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 BanacosAnd

Michael Ekster

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 Elevated Mixed Layer? (EML)

What is the 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

Why EML?

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.

Lapse Rate Tendency Equation

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

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.

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.

Data and Methodology

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.

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

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.

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 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.

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.

Eastern New York-western Massachusetts tornadoes: 28 August 1973

350 km swath of severe weather

Associated with an EML event

Case Studies

Case Studies

Case Studies

Case Studies

• Northern New England and southern Quebec derecho: 5 July 1999

Case Studies

Case Studies

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

Conclusions: