Transcript Synoptic-Scale Weather Systems of the Intermountain West

Winter-Weather Forecasting Topics
at the
WDTB Winter-Weather Workshop
Dr. David Schultz
NOAA/National Severe Storms Laboratory
Norman, Oklahoma
[email protected]
http://www.nssl.noaa.gov/~schultz
Today’s Topics
• Rebuttal of Wetzel and Martin’s (2001) PVQ diagnostic
• My philosophy of diagnosis
• Frontogenesis
- Introduction
- Example 1: IPEX IOP 5
- Example 2: Elevated convection
- Example 3: Midlevel NWly flow frontogenesis
• Following fronts through topography (Western Region)
• The melting effect (Kain et al. 2000)
• One final note to cheer you up. . . .
Thoughts on Wetzel and Martin’s
Ingredients-Based Methodology
Specifically, PVQ.
QG thinking limitations:
- small grid spacings need filtered in order to interpret QG diagnostics
- many nonQG processes: lake-effect, topography, convection
The PVQ diagnostic:
- assumes collocation of negative PV and convergence of Q
(horizontally and vertically)
- not clear what magnitude of PVQ means
- no mathematical expression/relationship with PVQ
- time constraints: if you’re going to look at the two components
independently, then why look at PVQ?
(cf. moisture flux convergence)
A Philosophy of Diagnosis
How do we assess weather features in the atmosphere?
Suppose you see something on the radar and you don’t know
what is causing it.
First attempt should be QG thinking: vorticity advection,
warm advection, etc.
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.
Petterssen (1936) Frontogenesis
F =
F =
1/
2
d/
dt
|q|
|q| ( E cos2b - D)
q = potential temperature
E = resultant deformation
b = angle between the isentrope and the axis of dilatation
D = divergence
Frontogenesis Facts
• Frontogenesis is “following the flow” (Lagrangian).
• Fronts that are weakening still possess frontogenesis.
• Note that tilting effects are not included in Petterssen’s
(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.
Frontogenesis: Example 1
• Frontogenesis can occur even in the
presence of strong topographic
contrasts
• In this case, from the Intermountain
Precipitation Experiment, we’ll see that
synoptic-scale influences can dominate
over topographic influences.
IPEX IOP 5: 17 February 2000
SURFACE
•
•
•
•
Surface cyclone south of SLC
Weak flow field at all levels
Snowband northwest of cyclone
4–12 in. snow in Tooele Valley
500 hPa
6-h median
reflectivity from
KMTX
yellow maxima
are 20-25 dBZ
700-hPa FRONTOGENESIS
500-hPa omega
L
700-hPa theta
shading
700-hPa frontogenesis
700-hPa winds
RUC-2: 1500 UTC
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
OUN
SEP
Elevated Convection and Frontogenesis
Frontogenesis: solid lines
CAPE: shading
Theta-e: thin solid lines
80% RH: dotted line
Heavy snow location: *
Frontogenesis at 1000 mb
(dotted) and 600 mb (dashed)
CAPE at 1000 mb (shading)
and 600 mb (overprinted
shading)
Elevated Convection and Frontogenesis
circulation within plane of
cross section
(i.e., frontal circulation)
Vertical motion: shaded
Theta-es: solid lines
circulation normal to plane of
cross section
(i.e., synoptic-scale circulation)
Vertical motion: shaded
Theta: solid lines
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.
1300 UTC 13 Sept. 2001
surface observations, CAPE, and radar
753 J/kg CAPE
482 J/kg CIN
700-hPa Frontogenesis and Theta
Western U.S. Issues:
Fronts and Cyclones
Tracking Cyclones and Upper-Level Forcing
• Lows typically don’t move through the West continuously.
• Schultz and Doswell (2000) suggested that tracking the
occurrence of a mobile pressure minimum (a signal
of the upper-level forcing) may assist in analysis.
L2
L1
primary low
Fraser River
trough
lee low
L3
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
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
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.
lR=Nh/f
lR is about 100–200 km for the Wasatch.
Blazek thesis
Steenburgh
and Blazek
(2001)
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.
Failure of the Norwegian Cyclone Model
in 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 lowlevel systems
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.
1800 UTC 3 February 1998
BNA
Nashville, TN
42 R-
44 R
Sfc maps
41 R
38 R-
37 R
34 S-
BNA Sounding
Near-freezing
isothermal layer
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).
Shrinking bright band
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 0°C.
• Radar bright band was shrinking.
• Surface temps did not fall below 0°C.
D
T
D
P
D=–
500
• D is the depth of precipitation needed to
eliminate the melting layer (inches)
 DP is the pressure depth of the abovefreezing layer (mb)
 DT is the mean temperature difference
between the freezing point and the wetbulb temperature of the environment (°C)
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.
Even if you were able to predict the
liquid equivalent perfectly
• . . . you’d 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).
• The following graph is snow ratios from
2273 snowfall events greater than 2 mm
liquid from 1980–1989 for 29 U.S.
stations.
percent
10 to 1 ratio
ratio of snow to liquid equivalent
percent
ratios of 5–15
account for 50.8%
of events
ratio of snow to liquid equivalent