Warm-Season Elevated

Thunderstorms with Heavy Rainfall:

A Composite StudyDr. Scott M. Rochette

SUNY Brockport

Basis of Presentation

• Background Review• Methodology of Composite Study• Kinematic and Thermodynamic Fields• Stability and Moisture Fields• Vertical Profiles and Hodographs• Correlations• Conceptual Model• Summary

Background Review

Elevated Thunderstorms 1(Colman 1990)

• An elevated thunderstorm occurs above a frontal inversion

• Isolated from surface diabatic effects • Colman’s criteria

– observation must lie on the cold side of an analyzed front, showing a clear contrast in temperature, dew point, and wind

– station’s temperature, dew point, and wind must be qualitatively similar to immediately surrounding values

– surface air on warm side of analyzed front must have higher e than air on cold side

Elevated Thunderstorms 2(Colman 1990)

• Cold-sector MCSs generally fit Maddox frontal or meso-high type flash flood scenarios

• Elevated thunderstorms can occur during any time of year– usually associated with heavy rain/snow or

hail– nearly all winter-season thunderstorms over

the U.S. east of the Rockies (excluding Florida) are elevated

Elevated Thunderstorm Climatology 1

• Climatology of elevated thunderstorms reveals bimodal variation – primary maximum in April– secondary maximum in September

(Colman 1990)

Elevated Thunderstorm Climatology 2

(Colman 1990)

Elevated Thunderstorm Climatology 3

(Colman 1990)

Max-e CAPE

Use max-e CAPE when lifting is at/above frontal zone

(stable PBL)

Elevated Convective Instability 1

• Convectively stable PBL e increases w/height

– Convective environment insulated from local surface diabatic effects

• Convective instability above frontal zone e decreases w/height

– Vertical profile helpful for diagnosis

Elevated Convective Instability 2(Trier and Parsons 1993)

Methodology

Composite Study of WSElevated Thunderstorms 1• 21 Cases

– 35 Events– Some occurred over multiple time periods 4 in (24 h)-1 of rain over (100 km x 100 km) area

• Diagnostic fields computed for each event– Thermodynamic– Kinematic– Stability– Moisture– Pre-convective environment ( 4 h of 0000/1200

UTC)

Composite Study of WSElevated Thunderstorms 2• MCS centroid identified for each event

– Initiation point– Point of most intense convection

• 11 x 11 grid defined wrt centroid x = 190.5 km– Grid computed for each parameter/event

• Composite fields created by averaging objectively analyzed fields for individual parameters

• Storm-relative composites– Geography shows spatial orientation/relative magnitudes– Not meant to signify specific geographic location

Elevated Thunderstorm Distribution (1993-1998)

Elevated +TSRA Events 1993-1998

Table 1. List of Heavy Rainfall Events

Event # StartingTime/Date

EndingTime/Date

MaximumRainfall(inches)

Location ofMaximumRainfall

# of SynopticData TimesAnalyzed

1 12UTC 6/6/93 22UTC 6/6/93 6.0 Central MO 1

2 00UTC 6/17/93 12UTC 6/17/93 6.4 Southwest MN 2

3 00UTC 7/13/93 13UTC 7/13/93 7.0 Southeast SD 2

4 20UTC 9/21/93 15UTC 9/22/93 8.5 Northwest MO 2

5 05UTC 9/24/93 19UTC 9/24/93 5.5 Southwest MO 1

6 03UTC 9/25/93 11UTC 9/25/93 11.0 Southwest MO 2

7 21UTC 4/10/94 17UTC 4/11/94 10.65 Southwest MO 2

8 21UTC 4/27/94 17UTC 4/28/94 10.0 Southeast KS 2

9 03UTC 4/29/95 20UTC 4/29/95 5.7 South-cent. KS 2

10 09UTC 5/16/95 13UTC 5/16/95 4.0 East-cent. MO 1

11 12UTC 5/17/95 04UTC 5/18/95 5.9 Central MO 2

12 03UTC 4/28/96 08UTC 4/29/96 8.8 South-cent. IL 3

13 01UTC 5/7/96 08UTC 5/7/96 6.1 North-cent. MO 1

14 03UTC 5/8/96 17UTC 5/8/96 5.4 North-cent. MO 2

15 17UTC 5/14/96 05UTC 5/15/96 6.0 East-cent. MO 1

16 22UTC 7/16/96 15UTC 7/17/96 11.9 Western IA 2

17 22UTC 7/17/96 14UTC 7/18/96 16.9 Northeast IL 2

18 22UTC 7/20/96 11UTC 7/21/96 7.9 West-cent. MO 2

19 06UTC 8/19/97 13UTC 8/19/97 5.4 Southwest MO 1

20 07UTC 7/26/98 15UTC 7/26/98 7.9 South-cent. MO 1

21 07UTC 7/27/98 13UTC 7/27/98 5.5 South-cent. MO 1

MCS Centroid Locations

Kinematic and Thermodynamic Fields

Composite Surface Conditions

Composite 925-hPa h/T

Composite 925-hPa Winds

Composite 925-hPa e

Composite 925-hPa Moisture Convergence

Composite 850-hPa h/T

Composite 850-hPa Winds

Composite 850-hPa e

Composite 850-hPa -V•e

Composite 850-hPa -•(qV)

Composite 850-hPa w

Composite 850-hPa qV

Composite 850-hPa -V•T

925- & 850-hPa Proximity Frontogenesis

Composite 700-hPa Winds

Composite 700-hPa T

Composite 700-hPa -V•T

Composite 700-hPa -•(qV)

Composite 500-hPa Winds

Composite 500-hPa h/

Composite 250-hPa Winds

Composite 250-hPa •V

Stability and Moisture Fields

Composite Lifted Index

Composite Showalter Index

Composite Mean-Parcel CAPE

Composite Mean-Parcel CIN

Composite Max-e CAPE

Composite Max-e CIN

Composite Convective Instability (e850 - e500)

Composite K Index

Composite Precipitable Water

Composite Surface-500 hPa Mean RH

Vertical Profiles and Hodographs

Composite Active MCS Sounding

Composite Active MCS Hodograph

Composite Active MCS e Profile

Composite LL Inflow Sounding

Composite LL Inflow Hodograph

LLJ

14 m s-1

Composite LL Inflow e Profile

Correlations Between Individual Cases and

Composites

Kinematic Field Correlations

.34

.88 .84.87 .89.97

.46 .45 .45 .41

.71

.52

(red = median)

Stability/Moisture Field Correlations

.87.77 .81 .84 .87

.58

(red = median)

Conceptual Model of Elevated +TSRA

Low-Level Features

Shaded orange: max 925-850 e advection

Dashed lines = 925 hPa e

Dashed-X lines = 925-850 hPa MCON

Green arrow = low-level jet (LLJ)

Circled X = active MCS site

Mid/Upper-Level Features

Solid lines = 500 hPa heights

Dashed lines = 250 hPa isotachs

Stippled area = surface-500 hPa mean relative humidity > 70%

Green arrow = 700 hPa jet

Circled X = active MCS site

Cross-Sectional View

Summary

Summary 1

• Elevated +TSRA tend to form:– ~160 km north of surface frontal

boundary– within east-west zone of 925-hPa

moisture convergence– ~400 km downstream of 850-hPa LLJ

– on cool side of strong LL e gradient

– within maxima of 850-hPa e advection and moisture convergence

Summary 2

• Elevated +TSRA tend to form:– along inflection point in 500-hPa

height field (~800 km downstream of weak S/W)

– underneath entrance region of ULJ, southwest of maximum divergence

– Above stable boundary layer• positive LI (~4C)• smaller positive SI (~1.4C)

Summary 3

• Elevated +TSRA tend to form:– in regions of positive max-e CAPE

(~1250 J kg-1)

– in regions of modest max-e CIN (<40 J kg-1)

– in regions of significant low-mid tropospheric moisture• Mean RH > 70%• PW > 1.2 in

Summary 4

• Composite Active MCS Region Characteristics– layer of conditional instability above

very stable boundary layer – convectively unstable from 800-650

hPa – strong veering over lowest 100 hPa

(SESW), modest shear aloft– clockwise-turning hodograph with

modest winds

Summary 5

• Composite LL Inflow Region Characteristics– drier, less stable boundary layer – higher CAPE values (well over 1000 J kg-1) – strong convective instability from 950-600

hPa (15 K decrease in e)

– modest veering over lowest 100 hPa, but strong speed shear

– modest clockwise turning on hodograph, max wind at 850 hPa

Cross-Sectional View

• SSW low-level jet transports high-e air northward over frontal zone

• SW mid-tropospheric flow transports lower-e air above warm moist air (creates CU layer)

• DTC associated with LL frontogenesis interacts constructively with DTC associated with ULJ’s entrance region (large-scale UVM)

• LL moisture convergence in LLJ’s exit region helps to initiate deep convection

• LLJ’s normal orientation to frontal boundary promotes cell training/high rainfall totals

Composite ‘Robustness’

• Computation of correlation coefficients between parameter fields for individual times and composite– strong correlations for basic fields

• thermodynamic• moisture• stability

– weaker correlations for derived fields• divergence/convergence• advection

Composite Caveats 1

• Composites developed for central US during warm season– apply during other times of year?– apply for other regions?– answer: a qualified maybe?

• Convection modifies its environment– rationale for selecting inflow points

and active MCS regions

Composite Caveats 2

• Smoothing of fields– Barnes objective analysis– composite = average– nevertheless, correlations indicate reliable results– Pay more attention to patterns, less to

magnitudes

• Elevated +TSRA are sneaky– form in ‘unfavorable’ environments– pay attention to cool sectors– look out for elevated convective instability

References

• Colman, B. R., 1990: Thunderstorms above frontal surfaces in environments without positive CAPE. Mon. Wea. Rev., 118, 1103-1121.

• Moore, J. T., F. H. Glass, C. E. Graves, S. M. Rochette, and M. J. Singer, 2003: The environment of warm-season elevated thunderstorms associated with heavy rainfall over the Central United States. Wea. Forecasting, 18, 861-878.

• Trier, S. B., and D. B. Parsons, 1993: Evolution of environmental conditions preceding the development of a nocturnal mesoscale convective complex. Mon. Wea. Rev., 121, 1078-1098.

Acknowledgments

• Mr. Thomas A. Niziol, NWSFO Buffalo

• Cooperative Institute for Precipitation Systems (CIPS), Saint Louis University