The Well Mixed Boundary Layer as Part of the Great Plains Severe Storms Environment

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

2. 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

3. 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?

4. The EML and Lid 850 mb 29 May 2004 00Z 700 mb 29 May 2004 00Z 850 mb 29 May 2004 12Z 700 mb 29 May 2004 12Z

5. The EML and Lid Carlson et al. (1983)

6. 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

7. 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

15. 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

16. Analog: 22 May 2004 Analog: 12 May 2004 Analog: 11 June 2008

17. 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

18. Ageostrophic Circulation Keyser and Carlson (1984)

20. 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

23. 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.

24. 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

25. 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

26. Importance of Weak LLLR in the Moist Warm Sector Maddox et al. 1980

27. 24 April 2009

31. 10 May 2010

35. 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

36. Downstream Supercell Environment

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43. 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

44. 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?