Observations of the Surface Boundary Structure within the

May 23rd Perryton, TX Supercell

Patrick S. Skinner

Dr. Christopher C. Weiss

Texas Tech University

Motivation

“This forward flank downdraft forms a relatively weak discontinuity at the surface on the forward and right flanks of the radar echo”

– Lemon and Doswell (1979)

Definition

• Forward-Flank Reflectivity Gradient (FFRG)

Numerical Modeling Studies

• Forward Flank Gust Front (FFGF) defined

by -1 K perturbation isotherm of θ or θv

Numerical Modeling Studies

• Many studies identify baroclinic generation of horizontal streamwise vorticity across the FFGF as an important source of rotation in low-level mesocyclogenesis – Klemp and Rotunno (1983)– Rotunno and Klemp (1985)– Wicker and Wilhelmson (1995)– Adlerman et al. (1999)

• Most simulations produce strong thermodynamic gradients across the FFGF

• If a wind shift is detected it often extends roughly northwards from low-level mesocyclone

Observations of the FFD• Shabbott and Markowski (2006) analyzed data collected within

the FFD of 12 supercells in Project VORTEX and subsequent campaigns– Tornadogenesis was more likely in storms exhibiting modest deficits of

θv and θe across the FFGF

– Storms with the strongest potential for baroclinic generation of horizontal vorticity had the weakest low-level mesocyclones

Markowski et al. 2003• Warm axisymmetric downdraft:

– Enhanced convergence– Stronger low-level vertical acceleration– Stronger rotation at the surface

Dowell and Bluestein 2002

• Different origin of parcel trajectories at different times in tornado lifecycle

• Parcels can originate near the surface within the inflow or from within the FFRG

• No discernable wind shift across FFRG

Beck and Weiss 2008

• Simulations produce modest deficits of θv (left) and no change in θe (right) across FFRG

MOBILE-07

• Objectives:– Add to dataset of Shabbott and Markowski (2006)– Are cohesive kinematic and thermodynamic

gradients present across the FFRG?

• 22 durable, unmanned, rapidly deployable probes available for MOBILE-07– Two probe designs

utilized• 4 probe mobile mesonet

StickNet

May 23rd, 2007 Perryton, TX Supercell Timeline

• Initial storm (Storm A) initiated along a warm front in Canadian River Valley shortly after 21:00 UTC

• Deployment on Storm A was delayed due to lack of a suitable road network

• Shortly after probe deployment initiated on Storm A, convection (Storm B) developed along the southwestern flank of Storm A and became dominant

• Deployment focus shifted to Storm B at approximately 22:00 UTC

• 20 StickNet probes and 4 probe mobile mesonet observed Storm B as it crossed Hwy 83

Probe Coverage

22:39 UTC

22:50 UTC

Time Series Analysis

Time Series Analysis

Time Series Analysis

Time Series Analysis

Time Series Analysis

Time-to-Space Conversion and Objective Analysis

• Assumption:– Storm is steady-state for 10 minutes

• Reality:– It is not.

• Utilized in tandem to provide 2-D visualization of surface conditions within storm

Time-to-Space Conversion and Objective Analysis

• Broad region of positive θe perturbations apparent across FFRG

• Modest deficits of θv observed across FFRG

• No distinct kinematic shift across FFRG• Development of secondary RFGF apparent over

probes 16 and 19

Comparison with FFRG Observations in Other MOBILE-07 Cases

• 4 of 5 cases analyzed exhibit a surplus of θe greater than 1 K across the FFRG

• All 5 cases analyzed exhibit a small deficit of θv across the FFRG*

Virtual Potential Temperature

• Representative of parcel buoyancy

• Significant baroclinic generation of vorticity (10-2 s-1) can occur across modest buoyancy gradients given a long parcel residence time– Dowell and Bluestein (2002)– Shabbott and Markowski (2006)

• Southward-directed gradients of θv can only be strengthened by including the effects of liquid water mixing ratio in θv

Equivalent Potential Temperature

• Most commonly used as a parcel tracer– Best utilized as the height a parcel would originate

on an inflow sounding (Markowski et al. (2002)

• Can also be used as a measure of potential instability– Conserved for dry and moist adiabatic processes– Representative of a specific moist adiabat on a

skew T/log P diagram

– An increase in θe is analogous to a rightward shift in parcel trajectory

Potential Ramifications - Caveat

• Without parcel trajectories, it is impossible to know if the opposing thermodynamic perturbations observed in MOBILE-07 were being ingested by the low-level mesocyclone

Potential Ramifications• An Increase in θe coupled with a decrease in θv in air parcels

entering the low-level updraft would result in:– Ingestion of baroclinically generated streamwise horizontal

vorticity– An increase in surface-based CAPE– A potential decrease in surface-based CIN– A lower LFC

• All of these would result in enhanced low-level vorticity

Origin of FFRG Air

• θe is strongly coupled with mixing ratio

• θv is strongly coupled with temperature

Origin of FFRG Air

• In the absence of heterogeneity within the storm inflow, positive perturbations of θe from the base state cannot exist without diabatic effects

• Diabatic cooling and moistening of the air would result in opposing perturbations of θv and θe

Origin of FFRG Air

• Evaporation of hydrometeors falling into the inflow would provide both cooling and moistening

• It is therefore possible that the thermodynamic boundary across the FFRG is not a discontinuity between inflow and FFD outflow, but rather a transition from inflow air to storm-modified inflow air– The lack of wind shift across the FFRG supports

this hypothesis– Additionally, this possibility has been noted in the

simulations by Beck and Weiss (2008) and in Beck et al. (2006)

One More Caveat

• Opposing gradients only offer the potential for enhancing low-level vertical vorticity

Conclusions

• A thermodynamic boundary consisting of negative perturbations of θv and positive perturbation values of θe can exist across the FFRG

– This boundary is a potential source of low-level baroclinically-generated horizontal streamwise vorticity AND greater potential instability

– Air across this boundary is more representative of near-surface inflow air modified by evaporational cooling than outflow air originating aloft

Future Work

• Expand the dataset to allow statistical analysis of the prevalence of opposing thermodynamic gradients across the FFRG

• Determine parcel trajectories of air entering the low-level mesocyclone using dual-Doppler analysis or numerical modeling

• Observe thermodynamic characteristics of air above the surface across the FFRG to fully determine potential effect on parcel instability

ReferencesAdlerman, E. J., K. K. Droegemeier, and R. Davies-Jones, 1999: A numerical simulation of cyclic mesocyclogenesis. J.

Atmos. Sci, 56, 2045-2069.

Beck, J. R. and C. C. Weiss, 2008: The effects of thermodynamic variability of low-level baroclinity and vorticity within numerically simulated supercell thunderstorms. Preprints 24th Conf. on Severe Local Storms, Amer. Meteor. Soc., Savannah, GA, 15.4.

Beck, J. R., J.L. Schroeder, and J. M. Wurman, 2006: High-resolution dual-Doppler analysis of the 29 May 2001 Kress, Texas, cyclic supercell. Mon. Wea. Rev., 134, 3125-3148.

Dowell, D. C. and H. B. Bluestein, 2002: The 8 June 1995 McLean, Texas, storm: Part II: Cyclic tornado formation, maintenance, and dissipation. Mon. Wea. Rev., 130, 2649-2670.

Klemp, J. B. and R. Rotunno, 1983: A study of the tornadic region within a supercell thunderstorm. J. Atmos. Sci., 40, 359-377.

Lemon, L. R. and C. A. Doswell, 1979: Severe thunderstorm evolution and mesocyclone structure as related to tornadogenesis. Mon. Wea. Rev., 107, 1184-1197.

Markowski, P. M., J. M. Straka, and E. N. Rasmussen, 2002: Direct surface thermodynamic measurements within the rear-flank downdrafts of nontornadic and tornadic supercells. Mon. Wea. Rev., 130, 1692-1721.

____ 2003: Tornadogenesis resulting from the transport of circulation by a downdraft: Idealized numerical simulations. J. Atmos. Sci., 60, 795-823.

Rotunno, R. and J. B. Klemp, 1985: On the rotation and propagation of simulated supercell thunderstorms. J. Atmos. Sci., 42, 291-292.

Shabbott, C. J. and P. M. Markowski, 2006: Surface in situ observations within the outflow of forward-flank downdrafts of supercell thunderstorms. Mon. Wea. Rev., 134, 1422-1441.

Wicker, L. J. and R. B. Wilhelmson, 1995: Simulation and analysis of tornado development and decay within a three-dimensional supercell thunderstorm. J. Atmos. Sci., 52, 2675-2703.

Conclusions

• A thermodynamic boundary consisting of negative perturbations of θv and positive perturbation values of θe can exist across the FFRG

– This boundary is a potential source of low-level baroclinically-generated horizontal streamwise vorticity AND greater potential instability

– Air across this boundary is more representative of near-surface inflow air modified by evaporational cooling than outflow air originating aloft