characteristics and climatology of appalachian lee troughs

Characteristics and Climatology of Appalachian Lee Troughs

Daniel B. Thompson, Lance F. Bosart and Daniel Keyser

Department of Atmospheric and Environmental SciencesUniversity at Albany/SUNY, Albany, NY 12222

Thomas A. WasulaNOAA/NWS, Albany, NY

Matthew KramarNOAA/NWS, Sterling, VA

Northeast Regional Operational Workshop XIII, Albany, NY3 Nov 2011

NOAA/CSTAR Award # NA01NWS4680002

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:

Lee troughs Prefrontal troughs

Region of study: Mid-Atlantic

Outflow boundaries

Sea breezes

• Analyze the structure of Appalachian Lee Troughs (ALTs)

• Obtain an objective definition of ALTs

• Analyze the distribution of severe convection in the Mid-Atlantic

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

3. Categorized ALTs based on their relationship with synoptic-scale cold fronts

• 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)

NEXRAD 2-km Mosaic (dBZ)2056 UTC 22 July 2008Source: College of DuPage


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’

ALTs – Common Low-Level FeaturesPotential temperature (black, K), geostrophic relative vorticity

(fills, 10−5 s−1), winds (barbs, kt)

100 km



100 km

Geostrophic Relative Vorticity Maximum



100 km

Geostrophic Relative Vorticity Maximum

Warm Core

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


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


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


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)


-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


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



31.9%

18.8%16.0%

33.3%


0000060012001800

17.0%

23.0%

27.8%

25.0%

7.1%



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



31.9%

18.8%16.0%

33.3%


0000060012001800

17.0%

23.0%

27.8%

25.0%

7.1%



• 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

• ALTs can be grouped into four categories based on their relationship with synoptic-scale cold fronts– ALTs that occur in advance of cold fronts can

be considered prefrontal troughs (PFTs)– Categories:

1. Inverted2. No PFT: Non-prefrontal3. PFT, partial FROPA: Prefrontal without frontal

passage through entire ALT Zone 4. PFT, total FROPA: Prefrontal with frontal

passage through entire ALT Zone

ALT Categories

1. 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

2. No PFT – trough occurs in the absence of a synoptic cold front


0000 UTC 10 July 2000MSLP (black, hPa) and 1000–850-hPa thickness (fills, dam)

3. PFT, partial FROPA– Front must be south of the NY/PA border or east of

the western third of PA– Front does not pass through entire ALT Zone


0000 UTC 3 June 2000MSLP (black, hPa) and 1000–850-hPa thickness (fills, dam)

1800 UTC 13 May 2000

4. PFT, total FROPA– Front must be south of the NY/PA border or east of

the western third of PA– Front passes through entire ALT Zone within 24 h


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

ALT Categories – Climatology

4.5%

50.8%

36.8%

8.0%

ALT % of Occurrence by Category (n=1629)

1 (Inverted)

2 (No PFT)

3 (PFT, partial FROPA)

4 (PFT, total FROPA)

• Category 2 (No PFT) occurs most frequently

ALT Categories – Climatology

4.5%

50.8%

36.8%

8.0%

ALT % of Occurrence by Category (n=1629)

1 (Inverted)

2 (No PFT)

3 (PFT, partial FROPA)

4 (PFT, total FROPA)

• 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

30.8%

17.7%15.4%13.8%

22.3%

Category 4 ALTs (PFT, to-tal FROPA) by Month

(n=130)

10.9%

22.9%

29.5%

30.2%

6.5%

Category 3 (PFT, partial FROPA) ALTs by Month

(n=599)18.1%

24.2%29.9%

23.3%4.5%

Category 2 (No PFT) ALTs by Month (n=827)

• 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

Different domain, same procedure as Mid-Atlantic

ALT Climatology in the Northeast

NORTHEAST INTERMOUNTAIN REGION (NEI)

NORTHEAST COASTAL PLAIN (NECP)

0%15%30%

11.9% 1.3% 1.9%

14.7% 0.8%3.6%

ALT Occurrence (%) as a Function of Zone (n=6120)

NON-PFTPFT

ZONE

PERC

ENT

OCC

URR

ENCE

(M

AY-S

EP 2

000-

2009

)

• Most ALTs recorded in Mid-Atlantic – More favorable terrain?

• 39% of ALTs in NECP were postfrontal– Convection unlikely

• Caveats:– Smaller-scale troughs may be undetected– Does not represent complete climatology of PFTs

ALT Climatology in the Northeast – Results

NEI

NECP

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

• Examined 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

• 12,330 storm reports• 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

1–5

11–1

521

–25

31–3

541

–45

51–5

561

–65

71–7

581

–85

91–9

510

1–10

511

1–11

512

1–12

513

0+

0

50

100

150

200

250

300

350

0.00%

20.00%

40.00%

60.00%

80.00%

100.00%

120.00%

Histogram of Storm Reports in the ALT Zone

Frequency

Cumulative %

Storm Reports Per Day

Num

ber o

f Day

s

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


• Pronounced mid-afternoon/early evening maximum in storm reports between 2100 and 2300 UTC

• 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

• ALTs have a shallow, warm core• ALTs form preferentially during diurnal and

seasonal heating maxima• Monthly distribution of ALTs varies depending on

the ALT category– Classic, terrain-induced ALTs are more likely in June,

July and August– ALTs associated with complete FROPA are more

likely during May and September• ALTs are more likely in the Mid-Atlantic than the

Northeast• The ALT Zone has a distinct diurnal maximum in

storm reports

Summary – Key Points

characteristics and climatology of appalachian lee troughs

Documents

thermal vorticity

hpa thickness fills

winds barbs

categorized alts

hpa average

hpa wind directionchecked

kt nexrad

objective definition