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 TestIn 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