Characteristics and Climatology of Appalachian Lee Troughs Daniel B. Thompson, Lance F. Bosart and...
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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
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 Northeast Regional Operational
Workshop XIII, Albany, NY 3 Nov 2011 NOAA/CSTAR Award #
NA01NWS4680002
Slide 2
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
Slide 3
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
Slide 4
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 MaySeptember, 20002009 3.Categorized ALTs based on
their relationship with synoptic-scale cold fronts
Slide 5
PV = g(/p)( + f) (Static stability)(Absolute vorticity)
d(PV)/dt = 0 for adiabatic flow Flow across mountain barrier will
subside on lee side Advects higher downward warming g(/p) decreases
must increase low level circulation Adapted from Martin (2006)
Appalachians Lee Trough Formation: PV Perspective
Slide 6
ALTs Common Low-Level Features MSLP (black, hPa), 1000850-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
Slide 7
ALTs Common Low-Level Features MSLP (black, hPa), 1000850-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
Slide 8
ALTs Common Low-Level Features MSLP (black, hPa), 1000850-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 A A
Slide 9
ALTs Common Low-Level Features Potential temperature (black,
K), geostrophic relative vorticity (fills, 10 5 s 1 ), winds
(barbs, kt) 100 km
Slide 10
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
Slide 11
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
Slide 12
Vertical extent of warm core ranges between 850 hPa and 700 hPa
Average: 788 hPa Standard deviation: 61 hPa ALTs Common Low-Level
Features
Slide 13
Domain for Climatology DOMAIN WIND ZONE ALT ZONE
Slide 14
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
Slide 15
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
Slide 16
Climatology was based on the following 3 criteria:
3)1000850-hPa layer-mean temperature anomaly Averaged 1000850-hPa
layer-mean temperature along each 0.5 of latitude within domain
Checked for maximum 1000850-hPa layer-mean temperature along each
0.5 of latitude within ALT Zone Methodology for Climatology
Criterion: difference of maximum and zonal average 1000850-hPa
layer-mean temperature must be greater than a threshold value
DOMAIN WIND ZONE ALT ZONE
Slide 17
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
Slide 18
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
Climatology Results
Slide 19
MSLP anomaly 1C Climatology Results
Slide 20
MSLP anomaly 1C Climatology Results Over 75% of ALTs occur in
June, July and August
Slide 21
MSLP anomaly 1C Climatology Results 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
Slide 22
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.Inverted 2.No PFT: Non-prefrontal 3.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
Slide 23
1.Inverted trough extends northward from south of the ALT Zone
MSLP (black, hPa) and 1000850-hPa thickness (fills, dam) ALT
Categories Examples 0000 UTC 31 May 2001
Slide 24
2.No PFT trough occurs in the absence of a synoptic cold front
ALT Categories Examples 0000 UTC 10 July 2000 MSLP (black, hPa) and
1000850-hPa thickness (fills, dam)
Slide 25
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 2000 MSLP (black,
hPa) and 1000850-hPa thickness (fills, dam)
Slide 26
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 1000850-hPa thickness (fills, dam)
Slide 27
ALT Categories Climatology Category 2 (No PFT) occurs most
frequently
Slide 28
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
Slide 29
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
Slide 30
Different domain, same procedure as Mid-Atlantic ALT
Climatology in the Northeast NORTHEAST INTERMOUNTAIN REGION (NEI)
NORTHEAST COASTAL PLAIN (NECP)
Slide 31
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
Slide 32
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, 20002009 Storm Reports in
the ALT Zone Data and Methodology ALT ZONE climate.met.psu.edu
Slide 33
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
Slide 34
Slide 35
Pronounced mid-afternoon/early evening maximum in storm reports
between 2100 and 2300 UTC
Slide 36
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
Slide 37
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