Characteristics and Climatology of Appalachian Lee Troughs
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Characteristics and Climatology of Appalachian Lee Troughs

Daniel B. Thompson, Lance F. Bosart and . Daniel Keyser. Department of Atmospheric and Environmental Sciences. University at Albany/SUNY, Albany, NY 12222. Thomas A. Wasula. NOAA/NWS, Albany, NY. Matthew Kramar.

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Characteristics and Climatology of Appalachian Lee Troughs




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Presentation on theme: "Characteristics and Climatology of Appalachian Lee Troughs"— Presentation transcript:

Slide1

Characteristics and Climatology of Appalachian Lee Troughs

Daniel B. Thompson, Lance F. Bosart and Daniel KeyserDepartment of Atmospheric and Environmental SciencesUniversity at Albany/SUNY, Albany, NY 12222Thomas A. WasulaNOAA/NWS, Albany, NYMatthew KramarNOAA/NWS, Sterling, VANortheast Regional Operational Workshop XIII, Albany, NY3 Nov 2011NOAA/CSTAR Award # NA01NWS4680002

Slide2

Motivation

+

Weak synoptic-scale forcing

Ample instability

Increased importance of mesoscale features for triggering convection

Topography

Horizontal rolls

Surface boundaries

Mid-Atlantic warm season often characterized by:

Region of study:

Mid-Atlantic

Slide3

Analyze the structure of Appalachian Lee Troughs (ALTs)Obtain an objective definition of ALTsAnalyze the distribution of severe convection in the Mid-Atlantic

Objectives

Slide4

Data and Methodology

Analyzed 13

cases of ALT events associated with warm-season severe convection

Sterling

, VA (LWX) CWA

0.5

° CFSR (Climate Forecast System Reanalysis)

Identified common

features

and used them

as criteria to construct a climatology

May–September

,

2000–2009

Categorized ALTs based

on their relationship

with synoptic-scale

cold fronts

Slide5

PV = −g(∂θ/∂p)(ζθ + f) (Static stability)(Absolute vorticity) d(PV)/dt = 0 for adiabatic flowFlow across mountain barrier will subside on lee sideAdvects 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

Slide6

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)

NEXRAD 2-km Mosaic (dBZ)

2056 UTC 22 July 2008

Source: College of DuPage

Slide7

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)

NEXRAD 2-km Mosaic (dBZ)

2056 UTC 22 July 2008Source: College of DuPage

Slide8

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)

NEXRAD 2-km Mosaic (dBZ)

2056 UTC 22 July 2008Source: College of DuPage

A

A’

Slide9

ALTs – Common Low-Level Features

Potential temperature (black, K), geostrophic relative vorticity

(fills, 10

5 s−1), winds (barbs, kt)

100 km

Slide10

ALTs – Common Low-Level Features

Potential temperature (black, K), geostrophic relative vorticity

(fills, 10

5 s−1), winds (barbs, kt)

100 km

Geostrophic Relative Vorticity Maximum

Slide11

ALTs – Common Low-Level Features

Potential temperature (black, K), geostrophic relative vorticity

(fills, 10

5 s−1), winds (barbs, kt)

100 km

Geostrophic Relative Vorticity Maximum

Warm Core

Slide12

Vertical extent of warm core ranges between 850 hPa and 700 hPa Average: 788 hPaStandard deviation: 61 hPa

ALTs – Common Low-Level Features

Slide13

Domain for Climatology

DOMAIN

WIND ZONE

ALT ZONE

Slide14

Climatology was based on the following 3 criteria:925-hPa Wind DirectionChecked for wind component directions orthogonal to and downslope of AppalachiansAppalachians in the Mid-Atlantic are oriented ~ 43° right of true northSatisfactory 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

Slide15

Climatology was based on the following 3 criteria:MSLP AnomalyAveraged MSLP along each 0.5° of latitude within domainChecked 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

Slide16

Climatology was based on the following 3 criteria:

1000–850-hPa layer-mean temperature anomalyAveraged 1000–850-hPa layer-mean temperature along each 0.5° of latitude within domainChecked 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

Slide17

The three criteria must be met for six consecutive 0.5° latitudesAn 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

Slide18

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

Slide19

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

Climatology – Results

Slide20

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

Climatology – Results

Over

75% of

ALTs occur in June, July and August

Slide21

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

Climatology – Results

Over

75% of

ALTs occur in June, July and

August

Nearly 66% of ALTs occur at 1800 or 0000 UTCThe seasonal and diurnal heating cycles likely play a role in ALT formation

Slide22

ALTs can be grouped into four categories based on their relationship with synoptic-scale cold frontsALTs that occur in advance of cold fronts can be considered prefrontal troughs (PFTs)Categories:InvertedNo PFT: Non-prefrontalPFT, partial FROPA: Prefrontal without frontal passage through entire ALT Zone PFT, total FROPA: Prefrontal with frontal passage through entire ALT Zone

ALT Categories

Slide23

Inverted

– trough extends northward from south of the ALT Zone

MSLP (black, hPa) and 1000–850-hPa thickness (fills, dam)

ALT Categories – Examples

0000 UTC 31 May 2001

Slide24

No PFT

– trough occurs in the absence of a synoptic cold front

ALT Categories – Examples

0000 UTC 10 July 2000

MSLP

(black,

hPa) and

1000–850-hPa

thickness

(fills, dam)

Slide25

PFT, partial FROPA

Front must be south of the NY/PA border or east of the western third of PAFront does not pass through entire ALT Zone

ALT Categories – Examples

0000 UTC 3 June 2000

MSLP

(black,

hPa) and

1000–850-hPa

thickness

(fills, dam)

Slide26

1800 UTC 13 May 2000

PFT, total FROPA

Front must be south of the NY/PA border or east of the western third of PAFront passes through entire ALT Zone within 24 h

ALT Categories – Examples

MSLP

(black,

hPa) and

1000–850-hPa

thickness

(fills, dam)

Slide27

ALT Categories – Climatology

Category 2 (No PFT) occurs most frequently

Slide28

ALT Categories – Climatology

Category 2 (No PFT) occurs most frequently

PFTs account for 44.8% of ALTs

How does the spatial distribution of convection change between categories? How does this distribution change between PFTs and non-PFTs? To be determined

Slide29

Category 2 and 3 are more common in JJA, while category 4 is more common in May and September

Stronger westerlies, more FROPA during “transition months”

ALT Categories – Monthly Distribution

Slide30

Different domain, same procedure as Mid-Atlantic

ALT Climatology in the Northeast

NORTHEAST INTERMOUNTAIN REGION (NEI)

NORTHEAST

COASTAL PLAIN (NECP)

Slide31

Most ALTs recorded in Mid-Atlantic

More favorable terrain?39% of ALTs in NECP were postfrontalConvection unlikelyCaveats:Smaller-scale troughs may be undetectedDoes not represent complete climatology of PFTs

ALT Climatology in the Northeast – Results

NEI

NECP

Slide32

Severe local storm reports were obtained from the NCDC Storm Data publicationExamined 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

climate.met.psu.edu

Slide33

12,330 storm reports754 unique days with at least one storm report199 days with > 20 storm reportsMost active day: 13 May 2002 (207)

Day = 0400 to 0400 UTC

Storm Reports – Daily Distribution

Slide34

Storm Reports – Daily Distribution

Slide35

Storm Reports – Daily Distribution

Pronounced mid-afternoon/early evening maximum in

storm

reports between 2100 and 2300 UTC

Slide36

What influence does an ALT have on the distribution of convection, with respect to location, mode and severity?What influence do each of the ALT categories have on this distribution?To be determined

ALTs and Convection – Further Questions

Slide37

ALTs have a shallow, warm coreALTs form preferentially during diurnal and seasonal heating maximaMonthly distribution of ALTs varies depending on the ALT categoryClassic, terrain-induced ALTs are more likely in June, July and AugustALTs associated with complete FROPA are more likely during May and SeptemberALTs are more likely in the Mid-Atlantic than the NortheastThe ALT Zone has a distinct diurnal maximum in storm reports

Summary – Key Points