Authors: James T. Moore Fred H. Glass Charles E. Graves Scott M. Rochette Marc J. Singer

1. The Environment of Warm-Season Elevated Thunderstorms Associated with Heavy Rainfall over the Central United States Authors: James T. Moore Fred H. Glass Charles E. Graves Scott M. Rochette Marc J. Singer

2. Purpose of the article Twenty-one warm-season heavy rainfall events in the central United States that developed above and north of a surface boundary are examined to define the environmental conditions and physical processes associated with these phenomena. -MCS’s account for 30-70% of warm-season precipitation. -Summer 1993

3. Previous Research • Colman: Defined elevated thunderstorms as those that are isolated from sfc diabatic effects and occur above frontal surfaces. • Three criteria: • Must lie on the cold side of analyzed front • Winds, temperatures and dew points must be similar to surrounding values • Surface air on warm side of analyzed front must have higher equivalent potential temps than air on cold side

4. Colman cont’ • Colman identified 5 characteristics of elevated thunderstorms • Strong warm air advection at 850-mb • Strong low-level veering of winds with height, from east at sfc to SSW at 850-mb, to SW at 500-mb • Extremely stable sfc air with LI values of 7°C • Shallow front exhibiting strong frontal inversion of >5°C • Sharply defined front associated with strong horizontal thermal contrast

5. Dataset and Methodology • Local heavy rain events from 1993-1998 were examined • Must have produced at least 10 cm of rain in a 24 hr period • Been initiated or been on going ± 4 hrs 0000 UTC or 1200 UTC. • Must have met Colman’s 3 criteria for elevated thunderstorms • A total of 21 heavy-rain events met criteria

6. Dataset used during research

7. Dataset used during research

8. Surface and kinematic upper-air fields • A) On average the sfc boundary is located 160 km south of MCS centroid • B) Baroclinic zone is shifted more north at 850mb • C & D) Elevated MCS centroid is located within the low-level θe gradient, just to the east of a weak north-south ridge axis. • Maximum θe values to the S-SW of the active MCS. • Given the location of the elevated MCS with respect to the baroclinic zones, frontogenetical forcing likely plays a role in the existence of the MCS.

9. Surface and kinematic upper-air fields • A) 925-mb wind vectors and isotachs • B) 850-mb wind vectors and isotachs • C) 925-mb moisture convergence • D) 850-mb moisture convergence • The centroid is located about 600 km downstream from the 925-mb wind maximum. The favored location for the elevated MCS is just north of the maximum moisture convergence. • The 850-mb wind maximum is 400 km upstream of the MCS centroid • Composite LLJ is oriented normal to the moisture field in figure D. • Moisture convergence is maximized just south of the MCS centroid.

10. Surface and kinematic upper-air field • A) 850-mb mixing ratio • B) 850-mb moisture transport vectors and magnitudes • C) 850-mb θe advection • D) 850-mb temperature advection • There is a large region of positive θe advection that coincides with the MCS centroid • This is critical in the destabilization process by promoting elevated convective instability above the sfc boundary • Elevated MCSs tend to be located with a region of positive thermal advection at 850-mb

11. Surface and kinematic upper-air fields • A) Composite analysis of 250-mb wind vectors and isotachs • B) 250-mb divergence • The elevated MCS is located within a divergence maximum of greater than 2.5 x 10-5 s-1 • McNulty: severe convection tends to develop in the divergence gradient south of a divergence maximum aloft • Junker/Glass: location of heaviest rainfall tends to be the gradient region of the max 250-mb divergence. • MCS-induced divergence likely increased divergence values locally

12. Stability and moisture fields • Elevated thunderstorms form above the boundary layer therefore would expect sfc and low level based stability to be poor indicators of atm. stability • Lifted Index, showalter index and the horizontal distribution of the mean parcel CAPE is representative of the boundary layer moisture and temp stratification • The mean LI for the MCS centroid is +4, which is expected because the MCS is located north of the sfc boundary

13. Stability and moisture fields • Elevated MCS centroid is located within the N-S gradient of modest CAPE values (~600 J/kg) • Using the max-θe CAPE, the MCS centroid is located at the 1200 J/kg • The CIN values at the MCS are >110 J/kg • The MCS centroid is located in a valley of max θe CIN, thus requiring less forced upward vertical motion to overcome negative buoyancy • The max-θe CAPE value is twice that of the mean parcel CAPE, which illustrates that greater positive buoyancy is realized by lifting a parcel along or above the sloped frontal zone

14. Vertical profiles of wind shear and stability • Composite soundings were constructed at the centroid location and at the inflow point • At the MCS centroid, the near-sfc wind is from the E-SE at ~2.5 m/s and veers to the SW at ~ 10 m/s at 850-mb • In contrast, at the inflow site, near-sfc winds are from the south at 2 m/s and veer to the SW at 15 m/s at 800-mb. Above 800-mb winds weaken and have little to no veering • Elevated MCS form downstream from the LLJ situated over the inflow site

15. Vertical profiles of wind shear and stability • A) θe vertical profile for the MCS centroid • B) θe vertical profile over the inflow point • Centroid site is characterized by a convectively stable boundary layer, with convectively unstable on top. • At the inflow point, the θe profile reveals a shallow convective stable layer with a deep layer of convectively unstable air aloft • The vertical shift in the location of the θe maximum from 950-mb at the inflow site to 800-mb at the centroid location is consistent with the northward transport of high θe air above the frontal zone • The depth of the convectively unstable air also changes from 350 mb at the inflow site to 150 mb at the MCS centroid

16. Representativeness of composite fields • Because some mature MCSs were included in the dataset, it is important to quantify their impact • Composite fields were recomputed, using synoptic times that were either pre-MCS or less than 3 hrs after the MCS initiation, resulting in 15 events being composited • The majority of the composite fields revealed little to no difference from the full dataset

17. Representativeness of composite fields • To examine the strength of the composite fields, the linear spatial correlation coefficient between the individual cases and composite fields was computed • High values of the correlation coefficient indicate that there is agreement between the pattern of the composite field and that for individual analysis • In about 50% of the cases, at least 10 parameters, out of the 18, had correlation coefficients that exceeded the median correlation value for that parameter • This result provides evidence that the composite patterns presented are reliable signatures of the typical environmental conditions that are common for elevated MCSs

18. Summary & Conclusions • Cross-sectional schematic of the MCS environment • MCS centered 160 km north of an east-west oriented sfc front • The exact position is the function of the thermal gradient, magnitude and orientation of the low-level inflow, and moisture content • S-SW LLJ transports high-θe air northward along and above the cool, stable layer • SW midtropospheric flow advects lower-θe air over the warm, moist high-θe air, resulting in a layer of elevated convective instability • Moisture convergence within the left-exit region of the LLJ helps to initiate deep convection in the unstable layer along or above the frontal zone • The LLJ contributes to an axis of moisture convergence that’s nearly parallel to the sfc boundary, which promotes cell training and subsequently high rainfall totals

19. Summary & Conclusions • Schematic diagrams that summarize the typical conditions associated with warm-season elevated thunderstorms attended by heavy rainfall • Presence of a east-west quasi-stationary front • Moderate north-south θe gradient • S-SW LLJ directed nearly normal to the boundary • SW-NE elongated moisture convergence axis at 925-mb found on and along the cool side, upstream from the MCS centroid • Positive 850-mb θe advection max nearly centered over the MCS centroid • Broad SW midtropospheric flow, with MCS centroid over inflection point • Relatively high relative humidity • MCS centroid typically located in the right entrance region of the ULJ • MCS centroid is favored just east of the max θe, in a region of WAA and moisture convergence at 850-mb

20. Summary & Conclusions • Analysis of max-θe CAPE shows values that are 2 times that of the mean parcel CAPE over the MCS centroid • In the vicinity of the MCS centroid, values of max-θe CIN are 1/3 of the mean parcel CIN • Relatively high correlation coefficients of individual fields confirm that operational forecasters can apply the patterns/signals displayed in the composites with prognostic numerical model data to help diagnose regions that are favorable for organized elevated thunderstorms that produce heavy rainfall • It is important to note the spatial distribution of the variables

21. QUESTIONS?