WinterWeather Forecasting Topics at the WDTB WinterWeather Workshop

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Winter-Weather Forecasting Topics at theWDTB
Winter-Weather Workshop
Dr. David Schultz NOAA/National Severe Storms
Laboratory Norman, Oklahoma schultz_at_nssl.noaa.gov
http//www.nssl.noaa.gov/schultz
20 August 2002
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Todays Topics
My philosophy of diagnosis Frontogenesis -
Introduction - Example 1 IPEX IOP 5 - Example
2 Elevated convection - Example 3 Midlevel
NWly flow frontogenesis Climatology of banding
in cyclones (Novak et al.)  Following fronts
through topography (Western Region) The melting
effect (Kain et al. 2000) Snow density and the
future of snowfall forecasting (Roebber et al.
2002, submitted to WAF)
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A Philosophy of Diagnosis
How do we assess weather features in the
atmosphere? Suppose you see something on the
radar and you dont know what is causing
it. First attempt should be QG thinking
advection of vorticity by thermal wind (e.g.,
vorticity advection, warm advection) If not QG,
then try frontogenesis at different levels. If
not frontogenesis, then something else
topography, PBL circulations, diabatic effects,
etc. Note that assessing instability is also
important, but secondary to this philosophy.
Gravitational stability or moist symmetric
instability only modulates the response to the
given forcing.
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Petterssen (1936) Frontogenesis
F d/dt Ñq F 1/2 Ñq ( E cos2b -
D)
q potential temperature E resultant
deformation b angle between the isentrope and
the axis of dilatation D divergence
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Frontogenesis Facts
Frontogenesis is following the flow
(Lagrangian). Fronts that are weakening still
possess frontogenesis. Note that tilting
effects are not included in Petterssens (1936)
form of frontogenesis. Diagnosis of
frontogenesis results in a diagnosis of the
forcing for vertical motion on the frontal
scale. Ascent occurs on the warm side of a
maximum of frontogenesis and on the cold side
of a region of frontolysis.
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Frontogenesis Example 1

• Frontogenesis can occur even in the presence of
  strong topographic contrasts
• In this case, from the Intermountain
  Precipitation Experiment, well see that
  synoptic-scale influences can dominate over
  topographic influences.
• Schultz et al. (February 2002 BAMS)

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IPEX IOP 5 17 February 2000
SURFACE

• Surface cyclone south of SLC
• Weak flow field at all levels
• Snowband northwest of cyclone
• 412 in. snow in Tooele Valley

500 hPa
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6-h median reflectivity from KMTX yellow
maxima are 20-25 dBZ
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700-hPa FRONTOGENESIS
500-hPa omega
L
700-hPa theta
shading 700-hPa frontogenesis 700-hPa
winds RUC-2 1500 UTC
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Frontogenesis Example 2

• Snowstorm in Oklahoma not well forecast
• Most snowfall fell well to the north of the
  surface frontal boundary
• Trapp et al. (2001) in March 2001 MWR

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OUN
SEP
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Elevated Convection and Frontogenesis
Frontogenesis at 1000 mb (dotted) and 600 mb
(dashed) CAPE at 1000 mb (shading) and 600 mb
(overprinted shading)
Frontogenesis solid lines CAPE shading Theta-e
thin solid lines 80 RH dotted line Heavy snow
location
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Elevated Convection and Frontogenesis
circulation within plane of cross section
(i.e., frontal circulation)
circulation normal to plane of cross section
(i.e., synoptic-scale circulation)
Vertical motion shaded Theta-es solid lines
Vertical motion shaded Theta solid lines
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Frontogenesis Example 3

• Frontogenesis in northwesterly flow, apparently
  unrelated to surface frontogenesis.
• I am collecting a list of cases that look similar
  to this event.
• Often misinterpreted as associated with
  upper-level jet circulations.

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1300 UTC 13 Sept. 2001 surface observations,
CAPE, and radar
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753 J/kg CAPE 482 J/kg CIN
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700-hPa Frontogenesis and Theta
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http//www.atmos.albany.edu/student/dnovak/CSTAR.h
tml
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Climatology of Banding

• Northeast U.S. cold-season (Oct.April) cases
  April 1995, October 1996 April 2001
• gt 25 mm rain (12.5 mm liquid equivalent, if
  frozen) in 24 h at a surface station
• 111 cases identified, 88 had radar data
• NCEP/NCAR Reanalysis data and Eta model initial
  conditions and 6-h forecasts for composites

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Classification Scheme

• Nonbanded (13 events)
• Single band (48 events)
• Multiple band events (35 events)

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Location of Bands Relative to Cyclone Center
Single Multiple
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Non banded
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Single Band NW
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Multiply banded
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Single band
Multiply banded
Non- banded
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Western U.S. IssuesFronts and Cyclones
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Tracking Cyclones and Upper-Level Forcing

• Look for pressure-check signatures in time series
  of SLP or altimeter setting, or the location of
  the zero isallobar

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Frontal Passages in the West-I
Upstream topography tears fronts apart
Steenburgh and Mass (1996) Fronts passing
through the west can be poorly defined at the
surface for many reasons. TEMPERATURE
- trapped cold air in valleys masks
frontal movement aloft - diurnal
heating/cooling effects -
different elevations of stations (use potential
temperature) - frontal
retardation/acceleration by topography
- precipitation (diabatic) effects
- upslope/downslope adiabatic effects (e.g.,
Chinooks) PRESSURE -
diurnal pressure variations - sea
level pressure reduction problems WINDS
- diurnal mountain/valley circulations -
topography channels the wind down the pressure
gradient, therefore the wind is not nearly
geostrophic
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Modification of Geostrophic Balance by Topography
Rossby radius of deformation (lR) is a
measure of the horizontal extent to which
modification of the force balances takes
place. lRNh/f lR is about 100200 km for the
Wasatch.
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Blazek thesis
Steenburgh and Blazek (2001)
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Frontal Passages in the West-II
Warm-frontal passages are often not well
defined at the surface, although regions of warm
advection are likely to be occurring aloft.
(Williams 1972) The strength of the potential
temperature gradient associated with the front is
strongly modulated by differential sensible
heating across the front. An estimate of the
contribution to frontogenesis from differential
diabatic heating . . . shows that it is several
times greater than the contribution from the
surface winds alone. (Hoffman 1995)
Advection of postfrontal air through the complex
topography is difficult to accomplish. Therefore
you may not see classic frontal passages at the
surface, but the baroclinic zone may be advancing
aloft. The temperature decrease (if any) behind
the cold front may be a result of downward mixing
of the colder air. Isallobars may be useful to
follow these elevated frontal passages through
the west. Larry Dunn has described some
frontal passages in the West as split fronts.
This concept may be useful and is in qualitative
agreement with the results described above. In
these cases, the precipitation may be out ahead
of the surface position of the front.
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Failure of the Norwegian Cyclone Modelin Western
Region
lack of warm fronts occluded fronts
sometimes act as cold fronts deformation of
fronts by topography precipitation is often
unrelated to surface features disconnect
between upper-level systems and low-level systems
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The Melting Effect as a Factor in
Precipitation-Type Forecasting

• Kain et al. (2000) December 2000 Weather and
  Forecasting
• Frozen precipitation falling through an
  above-freezing layer melts and absorbs latent
  heat from the environment.
• If enough cooling occurs, melting precipitation
  can be inhibited and rain will change to snow.

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1800 UTC 3 February 1998
BNA Nashville, TN
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Sfc maps
42 R-
44 R
38 R-
41 R
34 S-
37 R
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BNA Sounding
Near-freezing isothermal layer
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Shrinking bright band
A shrinking bright band on radar represents a
lowering melting layer, where snow changes to
rain. Note how the bright band encircles the
radar site (KBNA).
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Important Observations

• Cold advection could not explain drop in
  temperature.
• Temperature falls were only in regions of
  persistent moderate precipitation.
• BNA sounding showed 75-mb deep isothermal layer
  near 0C.
• Radar bright band was shrinking.
• Surface temps did not fall below 0C.

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DT DP 500
D

• D is the depth of precipitation needed to
  eliminate the melting layer (inches)
• DP is the pressure depth of the above- freezing
  layer (mb)
• DT is the mean temperature difference between the
  freezing point and the wet-bulb temperature of
  the environment (C)

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Criteria That May Warrant Consideration of the
Melting Effect

• Low-level temperature advection is weak.
• Steady rainfall of at least moderate intensity is
  expected for several hours.
• Surface temperatures are generally within a few
  degrees of freezing at the onset of the event.

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Even if you were able to predict the liquid
equivalent perfectly

• . . . youd still have to know the snow density.
• Usually this is assumed to be 10 inches of snow
  to 1 inch of liquid water (snow ratio).
• This will vary, however, depending on ice-crystal
  habit (function of T/RH), degree of riming,
  surface compaction due to weight and wind.

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NWS snow-density conversion table
Developed as a guide for QC of observations
Not intended as substitute for obs or as a
forecast method
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How are we doing now?

• For diagnosing snow ratio class (heavy,
  average, light) in a test sample
• 101 rule 45.0 correct
• climo 41.7 correct
• NWS table 51.7 correct

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How are we doing now?

• For diagnosing snow ratio class (heavy,
  average, light) in a test sample
• 101 rule 45.0 correct
• climo 41.7 correct
• NWS table 51.7 correct
•  Ensemble of neural networks that are fed
  sounding parameters, surface windspeed, and
  liquid-equivalent amount
• 60.4 correct

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What does the future look like?

• Roebber et al. (2002)
• http//www.nssl.noaa.gov/schultz/snowdensity/pape
  r.shtml
• Further improvements?
• - in-cloud vertical motion
• - storm electrification
• - predict actual ratio
• Operational implementation?
• Problems arise because of quality of
  observations
• Lack of understanding of cloud microphysical
  and aerosol processes
•  Lack of understanding of how snow density
  varies with ice-crystal habit

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Questions or comments?Please contact me. . . .
David Schultz NOAA/National Severe Storms
Laboratory Norman, Oklahoma schultz_at_nssl.noaa.gov
http//www.nssl.noaa.gov/schultz