keywords:
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- 1854 Precipitation (3354)