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Precipitation over Narrow Mountain Ranges
Ethan Gutmann
Roy Rasmussen, Greg Thompson, David Gochis,
Kyoko Ikeda, Changhai Liu, Jimy Dudhia, Martyn Clark
Precipitation: snow
Narrow: ~10km(the physics apply to all ranges)
Mountain: >1km high
Problem Definition
The Question?
Why are PRISM and WRF different, and which is correct?
Wind
The Question?Or is the Linear Theory model correct? (Smith and Barstad 2004)
Wind
The Question?
Wind
Or is the Linear Theory model correct? (Smith and Barstad 2004)
With unrealistic parameters
The hypothesisStrong updrafts on the upwind side of the mountain carry snow to the downwind side, where opposing downdrafts hurry the snow to the ground.
… but no solid evidence
For Reference:
Colle et al., 2000; Medina et al., 2004; Garvert et al., 2007
… but no solid evidence
Enhanced RADAR reflectivity in lee
Enhanced Model precipitation in lee
Stepping back…
The TestMeasurements of snow in the Sangre de Cristo Mountains10km wide, 1.5km high, ~100km upwind fetch
Modeling
Updrafts in WRF reach >4m/s
Terminal velocity of snow is ~1m/s
Note: stronger downdrafts
W (m/s)
NCAR Snow depth
SNOTELSnow depth
The Test
In the trees : ~3400m
The Test
Wind
NCAR Snow depth
SNOTELSnow depth
Site Photos
NCAR Site SNOTEL Site
Initial Data
Initial Data
Wind direction
Wind direction
The Test
Wind
Other Mountains
Wind
Future Climate?Apparent decrease in the future (relatively more on upwind).
NOTE: PGW is rescaled to match NARR mean, it increases everywhere
Conclusions
Snow preferentially falls in the lee of narrow mountain ranges.
This effect is likely to decrease in the future, but this decrease is relatively small.
This may lead to more water in upwind watersheds.
Thanks
Photo:Greg Thompson