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Show (3) 107 ESSA local climatological stations in the western half of · Colorado, eastern Utah, and southern Wyoming were used for obtaining estimated average 24-hour precipitation totals centered about the selected map times. The data were stratified by the two 700 mb wind direction categories. All stations selected read 24-hour precipitation totals at sunset (near 0000Z) map times. (4) The ratio A/B was computed for each station from the 24-hour averages, where A = 5 yr avg. 24 hr. precipitation for 700 mb wind < 240° B = 5 yr avg. 24 hr. precipitation for 700 mb wind 240° There is undoubtedly a large . error band in the data due to inadequate time resolution of upper wind and surface precipitation observations. However, a surprisingly organized field of A/ B was found (Figure 71) which can be logically correlated to terrain barrier orientation. For instance, the ratio of 0. 57 at Steamboat Springs reflects both a rainshadow effect from upstream barriers and the non-perpendicularity of the flow to the Park Range, while the very high value of 4. 05 at Wolf Creek Pass in the San Juan Range typifies perpendicular incidence of flow and the absence of upstream shadowing barriers. The low values along the Sawatch Range would be expected due to the north-south orientation of this major barrier. Similarly, the high values just east of the Sawatch (in the upper Arkansas Valley) reflect the necessity for prevailing south or southeast winds in order for significant precipitation to occur to an almost total "rain shadow" effect from the Sawatch barrier for west and northwest flow. The high values in northeastern Utah probably reflect a similar relationship with respect to the Wasatch and the Uintah ranges. To obtain an estimate of the five year total contribution of precipitation by moist flow in each of the two 700 mb wind direction categories, the ratios computed above were multiplied by 0. 28 (the ratio of frequency of occurrenci of 700 mb flow < 240° to flow 240°). Using this technique, it was found thal only about 16% as much precipitation occurs in Steamboat Springs for flow < 240° as for more northerly flow, while in the San Juans about equal amounti are deposited for the two direction categories. A similar study was made using twelve 30° direction categories for a more localized area surrounding the Park Range. In this more localized study, the 24-hour average 700 mb wind direction for Grand Junction was used, and a precipitation day was defined as one on which measurable precipitation occurred at at least one of the ESSA climatological stations used in the studJ (shown in Figure 72). Using this definition of precipitation day, daily precipitation totals for each station were tabulated under each of the 12 wind ,114 |
