Brief summary of low-level buoyancy parameters and research relating to tornadoes

VORTEX and sub-VORTEX cases studied by Paul Markowski show that rear flank downdraft (RFD) parcels entering tornadic circulations have less surface-based CIN and more surface-based CAPE.   Some of these low-level thermodynamic characteristics are probably detectable in the broader "background" environment for many cases, although to what degree is unclear.  Markowski found that smaller surface dew point depressions in storm inflow, approximating lower LCL heights and cloud bases and indicating more humidity in low-levels, were a reasonable indicator of  favorable RFD characteristics.  In my own investigations using nearby model profiles modified by inflow air mass surface observations, I've also found that CIN tends to be less and LFC heights lower in tornadic supercell cases, along with some degree of CAPE present below 3 km.

Box and whisker diagrams for CIN, LFC height, and low-level CAPE from a database of more than 300 RUC-2 profiles (modified by actual surface observations) associated with supercells are shown below.  (Important Note: 146 of the profiles came from the RUC-2 database of profiles associated with supercells put together by Rich Thompson and Roger Edwards at SPC, who I gratefully acknowledge for their help.)   In general, there is some separation between the boxes representing the middle 50% of the datasets in each category (nontornadic vs. significant tornadic), suggesting that these parameters may add useful information in attempting to discriminate between tornadic and nontornadic environments:

< CIN distributions (RUC-2)       < LFC height distributions (RUC-2)      < 0-3 km CAPE distributions (RUC-2) 

Below are similar diagrams of these parameters using a larger database of nearly 1000 FD (forecast diagnostic) profiles associated with supercells.  The FD profiles are based on the NGM model, limited to surface parcels, and coarser in time and vertical resolution than the RUC-2 profiles, so the resulting averaging tends to under-represent CIN and inflate low-level CAPE.  But, nevertheless, similar signals are present:

<CIN (FD)      < LFC height (FD)      < 0-3 km CAPE (FD)  

In Rasmussen and Blanchard's 1992 baseline climotology, there was also a tendency for CIN to be less in observed soundings associated with significant supercell tornado cases.  Large CIN based on surface or near-surface parcels implies a significant layer of negative buoyancy and stability near the ground.  Storms occurring with large CIN distributed though a deep layer are probably "elevated" in nature and considered less likely to produce tornadoes because primary inflow would be from well above the stable boundary layer.

Lower LFC heights imply larger amounts of low-level CAPE and possibly increased potential for low-level parcel accelerations favorable for tornadoes, while higher LFC heights imply more CIN and a deeper low-level stable layer less conducive to low-level mesocyclone intensification and associated tornadoes.

Depictions of CIN and LFC height are widely available with skewT logp software plots and estimated parameter fields (e.g., SPC's mesoanalysis page), while 0-3 km CAPE is not widely computed.  At any rate, in addition to assessing surface dew point depression or LCL height (from Markowski's work), assessment of CIN and LFC height (and low-level CAPE when available) may help in detecting environments that are "elevated" enough to limit significant supercell tornado potential (see elevated environment examples).

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