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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
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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 • 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 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 → ζθmust 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 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)
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