environmental conditions associated with cool season significant tornadoes over the north central...
TRANSCRIPT
Environmental Conditions Associated with Cool Season
Significant Tornadoes over the North Central United States
Mark F. Britt and Fred H. Glass
National Weather ServiceSt. Louis, MO
Hey Mark, It’s Winter! Why are you talking about tornadoes?
Based on the LSX dataset, 43% of all tornadoes are >F2 during the cool season compared to 26% throughout the year.
Produced by Jonathan Finch, WFO DDC, http://bangladeshtornadoes.org/UScasesSTL.html
Why Am I Here?
To increase situational awareness and increase knowledge of parameters of winter tornado events by examining past event’s:
Temporal and spatial distribution.
Thermodynamic and shear parameters.
Synoptic scale characteristics.
The Database
Was compiled using the SPC Storm Events Database and Storm Data.
“Cool Season” is defined as November 16th – end of February.
Covers the years from 1979 to 2005.
A total of 43 strong and violent tornadoes were identified over 18 separate tornado days.
Plotted using SeverePlot v2.0, 1999, Hart & Janish, NWS/NCEP/SPC
Area Studied
Environmental parameters were calculated using the tornadoes in RED.
Environmental DataSounding data was extracted from North American Regional Reanalysis (NARR) using NSHARP.
Every 50mb from the lowest model level to 100mbThe NARR is a long-term, consistent, high-resolution climate dataset for the North American domain, as a major improvement upon the earlier global reanalysis datasets in both resolution and accuracy (Mesinger et al., 2006)
Uses the old 32km Eta Model with the Regional Data Assimilation System (RDAS) 29 Vertical LevelsAnalysis every three hours (eight times a day)
Environmental Data
Thermodynamic and kinematic parameters currently deemed important to severe storm and tornadic environments were computed for each sounding. A composite sounding and hodograph were generated using the means of temperature, dewpoint, wind speed and wind direction from each level.
Composites
8 7
The Skew-T depicts a sounding with a relatively moist lower troposphere, small CAPE, and a low equilibrium level. Most of the instability is below 500mb.
The hodograph shows some veering of the winds from south to southwesterly in the lowest kilometer. Above that, winds are generally southwesterly, unidirectional, and increase in speed with height.
Thermodynamic Parameters
90th Percentile
75th Percentile
Median
25th Percentile
10th Percentile
Median from Thompson et
al. (2003)
8 7
2152 J/kg
# of Cases
Thermodynamic Parameters
16 16
Thermodynamic Parameters
7 9
Thermodynamic Parameters
18
Thermodynamic ParametersLittle convective inhibition in these environments with median values to ML=17 J/kg and MU=6 J/kg.Middle level (500-700mb) lapse rates were in a narrow range between 6.0 and 6.6oC/km.
Kinematic Parameters
15 12 12
(m/s)
Kinematic Parameters
14
Bunker’s RM Storm Motion = 237o/47kts.
Mean CompositesImages provided by the NOAA-CIRES Climate Diagnostics Center, Boulder Colorado from their Web site at http://www.cdc.noaa.govComposites of three-hourly mean and daily mean anomalies for sixteen of the cases were generated from the NARR dataset.
300 mb Mean Zonal Winds
T-24 T-12
Tornado Time
Anomaly
500 mb Mean Heights
T-24 T-12
Tornado Time
Anomaly
700 mb Mean Omega
T-24 T-12
Tornado Time
Anomaly
850 mb Mean Meridional Wind
T-24 T-12
Tornado Time
Anomaly
850 mb Mean Temperature
T-24 T-12
Tornado Time
Anomaly
850 mb Mean Specific Humidity
T-24 T-12
Tornado Time
Anomaly
1000 mb Mean Heights
T-24 T-12
Tornado Time
Anomaly
Climatology SummaryA larger percentage of tornadoes that occur
in the cool season are strong or violent compared to the year as a whole.
Strong and violent tornadoes can occur at anytime during the winter months, including during the late evening and overnight hours.
Sounding SummaryWhen comparing these cases to the significant
tornadic supercells in the Thompson et al dataset.
The MLCAPE in these cases are significantly lower.
The 0-1km and 0-6km bulk shear is noticeably higher, and the 0-1km SRH is similar.
The lower ML LCLs imply a relatively high amount moisture in the sub cloud layer.
The 700-500mb lapse rates occur in a narrow range between the 6.0-6.6oC/km.
All of these cases have their CAPE concentrated below 500mb.
Bunker’s Supercell Motion (2000) is 240o at 24 m/s.
Composite SummaryAll values were anomalously high for the time of year.
These cases occurred with strong large scale forcing (upward omega) caused by coupled jet structure, relatively deep trough, and low-mid level warm air advection. This likely offsets the weak thermodynamics.
The wind fields for these cases are quite strong which contributes to the observed enhanced vertical shear. The mean zonal wind speeds associated with jets streaks is 30 m/s at 500 mb and 42 m/s at 300 mb.
Composite SummaryA strong low level jet is responsible of for
significant lower tropospheric moisture transport and warm thermal advection.
The mean 1000 mb height field suggests that surface lows move northeastward from the Central Plains into the Great Lakes.
The low-level means of temperature and specific humidity are indicative of each of the individual cases; which possess a sharply baroclinic, low level boundary.
ReferencesMatthew J. Bunkers, Brian A. Klimowski, Jon W. Zeitler, Richard L. Thompson and Morris L. Weisman. 2000: Predicting Supercell Motion Using a New Hodograph Technique. Weather and Forecasting: Vol. 15, No. 1, pp. 61–79.
Mesinger, Fedor, DiMego, Geoff, Kalnay, Eugenia, Mitchell, Kenneth, Shafran, Perry C., Ebisuzaki, Wesley, Jovi, Dušan, Woollen, Jack, Rogers, Eric, Berbery, Ernesto H., Ek, Michael B., Fan, Yun, Grumbine, Robert, Higgins, Wayne, Li, Hong, Lin, Ying, Manikin, Geoff, Parrish, David, Shi, Wei, 2006: North American Regional Reanalysis Bulletin of the American Meteorological Society, 87, 343-360
Thompson R.L., Edwards, J. A. Hart, K. L. Elmore, and P. M. Markowski, 2003: Close proximity soundings within supercell environments obtained from the Rapid Update Cycle. Wea. Forecasting, 18, 1243-1261.