Transcript Development and Propagation of a Narrow Cold Frontal Rain Band

Development and
Propagation of a Narrow Cold
Frontal Rain Band in
Northern California
KC King
M.S. Student
Desert Research Institute
February 11, 2009
Introduction
► On
January 4, 2008, the Truckee/Reno area
experienced a precipitation event of heavy
rain that turned to snow late in the day
► Forecast from the National Weather Service
missed the timing of the change from rain
to snow and over forecasted the snowfall
► This presentation will describe the dynamics
and thermodynamics of the event on both
synoptic and mesoscales using both
observations and model generated data.
Presentation Outline
► Forecast
for January 4, 2008
► Quick Dynamics Review
► Observations of the event
 Synoptic Scale
 Mesoscale
► Modeled
Results
► Hypotheses on Model Error
► Conclusions
Forecast for January 4, 2008
► On
Jan 3, 2008 at
1:30 pm, the NWS
issued a blizzard
warning over the
northern Sierras
and Carson Range
NWS Products
► Blizzard
► High
Conditions:
Wind Warnings
The “Blizzard” of 2008
Northstar Resort Weather Blog
December 31, 2007 - 5:51 PM
► “ Took a long hard look at the four forecast
models. They are all telling the same story and it's
not bobbling around: POTENTIALLY EPIC SNOW
COMING TO TAHOE THIS WEEKEND.”
► “Ok, let me see if I can explain it in a way that
everyone can understand:
 Short Term (New Years Day, Wednesday, Thursday) BEAUTIFUL SKI/RIDE DAYS
 Extended (This Weekend) - POWDER BEYOND YOUR
WILDEST DREAMS”
But it rained… and rained.
So what happened?
Jan. 4 Observations at Reno
Narrow Cold Frontal Rain Bands
(NCFR)
► Narrow
focused
precipitation
associated with a cold
front
► Characterized by large
updrafts and intense
precipitation
► Strong cross-frontal
gradients over a small
area
More on NCFRs
► Caused
by forced convection of the density current
associated with the low-level leading edge of a cold
front
Jet Dynamics
► Strong
upper level jet
off the west coast
► Based on the 4quadrant jet streak
model:
 Divergence/Rising
motion (shown in blue)
in the right exit region
 Ageostrophic return
branch circulation
(shown in red) at lower
levels
Observations of Jan. 4 Event
250mb height/wind speed (m/s) 1/4/2008 at 00 Z
Upper Air Observations
Jan. 4, 2008 00Z
500 mb heights,
absolute vorticity
(x10-5/sec)
850 mb heights, winds
(knots)
Upper Air Dynamics
Jan. 4, 2008 12Z – 250 mb
Upper Air Observations
Jan. 4, 2008 12Z
500 mb heights,
absolute vorticity
(x10-5/sec)
850 mb heights,
winds (knots)
Upper Air Dynamics
Jan. 4, 00Z
IR Satellite View of Large Scale
Jan. 5 00Z
Surface Observations
Jan. 4, 2008
12Z
00Z
Stratospheric Ozone Map
High potential vorticity air masses from the
stratosphere were brought down to the troposphere
Observed Mesoscale Features
► MSLP
Map: 21Z
 Mesolow forming by
21Z over the Sierras
► Surface
Temperature: 21Z
 Pool of cold air over
the CA/NV/OR border
 Tongue of cold air is
pushing south almost
to Reno
Progression of Mesoscale Features
23Z
00Z
Radar Returns from Event
► 1410Z
– Jan 4, 2008
► Radar returns show
precipitation
occurring over most
of Northern CA and
into Reno by 14:10Z
(6:10 am PST)
► Red box highlights
large amounts of
precipitation over
area near
Bishop/Mono Lake
KDAX Radar at 20Z
► By
20Z we see the
intensification of
precipitation over
the Sierras south of
Tahoe
► Strong returns over
the foothills
► At this point, it was
still raining
Truckee Wind Profiler Data
Strong low level winds
(~10 mph) out of the
south from 12Z-23Z to a
height of at least 2.5 km
► Veering of wind with
height  warm
advection through the
column
► 00Z on January 5 surface
winds shift to
southwesterly
► Melt level also falls to
near surface level at 0Z
after rising for the 3
hours prior
►
Surface Meteograms
Summary of Observations
Upper level low pressure system associated with a straight
jet streak approaching the west coast of the United States
► Tropopause fold bringing an intrusion of high potential
vorticity stratospheric air
► Left jet exit region falling over Northern California and
Nevada
 Creates a transverse low level southerly jet
 Low level jet advects warm air (due to latent heating)
from the south keeping the North Tahoe and Reno areas
warm and in rain rather than snow
► Passage of the NCFR near 00Z leads to turning of the
winds from southerly to westerly allowing cold air to be
advected into the area and dropping the temperature
► Snow begins falling near 00Z
►
WRF Simulations
► Simulations
using the Weather Researching
and Forecast Model (WRF)
► Used North American Mesoscale Model
(NAM) analysis data to initialize the model.
► 27 km, 9 km, and 3 km grid size runs
► Model initialized at 12Z and 18Z on 1/4/08
► Used to better understand the mesoscale
dynamics
Data from 9 km grid
12Z initialization
► MSLP
and wind
barbs at 20Z
► Mesolow over the
Tahoe area is shown
► Winds turn too soon
from the south to
the southwest/west
Evolution of MSLP and Wind Fields
9 km run, 12Z initialization
21Z
22Z
9 km Simulation, 18Z Initialization
To try to better
simulate the southerly
winds seen in the
observations the
model was initialized
at 18Z
► Hoped that the later
initialization would
lead to a more
accurate simulation of
the ageostrophic low
21Z: 250 mb heights, winds
level return jet
►
9 km Simulation, 18Z Initialization
► By
00Z on Jan. 5,
the shortwave
trough over the
Sierras has
deepened and
extended southward
► Winds continue out
of the SW
00Z: 250 mb heights, winds
9 km Simulation, 18Z Initialization
MSLP, Winds
20Z
21Z
22Z
23Z
3 km Simulation, 18Z Initialization
►
►
►
►
To get an even closer view,
we used a 3 km grid over
the Reno/Tahoe region
Wanted to capture the
mesoscale dynamics that
kept the area warm and
the passage of the NCFR
MSLP, winds shown to the
right
Mesolow just to east of
Truckee as seen in obs
21Z
22Z
3 km Simulation, 18Z Initialization
MSLP, Winds
00Z
►
►
►
00:40Z
►
Winds begin to turn SW and
then W from 22Z – 0:40Z
Mesolow just to the east of
Truckee, California deepens
Surface convergence likely
leading to vertical motions,
condensation, precipitation
Flow is out of the south,
supporting the hypothesis
that warm advection of air
from the south keeps
temperatures relatively high
leading to rain, not snow
3 km Simulation, 18Z Initialization
Surface Temperature
21Z
00Z
In general, surface temperatures are too low in
comparison to the observations by 3-5 C
3 km Simulation, 18Z Initialization
20 Min Precipitation
► Well
defined band
of precipitation
extending from
west of Sacramento
to northwest of
Truckee
► This band is a
manifestation of the
NCFR ahead of the
cold front
22Z
3 km Simulation, 18Z Initialization
20 Min Precipitation
23Z
00Z
Distinct, thin line of high rainfall rate (NCFR) moves
SE and combines with another band of precipitation
forming to the south over the next four hours.
Hypotheses on Model Performance
►
►
►
WRF had large errors in both wind direction and surface
temperature
These errors were lessened by initializing the model closer
to the time of interest (12Z versus 18Z)
The secondary circulation set up in the jet exit region that
sustains the southerly winds was very sensitive to the
initial conditions
 This is likely due to the influence of terrain and the barrier jet.
Without proper forcing and initial state, the southerly winds seem
to be overwhelmed by westerlies / barrier jet.
 This may be due to insufficient data from the area over the ocean.
 Important for forecasting because the simulations were so sensitive
to the initial state/time.
 Very difficult to forecast an event like this due to the complex
interactions between the larger scale circulation, mesoscale
features, and the terrain.
Conclusions
►
►
►
►
►
►
The precipitation from the January 4 storm was likely due to a
intense rising motion in the left exit region of an upper level jet
streak and intensified by orographic lifting. The snowfall seems
to be associated with a narrow cold frontal rain band.
Both observations and model results support the conclusion that
ahead of the cold front a NCFR developed that brought intense
precipitation (snow).
The jet dynamics likely led to intensification of the front and
sustained southerly flow over the Northern Sierras.
The southerly wind brought warm air from latent heating due to
intense precipitation in the Sierras near Mammoth Lakes and
Yosemite, keeping the melt level high and preventing formation
of snow.
As the winds turned from out of the south to southwesterly
effectively cutting off the source of warm air, the cold front
progressed and the precipitation changed from rain to snow.
In order to effectively capture this event with the WRF, the initial
state was crucial to developing the correct wind structure.
References
Browning, K.A., Reynolds, R., 1994: Diagnostic Study of a Narrow Cold-Frontal Rainband and
Severe Winds Associated with a Stratospheric Intrusion. Quarterly Journal of the Royal
Meteorological Society, 120, p. 235-257.
Hobbs, P.V., Persson, O.G., 1982: The Mesoscale and Microscale Structure and Organization
of Clouds and Precipitation in Midlatitude Cyclones. Part V: The Substructure of Narrow
Cold-Frontal Rainbands. Journal of the Atmospheric Sciences, 39, p. 280-295.
Kaplan, M., 2008, Class Notes from Synoptic Meteorology Class.
Koch, S.E., Kocin, P.J., 1991: Frontal Contraction Processes Leading to the Formation of an
Intense Narrow Rainband. Meteorology and Atmospheric Physics, 46, p. 123-154.
Lin, Y.L., 2007. Mesoscale Dynamics. Cambridge University Press.
Parsons, David B., 1992: An Explanation for Intense Frontal Updrafts and Narrow ColdFrontal Rainbands. Journal of the Atmospheric Sciences, 49, p. 1810-1825.
Rutledge, S.A., Hobbs, P.V., 1984: The Mesoscale and Microscale Structure and Organization
of Clouds and Precipitation in Midlatitude Cyclones. Part XII: A Diagnostic Modeling Study
of Precipitation Development in Narrow Cold-Frontal Rainbands. Journal of the
Atmospheric Sciences, 41, p. 2949-2972.
References Con’t
National Climatic Data Center,
http://www.ncdc.noaa.gov/oa/ncdc.html.
Cable News Network, http://www.cnn.com.
Reno Gazette Journal, http://www.rgj.com.
Northstar Ski Resort, Unofficial Weather Blog,
http://northstarsnow.blogspot.com/.
Utah State Meteorology, Mesowest,
http://www.met.utah.edu/jhorel/html/mesonet/.
Plymouth State Weather Center,
http://vortex.plymouth.edu/make.html.
Thank you!
Special thanks to Michael Kaplan and Chris
Smallcomb for their time and assistance.
Any questions?