Thesis

Slide1 : Case Studies of Midwestern Thundersnow Events Christopher E. Halcomb To satisfy the degree requirements for Master of Science in Atmospheric Science University of Missouri-Columbia November 13, 2001


Introduction : Introduction Thundersnow (TS) characteristics Typically occurs in meso- bands Form of elevated convection Moist, stable boundary layer (Curran & Pearson 1971) Low CAPE values (< 200 J kg-1) Duration of TS observations are often short However, sustained TS  heavy snow totals Thanksgiving Week 2000 – Buffalo, NY 19-20 January 1995 – Columbia, MO 25 February 1979 – Cape Girardeau, MO


Introduction (cont…) : Introduction (cont…) In theory, byproduct of … Upright Instability - Positive MUCAPE (Moore et al. 1998) Gravitational instability  decreases with height Convective instability (CI) e decreases with height Conditional symmetric instability (CSI) In moist region (RH  80%) e surfaces more upright than Mg surfaces (PSI) es surfaces more upright than Mg surfaces (CSI) Negative equivalent potential vorticity (EPV) in an otherwise stable environment


Purpose, Objectives, and Statements of Thesis : Purpose, Objectives, and Statements of Thesis Purpose To determine when and where TS occurs most often To examine thermodynamic and dynamic characteristics of individual TS events Main objective To improve the accuracy of TS forecasting Statements of Thesis TS normally occurs in two locations NW of surface cyclone Due to CI in cyclonically-sheared environment NE of surface cyclone Due to CSI in anticyclonically-sheared environment


Methodology for Climatology : Methodology for Climatology Based on CD-ROM of surface data (SA) from 1961-1990 Reports of TS extracted with C program Characterized by Location State Actual count Normalized according to the land area of the average state Region Relative to cyclone center (Digital Atmosphere) Distance Direction Time of occurrence Month Time of day Intensity, TMPF, DWPF, SLP, winds Synoptic setting Digital Atmosphere Assoc. w/ cyclone Lake-effect Upslope Orographic Offshore cyclone Undetermined cyclone location Misc.


What is an event? : What is an event? A single observation of TS If more that one for a given time frame No more than 6 hours between multiple reports at a given station No more than 6 hours between reports at multiple, ‘nearby’ stations Distance between stations observing TS not more than 1100 km Halfway point of meso- scale To be associated as Type 1 event Within cloud shield/cyclone flow around cyclone Not an isolated precipitation report Clearly identifiable cyclone center


Methodology for Case Studies : Methodology for Case Studies 4 Cases examined 5 December 1999 – Wichita, KS 9 December 1999 – Lubbock, TX 11 March 2000 – St. Louis, MO 19 April 2000 – Ellsworth AFB, SD Analyses performed using GEMPAK (PCGRIDDS used for 5 December event) Surface analyses (METAR) Mandatory level analyses (RUC initial fields) Dynamics & Thermodynamics (RUC initial fields)


Methodology for Case Studies(cont…) : Methodology for Case Studies (cont…) Thermodynamics (stability) Cross section overlays of e, Mg, and RH (Moore & Lambert 1993) Alternative method also compared (Schultz & Schumacher 1999) 3-D EPV (McCann 1995) Mg not part of equation Any cross section line can be used Dynamics (forcing) Q vectors S-component to assess synoptic-scale forcing N-component to assess frontal-scale forcing Storm-relative moisture transport Isentropic maps Tropopause/PV maps Petterssen surface frontogenesis -  cross sections


Results of Climatology : Results of Climatology 375 TS events discovered for the period of 1961-1990 Preferences Regional Basin and Range Central Plains Mid-Atlantic Region to New England Temporal March and April peak Jet stream interaction Afternoon and evening hours No reason identified Classification Most TS associated with cyclone NW/NE quadrants 360 km from center Some Type 1 events may be lake-enhanced Great Lake and Great Salt Lake both contribute to Type 2


5 December 1999 : 5 December 1999 TS observed at KIAB, KICT at 0600 UTC Cloud-to-ground lightning strikes Bands of heavy precip indicated on radar imagery Synoptic setting West of surface cyclone (1009 hPa) Slight east-west tilt with height Cyclonic flow regime Under “strong” divergence axis Occluded characteristics aloft Tropopause/PV anomaly to the south


Slide25 : 300-hPa Heights and Isotachs


Slide26 : 300-hPa Divergence


Slide29 : Convergence of Qn Dashed areas = Convergence


Slide30 : Convergence of Qs


5 December 1999 Conclusions : 5 December 1999 Conclusions Upright instability present Inferred from 850-700-hPa LI <0 Released lower to middle level QG frontogenesis 700 hPa trowal co-located with precip. Upper tropospheric influences Trop/PV anomaly decreased static stability Qs field suggests presence of ducted gravity wave Divergence axis at 300 hPa


9 December 1999 : 9 December 1999 TSSN & TSIP observed at KLBB 90-minute duration 7 inches of snow/sleet in 6 hours Banded precipitation structure on radar Synoptic setting Northwest of surface cyclone (1009 hPa) Cyclonic flow regime and west-to east tilt Occluded structure aloft Tropopause/PV anomaly to south


Slide34 : 19991209 0900 UTC


9 December 1999Conclusions : 9 December 1999 Conclusions TS occurred as convective instability was released by 750 hPa frontogenesis over LBB in trowal airstream Additional vertical enhancement by nearby trop/PV anomaly and gravity wave Little difference in EPV3 plots


11 March 2000 : 11 March 2000 TS observed at KSTL and much of metro area Unconfirmed reports as far south as KCGI Cloud-to-ground lightning strikes observed Banded precipitation structure Northwest of open wave surface cyclone that was occluded aloft Nearly zonal flow regime above 500 hPa ahead of short wave Slight west-east tilt (nearly stacked) Tropopause/PV anomaly over N. Arkansas


11 March 2000Conclusions : 11 March 2000 Conclusions TS occurred in the presence of weak symmetric stability due to 700 hPa frontogenesis within the trowal However, there could be unresolved CSI Prominent divergence axis over E. MO Trop/PV anomaly and ducted gravity wave further enhanced UVV EPV3 plots very similar


19 April 2000 : 19 April 2000 TS reported at KRCA Up to 30” (75 cm) acc. in Black Hills Precip/snowfall records established NW of intense, occluded sfc cyclone System stacked at the lower levels Cyclonic shear Temperature gradient lessened 550 hPa Trop/PV anomaly to southeast


19 April 2000Conclusions : 19 April 2000 Conclusions TS occurred in region of WSS due to 700 hPa frontogenesis in strong easterly trowal airstream Orographic influences possible Prominent divergence axis at 300 hPa Trop/PV/gravity wave influences Enhanced upward motion Decreased static stability Differences observed between EPV3 fields, but diagnosis at KRCA not affected


Case Studies Overall Impressions : Case Studies Overall Impressions Each event in NW sector of cyclone Trowal airstream/frontogenesis are most important forcing mechanisms Q fields show that these mechanisms occur as a result of synoptic-scale forcing (Qs > Qn) Effects of trop/PV anomalies Enhance UVV & decrease static stability Create trowal as flow becomes more meridional in attempt to conserve PV (Hoskins et al. 1985) Gravity waves may also act to increase UVV Stability may be less important that forcing and vertical velocities in production of TS (Schultz 2000)


Evaluation of the Statement of Thesis : Evaluation of the Statement of Thesis Statements of Thesis TS normally occurs in two locations NW of surface cyclone Due to CI in cyclonically-sheared environment NE of surface cyclone Due to CSI in anticyclonically-sheared environment


Evaluation of the Statement of Thesis (cont…) : Evaluation of the Statement of Thesis (cont…) Evaluation None of the known TS events that season were NE of sfc cyclone  couldn’t test 2 of 4 events in NW quadrant exhibited upright instability 2 exhibited weak symmetric stability When a system becomes stacked Vertical shear/lapse rates decrease Stability increases Therefore, fine line between unstable and symmetrically stable environments NW quadrant hypothesis partially true, although oversimplified