Warm-Season Elevated Thunderstorms with Heavy Rainfall: A Composite Study Dr. 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 WS Elevated 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 WS Elevated Thunderstorms 2 MCS centroid identified for each event –Initiation point –Point of most intense convection 11 x 11 grid defined wrt centroid –  x = 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 ( )

Elevated +TSRA Events

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 (red = median)

Stability/Moisture Field Correlations (red = median)

Conceptual Model of Elevated +TSRA

Low-Level Features Shaded orange: max  e advection Dashed lines = 925 hPa  e Dashed-X lines = 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 (~4  C) smaller positive SI (~1.4  C)

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 hPa –strong veering over lowest 100 hPa (SE  SW), 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 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, 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, 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,

Acknowledgments Mr. Thomas A. Niziol, NWSFO Buffalo Cooperative Institute for Precipitation Systems (CIPS), Saint Louis University