OCR Text |
Show This echo line indicates convergence in the lee side trough located just north of Mt. Harris and explains the observed local t>acking of Mt. Harris upper winds as compared to the large scale flow. It also yields valuable clues as to the cause of the observed variation of target area -precipitation patterns with Mt. Harris wind direction, since Mt. Harris upper winds are observed to veer more northerly with the southward passage of the persistent east-west line echo. The ratio of Rabbit Ears Pass to Buffalo Pass precipitation rate has been found to be positively correlated with wind direction (measured in azimuthal degrees). Figure 70 gives a measure of the departure of Mt. Harris 10, 000 ft. MSL wind direction from the synoptic scale direction, with qualitative explanations the refor. Inserting the observed average direction difference for northwesterly flow into the divergence equation, and assuming uniform streamline divergence to be present from the surface (7400 ft. MSL) to 10, 000 ft. MSL, a positive vertic~l velocity of~ 10 cm sec- 1 was calculated for 10,000 ft. MSL near the lee-side trough. This vertical velocity is between 10% and 30% of the magnitude of the general orographic vertical velocity field across the Park Range. Thus, the presence of a superimposed east-west line of convergence zone precipitation on the general orographic precipitation pattern can be justified on the basis of comparative vertical velocity values. 3. 3o 4 Dependence of Precipitation Patterns on Synoptic Scale 700 mb Wind Direction One of the major problems in selecting cont!:_ol stations for weather modification operations i_n mountainous terrain is the fact that the meso-climatolo~ of precipitation in such terrain is so highly wind direction dependent. This wind direction dependence arises from complex interactions of differing orographic effects due to variations in terrain barrier orientations. A study of these effects for two 700 mb wind direction categories was made according to the following scheme: (1) For five snow seasons (November-April) map dates and times at which the synoptically-indicated temperature-dewpoint spread over the 5°C at 700 mb and/ or 500 mb, and 10°c at the other of Park Range was these two levels, were tabulated. (2) These dates and times were divided into two categories accord· ing to (a) synoptically-indicated 700 mb wind direction more southerly than 240°, and (b) synoptically-indicated 700 mb wind direction 2:: 240° over the Park Range. 112 |