COMET® TeletrainingForecasting Severe Thunderstorms

Version 1.0

Dr. Douglas Wesley UCAR/COMET

Ms. Wendy Schreiber-Abshire UCAR/COMET

Tuesday, 9 June 1998

© 1998 University Corporation for Atmospheric Research. All Rights Reserved

1

Forecasting Severe Thunderstorms

Main Menu

• Introduction and Objectives (Page 3)

• Section 1: Assessing Environmental Stability (Page 4)

• Section 2: Assessing Environmental Shear (Page 15)

• Section 3: Forecasting Supercells (Page 25)

• Section 4: Forecasting Mesoscale Convective Systems

(Page 30)

• Summary, Quiz, and Evaluation (Page 35)

2

Introduction and Overall Objectives of the Teletraining Session

• Describe the roles of environmental stability and wind

shear in the development of severe thunderstorms

• Describe important enviromental factors contributing to

the development of rotating updrafts in thunderstorms

• Distinguish between the different storm types and

severe weather associated with them

• Describe the formation and typical environments for

supercells

3

Introduction (cont.)

Emphasis of this teletraining session will be on these basic convective

processes: buoyancy, gust front, and dynamic interactions

Section 1: Assessing Environmental Stability

By the end of this section, you should be able to

• Identify important buoyancy processes that contribute

to updraft strength

• Describe the impact on thunderstorm potential for

buoyancy processes

• Describe the role of surface heating in enhancing

buoyant energy

5

Moore: 3 Lapse rates

Moore: pos/neg areas

Skwtcape.flc

Moore p. 8: 3 soundings; no instab., neutral, unstable

Cincape3.flc

CAPE/LI values and associateddegree of instability

CAPE LI(J/kg) (500 mb)

Stable: 0 >0marginally unstable <1000 0 to -3moderately unstable 1000-2500 -3 to -6very unstable 2500-3500 -6 to -9extremely unstable >3500 <-9

Moore p. 4: Conditional instability; lifting a layer

Moore p. 30: dry microburst sounding

Moore p. 36: wet microburst sounding

Moore p. 37: effect of elevated terrain on sounding

Non-supercell rotating updraft conceptual model

Summary: Assessing Environmental Stability

• Neutral, stable and unstable lapse rates

• Definitions of CAPE/CIN

• Modification of soundings by heating, terrain, layer lift

• Dry and wet microbursts, non-supercell rotation

Section 2: Assessing Environmental Shear

By the end of this section, you should be able to

• Identify important wind shear processes that affect

thunderstorm evolution

• Describe the role of directional shear in the evolution of

strong convective cells

18

The Role of Wind Shear

• Get bullets from Anticipating (Wendy?)

• see Moore p. 58

Moore p. 43: on synoptic scale, shear in various zones around fronts

Hodott02.flc

Hodott03.flc

Johnspat.gif

Importance of pre-severe convective storm hodograph

• Strong backing of winds with height in the lower troposphere favors development of left-moving anticyclonically rotating storms

• strong veering of winds with height in the lower troposphere favors development of right-moving cyclonically rotating storms

• wind profile with shear but little/no veering

Hodograph (cont.)

• or backing favors splitting storms where the right-moving cyclonically rotating cell is usually more severe

• warm air advection or low-level jets can increase clockwise turning

2dsqldev.gif

Note: need the flc here

Tiltvort.flc

Splitsqa.flc

Summary: Assessing Environmental Shear

32

Section 3: Forecasting Supercells

By the end of this section, you should be able to

• Identify important buoyancy processes that contribute

to supercell formation

• Describe the impact of wind shear on supercell

potential

33

Hodographs and Supercells

• Tornado proximity soundings show considerable clockwise turning in the lowest 1.5 km AGL with little curvature above that; speeds increasing with height, esp. in the first few km

• wind speeds (both ground and storm-relative) and rightward propagation increase significantly with tornado intensity

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(The notes page for this slide contains the first set of definitions to be completed by the learners as part of the exercise.)

36

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(The notes page for this slide contains the second set of definitions to be completed by the learners as part of the exercise.)

37

Frequency of Severity Reports

38

Exercise 2 Map

39

Summary: Icing Type and Severity

• Icing can be categorized into rime, clear, mixed and SLD types

• In general, rime icing tends to occur with T <-15° C, clear

when T > -10° C, and mixed ice at temperatures in between.

However, type varies depending on LWC, droplet size, and

aircraft-specific variables

• Severity is categorized by the rate of accumulation, the

effectiveness of available de-icing equipment, and the actions a

pilot must take to avoid or combat icing

• Algorithms and numerical model output have been developed

to correlate values of LWC, temperature, and droplet size with

icing type and severity. These algorithms and model output are

in the early stages of development and need further testing40

Section 4: Supercooled Large Drops (SLD)

By the end of this section you should be able to

• Describe two atmospheric processes and environments

that support SLD formation

• Describe cloud-top conditions most favorable to SLD

formation

• Identify favorable areas and layers for SLD formation

using skew-T diagrams, wind profiles, surface

precipitation observations, and 3.9 micron infrared

satellite imagery

41

Schematics Of Airflow Around An Iced Airfoil In An SLD Situation

42

Quiz and Evaluation

At this time, please complete the separate quiz. You may do so

anonymously. Also, please complete the evaluation form for

this teletraining session. This information will help us to

improve future sessions.

Please give your completed quiz to your site facilitator, who in

turn should mail them to:

Post: or FedEx:Dwight Owens Dwight OwensUCAR/COMET UCAR/COMETP.O. Box 3000 3450 Mitchell

Lane Boulder, CO 80307-3000 Boulder, CO

80301(303) 497-8469

43

Forecasting Aircraft Icing:Acknowledgements

Graphics from

• American Meteorological Society (AMS) modified from– Bernstein (1997)

– Kane, et al. (1998)

– Martner, et al. (1993)

– Politovich, Stankov, and Martner (1996)

– Rasmussen, et al. (1993)

– Sand, et al. (1984)

– Schultz & Politovich (1992)

– Willoughby and Protrowicz (1984)

• Additional images from– NASA-Lewis Research Center

– NOAA/NWS

– Omeron-Bernstein (1997) unpublished44