Characteristics and Climatology of Appalachian Lee Troughs


НазваCharacteristics and Climatology of Appalachian Lee Troughs
Дата конвертації16.04.2013
Розмір445 b.
ТипПрезентации


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

Motivation



Analyze the structure of Appalachian Lee Troughs (ALTs)

  • Analyze the structure of Appalachian Lee Troughs (ALTs)

  • Obtain an objective definition of ALTs

  • Analyze the distribution of severe convection in the Mid-Atlantic





PV = −g(∂θ/∂p)(ζθ + f)

  • 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 → ζθ must increase → low level circulation














Vertical extent of warm core ranges between 850 hPa and 700 hPa

  • Vertical extent of warm core ranges between 850 hPa and 700 hPa

    • Average: 788 hPa
    • Standard deviation: 61 hPa




Climatology was based on the following 3 criteria:

  • 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°


Climatology was based on the following 3 criteria:

  • 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


Climatology was based on the following 3 criteria:

  • 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


The three criteria must be met for six consecutive 0.5° latitudes

  • 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)





MSLP anomaly < −0.75 hPa Temperature anomaly > 1°C

MSLP anomaly < −0.75 hPa Temperature anomaly > 1°C

MSLP anomaly < −0.75 hPa Temperature anomaly > 1°C

MSLP anomaly < −0.75 hPa Temperature anomaly > 1°C

MSLP anomaly < −0.75 hPa Temperature anomaly > 1°C

MSLP anomaly < −0.75 hPa Temperature anomaly > 1°C

ALTs can be grouped into four categories based on their relationship with synoptic-scale cold fronts

  • 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


Inverted – trough extends northward from south of the ALT Zone

  • Inverted – trough extends northward from south of the ALT Zone



No PFT – trough occurs in the absence of a synoptic cold front

  • No PFT – trough occurs in the absence of a synoptic cold front



PFT, partial FROPA

  • 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


PFT, total FROPA

  • 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


Category 2 (No PFT) occurs most frequently

  • Category 2 (No PFT) occurs most frequently



Category 2 (No PFT) occurs most frequently

  • 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


Category 2 and 3 are more common in JJA, while category 4 is more common in May and September

  • 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”


Different domain, same procedure as Mid-Atlantic

Different domain, same procedure as Mid-Atlantic

Most ALTs recorded in Mid-Atlantic

  • 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


Severe local storm reports were obtained from the NCDC Storm Data publication

  • 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



12,330 storm reports

  • 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)





Pronounced mid-afternoon/early evening maximum in storm reports between 2100 and 2300 UTC

  • Pronounced mid-afternoon/early evening maximum in storm reports between 2100 and 2300 UTC



What influence does an ALT have on the distribution of convection, with respect to location, mode and severity?

  • 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


ALTs have a shallow, warm core

  • 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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Герундій має такі дієслівні властивості: а Герундій перехідних дієслів вживається з прямим додатком

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