WMOD00383

• Title: Simulation of Winter Precipitation in Western Mountain Watersheds with a Local-Scale Model
• Date: 11/1/1992
• Weather Modification - Doc Type: Report

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because comparisons of 100-yr type, base, and doubled-C02 precipitation,
are likely to be most meaningful with monthly or seasonal totals as
opposed to daily amounts. However, the nested models need to, at least,
moderately approximate the watershed daily precipitation distribution to
properly study such features as extreme events and various impacts on
ground cover. Consequently, nested modeling of historical periods needs
to be followed by comparison studies of precipitation amounts and also
precipitation distribution.

Only a brief description of the LSPM is given in the following
because Rhea (1977) 'presents thorough coverage and Medina (1991)
discussed some of the more important moc~l features. The model domain
and input data are described. Results are given separately for winter
periods and for summers because the mode!l performance depends on storm
type. Additional sections discuss input data and present some
conclusions.

MODEL DESCRIP~rION

The model simulates precipitation primarily through an accounting
of water condensed and evaporated as moist layers interact with the
orography. It can simulate only the most important physical processes
that lead to orographic precipitation, not the more complex cloud
processes. One advantage of such a simple model is the ease of
performing many runs on a relatively inexpensive microcomputer or work
station.

An important shortcoming of the modE:!l is its inability to properly
simulate convective precipitation. Consequently, estimates are more
accurate during winter, when precipitation occurs largely as a response
to the orography, than summer, when convection plays a dominant role.

As inputs, the model requires only routinely collected rawinsonde
data interpolated at 50-rob pressure levels from the surface to 300 rob.
For each sounding period, the outputs grenerated include estimates of
precipi tation for each domain grid point and selected locations, such as
gauge sites. The model integrates precipitation over selected time
periods such as daily, monthly, or seasonal. Volume precipitation is
developed for designated areas such as a particular watershed. Volume
precipitation integrated over selected periods can be employed in
regressions to estimate runoff (Rhea, 1977; Medina, 1981).

The equation (Rhea, 1977) employed by the model for computation of
a mean precipitation rate, f~I+l' between grid points I and I+l that are
separated by distance, ~x, is

-

(~/,l+l)/ =

EV/I:1P,
A (Q/ + I:1Cl/+1)/
pwg~x .

(1)

where
1 =
V, =
M =

Computational layer in question
Horizontal wind speed in x direction at grid upwind edge
Pressure thickness of the inflowing layer at upwind edge