cp-25 elevated thunderstorms impacts on surface...

CP-25 Elevated Thunderstorms Impacts on Surface Frontal Boundaries During

PRECIP

Josh Kastman and Patrick S. Market

Department of Soil, Environmental, and Atmospheric Sciences

University of Missouri

Introduction and BackgroundElevated Convection, or thunderstorms that form over a cool stable layer, have been observed to alter frontal propagation during the

Program for Research on Elevated Convection with Intense Precipitation (PRECIP) field campaigns. This phenomena has lead to frontal

stalling, or moving into unanticipated locations which can result in heavy rainfall in unprepared areas. Currently, operational models

struggle to recognize influences on synoptic/mesoscale boundaries by elevated MCSs. Short term, high resolution model forecast, from a

customized Weather Research and Forecast model (WRF) will be compared to RAP initial fields, which will be treated as analysis, and to

GFS forecasts. The GFS model that is used is the 80 km version. This model is shown for comparison because it was one of the models

PRECIP forecasters used to make determinations on field deployments. Other models, including the NAM, SREF and FIM, were also

used in the project, but for space considerations only the GFS is shown.

AcknowledgementsThis work is supported in part by the National Science Foundation (NSF),

Award No. AGS-1258358. Any opinions, findings, conclusions or

recommendations expressed herein are those of the author(s) and do not

necessarily reflect the views of NSF. Observed precipitation data was

provided by the National Weather Service Advanced Hydrologic Prediction

Service. Special thanks to Scott Rochette, Dr. Neil Fox and Dr. Bo Svoma

for their contributions and suggestions.

Data and MethodologyData for this project came from the University Corporation for Atmospheric Research (UCAR). Initially 5-minute radar files were observed

and compared to Weather Prediction Center (WPC) surface analyses to track frontal movement with MCS movement. From there, 2 m

temperature, 950-mb 𝜃𝑒, and frontogenesis initial RAP fields were juxtaposed with radar files to see if frontal motion was indeed

retreating southward. This method was used during previous PRECIP cases to track frontal motion along with MCS motion.

The WRF model used consisted of a 9 km outer domain with a 3 km inner nest. Initial conditions came from the 13 km RAP model,

using only initial (or 00 hour forecasts) conditions. This was done because the 00 hour forecast are close to analysis. At 9 km the Kain-

Fritsch cumulus parametrization is used, while at 3 convection is explicit. The Thompson 6-class microphysics scheme was used

throughout.

9-10 September 2014: Southern Iowa and Northern Missouri Analysis9-10 September 2014 featured two boundaries, each spawning convection. A cold front was in place across the Northern Plains while a

developing warm front was located over Iowa. Convection formed between the fronts. Deterministic solutions did not handle this well. A

large MCS was expected to form over Iowa and ride southwesterly flow in Wisconsin, where flash flood watches had been issued.

Instead convection remained locked onto the developing warming front and never reached Wisconsin. Eventually this convection, and

the boundary, moved into northern Missouri where it produced over 200 mm of rainfall. The convection cooled the area between fronts

and made it appear as though the front jumped hundreds of kilometers in a few hours as the air was cooled by the convection. The GFS

never really develops the warm front and does not account for the impendence of southwesterly flow. The RAP analysis and WRF

simulation clearly show a warm front in southern Iowa that retrogrades southward into northern Missouri. This feature acted as a focal

point for convection. As in the 3 June 2014 case, Corfidi vectors turn cross frontal as the system strengthens and frontogenesis is

analyzed on the warm flank of the convective cold pools.

3-4 June 2014: Southern Iowa and Northern Missouri AnalysisThe 3-4 June 2014 case is an example of strong convective cold pools expanding in a cross-frontal fashion (i.e. the gust front is normal

to the established boundary) acting to stop poleward progression of a warm front. Frontogenesis fields are seen developing farther and

farther south along the leading edge of the convective cold pool, contradictory to deterministic model forecasts. This forces the warm front

to stall and is pushed southward as strong convection continues. Frontogenesis may aid in the redevelopment of convection and in this

case acted as precursor to southward shift of the MCS. Corfidi vectors turn perpendicular to the front as new convection develops along

the frontogenesis band. Prior to the cold pool expansion, Corfidi vectors were pointed along the front. The GFS tires to have the warm

front advance poleward, with frontogenesis bullseyes remaining in the center of the 𝜃𝑒 gradient. Temperature due cool with time in the

GFS, however, they lack a thermal omphalos reflecting convective cold pool influence. The WRF simulation thermal depiction is closer to

the analysis fields. The 𝜃𝑒 gradient is sharper with many more local perturbations due to model generated convection than analysis grids,

but does show the front moving southward due to convective influences.

Summary and Conclusion Analysis of all the PRECIP field cases has revealed a pattern of low-level frontogenesis forming on the warm flank of convective cold pools among cases that cause frontal motion to stall and/or retrograde

equatorward. This pattern is best highlighted in the 3-4 June 2014 case. This phenomena appears to be strongly connected to equatorward formation of thunderstorms and may aid in the redevelopment of

convection, prolonging flash flooding conditions. Additionally, Corfidi vectors also may signify when storm motions is transitioning from parallel to cross-frontal. WRF Simulations generally handled the events

better in regard to frontal location, but were sensitive to model generated convection and created false perturbations in the 𝜃𝑒 fields.

18z

00z

06z

80 km GFS Forecast Fields

𝜃𝑒 Every 3 K (brown contours).

Reflectivity Every 5 dBZ (filled colors).

Petterssen Frontogenesis every 10-1

K/100km/3hr (white dashes).

Corfidi Vectors (white arrows).

2 m Temperature every 2 °C (filled colors).

𝜃𝑒 Every 3 K (red contours).


K/100km/3hr (purple dashes).

13 km RAP Initial Fields 3 km WRF-PRECIP

Reflectivity from KDMX every 5 dBZ

(filled colors).

Corfidi Vectors (White Arrows).


𝜃𝑒 Every 3 K (red contours). Petterssen

Frontogenesis every 10-1 K/100km/3hr

(purple dashes).

13 km RAP Initial Fields 80 km GFS Forecast Fields 3 km WRF-PRECIP Initial Fields

18z

00z

06z

𝜃𝑒 Every 3 K (brown contours).








K/100km/3hr (purple dashes).










Observed rainfall (mm) Surface Analysis at Event Beginning (1800 UTC) Surface Analysis at Event Mid-Point (0000 UTC) Surface Analysis at Event Ending (0900 UTC) Observed rainfall (mm) Surface Analysis at Event Beginning (1800 UTC) Surface Analysis at Event Mid-Point (0000 UTC) Surface Analysis at Event Ending (0900 UTC)





(purple dashes).





(purple dashes).

Reflectivity from KDMX every 5 dBZ

(filled colors).Reflectivity from KDMX every 5 dBZ

(filled colors).

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