WMOD00494

• Title: Nested Modeling of Wateshed Precipitation
• Date: 1/1/1993
• Weather Modification - Doc Type: Report

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in three elevation zones from October 1, 1982, to May 15, 1983. The striking
difference between the high elevation SNOTEL data and the low and moderately high
observation sites illustrates the problem of model verification over complex terrain.
In the model validation by Giorgi et al. (1992a), few precipitation observations were
available at high elevations such as the SNOTEL sites; consequently, their analyses
may have overestimated the model's positive bias (overprediction of precipitation) in
these regions. Figure 2a shows that MM4 predicted the high elevation cumulative
precipitation remarkably well through winter and spring in 1982-83; however, it
significantly overpredicted warm season convective precipitation.

a

OCTOBER - DECEMBER 1962-63

2500

- f,.(M( MoooI

E2OQO .
.s ' -H- Gauges> 2700 m msl
c
.2 - Gauges 2 t 00 10 2700 m msl
~ 1500
:g. -t- Gauges < 21oo'm msl
...
ci:
... 1000
.2:
c;;
"5
5500
o

o
DateO N 0 J

F M A M J J
i

A S

Month

b

JANUARY. DECEMBER 1968

800

~ 700 - f,.(M( Mo6eI
E
.s. 600 -+t- Qauges" 2700 m msl
c
o
'ii5OO
-&. . -t- .Gauges < 2100 m msl .
"2<(00
ci:
1300
Oi .
~200

8,00

o J F M A M MonI.hJ A SON D

Figure 2. Time-series of cumulative precipitation predicted by MM4' and
corresponding observations in the Gunnison Basin during: (a) the EI Niijo 1982-83
and (b) dry 1988-89 periods. Three elevation zones,stratify the observed data clearly
demonstrating the effect of elevation on precipitation during both ~riods. MM4
predictions closely matched observed cumulative precipitation above - 2700 m msl
in 1983 from October to mid-May; however, the model overestimated precipitation
during the convective summer months. In 1988, MM4 predicted light precipitation
that matched the low elevation observations from January to March, but still
overstimated convective precipitation.

6

Matthews et al.