characteristics and climatology of appalachian lee troughs
DESCRIPTION
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 PresentationTRANSCRIPT
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