- Slides: 21
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) � 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
What is the EML?
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 >F 2 intensity
Lapse Rate Tendency Equation �
� As well as differential ageostrophic temperature advection being positive (downstream of amplifying baroclinic wave, jet entrance, or frontogenetic circulation)
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 h. Pa 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 19702006 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.
Data and Methodology � Constructed composite analyses of 700 h. Pa heights and temperatures, 700 -500 h. Pa lapse rates, LI, and MSLP. � Backward Lagrangian trajectories at 250 h. Pa AGL, and every km up to 6 km for a duration of 96 hours.
Composite Analysis • 700 h. Pa 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.
Composite Analysis • 700 h. Pa temperature anomalies • Positive anomaly maxima moves from central plains to the Northeast. • Anomalies of +3°+3. 5°C exist.
Composite Analysis • 750 -500 h. Pa 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.
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.
Trajectory Analysis • Anticyclonic curvature • Consistent with the 700 h. Pa ridge • Most originate over the Intermountain West • Similar paths to 700 h. Pa temperature anomalies and 750 -500 h. Pa lapse rate composites. • (b) represents 10 trajectories showing three-quarter or greater anticyclonic loops.
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.
Case Studies � Eastern New Yorkwestern Massachusetts tornadoes: 28 August 1973 � 350 km swath of severe weather � Associated with an EML event
Case Studies • Northern New England southern Quebec derecho: 5 July 1999