Presentation on theme: "The Environment of Warm-Season Elevated Thunderstorms Associated with Heavy Rainfall over the Central United States Authors: James T. Moore Fred H. Glass."— Presentation transcript:
1 The Environment of Warm-Season Elevated Thunderstorms Associated with Heavy Rainfall over the Central United StatesAuthors:James T. MooreFred H. GlassCharles E. GravesScott M. RochetteMarc J. Singer
2 Purpose of the articleTwenty-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 ResearchColman: 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 frontWinds, temperatures and dew points must be similar to surrounding valuesSurface 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 thunderstormsStrong warm air advection at 850-mbStrong low-level veering of winds with height, from east at sfc to SSW at 850-mb, to SW at 500- mbExtremely stable sfc air with LI values of 7°CShallow front exhibiting strong frontal inversion of >5°CSharply defined front associated with strong horizontal thermal contrast
5 Dataset and Methodology Local heavy rain events from were examinedMust have produced at least 10 cm of rain in a 24 hr periodBeen initiated or been on going ± 4 hrs UTC or 1200 UTC.Must have met Colman’s 3 criteria for elevated thunderstormsA total of 21 heavy-rain events met criteria
8 Surface and kinematic upper-air fields A) On average the sfc boundary is located 160 km south of MCS centroidB) Baroclinic zone is shifted more north at 850mbC & 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 isotachsB) 850-mb wind vectors and isotachsC) 925-mb moisture convergenceD) 850-mb moisture convergenceThe 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 centroidComposite 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 ratioB) 850-mb moisture transport vectors and magnitudesC) 850-mb θe advectionD) 850-mb temperature advectionThere is a large region of positive θe advection that coincides with the MCS centroidThis is critical in the destabilization process by promoting elevated convective instability above the sfc boundaryElevated 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 isotachsB) 250-mb divergenceThe elevated MCS is located within a divergence maximum of greater than 2.5 x s-1McNulty: severe convection tends to develop in the divergence gradient south of a divergence maximum aloftJunker/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. stabilityLifted Index, showalter index and the horizontal distribution of the mean parcel CAPE is representative of the boundary layer moisture and temp stratificationThe 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/kgThe CIN values at the MCS are >110 J/kgThe MCS centroid is located in a valley of max θe CIN, thus requiring less forced upward vertical motion to overcome negative buoyancyThe 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 pointAt 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-mbIn 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 veeringElevated 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 centroidB) θe vertical profile over the inflow pointCentroid 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 aloftThe 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 zoneThe 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 impactComposite 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 compositedThe 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 computedHigh values of the correlation coefficient indicate that there is agreement between the pattern of the composite field and that for individual analysisIn about 50% of the cases, at least 10 parameters, out of the 18, had correlation coefficients that exceeded the median correlation value for that parameterThis 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 frontThe exact position is the function of the thermal gradient, magnitude and orientation of the low-level inflow, and moisture contentS-SW LLJ transports high-θe air northward along and above the cool, stable layerSW midtropospheric flow advects lower-θe air over the warm, moist high-θe air, resulting in a layer of elevated convective instabilityMoisture convergence within the left-exit region of the LLJ helps to initiate deep convection in the unstable layer along or above the frontal zoneThe 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 & ConclusionsSchematic diagrams that summarize the typical conditions associated with warm- season elevated thunderstorms attended by heavy rainfallPresence of a east-west quasi-stationary frontModerate north-south θe gradientS-SW LLJ directed nearly normal to the boundarySW-NE elongated moisture convergence axis at 925-mb found on and along the cool side, upstream from the MCS centroidPositive 850-mb θe advection max nearly centered over the MCS centroidBroad SW midtropospheric flow, with MCS centroid over inflection pointRelatively high relative humidityMCS centroid typically located in the right entrance region of the ULJMCS centroid is favored just east of the max θe, in a region of WAA and moisture convergence at 850-mb
20 Summary & ConclusionsAnalysis of max-θe CAPE shows values that are 2 times that of the mean parcel CAPE over the MCS centroidIn the vicinity of the MCS centroid, values of max-θe CIN are 1/3 of the mean parcel CINRelatively 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 rainfallIt is important to note the spatial distribution of the variables
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