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

Motivation Development of the moist/unstable warm sector is important to monitor, but… 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 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 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… Carlson, Lanicci, Warner… Moist/unstable air emerges from beneath the “lid” through a process termed “under-running” Moist/unstable air emerges from beneath the “lid” through a process termed “under-running” Where is the under-running process focused? Where is the under-running process focused? Is under-running a random event occurring where- ever a local weakness in the cap exists? Is under-running a random event occurring where- ever a local weakness in the cap exists? …or, is this a predictable phenomenon? …or, is this a predictable phenomenon?

850 mb 29 May Z 850 mb 29 May Z700 mb 29 May Z 700 mb 29 May Z The EML and Lid

Carlson 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 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 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. 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 This deeper mixing then protrudes downstream along zones of low-level horizontal deformation These deformation zones are usually associated with frontal boundaries These deformation zones are usually associated with frontal boundaries

Composite Charts Frontal orientation/position influences where the low-level lapse rate axis will protrude 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 Fronts are closely associated with the upper- level height pattern and jet stream Several re-occurring large-scale patterns have been observed Several re-occurring large-scale patterns have been observed

Analog: 22 May 2004 Analog: 11 June 2008Analog: 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 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 Ascent occurs over the well mixed boundary layer Subsidence occurs over the moist potentially unstable sector 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 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 This appears to be the “under-running” process documented by past authors

Ageostrophic Circulation Keyser and Carlson (1984)

Processes Promoting Storm Development Strong diabatic heating occurs within the low- level lapse rate axis Strong diabatic heating occurs within the low- level lapse rate axis This heating is located adjacent to rich low-level moisture 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 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 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 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 Strong diabatic heating leads to pressure falls which accelerate low-level flow Increased moisture transport into the region Increased moisture transport into the region Convergence/ascent within exit region of accelerating low-level wind fields Convergence/ascent within exit region of accelerating low-level wind fields Modifies vertical wind shear 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 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 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 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 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: 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 Low-level winds tend to veer to southwesterly as boundary layer mixes out Reduces magnitude of low-level vertical wind shear Reduces magnitude of low-level vertical wind shear Well mixed boundary layer is drier (higher LCL) Well mixed boundary layer is drier (higher LCL) Stronger convective outflow/cold pools Stronger convective outflow/cold pools Colder RFD’s, and adverse interactions with surrounding storms/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 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 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 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 Storm then moves downstream, off the low-level lapse rate axis Supercell/tornado ingredients are focused downstream/adjacent to the 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 Of the few cases examined in detail, the ageostrophic circulation was centered on the low-level lapse rate axis Key Questions for Future Work: Key Questions for Future Work: Is this circulation present in additional cases? Is this circulation present in additional cases? Why is it focused there? Why is it focused there? Interaction between frontal circulation, jet streak circulation, diabatic heating and reduced static stability? Interaction between frontal circulation, jet streak circulation, diabatic heating and reduced static stability?