Winter-Weather Forecasting Topics at the

WDTB Winter-Weather Workshop

Dr. David SchultzNOAA/National Severe Storms Laboratory

Norman, Oklahoma

schultz@nssl.noaa.govhttp://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 MethodologySpecifically, 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 DiagnosisHow 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 = d/dt ||

F = 1/2 || ( E cos2 - D)

potential temperatureE = resultant deformation = angle between the isentrope and the axis of dilatationD = 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 cyclone south of SLC• Weak flow field at all levels• Snowband northwest of cyclone• 4–12 in. snow in Tooele Valley 500 hPa

SURFACE

6-h median 6-h median reflectivity from reflectivity from KMTXKMTX

yellow maxima yellow maxima are 20-25 dBare 20-25 dBZZ

700-hPa FRONTOGENESIS

500-hPa omega500-hPa omega

700-hPa theta700-hPa theta

shadingshading700-hPa frontogenesis700-hPa frontogenesis

700-hPa winds700-hPa winds

RUC-2: 1500 UTCRUC-2: 1500 UTC

LL

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 linesCAPE: shadingTheta-e: thin solid lines80% RH: dotted lineHeavy 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

Vertical motion: shadedTheta-es: solid lines

circulation within plane of cross section (i.e., frontal circulation)

Vertical motion: shadedTheta: solid lines

circulation normal to plane of cross section (i.e., synoptic-scale circulation)

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 CAPE482 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.

L1

L3L2

primary low

Fraser Rivertrough

lee low

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 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

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

BNANashville, TN

Sfc maps

42 R-

34 S-37 R

41 R 38 R-

44 R

BNA Sounding

Near-freezingNear-freezingisothermal layerisothermal layer

Shrinking bright bandA shrinking bright A shrinking bright band on radar band on radar represents a loweringrepresents a loweringmelting layer, where melting layer, where snow changes to rain. snow changes to rain.

Note how the bright Note how the bright band encircles the band encircles the radar site (KBNA).radar site (KBNA).

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 is the depth of precipitation needed to eliminate the melting layerinches

P is the pressure depth of the above- freezing layer (mb)

T is the mean temperature difference between the freezing point and the wet-bulb temperature of the environment (°C)

T T PP 500 500

DD = – = –

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.

ratio of snow to liquid equivalent

perc

ent

10 to 1 ratio

ratio of snow to liquid equivalent

perc

ent

ratios of 5–15 account for 50.8%

of events