Post on 18-Dec-2015
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
*
*
*
*
*
*
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?