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 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 # NA01NWS

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

PV = −g(∂θ/∂p)(ζ θ + 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 → ζ θ must increase → low level circulation Adapted from Martin (2006) 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 2008 Source: 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 2008 Source: 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 2008 Source: College of DuPage A A’

ALTs – Common Low-Level Features Potential temperature (black, K), geostrophic relative vorticity (fills, 10 −5 s −1 ), winds (barbs, kt) 100 km

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

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

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

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

MSLP anomaly 1°C Climatology – Results

MSLP anomaly 1°C Climatology – Results Over 75% of ALTs occur in June, July and August

MSLP anomaly 1°C 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

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

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 2000 MSLP (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 2000 MSLP (black, hPa) and 1000–850-hPa thickness (fills, dam)

1800 UTC 13 May 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 Category 2 (No PFT) occurs most frequently

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

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)

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

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