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Mesoscale Structure of
Precipitation Regions in
Northeast Winter Storms
Matthew D. Greenstein, Lance F. Bosart, and Daniel Keyser
Department of Earth and Atmospheric Sciences
University at Albany, Albany, NY 12222
David J. Nicosia
National Weather Service
Binghamton Weather Forecast Office, Johnson City, NY 13790
7 April 2006
CSTAR-II support provided by NOAA Grant NA04NWS4680005
Outline
• Introduction
• Case selection
• Radar classification
• Cross section analysis
• Summary of results
• Future work
Introduction
• Forecasters can predict likely areas of precipitation
• Forecasters cannot always skillfully predict
mesoscale features
• Forecasting mesoscale details adds value to a forecast:
• Prediction of snowfall amount and variability
• Differentiating between high-impact and
low-impact snows
Introduction
• Precipitation regions have multiple modes (patterns)
• Goal is to examine ingredients …
*
*
*
*
Lift
Instability
Moisture
Microphysics
… to find ways of distinguishing the modes
Introduction: Previous banded studies
• Matejka, Houze, and Hobbs (1980)
Surge
Postfrontal
Cold frontal
Warm sector
Warm frontal
Introduction: Previous banded studies
• Nicosia and Grumm (1999)
Introduction: Previous banded studies
• Novak et al. (2004)
Banded
Nonbanded
Introduction: Previous banded studies
• Novak et al. (2004)
Banded
Nonbanded
Case Selection
• Cases occur in area bounded by
36.5°N, 50°N, 65°W, and 85°W
• Within U.S. radar coverage
• 1 October – 30 April
• No warm sector precipitation
• P–type predominantly snow
• “Heavy snow” = 15+ cm in 12 h over area the size of CT
• No lake effect snows and enhancements
• Past three winters (2002–3, 2003–4, 2004–5)
Case Selection
• Data used
• NCDC national hourly mosaic reflectivity images
• Public Information Statements (PNS)
• Northeast River Forecast Center snowfall maps
• NCDC’s U.S. Storm Events Database
• ASOS reports
20 Cases
• 26–27 Nov 2002 • 5–8 Dec 2003
• 19–20 Jan 2005
• 4–6 Dec 2002
• 13–15 Dec 2003
• 22–23 Jan 2005
• 25–26 Dec 2002 • 14–15 Jan 2004
• 24–25 Feb 2005
• 2–5 Jan 2003
• 27–28 Jan 2004
• 28 Feb–2 Mar 2005
• 6–7 Feb 2003
• 16–17 Mar 2004
• 8–9 Mar 2005
• 15–18 Feb 2003 • 18–19 Mar 2004
• 11–13 Mar 2005
• 6 Mar 2003
• 23–24 Mar 2005
Radar Classification
• 2km WSI NOWrad mosaics
* 15-min resolution
* 3 levels of quality control
* Composite reflectivity
1. Uniform
2. Classic Band
3. Transient Band
4. Bandlets
5. Fractured
6. Unclassifiable
Multiple modes may exist
in a storm’s lifecycle and
at one time
Radar Classification: Uniform
1200 UTC 27 Nov 2002
Radar Classification: Classic Band
1900 UTC 7 Feb 2003
Radar Classification: Transient Band
1200–2100 UTC 16 Feb 2003
Evolving Band
Radar Classification: Transient Band
1600 UTC 6 Dec 2003
Broken Band
Radar Classification: Transient Band
2115 UTC 14 Dec 2003
Messy Band
Radar Classification: Bandlets
1500 UTC 17 Feb 2003
Radar Classification: Fractured
1500 UTC 16 Mar 2004
Cross Section Analysis
• Previous research:
frontogenesis in the presence of weak
moist symmetric stability yields bands
• Negative saturation equivalent potential vorticity
(EPV*) indicates conditional slantwise instability (CSI)
and/or conditional upright instability (CI)
EPV* = – g (ζ · θ*e), where ζ is the absolute vorticity vector
• CI dominates CSI
Cross Section Analysis
• 32–km North American Regional Reanalysis (NARR)
• Cross sections contain …
• Saturation equivalent potential temperature – θe* (K)
• Relative humidity (%)
• 2D Petterssen Frontogenesis (ºC 100 km-1 3 h-1)
• Saturation equivalent potential vorticity - EPV* (PVU)
(calculated with the full wind)
• Vertical motion (μb s-1)
• Dendritic growth zone,
i.e., −12ºC and −18ºC isotherms
Cross Section Analysis: Classic Band
2100 UTC 7 Feb 2003
Strong, steep,
surface-based
frontogenesis
Strong, tilted
ascent rooted
in the
boundary layer
Weakly positive
EPV*
CI unimportant
Cross Section Analysis: Uniform
2100 UTC 22 Jan 2005
Weak, flat
frontogenesis
Upright ascent
Ascent strength
not a factor
Weakly positive
& negative EPV*
has no effect
No CI
Cross Section Analysis: Transient Band
1500 UTC 16 Feb 2003
Weak,
decoupled
frontogenesis

Inhibits
continuous
boundary layer
moisture feed
Weakly positive
EPV* seen in all
modes
Cross Section Analysis: Bandlets
0000 UTC 1 Mar 2005
Frontogenesis
lifts air parcels
to CI region
Escalatorelevator
Cross Section Analysis: Fractured
1500 UTC 16 Mar 2004
Weak,
decoupled,
fragmented
frontogenesis
Separate
EPV mins and
ascent maxes
Lower RH
Summary of Results: Distinguishing features
Frontogenesis
Uniform
weak and/or flat
or none
Classic
Band
strong and steep;
surface-based
Transient decoupled
Band
Bandlets
Other
‡ no CI
ascent indicates frontal
circulation dominates
ω not well rooted in B.L.;
CI alters precip pattern
thin, weak, or none; ‡ CI / escalator–elevator;
surface-based
deep CI = messier pattern
Fractured fragmented;
decoupled
‡ = some look like a band
CI or frontogenesis enhances
precip; lower RH
CI enhances updrafts & downdrafts
Summary of Results: Nondistinguishing features
• Ascent strength
* Uniform: −4 to −24 μb s-1
* Classic band: ≤ −20 μb s-1
• Intersection of max ascent with DGZ
• Depth of DGZ (~50–100 hPa in most cases)
• Intersection of max ascent with CI region
• RH patterns
• Reduced EPV*
* All cases contain EPV* 0–0.25 PVU (WMSS) and CSI
* Shape and location of reduced EPV* regions
Summary of Results: Nondistinguishing features
• From plan-view analyses…
• QG–forcing ratio: DCVA / (DCVA + WAA)
• Depths of reduced EPV* satisfying various criteria
• EPV* ≤ 0, ≤ 0.25, 0–0.25, or ≤ −0.25
+ RH ≥ 70% + Ascent
• Max vertical speed shear
• 850–500 hPa vertical speed shear
Summary of Results: EPV* vs. EPV g*
• Reasons for EPV g*
• Symmetric instability theory: thermal wind balance
• Mg more accurately captures growing instability
• Reasons for EPV*
• Better representation of curved flow
• Assumption that time scale of convection <<
time scale for large-scale environmental changes
not valid?
 Potential for slantwise convection better found by
using an evolving and unbalanced environment?
EPV*
0600 UTC 23 Jan 2005
EPV g*
Summary of Results: EPV* vs. EPV g*
• EPVg* produces a messier pattern with more negative
values, especially in dry areas
• EPVg* “bull’s-eyes” line up with band positions
• Because…
1) Value does not seem to matter
2) WMSS is a necessary but not distinguishing factor
3) CI plays an important role
• Use EPV* because it produces a cleaner image
• If classic band is indicated, use EPV g* for position
Summary of Results: Conceptual Models
Summary of Results: Conceptual Models
Summary of Results: Conceptual Models
Summary of Results: Conceptual Models
Summary of Results: Conceptual Models
Summary of Results: Flowchart
Frontogenesis?
YES
NO
Decoupled frontogenesis?
CI?
YES
NO
YES
NO
Fragmented
frontogenesis?
Strong, steep
frontogenesis?
Bandlets
Uniform
YES
NO
YES
NO
Fractured
Transient
Band
Classic
Band
CI?
YES
NO
Bandlets
Uniform
Future Work
• Is the “fractured” mode really just a hybrid of
“bandlets” & “transient bands” but with drier spaces?
• Prove decoupled frontogenesis hypothesis
• Investigate band lag
• Examine the EPV g* “bull’s-eyes”
Special Thanks
• Lance and Dan
• David Ahijevych (NCAR)
• Kevin Tyle
• Alan Srock
• Anantha Aiyyer
• Keith Wagner
• Celeste, Sharon, and Lynn
• My parents
Questions? Comments?
e-mail: [email protected]