Deep Convection, Severe Weather, and Appalachian

Lee/Prefrontal Troughs

Daniel B. Thompson, Lance F. Bosart and Daniel Keyser

Department of Atmospheric and Environmental SciencesUniversity at Albany, State University of New York

NWS Focal Points: Thomas A. WasulaNOAA/WFO Albany, NY

Matthew KramarNOAA/WFO Sterling, VA

CSTAR Spring Meeting: 6 May 2011NOAA/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) Background and literature review

2) Data and methods

3) Preliminary results

4) Technology transfer

5) 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 May-

September 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

• 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

Schultz (2005)

• These PFTs have:

- Maximum surface relative vorticity

- Maximum surface convergence

- Minimum surface pressure

• But they lead temp. gradient

Data and Methods

• 14 cases of ALT convection from May-September – 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/

dthompso/docs.html

• dthompso@atmos.albany.edu

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