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Characteristics and Climatology of Appalachian Lee Troughs

Characteristics and Climatology of Appalachian Lee Troughs. Daniel B. Thompson, Lance F. Bosart and Daniel Keyser Department of Atmospheric and Environmental Sciences University at Albany/SUNY, Albany, NY 12222 Thomas A. Wasula NOAA/NWS, Albany, NY Matthew Kramar NOAA/NWS, Sterling, VA

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Characteristics and Climatology of Appalachian Lee Troughs

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  1. Characteristics and Climatology of Appalachian Lee Troughs Daniel B. Thompson, Lance F. Bosart and Daniel Keyser Department of Atmospheric and Environmental Sciences University at Albany/SUNY, Albany, NY 12222 Thomas A. Wasula NOAA/NWS, Albany, NY Matthew Kramar NOAA/NWS, Sterling, VA Northeast Regional Operational Workshop XIII, Albany, NY 3 Nov 2011 NOAA/CSTAR Award # NA01NWS4680002

  2. Motivation Region of study: Mid-Atlantic Mid-Atlantic warm season often characterized by: Topography Weak synoptic-scale forcing Increased importance of mesoscale features for triggering convection Horizontal rolls → + Ample instability Surface boundaries

  3. Objectives • Analyze the structure of Appalachian Lee Troughs (ALTs) • Obtain an objective definition of ALTs • Analyze the distribution of severe convection in the Mid-Atlantic

  4. Data and Methodology • Analyzed 13 cases of ALT events associated with warm-season severe convection • Sterling, VA (LWX) CWA • 0.5° CFSR (Climate Forecast System Reanalysis) • Identified common features and used them as criteria to construct a climatology • May–September, 2000–2009 • Categorized ALTs based on their relationship with synoptic-scale cold fronts

  5. Lee Trough Formation: PV Perspective • PV = −g(∂θ/∂p)(ζθ+ f) (Static stability)(Absolute vorticity) • d(PV)/dt = 0 for adiabatic flow • Flow across mountain barrier will subside on lee side • Advects higher θ downward → warming • −g(∂θ/∂p) decreases → ζθmust increase → low level circulation Appalachians Appalachians Adapted from Martin (2006)

  6. ALTs – Common Low-Level Features NEXRAD 2-km Mosaic (dBZ) 2056 UTC 22 July 2008 Source: College of DuPage MSLP (black, hPa), 1000–850-hPa thickness (fills, dam), thermal vorticity < 0 (white, 10−5 s−1), 10-m winds (barbs, kt)

  7. ALTs – Common Low-Level Features NEXRAD 2-km Mosaic (dBZ) 2056 UTC 22 July 2008 Source: College of DuPage MSLP (black, hPa), 1000–850-hPa thickness (fills, dam), thermal vorticity < 0 (white, 10−5 s−1), 10-m winds (barbs, kt)

  8. ALTs – Common Low-Level Features A NEXRAD 2-km Mosaic (dBZ) 2056 UTC 22 July 2008 Source: College of DuPage A’ MSLP (black, hPa), 1000–850-hPa thickness (fills, dam), thermal vorticity < 0 (white, 10−5 s−1), 10-m winds (barbs, kt)

  9. ALTs – Common Low-Level Features Potential temperature (black, K), geostrophic relative vorticity (fills, 10−5 s−1),winds (barbs, kt) 100 km

  10. ALTs – Common Low-Level Features Potential temperature (black, K), geostrophic relative vorticity (fills, 10−5 s−1),winds (barbs, kt) Geostrophic Relative Vorticity Maximum 100 km

  11. ALTs – Common Low-Level Features Potential temperature (black, K), geostrophic relative vorticity (fills, 10−5 s−1),winds (barbs, kt) Warm Core Geostrophic Relative Vorticity Maximum 100 km

  12. ALTs – Common Low-Level Features • Vertical extent of warm core ranges between 850 hPa and 700 hPa • Average: 788 hPa • Standard deviation: 61 hPa

  13. Domain for Climatology WIND ZONE ALT ZONE DOMAIN

  14. Methodology for Climatology • Climatology was based on the following 3 criteria: • 925-hPa Wind Direction • Checked for wind component directions orthogonal to and downslope of Appalachians • Appalachians in the Mid-Atlantic are oriented ~ 43° right of true north • Satisfactory meteorological wind directions exist between 223° and 43° • Criterion: wind direction computed from zonal average of wind components along each 0.5° of latitude within Wind Zone must be between 223° and 43° WIND ZONE ALT ZONE DOMAIN

  15. Methodology for Climatology • Climatology was based on the following 3 criteria: • MSLP Anomaly • Averaged MSLP along each 0.5° of latitude within domain • Checked for minimum MSLP along each 0.5° of latitude within ALT Zone • Criterion: difference of minimum and zonal average MSLP must be less than a threshold value WIND ZONE ALT ZONE DOMAIN

  16. Methodology for Climatology • Climatology was based on the following 3 criteria: • 1000–850-hPa layer-mean temperature anomaly • Averaged 1000–850-hPa layer-mean temperature along each 0.5° of latitude within domain • Checked for maximum 1000–850-hPa layer-mean temperature along each 0.5° of latitude within ALT Zone • Criterion: difference of maximum and zonal average 1000–850-hPa layer-mean temperature must be greater than a threshold value WIND ZONE ALT ZONE DOMAIN

  17. Methodology for Climatology • The three criteria must be met for six consecutive 0.5° latitudes • An algorithm incorporating the three criteria was run for the length of the climatology at 6-h intervals (0000, 0600, 1200 and 1800 UTC) • ALTs identified by this algorithm were manually checked for false alarms (e.g. frontal troughs, cyclones, large zonal pressure gradients)

  18. Climatology – Results ← Stricter ← Stricter • Each bubble denotes the percentage of time an ALT is recorded under a particular set of MSLP/temperature anomaly constraints • Boxesindicate the criteria adopted as the ALT definition

  19. Climatology – Results MSLP anomaly < −0.75 hPaTemperature anomaly > 1°C

  20. Climatology – Results MSLP anomaly < −0.75 hPaTemperature anomaly > 1°C • Over 75% of ALTs occur in June, July and August

  21. Climatology – Results MSLP anomaly < −0.75 hPaTemperature anomaly > 1°C • Over 75% of ALTs occur in June, July and August • Nearly 66% of ALTs occur at 1800 or 0000 UTC • The seasonal and diurnal heating cycles likely play a role in ALT formation

  22. ALT Categories • ALTs can be grouped into four categories based on their relationship with synoptic-scale cold fronts • ALTs that occur in advance of cold fronts can be considered prefrontal troughs (PFTs) • Categories: • Inverted • No PFT: Non-prefrontal • PFT, partial FROPA: Prefrontal without frontal passage through entire ALT Zone • PFT, total FROPA: Prefrontal with frontal passage through entire ALT Zone

  23. ALT Categories – Examples • Inverted – trough extends northward from south of the ALT Zone 0000 UTC 31 May 2001 MSLP (black, hPa) and 1000–850-hPa thickness (fills, dam)

  24. ALT Categories – Examples • No PFT – trough occurs in the absence of a synoptic cold front 0000 UTC 10 July 2000 MSLP (black, hPa) and 1000–850-hPa thickness (fills, dam)

  25. ALT Categories – Examples • PFT, partial FROPA • Front must be south of the NY/PA border or east of the western third of PA • Front does not pass through entire ALT Zone 0000 UTC 3 June 2000 MSLP (black, hPa) and 1000–850-hPa thickness (fills, dam)

  26. ALT Categories – Examples • PFT, total FROPA • Front must be south of the NY/PA border or east of the western third of PA • Front passes through entire ALT Zone within 24 h 1800 UTC 13 May 2000 MSLP (black, hPa) and 1000–850-hPa thickness (fills, dam)

  27. ALT Categories – Climatology • Category 2 (No PFT) occurs most frequently

  28. ALT Categories – Climatology • Category 2 (No PFT) occurs most frequently • PFTs account for 44.8% of ALTs • How does the spatial distribution of convection change between categories? • How does this distribution change between PFTs and non-PFTs? • To be determined

  29. ALT Categories – Monthly Distribution • Category 2 and 3 are more common in JJA, while category 4 is more common in May and September • Stronger westerlies, more FROPA during “transition months”

  30. ALT Climatology in the Northeast Different domain, same procedure as Mid-Atlantic NORTHEAST INTERMOUNTAIN REGION (NEI) NORTHEASTCOASTAL PLAIN (NECP)

  31. ALT Climatology in the Northeast – Results • Most ALTs recorded in Mid-Atlantic • More favorable terrain? • 39% of ALTs in NECP were postfrontal • Convection unlikely • Caveats: • Smaller-scale troughs may be undetected • Does not represent complete climatology of PFTs NEI NECP

  32. Storm Reports in the ALT Zone – Data and Methodology • Severe local storm reports were obtained from the NCDC Storm Data publication • Examined all tornado, severe thunderstorm wind and severe hail (>1”) for May–September, 2000–2009 ALT ZONE climate.met.psu.edu

  33. Storm Reports – Daily Distribution Day = 0400 to 0400 UTC • 12,330 storm reports • 754 unique days with at least one storm report • 199 days with > 20 storm reports • Most active day: 13 May 2002 (207)

  34. Storm Reports – Daily Distribution

  35. Storm Reports – Daily Distribution • Pronounced mid-afternoon/early evening maximum in storm reports between 2100 and 2300 UTC

  36. ALTs and Convection – Further Questions • What influence does an ALT have on the distribution of convection, with respect to location, mode and severity? • What influence do each of the ALT categories have on this distribution? • To be determined

  37. Summary – Key Points • ALTs have a shallow, warm core • ALTs form preferentially during diurnal and seasonal heating maxima • Monthly distribution of ALTs varies depending on the ALT category • Classic, terrain-induced ALTs are more likely in June, July and August • ALTs associated with complete FROPA are more likely during May and September • ALTs are more likely in the Mid-Atlantic than the Northeast • The ALT Zone has a distinct diurnal maximum in storm reports

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