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 – cellular mesoscale areas of assent Frontogenesis forcing – linear/mesoscale areas of assent  Frontogenesis Forcing is the focus of this talk.

Frontogenesis (definition) 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<0F>0

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

Horizontal Deformation (cont.) F<0 Need to consider orientation of isotherms relative to axis of dilatation.

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

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 mb 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 z00z

925mb 12z 15 OCT 01 Large-scale deformation field - eastern KS/western MO

18z 15 OCT 01 18z 600mb Frontogenesis 18z mosaic base reflectivity and surface observations

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 12z6 hr ETA 18z 1150z1805z Organization of precipitation increases as F orientation becomes aligned with lower levels.

Sloped Continuity of F 6hr ETA forecast valid 18z 15 OCT 01 Presence of parallel axes of positive frontogenesis sloping upward toward colder air is a common aspect of heavy banded precipitation areas. 600mb 700mb 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 INX 0903Z Montgomery Co.  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 mb 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 mb 700mb

mb Lapse Rates 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, C/km4.5 C/km SGF 12z 11/09/00 TOP 12z 10/15/01

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

ABR VAD WIND PROFILE

Upper-Air 00z 25 FEB mb700 mb

Upper-Air 12z 25 FEB mb500 mb

INL 12z 25 FEB 01

Anticipation of Banded Precip… ALB 1/7/ Z …leads to more accurate prediction of heavy precipitation. KALB Z 35007KT 1/4SM +SN FZFG OVC008 M01/M01 A2952 RMK AO2 TWR VIS 1/2 SLP998 SNINCR 4/011 P0020 T 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 Bosart, L. F., 1981 : The Presidents’ Day snowstorm of February 1979: A subsynoptic-scale event. Mon. Wea. Rev., 109, Keyser D., M. J. Reeder, and R. J. Reed, 1988 : A generalization of Petterssen’s frontogenesis function and its relation to the forcing of vertical motion. Mon. Wea. Rev., 116, Martin, J. E., 1998 : The structure and evolution of a continental winter cyclone. Mon. Wea. Rev., 126, Moore, J. T., and P. D. Blakley, 1988 : The role of frontogenesis forcing and conditional symmetric instability in the Midwest snowstorm of January Mon. Wea. Rev., 116, Sanders, 1986 : Frontogenesis and symmetric stability in a major New England snowstorm. Mon. Wea. Rev., 114, Schultz D. M., 2001 : Reexamining the cold conveyor belt. Mon. Wea. Rev., 129, Schultz D. M., and P. N. Schumacher, 1999 : The use and misuse of conditional symmetric instability. Mon. Wea. Rev., 127, Steigerwaldt, H., 1986 : Deformation zones and heavy precipitation. NOAA Technical Memorandum, NWS CR pp.