The Well Mixed Boundary Layer as Part of the Great Plains Severe Storms Environment Jonathan Garner Storm Prediction Center.

The Well Mixed Boundary Layer as Part of the Great

Plains Severe Storms Environment

Jonathan GarnerStorm Prediction Center

Motivation

Development of the moist/unstable warm sector is important to monitor, but…

Processes occurring within the hot/dry side of the severe storm environment are important too

Evolution of the well mixed boundary layer adjacent to the moist sector can provide important clues for the initiation of supercells

The EML and Lid

Carlson, Lanicci, Warner… Moist/unstable air emerges from

beneath the “lid” through a process termed “under-running”

Where is the under-running process focused?

Is under-running a random event occurring where-ever a local weakness in the cap exists?

…or, is this a predictable phenomenon?

850 mb 29 May 2004 12Z

850 mb 29 May 2004 00Z

700 mb 29 May 2004 00Z

700 mb 29 May 2004 12Z

The EML and Lid

The EML and LidCarlson et al. (1983)

The EML and Lid

Many significant supercell events over the Great Plains appear to emanate off of steep low-level lapse rate axes

Preliminary work suggests that these low-level lapse rate axes focus the EML “under-running” process

Formation of the Axis

Surface heating over the elevated terrain and high plains initially yields deep boundary layer mixing.

This deeper mixing then protrudes downstream along zones of low-level horizontal deformation

These deformation zones are usually associated with frontal boundaries

Composite Charts

Frontal orientation/position influences where the low-level lapse rate axis will protrude

Fronts are closely associated with the upper-level height pattern and jet stream

Several re-occurring large-scale patterns have been observed

Analog: 22 May 2004

Analog: 11 June 2008

Analog: 12 May 2004

Processes Promoting Storm Development

Several supercell environments examined in detail show that a pronounced ageostrophic circulation is focused above the low-level lapse rate axis Ascent occurs over the well mixed boundary

layer Subsidence occurs over the moist potentially

unstable sector Near surface transverse portion of the

circulation is directed from the moist side (beneath the cap) to the hot well-mixed airmass

This appears to be the “under-running” process documented by past authors

Ageostrophic CirculationKeyser and Carlson (1984)

Processes Promoting Storm Development

Strong diabatic heating occurs within the low-level lapse rate axis

This heating is located adjacent to rich low-level moisture

High surface theta-e values are favored within the transition zone, which aids in maximizing CAPE

Diabatic Heating within the LLR axis (Kaplan et

al. 1984) Model sensitivity study showed that upper-level divergence associated with jet streaks was enhanced with the inclusion of surface diabatic heating and subsequent development of a deep well-mixed PBL

• Cross section of MASS diabatic simulation (for 5 June 1980 00 UTC; i.e. Grand Island Tornado) of tangential wind component vectors, and potential temperature. Vectors are at each model grid point. Dashed lines represent upward motion. Thick vectors highlight the diabatically-induced circulation.

Diabatic Heating within the LLR axis (Kaplan et al.

1984) Strong diabatic heating leads to

pressure falls which accelerate low-level flow Increased moisture transport into the

region Convergence/ascent within exit region

of accelerating low-level wind fields Modifies vertical wind shear

Eastward component of ageostrophic circulation advects dry air above returning low-level moisture (i.e., differential moisture advection), which increases buoyancy

Processes Promoting Storm Development

Static stability and CINH are greatly reduced within the low-level lapse rate axis

Vertical motion occurring in response to upper disturbances, low-level deformation zone and strong diabatic heating are enhanced due to reduction in static stability

Therefore, environment in the vicinity of the lapse rate axis is dominated by ascent where the cap is weak and CAPE is large

Importance of Weak LLLR in the Moist Warm Sector

Maddox et al. 1980

24 April 2009

10 May 2010

Importance of Weak LLLR in the Moist Warm Sector

Steep low-level lapse rates over the moist side of the warm sector are detrimental to tornadic storms: Low-level winds tend to veer to

southwesterly as boundary layer mixes out

Reduces magnitude of low-level vertical wind shear

Well mixed boundary layer is drier (higher LCL)

Stronger convective outflow/cold pools Colder RFD’s, and adverse interactions with

surrounding storms/cold pools

Downstream Supercell Environment

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Summary Discrete storm development appears to be

favored near the “nose” of the low-level lapse rate axis Lapse rate axis virtually points to where

supercells will develop Hot well mixed boundary layer focused into a

narrow region is more conducive for discrete supercell development versus steep low-level lapse rates becoming widespread across the moist sector

Storm then moves downstream, off the low-level lapse rate axis Supercell/tornado ingredients are focused

downstream/adjacent to the lapse rate axis

Summary

Of the few cases examined in detail, the ageostrophic circulation was centered on the low-level lapse rate axis

Key Questions for Future Work: Is this circulation present in additional

cases? Why is it focused there?

Interaction between frontal circulation, jet streak circulation, diabatic heating and reduced static stability?