Recipe for Flood: Rainstorms falling on the Sierra Nevada Snowpack
1,2
Lundquist ,
2
Ralph ,
Jessica D.
F. Marty
Paul
1CIRES, University of Colorado, Boulder
2
Nieman ,
1,2
Kingsmill ,
1,2
White ,
David
Allen
Daniel
2NOAA ESRL Physical Sciences Division
1,2
Gottas
2005
Contact info: jessica.lundquist@noaa.gov, 303-497-4729
Radar brightband =
altitude where snow
melts
Ingredients:
1) Warm Heavy Precipitation
To cause a flood, a storm must carry a lot of moisture that falls as rain rather than snow. A
"pineapple express" or "atmospheric river" storm (pictured in SSM/I image at left) taps into
tropical moisture to produce heavy rains in California. However, similar amounts of rain can lead
to very different stream responses (see below.)
(a) During two storms in March 2005, most
precipitation fell in the warm sector of the
storm preceding a cold front. The melting
level elevation (colored markers) and
precipitation amounts (black line) were
similar between the storms. (b) However,
over 3 times as much streamflow (black line)
resulted from the second storm! In the
Sacramento valley, the same amount of
water fell from 19-21 March as from 22-24
March (table below). At higher elevations,
more water fell during the second storm.
Also, more snow was available to melt (right).
Atmospheric River
Can new technology
improve forecasts?
Directions:
Techniques:
Total Water
storm
(mm)
19 Mar 22 Mar
Sacramento precip
28
28
Blue Canyon precip
93
112
Sugar Pine precip
89
124
Blue Canyon melt
0 to 4
16
NF American flow
46
162
Hydrometeorological Testbed
Intensive Observations in the American River
915 MHz Profiler
Locations
American River Temperature Profiles
2) Snow and melt
Blue Canyon, at 1600 m in
the American River Basin
(lowest elevation on
temperature contours)
received both snow and
rain during this period.On
18 March, the snow pillow
measured no snow on the
ground, but snow
accumulated from 19 to 22
March. On 22 March, 16
mm of snow melted,
contributing to streamflow.
Increasing density indicates
that a mixture of rain and
snow fell even while snow
accumulated on the ground.
Blue Canyon, 1600m
1) Free-atmosphere vs.
Surface Temperatures
On average,
the melting
level (Bodega
Bay, red circles,
and Chowchilla,
blue triangles)
corresponds
with a free-air
temperature
between 0 and
2oC (shown for
Oakland
sounding,
right).
Mountain
surface
temperatures
(shown for 24
stations in the
American River
Basin) vary
with diurnal
heating,
landcover, and
cold air
drainage in
addition to freeair circulations.
Gin Flat, 2100m
Despite the high melting
level measured by radar,
Gin Flat, at 2100 m in
Yosemite (dashed line in
temperature profile),
accumulated snow and not
rain through both storms, as
indicated by decreasing
snow density. The Merced
River (not shown) received
little runoff during these
storms.
Sugarpine
3) High soil moisture
North Fork American River Streamgage
Alpha
Blue Canyon, 1600 m
Yosemite Hydroclimate Study
Intensive Observations in the Merced
and Tuolumne Rivers
In the maritime mountain ranges of North
America, most precipitation falls during the
winter months, with a large percentage falling in
the form of snow. Particularly warm storms
result in floods primarily because rain falls at
higher elevations and over a much larger
contributing area than during a typical storm.
Because of the different sizes and fall speeds of
rain and snow, Doppler radars are able to detect
the melting level in the atmosphere, and
automated algorithms are available to make this
information available to river forecasters. This
study compares free-atmosphere observations
from four vertically-profiling radars with
measurements of surface temperature, snowfall,
precipitation, soil moisture, and streamflow at
various elevations in the American and Merced
River Basins of California in an effort to improve
flood forecasting.
Over 75% of the American River basin
(above) lies below 2000 m, as opposed
to less than 10% of the Merced (below).
Thus, most storms bring a mixture of rain
and snow to the American, while only the
warmest storms bring rain and floods to
the Merced.
Surface measurements of temperature show that the
melting level on the ground is strongly modified by
diurnal variations in radiation, such that a warm
storm arriving at night will have a lower melting level
on the surface than aloft. The opposite may occur
during the day. Red circles show melting level at
Bodega Bay, and blue triangles at Chowchilla.
Merced River Temperature Profiles
Gin Flat, 2100 m
Happy Isles Gage
Merced Basin
2) Diurnal cycle matters
Rivers often respond more to
later storms in a series
because soils become
saturated. However, at Blue
Canyon, soil moisture levels
rose higher during the first
storm. The 22 March storm
was unique because soil
moisture rose first and more
dramatically at the deeper soil
sensor, suggesting that water
pooled on the granite bedrock
beneath the thin soil, and the
water table rose.
In most of the Sierra
Nevada, soils are
undersaturated only
during the summer
through early winter.
Mid-winter melt and
shallow soils
combine to make
soils hover near
saturation during
much of the flood
season. At Blue
Canyon (left) soils
were saturated by
mid-February.
The average free-air
temperature at the melting
level elevation is about
1.4oC, with no clear
dependence on time of
day (shown at left for
Grass Valley). Surface
temperatures at the
melting level, however,
vary from 0oC at midnight
to 2oC in the afternoon,
suggesting that the rainsnow elevation on the
ground should be
adjusted for time of day.
Conclusions:
*California basins are very sensitive to the altitude below which
precipitation falls as rain.
*Vertically-pointing radars can detect this altitude in the freeatmosphere, but corrections (such as time of day) may need to applied
to determine whether precipitation will stay frozen or melt at the surface.
*Key differences between the storms on 19 March and 22 March 2005
were more orographically-enhanced precipitation and more snow at low
elevations available to melt. Thus, storm structure and sequences of
storms, as well as the degree of soil saturation, are also important
factors in floods.