WMOD00288 (2) - Laserfiche WebLink

Home Browse Search

to have high elevation recording gage data and special storm sounding data for each massif. The actual data available for analysis, however. are somewhat limited. Table 1. 2- 3 lists, the recording raingage sites wi thin or near the individual massif s. A precipitation episode was specified as the occurrence of 0.10 inch of precipitation in 24 hours at one or more of the railngage sites within the massif. The beginning and ending time of the episodes were tabulated from the Climatological Data publica- tions of hourly precipitation data. In certain cases the investigation was hindered due to the change-over from the 0.01 inch resolution weighing raingage to the 0.10 inch resolution Porter-Fischer type gage at a number of NWS gages in the late 1960's. The meteorologically-determined time lag between sounding time and precipita- tion station makes it possible to relate a one hour precipitation record (zero hour) to the aerologically determined cloud top temperature. Alternatively. it may be desirable to extend the precipitation sample used in this relationship to minus one hour through zero hour to plus one hour. or to some other longer time interval. In this way the sample size is increased. but the meaningfulness of the physical rela- tionship of the precipitation to the sounding is decreased if it is assumed that an exact time lag has been specified. Since the time lags were determined from 24 hour synoptic maps. it is considered that the likelihood of the specification of an exact lag is not realistic and that a time interval other than zero hour should be considered. Tests were run on several years of data to determine the most sensitive time interval. During these tests it was discovered that a number of zero observed pre- cipitation hours occurred. It was possible to estimate. mostly from nearby stations. that some represented lulls in precipitation. when clouds were present that would have been good candidates for seeding. In other cases. the precipitation producing system had not arrived. or had passed by at some time during the time interval under test. It was largely on the basis of how best to include the cloudy zero precipitation hours. and exclude the non-cloudy zero hours. that a seven hour precipitation interval extending from minus three hours to plus three hours was chosen. 1.2.4 Methodology of Relating ]~awinsonde Data with Precipitation Episodes. The network of rawinsonde stations operated by the National Weather Service. either currently. or during some portion of the study period, are shown on Figure 1.2-5. Table 1.2-4 lists those stations for which data were not available for the entire study period. Soundings are taken. as a rule. every twelve hours without reference to storm conditions. Prior to June 1957. the regular sounding times were 0300 and 1500Z. sub- sequently the sounding times were 0000 and l200Z. In certain cases. therefore, it 1-15 "I