Transcript Deep Convection, Severe Weather, and Appalachian Lee

Deep Convection, Severe
Weather, and Appalachian
Lee/Prefrontal Troughs
Daniel B. Thompson, Lance F. Bosart and
Daniel Keyser
Department of Atmospheric and Environmental Sciences
University at Albany, State University of New York
NWS Focal Points:
Thomas A. Wasula
NOAA/WFO Albany, NY
Matthew Kramar
NOAA/WFO Sterling, VA
CSTAR Spring Meeting: 6 May 2011
NOAA/CSTAR Grant # NA01NWS4680002
Importance
• Proximity of convective initiation region to
densely populated Eastern Seaboard
– Aviation impacts
• Forecasting Appalachian Lee Trough (ALT)
convection can be challenging (especially in
summer months during weak upper flow) with
respect to:
– Location
– Mode
– Severity
• Weak synoptic forcing shifts focus to mesoscale
features (pressure troughs, boundaries, etc.)
Overview
1)
2)
3)
4)
5)
Background and literature review
Data and methods
Preliminary results
Technology transfer
Next steps in research
Relevant Literature (1 of 2)
• Weisman (1990) Mon. Wea. Rev.: An
observational study of warm season
southern Appalachian troughs. Part II:
Thunderstorm genesis zones
– ALT present 40% of time from MaySeptember 1984-1985
– Maximum in convection in late afternoon
– Convective events stratified by strength of
forcing
• Lee trough was not the focus for convection in 3
out of 4 forcing categories
Lee Trough Formation:
PV Perspective
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PV = g(ζθ + f)(-∂θ/∂p)
d(PV)/dt = 0 for adiabatic flow
Need component of flow normal to mountain barrier
Flow across mountain barrier will subside on lee side
– Advects higher θ downward → warming
– -∂θ/∂p decreases → ζθ must increase → low level circulation
Martin (2006)
Relevant Literature (2 of 2)
• Schultz (2005) Mon. Wea. Rev.: A review
of cold fronts with prefrontal troughs and
wind shifts
– Identified 10 mechanisms leading to prefrontal
trough (PFT) development
• External to front vs. directly associated with front
• Interaction with lee trough is one such mechanism
– PFTs can:
• Become dominant front while original front decays
• Maintain intensity, leading to two cold fronts
• Never develop frontal characteristics
Prefrontal Trough Formation:
Frontogenesis Acting On Along-front
Temperature Gradients
• One of ten formation mechanisms in Schultz (2005)
• Along-front warm advection induces pressure falls
– PFT and wind shift propagate eastward
– Can move eastward relative to front which moves at advective
wind speed
• These PFTs have:
- Maximum surface relative
vorticity
- Maximum surface
convergence
- Minimum surface
pressure
• But they lead temp. gradient
Schultz (2005)
Data and Methods
• 14 cases of ALT convection from MaySeptember
– Provided by Matt Kramar
– Area of focus: Southeastern PA through
Carolinas
• 0.5° CFSR (Climate Forecast System
Reanalysis) dataset
• Horizontal maps and vertical cross
sections analyzed in GEMPAK and
examined for common features
Preliminary Results (1 of 4)
• ALTs characterized by:
– Low-level wind component normal to
mountain barrier
– Low-level thickness ridge
– Low-level thermal vorticity minimum
– Low-level geostrophic vorticity maximum
• ALTs are shallow, warm-core features
• Climatology is being constructed based on
these common features
Preliminary Results (2 of 4)
22 July 2008
500 hPa 1800 UTC 22 July 2008
Height (dam, black), geo. vorticity (s^-1,
red) and wind (knots, barbs and fills)
Preliminary Results (3 of 4)
1800 UTC 22 July 2008
Preliminary Results (4 of 4)
1800 UTC 22 July 2008
Technology Transfer
• ALT climatology will pinpoint favored areas and
times of:
– ALT occurrence
– Convective initiation
– Severe thunderstorms
• Future case studies and composite analyses
will:
– Identify key flow patterns and mesoscale features
associated with certain classes of events
• Goal: synthesize this information into conceptual
model
– Develop “rules of thumb” for forecasters
Next Steps
• Develop objective criteria for defining ALTs
– Construct climatology based on this criteria
• Look for common flow patterns and vertical
temperature profiles when comparing:
– Days with common convective location, mode and
severity
– Active cases vs. null cases
• Analyze prefrontal trough cases over Northeast
(Tom Wasula)
– Identify similarities / differences with ALT cases
Contact Information
• Preliminary results are located at:
– http://www.atmos.albany.edu/student/dthomps
o/docs.html
• [email protected]
Questions or comments?
ALTs vs. PFTs
• PFTs are tied to frontal cyclone, while ALTs
may develop in absence of synoptic-scale
forcing
– But ALTs can be manifested as PFTs ahead of
a surface cold front
• ALTs are thought to preferentially develop
from SE PA to Carolinas, where terrain
orientation is more favorable
• Similarities may exist in structure and
formation of both ALTs and PFTs