WMOD00527

• Title: Quantitative Precipitation Forecasting for Improving Reservoir Operations
• Date: 4/1/1995
• Weather Modification - Doc Type: Report

Table 1 lists the 42 storm periods selected for this study. The 6-digit starting and ending
dates give the UTC (Universal Time Coordinated) year, month, and day (YYMMDD). UTC
starting and ending times give the first and last hour respectively for the l2-h periods in
which precipitation began and ended; each 12-h period is centered on either the 0000 or
1200 UTC atmospheric sounding time. The maximum precipitation accumulation measured
within the ARB always exceeded the greater than (>) values listed in the "Max Cum" column.

Preeipitation data for the major storm periods were compiled for 14 SCPP gauge sites and
8 Reclamation/CDWR gauge sites within the ARB. These 22 gauge sites, shown on figure 2,
are listed in table 2. Both 6- and 12-h precipitation accumulations were compiled; however,
only the 12-h values, centered on the UTC atmospheric sounding times, were used in this
study. Data were not always available for all 22 gauge sites because of instrument problems.

Precipitation data recorded at remote locations only provide indices of the actual areal
amounts. This limitation occurs because the gauges only represent point observations, and
some of the precipitation is not measured by the gauges because of wind effects, snow
capping, and gauge malfunctions.

Table 2 indicated two sites in the ARB where Reclamation/CDWR and SCPP gauges were
located very close to each other. These sites are S06IBLU and S50/GRE. Comparing
precipitation data from these two sets of collocated gauges provides insight into the accuracy
of point precipitation data. The results of comparisons of l2-h precipitation totals, taken
from the 1983/84 - 1985/86 SCPP field seasons, are listed in table 3. The data in this table
indicate that the 12-h precipitation data from the collocated gauges at both sites were in close
agreement; sample correlations were 0.98 and 0.97, respectively.

2.4.2 Rawinsonde Data

During SCPP, rawinsondes (upper-air atmospheric soundings) were taken at Sheridan,
California, during winter storm periods. Sheridan (SH) is located just to the west of the
ARB, and approximately 150 kilometers (93 miles) north-northeast of Oakland, California
(fig. 3). Oakland (OAK) is the rawinsonde station operated by the National Weather Service
that is closest to the ARB; both historic and real time sounding data are usually available
for Oakland for the two synoptic times each day (0000 and 1200 UTC). Because Sheridan
rawinsondes will not be available in the future as a real time data source, a comparative
study of Oakland and Sheridan sounding data was done to determine if Oakland data could
be substituted for Sheridan data in the orographic precipitation model.

For this study, atmospheric sounding times were selected from the 1981-82 and 1982-83
SCPP winter field seasons when good data were available from both Sheridan and Oakland
at least through the 400-mb level. These two winter seasons were used because both were
wetter than normal and had at least 10 storm periods where the cumulative precipitation
within the ARB exceeded 60 mm (2.34 in.) water equivalent over a significant portion of the
basin. A total of 120 Oakland/Sheridan sounding pairs was selected. These sounding pairs
were subdivided based on the 700-mb wind direction at Oakland, viz., NW (northwest) flow
(2700 - 3600) and SW (southwest) flow (1800 - 2690). The resulting sample sizes were 40
soundings for NW flow and 80 soundings for SW flow. Parameters compared were
temperature (T), relative humidity (RH), wind direction (WD), and wind speed (WS) at
selected pressure levels from the 1000-mb level through the 500-mb level. Table 4 lists the
mean values and mean difference computed for each parameter at each pressure level.

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