Appalachian Lee Troughs and their Association with Severe Thunderstorms

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

Spring CSTAR Meeting 4 May 2012

NOAA/CSTAR Award # NA01NWS4680002

Outline

• Climatology of Appalachian lee troughs (ALTs; review)

• Quantification of CAPE/shear at first daily storm report

• CAPE on ALT days vs. non-ALT days• Composites• Technology transfer

Outline

• Climatology of Appalachian lee troughs (ALTs; review)

• Quantification of CAPE/shear at first daily storm report

• CAPE on ALT days vs. non-ALT days• Composites• Technology transfer

Data and Methodology for Climatology

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– “ALT Zone”

ALTZONE

Methodology for Climatology

• Climatology of ALTs was based on the following 3 criteria:1) 925-hPa Wind Direction• Wind component normal to and downslope of

Appalachians

2) MSLP Anomaly• Anomaly with respect to zonal average < −0.75 hPa

3) 1000–850-hPa Mean Temperature Anomaly• Anomaly with respect to zonal average > 1°C

• All 3 criteria must be met for 3° latitude

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

Outline

Climatology of Appalachian lee troughs (ALTs; review)

• Quantification of CAPE/shear at first daily storm report

• CAPE on ALT days vs. non-ALT days• Composites• Technology transfer

CAPE/Shear at First Daily Storm Report• To quantify severe thunderstorm parameters

characteristic of ALT Zone, CAPE/shear was calculated at location of first daily storm report

• Dataset: 32 km NARR (8 analysis times daily)• Procedure:

– Find location and time of first severe report on a certain day (0400–0359 UTC)

– Calculate MUCAPE and Sfc–500-hPa shear at location of storm report using nearest analysis time at least 30 min prior to storm report

ALTZONE

CAPE/Shear at First Daily Storm Report• Only included days in which first storm report

occurred between 1530 and 0029 UTC– Diurnal cycle is the dominant mode of temporal

variabilityTime of 1st Daily

Storm Report (UTC)Corresponding

NARR analysis time (UTC)

1530–1829 1500

1830–2129 1800

2130–0029 210076.3%

23.7%

Time of 1st Daily Storm Report (UTC)

1530-00290030-1529

CAPE/Shear at First Daily Storm Report• ALT Zone was divided into sectors to minimize

the likelihood of the first daily storm report not being representative of the environment

CENTER

NORTH

SOUTH

CAPE/Shear at First Daily Storm Report

• South sector peaks earlier (1800 UTC) than north sector (2000 UTC)

• Center sector has flat peak between 1800–2100 UTC

NORTH

CENTER

SOUTH

CAPE/Shear at First Daily Storm Report

• Higher median CAPE (shear) for first daily storm report in south (north) sector

• Higher shear in north sector is likely because it is nearer to the mean warm-season upper jet

Whiskers: 10th and 90th percentiles // Box edges: 25th and 75th percentiles // Line: median

NORTH

CENTER

SOUTH

CAPE/Shear at First Daily Storm Report

• CAPE (shear) at first daily storm report maximized in June, July and August (May and September)

Whiskers: 10th and 90th percentiles // Box edges: 25th and 75th percentiles // Line: median

Outline

Climatology of Appalachian lee troughs (ALTs; review)

Quantification of CAPE/shear at first daily storm report

• CAPE on ALT days vs. non-ALT days• Composites• Technology transfer

Background

• What is the reason for increased number of storm reports with the presence of an ALT?– Background

conditions similar, ALT acts as trigger?

– ALTs associated with increased CAPE?

ALT at 18Z NO ALT at 18Z0

2

4

6

8

10

12

14

16

18 17.0

9.3

Average Severe Reports per Severe Period (1800 to 0000 UTC)

Background

• What is the reason for increased number of storm reports with the presence of an ALT?– Background

conditions similar, ALT acts as trigger?

– ALTs associated with increased CAPE?

ALT at 18Z NO ALT at 18Z0

2

4

6

8

10

12

14

16

18 17.0

9.3

Average Severe Reports per Severe Period (1800 to 0000 UTC)

• Compare 0000 UTC observed MUCAPE values at GSO, RNK, WAL and IAD on ALT and non-ALT days– Data obtained from SPC sounding archive

(courtesy Rich Thompson)• Only use the times when observed MUCAPE >

0 and observed lifted parcel level is within 180 hPa of the surface

• Generate box and whisker plots for comparison

Methodology

0000 UTC Observed CAPE: ALT vs. Non-ALT Days

• All four stations have significantly greater median, 25th, 75th, and 90th percentile MUCAPE on ALT days

• Intuitive since ALTs contain low-level thermal maxima (by definition)

ALT DAYSNON-ALT DAYS

Whiskers: 10th and 90th percentiles // Box edges: 25th and 75th percentiles // Line: median

Outline

Climatology of Appalachian lee troughs (ALTs; review)

Quantification of CAPE/shear at first daily storm report

CAPE on ALT days vs. non-ALT days• Composites• Technology transfer

Review of ALT CategoriesCat 1 (Inverted) Cat 2 (No PFT)

Cat 3 (PFT, Partial FROPA) Cat 4 (PFT, Total FROPA)

Composite Methodology• Made composites for 3 of the 4 ALT categories

– Category 1 (Inverted) was omitted due to low frequency of occurrence

• Two composites of each category were created– Severe– Non-severe

Composite Methodology: Severe/Non-Severe Partitioning

• “Clustering” – attempt to control for population bias in Storm Data– Overlay a 0.5° by 0.5° grid box over the domain– If a storm report occurs within a certain grid box

on a certain day, that grid box is considered “active” for the day• Any subsequent storm reports occurring within the

active box are discarded for the day• The number of active grid boxes for each day are tallied

to measure how widespread the severe weather was on that day

Composites

CATEGORY (Icon)

# Members # Active Boxes

Time (UTC)

2N 22 0 1800

2S 22 > 20 1800

3N 17 0 1800

3S 17 > 20 1800

4N 17 < 4 Assorted

4S 17 > 9 Assorted

• “N”: Non-severe category• “S”: Severe category• Cat 2: No PFT• Cat 3: PFT, partial FROPA• Cat 4: PFT, total FROPA

Category 3: PFT, Partial FROPAMSLP (black, hPa), 2-m dewpoint (fills, °C; 20°C isodrosotherm in white), 10-m streamlines (arrows)

Non-severe (N=17)

Severe (N=17)

• Increased dewpoints over ALT Zone in severe composite

Category 3: PFT, Partial FROPAMSLP (black, hPa), 2-m dewpoint (fills, °C; 20°C isodrosotherm in white), 10-m streamlines (arrows)

Non-severe (N=17)

Severe (N=17)

• Increased dewpoints over ALT Zone in severe composite

• Surface low center position is different between the two composites

L

L

Category 3: PFT, Partial FROPA500-hPa heights (black, dam), Q-Vectors (arrows), Q-Vector divergence (fills)

Non-severe (N=17)

Severe (N=17)

• 500-hPa trough upstream of ALT Zone in severe composites

• Strong QG forcing for ascent does not affect ALT Zone in either composite

Category 3: PFT, Partial FROPASurface to 500-hPa vertical wind shear (black, kt), MUCAPE (fills, J/kg)

Non-severe (N=17)

Severe (N=17)

• MUCAPE values are 500–1000 J/kg greater over the ALT Zone in the severe composite

• Weak shear and weak QG forcing suggests severe category 3 events are not well organized/focused by synoptic-scale forcing

Category 3: PFT, Partial FROPA

Maximum difference in θe

from surface to mid-levels (lines, K), maximum mid-level lapse rate over a 200-hPa-deep layer (fills, K/km)

Severe (N=17)

• Wet microbursts are favored when vertical difference in θe

from surface to mid-levels is > 20 K

Atkins and Wakimoto (1991); Fig. 10

Category 4: PFT, Total FROPA

Surface to 500-hPa vertical wind shear (black, kt), MUCAPE (fills, J/kg)

Severe (N=17)

• Favorable juxtaposition of MUCAPE and shear exists over the northern ALT Zone

Category 4: PFT, Total FROPA

Surface to 500-hPa vertical wind shear (black, kt), MUCAPE (fills, J/kg)

Severe (N=17)

• Favorable juxtaposition of MUCAPE and shear exists over the northern ALT Zone

• Higher shear values and spatial distribution of storm reports suggest that category 4 severe events may be more organized and favor the DC to Philadelphia corridor

Percentage of category 4 days (n=130) with at least one active grid box

Outline

Climatology of Appalachian lee troughs (ALTs; review)

Quantification of CAPE/shear at first daily storm report

CAPE on ALT days vs. non-ALT daysComposites• Technology transfer

Technology Transfer: CAPE/Shear at First Daily Storm Report

• Boxplots of CAPE/shear at first daily storm report can be used to put an expected severe event into climatological context– Boxplots need to be re-done with obs in order to get more accurate values,

since NARR MUCAPE is underdone compared to obs

Whiskers: 10th and 90th percentiles // Box edges: 25th and 75th percentiles // Line: median

Technology Transfer: Conceptual Models of Composite ALT Categories

• Application of conceptual models can allow forecasters to quickly identify environments that are conducive to severe weather on ALT days

L> 25 kt Sfc.–

500-hPa Shear

Axis of High MUCAPE

Δθe = 20 K

Δθe = 20 K

Td = 20°C

500-hPa Trough Axis

Td = 20°C

Category 3 Severe Conceptual Model

For More Information

• This presentation, as well as past presentations, can be found at my website:– http://www.atmos.albany.edu/student/dthompso/• (Note: No “n” in “dthompso”)

• Email:– dthompson@albany.edu• (Note: Now there is an “n”)

Thank you for your time and suggestions.

Spare slides

• 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

Domain for Climatology

DOMAIN

WIND ZONE

ALT ZONE

-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

Location of First Storm Report by ALT Category

• Majority of first daily storm reports occur west of composite ALT– Orographic forcing for thunderstorm initiation?

Relevant Papers

• Koch and Ray (1997): Convectively active boundaries in NC

• Murphy and Konrad (2005): Spatial and temporal patterns of lightning in the southern Appalachians

• Parker and Ahijevych (2007): Radar-based climatology of convection in the mid-Atlantic

Location of First Storm Report by ALT Category

• Possible reasons for disparity– Differing methodology in MUCAPE calculation?– Boundary layer parameterization in reanalyses is not too good?