Transcript Evaluation of “Climatology of Storm Reports Relative to

Evaluation of “Climatology of
Storm Reports Relative to UpperLevel Jet Streaks” by Clark et al.
BY: KRISTIN BREMER
&
SEAN ARCHER
Outline
 Introduction
 Background
 Previous work
 Extended work
 Data & Methodology
 Data retrieval
 How jet streak was defined
 How data was analyzed
 Results
 Jet streak position relative to storm reports
 Jet streak composite fields
 Jet streak curvature
 Jet streak direction
 Discussion & Conclusions
Introduction
 Ageostrophic winds result in upper-level divergence
in the right-entrance and left exit regions of a jet
streak
 Jet streaks are said to be important in the
development of severe wx because they help to:
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Decrease static stability
Cool air to saturation in a layer of air
Release CAPE
Enhance moisture convergence at low-levels
Enhance vertical wind shear
Background
 Based on paper by Rose et al.
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10 year climatological study of F1 and above tornado occurrences
relative to 250 hPa jet streaks
Used 4 quadrant model
Did not considered other major jet dynamics
Found that the majority of the tornado reports were in the exit
quadrants of jet, specifically the left-exit
Concluded that there were errors when using the 4 quadrant model
 Clark et al. extended these results by not only looking at
all tornado reports in U.S. but also hail reports and wind
reports
 Clark et al. will also be looking at other cloud dynamics
such as upper-level divergence, curvature, and direction
of jet streaks
Data & Methodology
 Upper-level jet streaks: North American Regional
Reanalysis (NARR)
 Storm reports: National Climatic Data Center
(NCDC) on the Storm Prediction Center Website
 March-September 1994-2004
 Only jet streaks that occurred at 0000 UTC with
storm reports being within 3 hour period
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Any storm report within this time period but was outside of the
jet streak was deemed a “non-jet-related” report
Data & Methodology
 How jet was defined:
 Enclosed area of 25 m/s+ winds
 Mark jet core and jet endpoints
 Jet core defined as point within the area of max wind speed
where the acceleration became zero
 Max major jet axis at 1000 km
 After each jet was defined, it was placed on 77 x 37
grid with the jet core at the center using Grid
Analysis and Display System (GrADS)
(Clark et al.)
Results
 A) Jet streak position relative to storm reports
 105,987 storm reports out of 126, 864 analyzed fell within jet
streak-84%
 Peak month for both jet-related and non-jet-related was June
and May was the peak month for jet-related storm reports
 Most likely due to north migration of jet stream
 Non jet-related reports were most likely due to MCS which
usually occur outside a jet streak and during July and August
when these reports were at its peak
Results
 A) Jet streak position relative to storm reports cont’d
 March and April-Storm reports majority in left-exit
 April-Storm reports majority in right-entrance
 After April the right-exit region was favored which is not
consistent with 4QM
 Tornado reports-Exit region favored, specifically the left exit
 Hail reports-More evenly distributed but a majority were
found in the left exit
 Wind reports-majority in right entrance
Results
 B) Jet streak Composite fields
 Upper-level divergence does not exactly coincide with the
4QM-slight displacement toward cyclonically sheared side of
jet streak
 Number of storm reports matched up with areas of strong
upper-level divergence in entrance regions, specifically rightentrance
 Exit regions had most reports just south of area of high upperlevel divergence (right-exit)
 Exit-regions seemed to coincide better with low-level
convergence
Results
 C) Jet Streak Curvature
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Radius of curvature
R = 𝐿/∆𝑑
R* = (1/R) x 〖10〗^4
Linear (-1.5≤R*≤1.5)
Cyclonic (R*≥5.0)
Anticyclonic (R*≤-5.0)
Jet streaks with values
outside above intervals are
not used
Jet Streak Curvature: Storm Report Distribution
Results
 C) Jet Streak Curvature (Distribution Results)
 Matched conceptual models (Beebe and Bates, 1955)
 Matched numerical simulations (Moore and VanKnowe, 1992)
 Anticyclones – stronger max divergence/convergence and
stronger vertical velocity contradicts Moore and Vanknowe
 Areas of storm report maxima experienced max divergence
aloft and max convergence at the surface with a low pressure
centered in the region
Jet Streak Curvature: Atmospheric Conditions
Results
 D) Jet streak Direction
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Jet streaks categorized
into 6 ranges of directions
Easterly jet streaks were
ignored
SW and WSW were the
most active and had the
most storm reports
Jet Streak Direction: Storm Report Distribution
Jet Streak Direction: Atmospheric Conditions
Discussion & Conclusions
 The left exit (right entrance) region contained the
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most storm reports in April and May (after April)
Storm reports were concentrated on the axis in the
exit region and the center of the right entrance
region
Anticyclonic curvature had the stronger vertical
velocity while linear had the weakest
More storm reports in WSW and SW oriented jet
streaks
Based on the results by Clark et al., jet streaks play a
major role in the development of severe wx
Questions?