WMOD00505 - Laserfiche WebLink

Home Browse Search

..- I A = 5 yr avg. 24 hr. precipitation for 700 mb wind < 2400 B = 5 yr avg. 24 hr. precipitation for 700 mb wind;::: 2400 (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 cate- gories. All stations selected read 24-hour precipitation totals at sunset (near OOOOZ) map times. (4) The ratio A/B was computed for each station from the 24-hour averages, where 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 O. 57 at Steamboat Springs reflects both a rain- shadow 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 t1rain shadow" effect from the Sawatch barrier for west and northwest flow. The high values in northeastern Utah probably reflect a similar relationship 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 O. 28 (the ratio of frequency of occurrence of 700 mb flow < 2400 to flow :<:: 2400). Using this technique, it was found that only about 160/0 as much precipitation occurs in Steamboat Springs for flow < 2400 as for more northerly flow, while in the San Juans about equal amounts' are deposited for the two direction categories. A similar study was made using twelve 300 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 studY; (shown in Figure 72). Using this definition of precipitation day, daily pre~ · cipitation totals for each station were tabulated under each of the 12 wind -114