WMOD00317

• Contract/Permit #: 14-06-D-6999
• Project Name: CAWRP
• Title: Cascades Atmosphere Water Resources Program - Volume I
• Prepared For: US Department of the Interior Bureau of Reclaimation
• Date: 7/1/1970
• Weather Modification - Doc Type: Report

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SECTION 3
A THEORE;TICAL_ MOD~~IOR OROGRAPHIC PRECIPITATION

3.1 Introduction

Concurvently witb our field studies of the structure of winter orographic

~Jouds over the Cascade Mountains, efforts are bein~ made to develop a

theoretical model which describes the essential features of the airflow over

the mountain and the development of orographic ctouds and precipitation. In

'this section we present an account of the state of the model at the end of

19?1 and some of the theoretical predictions.

3.2 The Airflow Model

There are two main ways in which a mountain can modify the flow of air.

Firstly, it acts as a barrier which deflects the air over and around it or

blocks the air (the barrier effect). Secondly, a mountain can serve as a

source or sink of heat (the diabatic effect). Since much of the precipitation

which is deposited on the Cascade nountains is produced by the stable

overrunning of warm fronts, we have concentrated our attention on the barrier

effect. Therefore, in the model described below, it is assumed that the

atmosphere is stably stratified and that the only heat sources are due to the

release of latent heat within the cloud.

The equations describing the flow of dry air over a mountain are

considered first. These then serve as a guideline in developing equations to

describe the flow of cloudy or partly-cloudy air over a mountain.

3.2.1 Airflow in a Dry Adiabatic AtMOsphere

We consider first the two dimensional flow of dry air over a mountain in

the form of a long ridge. The x-axis is taken perpendicular to the len~h of

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