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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 NOAA/NWS, Sterling, VA - PowerPoint PPT Presentation
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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 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
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
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’
ALTs – Common Low-Level FeaturesPotential temperature (black, K), geostrophic relative vorticity
(fills, 10−5 s−1), winds (barbs, kt)
100 km
ALTs – Common Low-Level FeaturesPotential temperature (black, K), geostrophic relative vorticity
(fills, 10−5 s−1), winds (barbs, kt)
100 km
Geostrophic Relative Vorticity Maximum
ALTs – Common Low-Level FeaturesPotential temperature (black, K), geostrophic relative vorticity
(fills, 10−5 s−1), winds (barbs, kt)
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
ALTs – Common Low-Level Features
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
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
• 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
ALT Categories – Examples
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
ALT Categories – Examples
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
ALT Categories – Examples
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
Storm Reports – Daily Distribution
Storm Reports – Daily Distribution
• 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