WMOD00527

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

(orientation is skewed from true north). The gridded field data for these two models are
available in 50-mb intervals, which is the vertical spacing required by the orographic
precipitation model.

Because the NGM and Eta models include the initial gridded field data, the OAK (or any
other site's) upper-air sounding should no longer be necessary for the first QPF period. The
grid point closest to the watershed for which the QPF is desired can be used to represent the
input upper-air sounding. However, the available time of the model gridded field data
presently precludes the use of the initial data when significant storms are moving into the
forecast area; the Eta model data are not available until about 3 h after upper-air sounding
observations are available (the NGM data are available about 30 min later). Consequently,
actual sounding data must be used as orographic precipitation model input for the first 3- to
6-h QPF. The time delay in the availability of the NMC model data is expected to decrease
in the future.

The A VN model data are available for 12-h periods out to 72 h, and the MRF model data are
available for 12-h periods out to 120 h into the future. The grid points selected for the A VN
and MRF data are spaced about 320 km apart. These models have data for only the
mandatory pressure levels (1000, 850, 700, 500,400,300,250,200, 150, and 100 mb) and not
in 50-mb intervals as do the NGM and Eta models. Dr. Rhea wrote a program that does both
temporal and spatial interpolations of the A VN gridded field data to obtain representative
input upper-air data at 6-h intervals for each watershed of interest; the data are then
interpolated vertically to 50-mb intervals. The MRF model data are presently being used,
without temporal interpolation, for days 4 and 5 for the ARB only.

Use of the NGM, Eta, and A VN model gridded field data was not limited to the ARB. As a
first step in expanding the use of the orographic precipitation model to other areas by making
use of the gridded field data, a procedure was set up along similar lines to the way the model
had been used (for QPF purposes) in the past, prior to the operational availability of the
gridded field data.

2.7.1 Past Method of Using the Orographic Precipitation Model

The past method of QPF use of the orographic precipitation model can be briefly described
in the following steps:

1. The orographic precipitation model was set up for each watershed of interest with a 5-km
topographic grid. Using this topographic grid and a hypothetical "reference sounding" as
input, reference average watershed amounts of orographic model precipitation were
computed for each 10 degrees of 700-mb direction. This computation was done one time
only and the reference average watershed precipitation output data were written into a
"reference table" for repeated later use. (In addition to the average values, the model
could also output precipitation amounts for specific locations within the watershed.)

The hypothetical "reference sounding" was selected to represent a warm, wet, windy, very
heavy precipitation event, and included the following: (1) a 700-mb temperature of 0 cC,
(2) a 50-kn wind speed at the 700-mb level, (3) saturated moisture from the surface
upward to the 450-mb level, and (4) an assumed 6-h duration for these conditions.

2. Wind direction and speed were inferred from the 700-mb prognostic charts (out to 60 h).

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