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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 Troughsand 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 Spring CSTAR Meeting 4 May 2012 NOAA/CSTAR Award # NA01NWS4680002

  2. Outline • Climatology of Appalachian lee troughs (ALTs; review) • Quantification of CAPE/shear at first daily storm report • CAPE on ALT days vs. non-ALT days • Composites • Technology transfer

  3. Outline • Climatology of Appalachian lee troughs (ALTs; review) • Quantification of CAPE/shear at first daily storm report • CAPE on ALT days vs. non-ALT days • Composites • Technology transfer

  4. Data and Methodology for Climatology 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 • “ALT Zone” ALT ZONE

  5. Methodology for Climatology • Climatology of ALTs was based on the following 3 criteria: • 925-hPa Wind Direction • Wind component normal to and downslope of Appalachians • MSLP Anomaly • Anomaly with respect to zonal average < −0.75 hPa • 1000–850-hPa Mean Temperature Anomaly • Anomaly with respect to zonal average > 1°C • All 3 criteria must be met for 3° latitude

  6. 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

  7. Outline • Climatology of Appalachian lee troughs (ALTs; review) • Quantification of CAPE/shear at first daily storm report • CAPE on ALT days vs. non-ALT days • Composites • Technology transfer

  8. 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 ALT ZONE

  9. CAPE/Shear at First Daily Storm Report • Only included days in which first storm report occurred between 1530and 0029 UTC • Diurnal cycle is the dominant mode of temporal variability

  10. 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

  11. 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

  12. 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

  13. 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)

  14. Outline • Climatology of Appalachian lee troughs (ALTs; review) • Quantification of CAPE/shear at first daily storm report • CAPE on ALT days vs. non-ALT days • Composites • Technology transfer

  15. Background • What is the reason for increased number of storm reports with the presence of an ALT? • Background conditions similar, ALT acts as trigger? • ALTs associated with increased CAPE?

  16. Background • What is the reason for increased number of storm reports with the presence of an ALT? • Background conditions similar, ALT acts as trigger? • ALTs associated with increased CAPE?

  17. Methodology • Compare 0000 UTC observed MUCAPE values at GSO, RNK, WAL and IAD on ALT and non-ALT days • Data obtained from SPC sounding archive (courtesy Rich Thompson) • Only use the times when observed MUCAPE > 0 and observed lifted parcel level is within 180 hPa of the surface • Generate box and whisker plots for comparison

  18. 0000 UTC Observed CAPE: ALT vs. Non-ALT Days Whiskers: 10th and 90th percentiles // Box edges: 25th and 75th percentiles // Line: median • All four stations have significantly greater median, 25th, 75th, and 90th percentile MUCAPE on ALT days • Intuitive since ALTs contain low-level thermal maxima (by definition) ALT DAYS NON-ALT DAYS

  19. Outline • Climatology of Appalachian lee troughs (ALTs; review) • Quantification of CAPE/shear at first daily storm report • CAPE on ALT days vs. non-ALT days • Composites • Technology transfer

  20. Review of ALT Categories Cat 1 (Inverted) Cat 2 (No PFT) Cat 4 (PFT, Total FROPA) Cat 3 (PFT, Partial FROPA)

  21. Composite Methodology • Made composites for 3 of the 4 ALT categories • Category 1 (Inverted) was omitted due to low frequency of occurrence • Two composites of each category were created • Severe • Non-severe

  22. Composite Methodology: Severe/Non-Severe Partitioning • “Clustering” – attempt to control for population bias in Storm Data • 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

  23. Composites • “N”: Non-severe category • “S”: Severe category • Cat 2: No PFT • Cat 3: PFT, partial FROPA • Cat 4: PFT, total FROPA

  24. Category 3: PFT, Partial FROPA MSLP (black, hPa), 2-m dewpoint (fills, °C; 20°C isodrosotherm in white), 10-m streamlines (arrows) Non-severe (N=17) • Increased dewpoints over ALT Zone in severe composite Severe (N=17)

  25. Category 3: PFT, Partial FROPA MSLP (black, hPa), 2-m dewpoint (fills, °C; 20°C isodrosotherm in white), 10-m streamlines (arrows) L L Non-severe (N=17) • Increased dewpoints over ALT Zone in severe composite • Surface low center position is different between the two composites Severe (N=17)

  26. Category 3: PFT, Partial FROPA 500-hPa heights (black, dam), Q-Vectors (arrows), Q-Vector divergence (fills) Non-severe (N=17) • 500-hPa trough upstream of ALT Zone in severe composites • Strong QG forcing for ascent does not affect ALT Zone in either composite Severe (N=17)

  27. Category 3: PFT, Partial FROPA Surface to 500-hPa vertical wind shear (black, kt), MUCAPE (fills, J/kg) Non-severe (N=17) • MUCAPE values are 500–1000 J/kg greater over the ALT Zone in the severe composite • Weak shear and weak QG forcing suggests severe category 3 events are not well organized/focused by synoptic-scale forcing Severe (N=17)

  28. Category 3: PFT, Partial FROPA Atkins and Wakimoto (1991); Fig. 10 Maximum difference in θe from surface to mid-levels (lines, K), maximum mid-level lapse rate over a 200-hPa-deep layer (fills, K/km) • Wet microbursts are favored when vertical difference in θe from surface to mid-levels is > 20 K Severe (N=17)

  29. Category 4: PFT, Total FROPA Surface to 500-hPa vertical wind shear (black, kt), MUCAPE (fills, J/kg) • Favorable juxtaposition of MUCAPE and shear exists over the northern ALT Zone Severe (N=17)

  30. Category 4: PFT, Total FROPA Percentage of category 4 days (n=130) with at least one active grid box Surface to 500-hPa vertical wind shear (black, kt), MUCAPE (fills, J/kg) • Favorable juxtaposition of MUCAPE and shear exists over the northern ALT Zone • Higher shear values and spatial distribution of storm reports suggest that category 4 severe events may be more organized and favor the DC to Philadelphia corridor Severe (N=17)

  31. Outline • Climatology of Appalachian lee troughs (ALTs; review) • Quantification of CAPE/shear at first daily storm report • CAPE on ALT days vs. non-ALT days • Composites • Technology transfer

  32. Technology Transfer: CAPE/Shear at First Daily Storm Report Whiskers: 10th and 90th percentiles // Box edges: 25th and 75th percentiles // Line: median • Boxplots of CAPE/shear at first daily storm report can be used to put an expected severe event into climatological context • Boxplots need to be re-done with obs in order to get more accurate values, since NARR MUCAPE is underdone compared to obs

  33. Technology Transfer: Conceptual Models of Composite ALT Categories Category 3 Severe Conceptual Model L > 25 kt Sfc.–500-hPa Shear 500-hPa Trough Axis Δθe = 20 K Td= 20°C Δθe = 20 K Td= 20°C Axis of High MUCAPE • Application of conceptual models can allow forecasters to quickly identify environments that are conducive to severe weather on ALT days

  34. For More Information • This presentation, as well as past presentations, can be found at my website: • http://www.atmos.albany.edu/student/dthompso/ • (Note: No “n” in “dthompso”) • Email: • dthompson@albany.edu • (Note: Now there is an “n”) Thank you for your time and suggestions.

  35. Spare slides

  36. 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)

  37. Domain for Climatology WIND ZONE ALT ZONE DOMAIN

  38. 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

  39. Location of First Storm Report by ALT Category • Majority of first daily storm reports occur west of composite ALT • Orographic forcing for thunderstorm initiation?

  40. Relevant Papers • Koch and Ray (1997): Convectively active boundaries in NC • Murphy and Konrad (2005): Spatial and temporal patterns of lightning in the southern Appalachians • Parker and Ahijevych (2007): Radar-based climatology of convection in the mid-Atlantic

  41. Location of First Storm Report by ALT Category • Possible reasons for disparity • Differing methodology in MUCAPE calculation? • Boundary layer parameterization in reanalyses is not too good?

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