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The Association of the Elevated Mixed Layer with Significant Severe Weather Events in the Northeastern United States

The Association of the Elevated Mixed Layer with Significant Severe Weather Events in the Northeastern United States. By: Peter Banacos And Michael Ekster. What is Elevated Mixed Layer? (EML).

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The Association of the Elevated Mixed Layer with Significant Severe Weather Events in the Northeastern United States

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  1. The Association of the Elevated Mixed Layer with Significant Severe Weather Events in the Northeastern United States By:Peter Banacos And Michael Ekster

  2. What is Elevated Mixed Layer? (EML) • The horizontally advected hot, dry, and deeply mixed boundary layer air over areas of lower terrain. • Lies atop an interface of strong static stability, creating CIN • Similar to a “loaded gun” sounding. • Late spring / summer phenomenon

  3. What is the EML?

  4. Why EML? • EML formation is well understood, however its movement, evolution, and inevitable dissipation has received less attention • Johns and Dorr (1996) • Farrell and Carlson (1989) • Solely focused on tornadoes and not derechos or large hail events • Performed prior to high-powered compositing techniques • This paper attempts to catalog significant severe weather • Hail >2 in. (5.1 cm) in diameter, • Convective wind gusts >65 kt (33 m s^-1) • Tornadoes of >F2 intensity

  5. Lapse Rate Tendency Equation • A B C D E • A = Diabatic Heating Term, where a decrease (increase) in diabatic heating with height yields a steepening (lessening) lapse rate • B = Horizontal Lapse Rate Advection term • C = Vertical Lapse Rate Advection term • D = Differential Temperature Advection term, where a steepening (lessening) lapse rate would occur within a thermally indirect (direct) circulation • E = Vertical Stretching term.

  6. As well as differential ageostrophic temperature advection being positive (downstream of amplifying baroclinic wave, jet entrance, or frontogenetic circulation)

  7. Data and Methodology • 929 significant severe reports (3% of original 30,617 reports) • EML soundings were chosen based on: • An elevated lapse rate ≥ 8°C/km through a depth of 200 hPa or greater • An increase in environmental relative humidity with height from a minimum at the bottom of the layer of steep lapse rate, through the depth of the steep lapse rate layer • 34 significant severe weather days from 1970-2006 that met an “EML” criterion. • This accounted for only 0.1% of severe weather reports in the NE, but owed to 52.9% of fatalities and 45% of injury totals.

  8. Data and Methodology • Constructed composite analyses of 700 hPa heights and temperatures,700-500 hPa lapse rates, LI, and MSLP. • Backward Lagrangian trajectories at 250 hPa AGL, and every km up to 6km for a duration of 96 hours.

  9. Composite Analysis • 700 hPa height anomalies • EML events have a stronger northerly displaced ridge and heights anomalies of 36 m. • Placement of ridge and trough allow for a westerly geostrophic flow.

  10. Composite Analysis • 700 hPa temperature anomalies • Positive anomaly maxima moves from central plains to the Northeast. • Anomalies of +3°-+3.5°C exist.

  11. Composite Analysis • 750-500 hPa lapse rate • The EML composite mean value is 7.5°-8°C km • Anomalies are +1°C/km • Plume extends across Great Lakes region. • Important because EML plume shows steeper lapse rates.

  12. Composite Analysis • LI and MSLP • MSLPs are similar in EML and non-EML cases • Illustrates importance of atmosphere aloft-EML Plume. • Greater potential instability in the EML composite.

  13. Trajectory Analysis • Anticyclonic curvature • Consistent with the 700 hPa ridge • Most originate over the Intermountain West • Similar paths to 700 hPa temperature anomalies and 750-500 hPa lapse rate composites. • (b) represents 10 trajectories showing three-quarter or greater anticyclonic loops.

  14. Trajectory Analysis • After t-51 h, parcel begins subsiding. • Dp/Dt shows subsidence 1-2 prior to event followed by accent. • Subsidence strengthens capping inversion • Maintains EML plume by not allowing DMC. • Vertical velocity switches sign at t-14 h. • Rising motion allows cap to weaken and DMC fires.

  15. Case Studies • Eastern New York-western Massachusetts tornadoes: 28 August 1973 • 350 km swath of severe weather • Associated with an EML event

  16. Case Studies

  17. Case Studies

  18. Case Studies • Northern New England and southern Quebec derecho: 5 July 1999

  19. Case Studies

  20. Case Studies

  21. Conclusions: • Hypothesize V ∙ and ω > 0 within the EML plume are both essential for maintenance and transport over large distances. Release of instability over the NE is essential. • Mean 700hPa anticyclonic flow featured over Tennessee River valley. • 700hPa trough along the West Coast that results in ejection of the EML plume across the plains. • Anticyclonic flow across the plains, yielding mean ω > 0 and limited convection for plume maintenance until it moves downstream of the ridge axis. • Shortwave trough embedded in enhanced to moderately strong WNW mid-level flow across Great Lakes, which leads to entrained EML air, leading to convection and ω< 0

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