Abstract

Estimates of precipitation distribution in complex mountain terrain demonstrate a high level of uncertainty due to a lack of spatial and temporal measurements representing the variability and processes of rain and snowfall across an elevation gradient. In this study we analyzed mountain front precipitation patterns during the 2010 water year accumulation period from the rain dominated foothills to the consistently snow covered alpine basins above tree-line, across a 3300 m elevation gradient of the Kaweah River watershed, Sierra Nevada, California. On the lower mountain front, 140 m - 1700 m, we utilized distributed precipitation measurements and atmospheric soundings to determine the rain snow transition zone spanned an elevation range of 200 m to > 2100 m and was temporally dependent on saturation vapor pressure and temperatures specific to each storm event. For elevations above 1700 m we carefully analyzed filtered paired snow on and snow off scanning LiDAR measurements to an elevation of 3495 m using a spatial analysis of 1 m elevation bands. The March 21-22, LiDAR derived, snow depths showed three distinct elevation dependant patterns: i) elevations below 2100 m were strongly influenced by the mixture of rain and snow, ii) areas not under forest canopy between the elevations 2100 m and 3300 m showed a strong linear relationship with elevation (R2 .97, p < .001) and, iii) elevations above 3300 m showed evidence of wind redistribution and mass movement from steep terrain resulting in snow depths much less than those predicted by the linear trend established for elevations between 2100 m and 3300 m. To further investigate the conditions before and after the LiDAR flights we analyzed data from 14 SWE and depth sensor suites. These data indicated a mean snow density of 38%, on the date of the flights, and that peak accumulation occurred between March 14th and April 30th. While density data from all 14 sensor suites showed good agreement (R2 .53) no clear trends of variance by latitude, elevation or physiography was found suggesting it may not be possible to further constrain the spatial and temporal variability in snow density without additional in-situ measurements.

Citation

Kirchner, P.B., Bales, R.C. Flanagan, J., Musselman, K.N., Molotch, N.P. (2011): Mountain front precipitation accumulation over a 3300m elevation gradient from scanning LiDAR? snow depth and in-situ instrumental measurements, southern Sierra Nevada, California. Fall meeting, American Geophysical Union, December 2011, Room 3011, 2:10 PM - 2:25 PM. Abstract C23F-03. .