Transcript The Environment of Warm-Season Elevated Thunderstorms

The Environment of WarmSeason 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
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 500mb
▫ 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 250mb 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 250mb 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
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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
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Schematic diagrams that summarize the
typical conditions associated with warmseason 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?