PRELIMINARY STUDY RESULTS
Rain versus Snow in the Sierra Nevada, CA:
Comparing free-air observations of melting level with surface measurements
Jessica Lundquist 1 , Paul Neiman 3 , Brooks Martner 2,3 ,
Allen White 2,3 , Dan Gottas 2,3 , Marty Ralph 3
1 Civil and Env. Engineering, University of Washington, Seattle, Washington
2 Cooperative Institute for Research in Environmental Science, University of
Colorado, Boulder, Colorado
3 NOAA Earth Sciences Research Laboratory, Physical Sciences Division,
Boulder, Colorado

Vertical Profilers detect the altitude at which falling snow melts and changes to rain. In California, this altitude varies from 1 to 3 km.
American
River
2005

The altitude of the rain-snow line determines how much basin area contributes to runoff & floods.

American River
Basin
Computed from
UW’s
VIC model daily
INPUTS
(Bales et al, in press)
“ “ Rain vs Snow ” ”
Detailed snow vs rain model
(Dettinger et al., in prep) each year ’ ’ s

Floods in the
American River
Basin come from warm storms with high snow lines.
Accurate knowledge of the rain-snow line
(which changes hourly!) helps flood forecasts.
Photo by Bryan Patrick, Sacramento Bee

Test how well radar melting level represents what happens on the surface.
6) 2190 m
5) 2100 m
1) 1610 m
4) 2010 m
2) 1710 m
3) 1800 m
7) 2320 m
8) 2440 m
SWE & depth
SWE only

We need colocated precip and snow depth to tell rain from snow.

T>2ºC
T<0ºC
Grass
Valley melt level Bodega Bay melt level snow rain

1) On rainy days, what is the probability of snow accumulating or melting at each temperature?
2) How well does the radar-measured melting level correspond with a melting temperature at the surface?

Probability contours for ∆ SWE/ ∆ precip ratio at
10 AR snow pillows during 2001-2005
Below 0ºC, all precip falls as snow
(with some undercatch and overcatch at gages)
1 2

Probability contours for ∆ SWE/ ∆ precip ratio at
10 AR snow pillows during 2001-2005
1 2
From 0 to 2ºC, an increasing portion of rain falls with the snow

Probability contours for ∆ SWE/ ∆ precip ratio at
10 AR snow pillows during 2001-2005
Above 2ºC, snow is likely to melt, but the distribution is broad and uncertainty is large!
Amount of melt depends on turbulent energy transfer
(winds).
1 2
2.6ºC = 50% chance snow gain, 50% snow loss

Melting level averages 1.8ºC in free-air but varies diurnally at surface
Bodega Bay melting level (red circle)
Grass Valley melting level
(blue triangle)

Height adjustment from radar brightband level to AR surface snow change (2.6
° C)
Bodega Bay
Chico Grass Valley
Chowchilla
All have diurnal variations and large standard deviations.
hour of day
Stats from 2001 to 2005.

Height adjustment from radar brightband level to surface snow change (2.6
° C)
Chico is to the north, where temperatures are cooler, = lower brightband at level of warmer
AR T’s, and less adjustment.
hour of day

Height adjustment from radar brightband level to surface snow change (2.6
° C)
Chowchilla is to the south = warmer T’s, higher melting levels, = cooler
AR T’s and greater adjustments.
hour of day

Height adjustment from radar brightband level to surface snow change (2.6
° C)
Bodega Bay is to the west, on the coast, = temperatures are not so influenced by radiation, = largest diurnal changes and variance.
hour of day

• The altitude where rain changes to snow is crucial for flood forecasting.
• Profiling radars detect the rain-snow altitude in the free atmosphere.
• Surface temperatures provide current best estimate of how and when that altitude may differ on the surface.
• Correcting by profiler location, time of day, and surface stats can improve elevation estimate
RMSE by 150 m = 7.5% contributing area = 12 m 3 /s (850 ft 3 /s) in a medium-large AR storm

Warm rain on 22
March = loss of snow depth and SWE at
Blue Canyon.

T>2ºC
T<0ºC
Bodega
Bay melt level rain snow
Grass Valley melt level

C a
l i f o
r n
i a
v a d a