investigating mesoscale precipitation banding in two east coast snowstorms adam frumkin
TRANSCRIPT
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Investigating Mesoscale Precipitation Banding in Two East Coast SnowstormsAdam Frumkin
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Outline1. Motivation2. Introduction and Background3. Objectives/Methods4. Introduction to the Two Cases5. Results6. Conclusions
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Motivation• Snowfall rates in mesoscale snowbands can exceed
4in/h• Due to their small scale, mesoscale snowbands can
result in large gradients in snow accumulation• They are a challenge to diagnose and predict▫ Few operational models use a high enough resolution
to resolve individual bands
• Bands are the result of many factors (e.g. CSI)• The conditions in midlatitude cyclones are often
ideal for the generation and release of CSI▫Operational models can resolve the conditions
known to produce bands
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Introduction/Background•CSI is a form of Moist Symmetric Instability
(MSI) and is released through moist slantwise convection
•The ingredients for MSI are moisture, instability, and lift.
•MSI is released when a parcel is lifted slantwise past the LCL to the level of free slantwise convection
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Introduction/Background•Frontogenetical forcing is an efficient way
to lift a parcel and to release MSI▫Topographic forcing is another way
•Frontogenesis and CSI can coexist▫Difficult to distinguish CSI vs. frontogenesis
bands•Heavy bands can also be due to:▫Conditional Instability (CI), a type of
gravitational instability•CI will dominate over CSI▫CSI can induce CI
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Introduction/BackgroundSchultz and Schumacher (1999) suggest using MPVg
*
instead of MG (more versatile)
Novak et al., (2005)
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Objective/Methods• Objective: Investigate the instabilities and forcing mechanisms in two
significant mesoscale snowband events on the East Coast
• Methods: Following the suggestions of Schultz and Schumacher (1999)
1. Identify the location of mesoscale snowbands in the two events on radar
▫ Using RUC 0hr forecasts2. Plot spatial maps of:
1. MSLP -> Track low pressure center2. 800hPa frontogenesis -> diagnose regions of enhanced lifting
3. Plot cross sections of 1. MPVg* -> Where do Θe
* lines slope more steeply than Mg lines
2. Θe* -> diagnose CI vs CSI
3. RH -> saturation is necessary for the release of the instability4. Frontogenesis -> do frontogenesis and instability overlap?
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Two Case StudiesNJ 26 Dec 2010 MA 09 Dec 2005
• Low pressure deepened as it tracked northward along the eastern seaboard▫ Hurricane force winds▫ Thunder and lightning
• Low pressure rapidly deepened over Cape Cod▫ Hurricane force winds▫ Thunder and lightning
http://www.nohrsc.nws.gov/
http://www.cocorahs.org/
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KDIX Radar (21Z)
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KDIX Radar (00Z)
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KDIX Radar (03Z)
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KDIX Radar (08Z)
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MSLP and 800mb Frontogenesis (18Z)
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MSLP and 800mb Frontogenesis (21Z)
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MSLP and 800mb Frontogenesis (00Z)
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MSLP and 800mb Frontogenesis (03Z)
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CSI Cross Section (18Z)
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CSI Cross Section (21Z)
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CSI Cross Section (00Z)
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CSI Cross Section (03Z)
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KBOX Radar (18Z)
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KBOX Radar (1830Z)
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KBOX Radar (19Z)
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KBOX Radar (1930)
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MSLP and 800mb Frontogenesis (15Z)
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MSLP and 800mb Frontogenesis (18Z)
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MSLP and 800mb Frontogenesis (21Z)
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MSLP and 800mb Frontogenesis (00Z)
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CSI Cross Section (15Z)
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CSI Cross Section (18Z)
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CSI Cross Section (21Z)
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CSI Cross Section (00Z)
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Summary•2 strong midlatitude cyclones developed•Well-defined frontogenesis regions in the
NW quadrant•Banding is present in both cases• In MA case, banding appears around the
time the CSI is evident▫Occurs later in NJ case
•CSI and CI are present at the same time at nearly every time step
•CSI (also CI) and frontogenesis nearly always overlap
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Questions/References•Novak. R. D., J. S. Waldstreicher, D. Keyser, and L. F. Bosart, 2006: A forecast strategy for anticipating cold season mesoscale band formation within eastern U.S. cyclones. Wea. Forecasting, 21, 3–23.
•Nicosia, D. J., and R. H. Grumm, 1999: Mesoscale band formation in three major northeastern United States snowstorms. Wea. Forecasting, 14, 346–368.
•Schultz, D. M., and P. N. Schumacher, 1999: The use and misuse of conditional symmetric instability. Mon. Wea. Rev., 127, 2709–2732.
•COMET Module, http://www.meted.ucar.edu/export/csi/
•The Conditional Symmetric Instability (CSI) Homepage, David Schultz and Phil Schumacher