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Appalachian Lee Troughs and their Association with Severe Thunderstorms

Appalachian Lee Troughs and their Association with Severe Thunderstorms. 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

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Appalachian Lee Troughs and their Association with Severe Thunderstorms

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  1. Appalachian Lee Troughs and their Association with Severe Thunderstorms 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 37th Northeastern Storm Conference, Rutland, VT 3 Mar 2012 NOAA/CSTAR Award # NA01NWS4680002

  2. Motivation • Region of study: Mid-Atlantic • Accurately forecasting location, mode and severity of thunderstorms is important, due to proximity of Eastern Seaboard • Region is often characterized by weak forcing and ample instability • Mesoscale boundaries important • Sea breeze boundary • Outflow boundaries • Lee trough

  3. Objectives • Analyze the structure of Appalachian Lee Troughs (ALTs) • Construct a climatology of warm-season ALTs • Analyze the distribution of severe convection in the Mid-Atlantic • Spatial distribution • Temporal distribution • Characteristic CAPE/shear

  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

  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 MSLP (black, hPa), 1000–850-hPa thickness (fills, dam), thermal vorticity < 0 (white, 10−5 s−1), 10-m winds (barbs, kt) 1800 UTC Composite (n=13)

  7. ALTs – Common Low-Level Features Winds orthogonal to mountains MSLP (black, hPa), 1000–850-hPa thickness (fills, dam), thermal vorticity < 0 (white, 10−5 s−1), 10-m winds (barbs, kt) 1800 UTC Composite (n=13)

  8. ALTs – Common Low-Level Features Thermal ridge Winds orthogonal to mountains MSLP (black, hPa), 1000–850-hPa thickness (fills, dam), thermal vorticity < 0 (white, 10−5 s−1), 10-m winds (barbs, kt) 1800 UTC Composite (n=13)

  9. ALTs – Common Low-Level Features Thermal ridge Winds orthogonal to mountains Negative thermal vorticity MSLP (black, hPa), 1000–850-hPa thickness (fills, dam), thermal vorticity < 0 (white, 10−5 s−1), 10-m winds (barbs, kt) 1800 UTC Composite (n=13)

  10. ALTs – Common Low-Level Features Thermal ridge Winds orthogonal to mountains Negative thermal vorticity MSLP (black, hPa), 1000–850-hPa thickness (fills, dam), thermal vorticity < 0 (white, 10−5 s−1), 10-m winds (barbs, kt) 0000 UTC Composite (n=13)

  11. Domain for Climatology WIND ZONE ALT ZONE DOMAIN

  12. 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

  13. 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

  14. 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

  15. 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)

  16. 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

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

  18. 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

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

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

  21. Controlling for Dataset Inconsistencies • “Clustering” – attempt to control for population bias • Overlay a 0.5° by 0.5° grid box over the domain • If a storm report occurs within a certain grid box on a certain day, that grid box is considered “active” for the day • Any subsequent storm reports occurring within the active box are discarded for the day • The number of active grid boxes for each day are tallied to measure how widespread the severe weather was on that day

  22. Storm Reports – Spatial Distribution n=706 CFSR composite of top 10% of severe ALT days. MUCAPE (fills, J/kg) and surface to 500-hPa shear (black, kt) n=48 Percentage of ALT days with >0 active grid boxes (smoothed)

  23. Storm Reports – Spatial Distribution n=706 CFSR composite of top 10% of severe ALT days. MUCAPE (fills, J/kg) and surface to 500-hPa shear (black, kt) n=48 Percentage of ALT days with >0 active grid boxes (smoothed) • Storm report max near D.C. coincides with CAPE/shear maxima • NC local max more difficult to explain

  24. CAPE/Shear at First Daily Storm Report • To quantify severe thunderstorm parameters characteristic of ALT Zone, CAPE/shear was calculated at location of first daily storm report • Dataset: 32 km NARR (8 analysis times daily) • Procedure: • Find location and time of first severe report on a certain day (0400–0359 UTC) • Calculate MUCAPE and Sfc–500-hPa shear at location of storm report using nearest analysis time at least 30 min prior to storm report

  25. CAPE/Shear at First Daily Storm Report • Only included days in which first storm report occurred between 1530and 0029 UTC

  26. CAPE/Shear at First Daily Storm Report • ALT Zone was divided into sectors to minimize the likelihood of the first daily storm report not being representative of the environment NORTH CENTER SOUTH

  27. CAPE/Shear at First Daily Storm Report NORTH • South sector peaks earlier (1800 UTC) than north sector (2000 UTC) • Center sector has flat peak between 1800–2100 UTC CENTER SOUTH

  28. CAPE/Shear at First Daily Storm Report Whiskers: 10th and 90th percentiles // Box edges: 25th and 75th percentiles // Line: median NORTH • Higher median CAPE (shear) for first daily storm report in south (north) sector • Higher shear in north sector is likely because it is nearer to the mean warm-season upper jet CENTER SOUTH

  29. CAPE/Shear at First Daily Storm Report • First daily storm report does not concentrate well in CAPE/shear phase-space

  30. CAPE/Shear at First Daily Storm Report • First daily storm report does not concentrate well in CAPE/shear phase-space No storm reports occurred in this phase-space

  31. CAPE/Shear at First Daily Storm Report Whiskers: 10th and 90th percentiles // Box edges: 25th and 75th percentiles // Line: median • CAPE (shear) at first daily storm report maximized in June, July and August (May and September)

  32. Summary – Key Points • ALTs form preferentially during diurnal and seasonal heating maxima

  33. Summary – Key Points • Distribution of storm reports in ALT Zone varies by latitude • First daily storm report occurs 2 h earlier in south sector compared to north sector

  34. Summary – Key Points • CAPE and shear at first daily storm report vary by latitude and month • Greater median CAPE (shear) occurs in • June, July and August (May and September) • South (north) sector GREATER SHEAR May, Sep GREATER CAPE Jun, Jul, Aug

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