Frontogenesis Forcing and Banded Precipitation

Peter Banacos NOAA/NWS/NCEP/SPC WDTB Winter Weather Workshop 09 October 2002

Presentation Outline

• Ingredients based methodology for winter precipitation • Elements of frontogenesis • Favorable synoptic patterns • Case study examples / inferring frontogenesis • Summary

Ingredients Based Forecasting

Purpose: to focus the forecaster on the necessary conditions (“ingredients”) needed for a specific meteorological event to take place.

Making Forecast Assessments

• • • Diagnose three ingredients first:

Moisture

– sufficiency, potential for evaporation in boundary layer, dry slots.

Thermal Stratification

– precipitation type, melting/evaporative cooling/other diabatic effects.

Lift

– examine forcing for assent.

Lastly, assess microphysical aspects and instability for possible enhancement to precipitation rate

5 Lift Mechanisms Commonly Assessed for Winter Precip.

• Traditional,large-scale QG forcing (temperature advection / isentropic lift, differential vorticity advection) – precipitation trends often mesoscale • Convective mechanisms – lake-effect or snow squalls in cold advection regimes, elevated instability • Orographic lift – requires knowledge and familiarity of local terrain influences • Jet Streaks/coupled jets assent – cellular mesoscale areas of • Frontogenesis forcing assent  – linear/mesoscale areas of Frontogenesis Forcing is the focus of this talk.

Frontogenesis (definition)

F



D Dt



p

 • The 2-D frontogenesis function (

F

) – quantifies the change in (potential) horizontal temperature gradient following air parcel motion

F>0 frontogenesis, F<0 frontolysis

• Conceptually, the local change in horizontal temperature gradient near an existing front, baroclinic zone, or feature as it moves.

Kinematics of Frontogenesis

Examine separate contributions of horizontal divergence , deformation , and vorticity to the field of frontogenesis.

Horizontal Divergence

• Divergence (Convergence) acts frontolytically (frontogenetically), always, irrespective of isotherm orientation.

F

<0

F

>0

Horizontal Deformation

F

>0 Flow fields involving deformation acting frontogenetically are prominent in many cases.

Horizontal Deformation (cont.)

F

<0 Need to consider orientation of isotherms relative to axis of dilatation.

Horizontal Vorticity

F

=0 Acts to rotate isotherms, cannot tighten or weaken them.

Other Contributing Factors to Frontogenesis

The kinematic field, and deformation in particular, plays an important role in frontogenesis. Other processes such as diabatic heating and tilting effects may also contribute in an important way to frontogenesis.

Examples: differential solar heating Latent heating with convective motions (documented in coastal frontogenesis process).

Dynamics of Frontogenesis (vertical circulation)

Flow field dominated by deformation.

Dynamics of Frontogenesis (cont.)

Ageostrophic circulation develops as a response to increasing temperature gradient.

Dynamics of Frontogenesis (cont.)

When we talk about frontogenesis forcing, it’s the resulting ageostrophic circulation we are most interested in for precipitation forecasting.

Use of Frontogenesis in

• •

Forecasting

• Presence of

F

in 800-600mb layer can help diagnose and predict areas of heavy banded precipitation.

F>0

contributes toward symmetric instability (SI), but heavy precipitation can occur without the presence of SI.

F

DOES NOT require a strong surface cyclone, only a baroclinic zone (this fact can lull the forecaster into neglecting heavy snow potential (see Moore and Blakley, 1988). This is also why

F

rarely plays an important role as a forcing mechanism in the warm season.

Frontogenesis and Symmetric Instability

Common Synoptic Patterns

• Generally, look for situations leading to a strong baroclinic zone in the low to middle troposphere.

• Although

F

can manifest itself in many ways, there are a few common synoptic patterns that occur relatively frequently.

NE of Weak Surface Cyclone with Southward Moving Cold Front

• This synoptic pattern is examined in detail in case #1.

NW of Strong Cyclone “wrap around precipitation”

NW of Strong Cyclone 1/6/02

North of Strong E-W Frontal Zone

North of Strong E-W Frontal Zone 3/13/02

Example Case of Frontogenesis and Banded Precipitation

Date: 15 October 2001 (Case #1)

• Narrow band (1-2 counties wide) of moderate to heavy rainfall from eastern KS to central IL.

• Associated with weak surface features but a moderately strong baroclinic zone and frontogenesis forcing.

700mb 00z 15 OCT 01

Surface 15 OCT 2001 00z 12z

925mb 12z 15 OCT 01

Large-scale deformation field - eastern KS/western MO

18z 15 OCT 01

18z mosaic base reflectivity and surface observations 18z 600mb Frontogenesis

Kirksville, MO (ASOS) Hourly Rainfall 15 October 2001 0.30

0.25

0.20

0.15

0.10

0.05

0.00

14 15 16 17 18 TIME (UTC) 19 20 21 22

• Rainfall rates between 0.10” and 0.25” occurred for a 6 hour period from 15-20z. • Moderate to heavy precipitation can persist longer (12+ hours) with slower moving systems or mature extratropical cyclones.

Topeka, KS 12z 15 OCT 01

700mb Frontogenesis / Base Reflectivity

0 hr ETA 12z 6 hr ETA 18z 1150z 1805z • Organization of precipitation increases as F orientation becomes aligned with lower levels.

600mb

Sloped Continuity of F

6hr ETA forecast valid 18z 15 OCT 01 700mb • Presence of parallel axes of positive frontogenesis sloping upward toward colder air is a common aspect of heavy banded precipitation areas. 850mb

Sloped Continuity of Frontogenesis Forcing (cont.)

• The previous two slides have several important implications: 1) Several levels should be assessed for spatial continuity and orientation of

F

, to see if banding is likely to occur at a given time. 2) Vertical averaging should probably be avoided.

3) The sloped continuity tells us something about the structure of the wind field we can use to infer frontogenesis from single sounding (observed or model derived), VAD, or wind profiler data, and large-scale flow fields.

Role of Deep-Layer Shear Profile

Nature of environmental wind profile may be conducive to “seeder-feeder” mechanism and rapid precipitation generation / elongation of bands during initial development phase.

Role of Deep-Layer Shear (cont.)

Martin (1998) • Note banding orientation (parallel to isotherms).

Case #2: 9 NOV 00

Montgomery Co.  INX 0903Z Unlike Case #1, this case shows narrow multiple banded precipitation. Lower stability likely played a role.

9 NOV 00 CASE

“…A stripe of snow in se KS on Nov 9th with accumulations of 6-12" in 3 hours . The stripe was oriented n-s and was about 4 miles wide between the two 6" contours. We had one place that got 2 inches and about 2 to 3 miles away they got 9-12 inches . We didn’t even know it occurred until I placed about 30 phone calls into that county from the rural directory telephone book. The 00z ETA had the –20C 500 temp bullseye to the south over se Oklahoma. But the 12z raobs verified it much further north at SGF. Looks like the inversion was around 700mb with neutral lapse rate at SGF above 700. I have a feeling the snow was convective. The snow fell from 8-11z right as the 700-500mb trough was moving through that area. One guy told me it was the biggest snow in over 20 years . The area only covered about 10% of Montgomery county!”

- Forecaster, WFO ICT

00z 9 NOV 2000 – Intense Baroclinic Zone Surface 925mb

00z 9 NOV 2000 850mb 700mb

SGF 12z 11/09/00

700-500mb Lapse Rates

TOP 12z 10/15/01 7.8 C/km 4.5 C/km Near neutral or unstable lapse rates (with respect to a moist adiabat) implies sharp/narrow, stronger, and multiple banded precipitation organization. Resulted in 2-3”/hr snowfall rates on Nov 9, 2000.

Modulation of Band Intensity by Instability for a constant value of

F

As gravitational or symmetric stability decreases, the horizontal scale of the band decreases while the intensity of the band increases. Multiple bands become established in an unstable regime.

Case #3: 00z 25 Feb 01

Deformation zones associated with strong cyclones

Unidirectional Shear Profile

ABR 00z BIS 00z H7-H5 lapse rate: 6.4 C/km H7-H5 lapse rate: 4.5 C/km

ABR VAD WIND PROFILE

Upper-Air 00z 25 FEB 2001 500 mb 700 mb

Upper-Air 12z 25 FEB 2001 500 mb 700 mb

INL 12z 25 FEB 01

Anticipation of Banded Precip…

ALB 1/7/02 0242Z …leads to more accurate prediction of heavy precipitation. KALB 070451Z 35007KT 1/4SM +SN FZFG OVC008 M01/M01 A2952 RMK AO2 TWR VIS 1/2 SLP998 SNINCR 4/011 P0020 T10061006 400391011 RVRNO=

SUMMARY

• When applied within the context of ingredients based forecasting, frontogenesis is useful for assessing potential for banded winter precipitation, which is generally a good candidate for SPC mesoscale discussions.

• Doesn’t require a strong cyclone, only a strong baroclinic zone, often developed through horizontal deformation and associated largely w/ unidirectional vertical shear in the low to mid levels. • “Sloped continuity” in time and space of

F

leads most directly to strong banded precipitation.

• Examine lapse rates in precipitation generating layer to assess modulating role of instability (upright/symmetric).

References

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116

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