BY: KRISTIN BREMER&

SEAN ARCHER

Evaluation of “Climatology of Storm Reports Relative to Upper-Level Jet Streaks” by Clark et al.

Outline

IntroductionBackground

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: 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. 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 1994-2004Only jet streaks that occurred at 0000 UTC

with storm reports being within 3 hour period 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 right-entrance

Exit regions had most reports just south of area of high upper-level divergence (right-exit)

Exit-regions seemed to coincide better with low-level convergence

Results

C) Jet Streak Curvature 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 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 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?