Mesoscale Features of the 31 January 2011 Oklahoma StormJennifer NewmanSchool of Meteorology, University of Oklahoma

Event OverviewFrom 31 January to 1 February 2011, regions of thunder-sleet, freezing rain, and snow moved through Oklahoma

>100 car accidents

Close to 500 motorists stranded

4 fatalities

Mesoscale FeaturesConvective activity persisted in Oklahoma, despite the fact that the atmosphere was extremely stable CSI?

Bands of snow formed in SW OK and moved throughOklahoma during the early morning of 1 February.Located 100 km apart Ducted gravity waves?

Beginning of Event: Overnight ConvectionStations in Oklahoma reported thundershowers, lightning, freezing rain, and ice pellets0300 UTC (10 pm LT) 0.5 reflectivity

Beginning of Event: Overnight Convection0300 UTC mesoanalysis indicated no MLCAPE in OklahomaPossible explanation: Conditional Symmetric Instability (CSI) MLCAPE (contoured) and CIN (shaded)

Background: CSI

Background: CSI

Evaluation for CSIVertical cross-section was taken using soundings oriented along a SW-NE line. Soundings used: 00Z and 12Z 1 February 2011

CSI Cross-section: 00ZPurple lines: eBlue lines: mg

CSI Cross-section: 00ZPurple lines: eBlue lines: mgCSI possible

CSI Cross-section: 12ZPurple lines: eBlue lines: mg

CSI: Brief Conclusions

Background: Gravity WavesWaves are created when a boundary between materials of different densities is perturbed

-Pressure oscillations-Shifts in wind speed and direction-Vertical motion can modulate or induce precipitation

Reference: Bosart, L. F. and F. Sanders, 1986: Mesoscale structure in the Megalopolitan Snowstorm, 1112 February 1983. Part III: A large-amplitude gravity wave. J. Atmos. Sci., 43, 924939.

Background: Ducted Gravity WavesIf a stable layer is located underneath a convectively unstable layer, gravity wave energy can be reflected and remain in the lower layer (Lindzen and Tung 1976)

Reference: Lindzen, R. S. and K.-K. Tung, 1976: Banded convective activity and ducted gravity waves. Mon. Wea. Rev., 104, 16021617.

Observations from the Oklahoma Mesonet

Mesonet consists of over 100 surface observation stations across the state of Oklahoma

Jointly operated by OU and Oklahoma State University

Reports surface variables (temperature, wind speed, pressure, etc. every 5 min

Mesonet Observations of Gravity WaveObserved at all stations enclosed in red circlesData from stations in blue circles shown on next slide

Mesonet Data: Pressure OscillationsPhase speed: 28.9 m s-1Wavelength: 200 kmSea-Level Pressure Time SeriesPressure drop of 2-3 mb in approx. 40 minutes

Ninnekah Mesonet StationSea-Level Pressure and 10-m Wind Time Series

KTLX Radar: 1200 UTC0.5 Reflectivity

KTLX Radar: 1230 UTC0.5 Reflectivity

KTLX Radar: 1300 UTC0.5 Reflectivity

KTLX Radar: 1330 UTC0.5 Reflectivity

KTLX Radar: 1400 UTC0.5 Reflectivity

Mesonet vs. Radar ObservationsLowest pressure occurred just ahead of precip. band (area of rising motion)

From conceptual model: Greatest rising motion should occur halfway between pressure ridge and trough

Possible that speed of precip. bands was different than phase speed of gravity wave

Wavelength of gravity wave based on pressure fluctuations was different than wavelength of precip. bands also suggests propagation speeds were different

Phase Speed CalculationsReference: Lindzen, R. S. and K.-K. Tung, 1976: Banded convective activity and ducted gravity waves. Mon. Wea. Rev., 104, 16021617.

Phase Speed Calculations

Phase Speed CalculationsStable layer Unstable layer

Phase Speed CalculationsStable layer Unstable layer

Summary and ConclusionsSeveral mesoscale features were evident from radar and satellite imagery during the Oklahoma storm of 31 January 1 February 2011

Based on observed soundings, CSI was possible at low levels over Norman at 00Z 1 February - There were likely other processes occurring (frontogenesis, vorticity advection) that enhanced convection- Difficult to isolate effects of CSI

Snow bands on 1 February appear to be associated with a gravity wave, which was observed at the surface in great detail

Theoretical gravity wave phase speed agrees well with timing of pressure oscillations

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