The Environment of Warm- Season Elevated Thunderstorms Associated with Heavy Rainfall over the Central United States Authors: James T. Moore Fred H. Glass.

The Environment of Warm-Season Elevated Thunderstorms Associated with Heavy Rainfall over the Central United States

Authors: James T. MooreFred H. GlassCharles E. GravesScott M. RochetteMarc J. Singer

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

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

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

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

Dataset used during research

Dataset used during research

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.

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.

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

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

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

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

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

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

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

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

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

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

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

QUESTIONS?