appalachian lee troughs and their association with severe thunderstorms

Appalachian Lee Troughs and their Association with Severe

ThunderstormsDaniel B. Thompson, Lance F. Bosart and

Daniel KeyserDepartment of Atmospheric and Environmental Sciences

University at Albany/SUNY, Albany, NY 12222

Thomas A. WasulaNOAA/NWS, Albany, NY

Matthew KramarNOAA/NWS, Sterling, VA

37th Northeastern Storm Conference, Rutland, VT3 Mar 2012

NOAA/CSTAR Award # NA01NWS4680002

Motivation• Region of study: Mid-Atlantic• Accurately forecasting location, mode and

severity of thunderstorms is important, due to proximity of Eastern Seaboard

• Region is often characterized by weak forcing and ample instability→ Mesoscale boundaries important

• Sea breeze boundary• Outflow boundaries• Lee trough

• Analyze the structure of Appalachian Lee Troughs (ALTs)

• Construct a climatology of warm-season ALTs

• Analyze the distribution of severe convection in the Mid-Atlantic– Spatial distribution– Temporal distribution– Characteristic CAPE/shear

Objectives

Data and Methodology

1. Analyzed 13 cases of ALT events associated with warm-season severe convection

─ Sterling, VA (LWX) CWA ─ 0.5° CFSR (Climate Forecast System

Reanalysis)2. Identified common features and used

them as criteria to construct a climatology– May–September, 2000–2009

• PV = −g(∂θ/∂p)(ζθ + f)

(Static stability)(Absolute vorticity) • d(PV)/dt = 0 for adiabatic flow• Flow across mountain barrier will subside on lee side

– Advects higher θ downward → warming– −g(∂θ/∂p) decreases → ζθ must increase → low level circulation

Adapted from Martin (2006)

Appalachians Appalachians

Lee Trough Formation: PV Perspective

ALTs – Common Low-Level Features

MSLP (black, hPa), 1000–850-hPa thickness (fills, dam), thermal vorticity < 0 (white, 10−5 s−1), 10-m winds (barbs, kt)

1800 UTC Composite (n=13)

ALTs – Common Low-Level Features

MSLP (black, hPa), 1000–850-hPa thickness (fills, dam), thermal vorticity < 0 (white, 10−5 s−1), 10-m winds (barbs, kt)

1800 UTC Composite (n=13)

Winds orthogonal to

mountains

ALTs – Common Low-Level Features

MSLP (black, hPa), 1000–850-hPa thickness (fills, dam), thermal vorticity < 0 (white, 10−5 s−1), 10-m winds (barbs, kt)

1800 UTC Composite (n=13)

Winds orthogonal to

mountains

Thermal ridge

ALTs – Common Low-Level Features

MSLP (black, hPa), 1000–850-hPa thickness (fills, dam), thermal vorticity < 0 (white, 10−5 s−1), 10-m winds (barbs, kt)

1800 UTC Composite (n=13)

Winds orthogonal to

mountains

Thermal ridge

Negative thermal vorticity

ALTs – Common Low-Level Features

0000 UTC Composite (n=13)

MSLP (black, hPa), 1000–850-hPa thickness (fills, dam), thermal vorticity < 0 (white, 10−5 s−1), 10-m winds (barbs, kt)

Negative thermal vorticity

Winds orthogonal to

mountains

Thermal ridge

Domain for Climatology

DOMAIN

WIND ZONE

ALT ZONE

• Climatology was based on the following 3 criteria:1) 925-hPa Wind Direction

– Checked for wind component directions orthogonal to and downslope of Appalachians

– Appalachians in the Mid-Atlantic are oriented ~ 43° right of true north

→ Satisfactory meteorological wind directions exist between 223° and 43°

DOMAIN

WIND ZONE

ALT ZONE

Criterion: wind direction computed from zonal average of wind components along each 0.5° of latitude within Wind Zone must be between 223° and 43°

Methodology for Climatology

• Climatology was based on the following 3 criteria:2) MSLP Anomaly

– Averaged MSLP along each 0.5° of latitude within domain– Checked for minimum MSLP along each 0.5° of latitude

within ALT Zone

DOMAIN

WIND ZONE

ALT ZONE

Methodology for Climatology

Criterion: difference of minimum and zonal average MSLP must be less than a threshold value

• Climatology was based on the following 3 criteria:3) 1000–850-hPa layer-mean temperature anomaly

– Averaged 1000–850-hPa layer-mean temperature along each 0.5° of latitude within domain

– Checked for maximum 1000–850-hPa layer-mean temperature along each 0.5° of latitude within ALT Zone

Methodology for Climatology

Criterion: difference of maximum and zonal average 1000–850-hPa layer-mean temperature must be greater than a threshold value DOMAIN

WIND ZONE

ALT ZONE

• The three criteria must be met for six consecutive 0.5° latitudes

• An algorithm incorporating the three criteria was run for the length of the climatology at 6-h intervals (0000, 0600, 1200 and 1800 UTC)

• ALTs identified by this algorithm were manually checked for false alarms (e.g. frontal troughs, cyclones, large zonal pressure gradients)

Methodology for Climatology

-2 -1.75 -1.5 -1.25 -1 -0.75 -0.5 -0.25 00

0.5

1

1.5

2

2.5

3

3.5

26.6

ALT Occurrence (%) as a Function of MSLP/Temperature Anomaly Thresholds (n=6120)

MSLP Anomaly Threshold (hPa)1000

-850

-hPa

Mea

n Te

mpe

ratu

re

Ano

mal

y Th

resh

old

(° C

)

• Each bubble denotes the percentage of time an ALT is recorded under a particular set of MSLP/temperature anomaly constraints

• Boxes indicate the criteria adopted as the ALT definition

← Stricter

← Stricter

Climatology – Results

MSLP anomaly < −0.75 hPa Temperature anomaly > 1°C

Climatology – Results

31.9%

18.8%16.0%

33.3%

ALTs by Time (UTC, n=1629)

0000060012001800

17.0%

23.0%

27.8%

25.0%

7.1%

ALTs by Month (n=1629)

MayJuneJulyAugustSeptember

• Over 75% of ALTs occur in June, July and August

MSLP anomaly < −0.75 hPa Temperature anomaly > 1°C

Climatology – Results

31.9%

18.8%16.0%

33.3%

ALTs by Time (UTC, n=1629)

0000060012001800

17.0%

23.0%

27.8%

25.0%

7.1%

ALTs by Month (n=1629)

MayJuneJulyAugustSeptember

• Over 75% of ALTs occur in June, July and August• Nearly 66% of ALTs occur at 1800 or 0000 UTC

– The seasonal and diurnal heating cycles likely play a role in ALT formation

• Severe local storm reports were obtained from the NCDC Storm Data publication

• Included all tornado, severe thunderstorm wind and severe hail (>1”) for May–September, 2000–2009

Storm Reports in the ALT Zone – Data and Methodology

ALT ZONE

• 754 unique days with at least one storm report

• 199 days with > 20 storm reports• Most active day: 13 May 2002 (207)

Day = 0400 to 0400 UTC

Storm Reports – Daily Distribution

776; 51%

555; 36%

199; 13%

Storm Reports in the ALT Zone

Days with no storm reports

Days with 1-20 storm reports

Days with > 20 storm reports

Controlling for Dataset Inconsistencies• “Clustering” – attempt to control for

population bias– Overlay a 0.5° by 0.5° grid box over the

domain– If a storm report occurs within a certain grid

box on a certain day, that grid box is considered “active” for the day• Any subsequent storm reports occurring within the

active box are discarded for the day• The number of active grid boxes for each day are

tallied to measure how widespread the severe weather was on that day

Storm Reports – Spatial Distribution

CFSR composite of top 10% of severe ALT days. MUCAPE (fills, J/kg) and surface to

500-hPa shear (black, kt)

n=706

n=48

Percentage of ALT days with >0 active grid boxes (smoothed)

Storm Reports – Spatial Distributionn=706

n=48

• Storm report max near D.C. coincides with CAPE/shear maxima

• NC local max more difficult to explain

Percentage of ALT days with >0 active grid boxes (smoothed)

CFSR composite of top 10% of severe ALT days. MUCAPE (fills, J/kg) and surface to

500-hPa shear (black, kt)

CAPE/Shear at First Daily Storm Report• To quantify severe thunderstorm parameters

characteristic of ALT Zone, CAPE/shear was calculated at location of first daily storm report

• Dataset: 32 km NARR (8 analysis times daily)• Procedure:

– Find location and time of first severe report on a certain day (0400–0359 UTC)

– Calculate MUCAPE and Sfc–500-hPa shear at location of storm report using nearest analysis time at least 30 min prior to storm report

CAPE/Shear at First Daily Storm Report• Only included days in which first storm report

occurred between 1530 and 0029 UTC

Time of 1st Daily Storm Report

(UTC)

Corresponding NARR analysis

time (UTC)

1530–1829 1500

1830–2129 1800

2130–0029 210076.3%

23.7%

Time of 1st Daily Storm Report (UTC)

1530-00290030-1529

CAPE/Shear at First Daily Storm Report• ALT Zone was divided into sectors to

minimize the likelihood of the first daily storm report not being representative of the environment

CENTER

NORTH

SOUTH

CAPE/Shear at First Daily Storm Report

• South sector peaks earlier (1800 UTC) than north sector (2000 UTC)

• Center sector has flat peak between 1800–2100 UTC

NORTH

CENTER

SOUTH

CAPE/Shear at First Daily Storm Report

• Higher median CAPE (shear) for first daily storm report in south (north) sector

• Higher shear in north sector is likely because it is nearer to the mean warm-season upper jet

Whiskers: 10th and 90th percentiles // Box edges: 25th and 75th percentiles // Line: median

NORTH

CENTER

SOUTH

CAPE/Shear at First Daily Storm Report

• First daily storm report does not concentrate well in CAPE/shear phase-space

CAPE/Shear at First Daily Storm Report

No storm reports

occurred in this phase-space

• First daily storm report does not concentrate well in CAPE/shear phase-space

CAPE/Shear at First Daily Storm Report

• CAPE (shear) at first daily storm report maximized in June, July and August (May and September)

Whiskers: 10th and 90th percentiles // Box edges: 25th and 75th percentiles // Line: median

• ALTs form preferentially during diurnal and seasonal heating maxima

Summary – Key Points

31.9%

18.8%16.0%

33.3%

ALTs by Time (UTC, n=1629)

0000060012001800

17.0%

23.0%

27.8%

25.0%

7.1%

ALTs by Month (n=1629)

MayJuneJulyAugustSeptember

• Distribution of storm reports in ALT Zone varies by latitude– First daily storm report occurs 2 h earlier in

south sector compared to north sector

Summary – Key Points

Summary – Key Points

• CAPE and shear at first daily storm report vary by latitude and month– Greater median CAPE (shear) occurs in

• June, July and August (May and September)• South (north) sector

GREATER SHEAR

GREATER CAPE

Jun, Jul, Aug

May, Sep