The Environment of Warm Season Elevated Thunderstorms

download report

Transcript The Environment of Warm Season Elevated Thunderstorms

AnneMarie Giannandrea
ESC 452 Mesoscale Meteorology
4/22/10
Introduction
 MCS rain accounts for 30-70% of the Central United
States growing season rainfall
 They also lead to flash flooding and property
destruction.
 Study was produced “the focus of this study is to
quantify the synoptic–meso-α-scale environment and
physical processes favorable for the production of
organized elevated thunderstorms (i.e., MCSs) that
produce copious rainfall.” (Moore, et.al, 2003)
Definitions
 Composite Study: Study which attempts to look at representative cases
of a phenomenon and discern the common characteristics of that
phenomenon.
 Elevated Convection: Convection occurring within an elevated layer,
i.e., a layer in which the lowest portion is based above the earth's
surface. Elevated convection often occurs when air near the ground is
relatively cool and stable, e.g., during periods of isentropic lift, when an
unstable layer of air is present aloft.
In cases of elevated convection, stability indices based on near-surface
measurements (such as the lifted index) typically will underestimate the
amount of instability present. Severe weather is possible from elevated
convection, but is less likely than it is with surface-based convection.
(From http://www.crh.noaa.gov/glossary.php?letter=e)
What Creates Elevated Convection?
Elevated
Convection
WAA
Stable Surface
Air
Shallow, Sharply
Defined Front
LL Veering
From Coleman, 1990
Previous Research
 Coleman defined the elevated thunderstorm and MCS
in his papers in the 1990s
 More recently, there has been an attempt at using a
composite approach to determine the characteristics
of these storms.
Dataset
 Storms must have produced at least 10 cm of rain, and
have been ongoing within 4 hours of either the 00Z or
the 12Z sounding
 A total of 21 events were identified, covering 35
sounding periods.
Method
 Satellite imagery was used to determine the existence of
the MCS in the window around 0000Z and 1200Z
 Plots were generated and used to determine whether or
not the MCS was elevated
 Rawinsonde data was processed using documented
schemes, and derived parameters were output. They were
plotted on grids centered around the respective MCS
centroid.
 Centroids were determined by the highest reflectivities or
coldest cloud tops, if radar data was unavailable.
Surface and Kinematic Upper Air
Fields
 Surface:
 The surface boundary is oriented west to east, 160 km South of the MCS
Centroid
 The centroid is located in the θe gradient, to the east of a ridge axis.
 925 hPa:
 West- east baroclinic zone of moderate strength
 The centroid is located 600 km downstream of the wind max.
 Located north of the max. moisture convergence
 850 hPa:
 Baroclinic zone is farther north
 Heaviest rain is located in the left exit region of the southwesterly wind
maximum
 The wind maximum is approximately 400 km upstream of the MCS
centroid.
 Large region of θe advection close to the centroid.
 Moisture convergence values at 850 hPa (0.8 g (kg h)-1)
 Region of positive thermal advection (about 0.4 C h-1)
Surface and Kinematic Upper Air
Fields
 700 hPa
 Similar to 850 hPa in distances, but weaker in magnitudes
 500 hPa
 A weak short wave trough in the height and vorticity fields is
found approximately 100 km upstream.
 Continued veering of winds
 Midlevel jet axis found west of the centroid.
 250 hPa
 Anticylclonically curved jet streak ( approximately 80 kts) well
to the northeast
 The centroid is within a divergence maximum.
Stability and Moisture Fields
 Stability
 The mean LI is around 4 C at the MCS centroid, and the
Showalter index is similar
 The mean surface CAPE is overwhelmed by the mean
surface CIN (110 J kg-1)
 The max θe CAPE values are on the order of 1250 J kg-1
at the centroid, and it lies within a max θe CIN valley
Stability and Moisture Fields
 Moisture:
 Precipitable water values range from 1.2 to 1.3 inches at
the centroid, with higher values to the south
 The Mean Relative Humidity (MERH) has values greater
than 70% at the centroid, and a distribution that mirrors
the shape of the MCS.
Vertical Profiles of Wind Shear and
Instability
 Composite soundings were constructed for both the MCS
centroid and an inflow point sufficiently far to the south.
 At the centroid, the near surface wind is from the eastsoutheast at about 2.5 ms-1 and veers to the southwest at
about 10 ms-1 at 850 hPa. Above 850 hPa, the winds
gradually increase in speed to 25 ms-1 at 300 hPa, with little
directional change.
 At the inflow site, the near-surface winds are from the south
at 2 ms-1 and veer to the southwest at 15 ms-1 at 800 hPa.
Vertical Profiles of Wind Shear and
Instability
 The centroid has a convectively stable boundary layer
with a 150 hPa (800-650 hPa) unstable layer.
 The inflow site has a shallow layer of convectively
stable air, and a 350 hPa (950-600 hPa) layer of
unstable air above it.
Representativeness
 The 15 of the 21 cases were recomputed to remove the
environmental modification the occurs in mature MCSs
 When the fields were recomputed, in most cases the
magnitudes were different, but the spatial distribution was
the same.
 Basic parameters had high correlation coefficient, derived
parameters were less well correlated.
 In about 50% of the cases at least 10 out of the 18
parameters studied were above the median correlation
coefficient for the respective parameter.
Crosssectional
View
Schematic cross-sectional view taken parallel to the LLJ across the frontal zone. Dashed lines
represent typical θe values, the large stippled arrow represents the ascending LLJ, the thin
dotted oval represents the ageostrophic direct thermal circulation associated with the upperlevel jet streak, and the thick dashed oval represents the direct thermal circulation associated
with the low-level frontogenetical forcing. The area aloft enclosed by dotted lines indicates
upper-level divergence; the area aloft enclosed by solid lines denotes location of upper-level jet
streak. Note that in this cross section the horizontal distance between the MCS and the location
of the upper-level jet maximum is not to scale.
Plan View
Schematic diagrams that summarize
the typical conditions associated with
warmseason elevated thunderstorms
attended by heavy rainfall: (a) low-level
plan view and (b) middle– upper-level
plan view. In (a), dashed lines are
representative θe values decreasing to
the north, dashed–cross lines represent
925–850-hPa moisture convergence
maxima, the shaded area is a region of
maximum θe advection, the broad
stippled arrow denotes the LLJ, the
encircled X represents the MCS
centroid location, and the front is
indicated using standard notation. In
(b), dashed lines are isotachs
associated with the upper-level jet,
solid lines are representative height
lines at 500 hPa, the stippled arrow
denotes the 700-hPa jet, and the
shaded area indicates where the mean
surface-to-500-hPa relative humidity
exceeds 70%.
Conclusions
 The applicability of these results to other parts of the
United States has not been established.
 Although quantifying the significant parameters used to
diagnose a favorable region of elevated convection leading
to heavy rain is important, it is equally as important to note
the spatial distribution of these variables. In this way a
conceptual model can be constructed that depicts how
critical physical processes synergistically interact to create
a mesoscale environment favorable for the development
of elevated thunderstorms. (Moore, et. al., 2003)
Works Cited
 Colman, B. R., 1990a: Thunderstorms above frontal
surfaces in environments without positive CAPE. Part
I: A climatology. Mon. Wea. Rev., 118, 1103–1121.
 Moore, et. al., 2003: The Environment of Warm
Season Elevated Thunderstorms Associated with
Heavy Rainfall over the Central United States.
Weather and Forecasting, 18, 861-878.