Background and Definitions Mesoscale Convective System (MCS): an isolated, nearly contiguous region of thunderstorms, sometimes surrounded by an extensive region of moderate rainfall. Total size is usually 100-300 km across. Bow-echo: a bow-shaped line of thunderstorms often containing strong surface winds. Mesoscale Convective Vortex: a lower-mid-tropospheric horizontal wind circulation derived from an area of convection (often an MCS). 0600 UTC 10 June, 2003 X 200 km 0540 UTC 10 June, 2003 11 June, 2003

Houze et al. (BAMS, 1989)

Houze et al. (BAMS, 1989)

Johnson and Hamilton (1988)

⁄ Basic Equations: 2D Squall Line *Also, no vortex tilting or stretching -- Or, more simply, consider the 2D horizontal vorticity equation: where

“Optimal” condition for cold pool lifting C/∆u = 1 RKW Theory Rotunno et al. (JAS, 1988) C/∆u > 1 “Optimal” condition for cold pool lifting C/∆u = 1 C/∆u < 1

2D Convective System Evolution: C/∆u << 1 C/∆u ~ 1 C/∆u > 1 Weak shear, strong cold pool: rapid evolution Strong shear, weak cold pool: slow evolution

Rear-inflow jet intensity increases with increasing CAPE!

RKW Theory: all other things being equal (e. g RKW Theory: all other things being equal (e.g., same external forcing), squall line strength/longevity is “optimized” when the circulation associated with the system-generated cold pool remains “in balance” with the circulation associated with the low-level vertical wind shear. Issue: Squall-lines are observed to be strong and long-lived for a wider range of environments than suggested by the models (e.g., weaker shears, deeper shears,….). “Optimality” depends on metric being applied… e.g., updraft strength, rainrate, total rainfall, maximum surface winds....

Now Consider a 3D Squall Line….without Coriolis: ⁄ ⁄ --

Weisman and Davis (1998) f=0

How can we systematically produce the observed line-end vortex pattern?

Line-end vortex mechanisms: Mature Phase:

⁄ Vertical Vorticity: ⁄ …flux form ⁄ Circulation:

⁄ ⁄

…tilting of system-generated horizontal vorticity Rear-inflow jet (Davis and Weisman, 1994; Weisman and Davis, 1998; Davis and Galarneau, 2009)

Role of Line-End Vortices Focuses and Intensifies Rear-Inflow Jet

Now Consider a 3D Squall Line….with Coriolis: --

   f-flux  ⁄ f-flux

Derechos: Severe Lines of Thunderstorms Damage from straight-line wind Long swaths (> 400 km), long duration (> 6 h) Wide damage swaths (100-500 km) Rapid movement: 20-30 m/s Earthsky.org NOAA Storm Prediction Center Csmonitor.com

Derecho: (Johns and Hirt 1987) Large CAPE Moderate Shear

29 June 2012 Derecho: Composite Radar 03 UTC 18 UTC 21 UTC 00 UTC SPC Storm Reports

29 June 2012 Derecho: 3 km WRF-ARW Forecast: DART Analysis, MYJ, Morrison 18 UTC (6 h) 21 UTC (9 h) 00 UTC (12 h) 03 UTC (15 h) Composite Radar 18 UTC 21 UTC 00 UTC 03 UTC

29 June 2012 21 UTC Reflectivity, Sfc winds *Cold Pool : -14 to -16 C *Strong Rear Inflow Jet *No cyclonic vorticity along leading line 975 hPa Theta 850 hPa Theta

29 June 2012 21 UTC Vertical X-sections *Cold Pool : 4 km deep, -14 to -16 C *Deep Rear Inflow Jet 8 km 4 km 0 km Reflectivity, Theta Winds, Theta-E

The 8 May 2009 “Super Derecho”: SUNY Albany 9 April 2014 The 8 May 2009 “Super Derecho”: 8-10 h of Hurricane-Force Winds, Extensive Damage… Radar 17:56 UTC 05/08/09 (Paducah) Morris Weisman NCAR/MMM Also: Lance Bosart, Clark Evans

Base Reflectivity 1334z KSGF 100-110KT winds at ~1kft Here is the corresponding reflectivity image. Base Reflectivity 1334z KSGF

Storm-Relative Velocity 1334z KSGF A close-up view of the Doppler velocity data from Springfield, Missouri WSR-88D. Doppler wind speeds were in excess of 100kt at about 1000 feet AGL. All sorts of interesting couplets are also evident along the line of deep convection ahead of the circulation. Storm-Relative Velocity 1334z KSGF

Occluding Stage: 09 UTC (21 h) 11 UTC (23 h) 13 UTC (25 h)

850 mb W (contoured) and Vertical Vorticity (shaded) 06 UTC 07 UTC 09 UTC 11 UTC 12 UTC 13 UTC

Vorticity Equation: Vertical Vorticity: tilting stretching

07 UTC 850 hPa Vorticity …Tilting… Stretching

Circulation: (other)

 f-flux   vort-flux  ⁄ + (other) tilting vort-flux f-flux

900 hPa Horizontal Vorticity, SR Flow, W (shaded) 08 May 2009 Derecho 900 hPa Horizontal Vorticity, SR Flow, W (shaded) With low-level jet from SW, streamwise horizontal vorticity evident in low-level environment….

So, is this a “Landicane”??

29 June 2012 versus 08 May 2009 Derechoes …Cold-pool dominant …Descending rear-inflow …Cyclonic mid-level vortex Radar Reflectivity Model Reflectivity 08 May 2009 …Mesovortex dominant …Elevated rear-inflow jet …Warm-core vortex extending to surface

Cape/Shear Intercomparison: 29 June 2012 08 May 2009 CAPE: 2500-3500 j/kg Shear: 30-50 kts (15-25 ms-1) CAPE: 5000-6000 j/kg Shear: 20-30 kts (10-15 ms-1)

850 hPa Intercomparison:   08 May 2009 29 June 2012 NO Low-Level Jet, NO west-east boundary Low-Level Jet, west-east boundary, Lee trough

Summary: ….3 km WRF-ARW was capable of not only predicting the potential for two high-end Derecho events, but also was capable of distinguishing the differing mechanisms… 29 June 2012: Cold Pool dominant 08 May 2009: Mesovortex dominant ….These two cases may help clarify the differing environmental characteristics that contribute to these two archetypes: 29 June 2012: Extreme instability, modest unidirectional low-level shear 08 May 2009: Mid-trop baroclinicity, low-level jet, strong directional shear (streamwise at low-levels)

Atkins et al. MWR (2004)

Atkins et al. MWR (2004)

Wakimoto et al. MWR (2006)

f=0 US = 20/2.5 t = 4 hr w, V z=3 km qr, V θ’ z=250 m 50 km no meso- vortices! continuous updraft 50 km

US = 20/2.5 t = 4 hr w, V z=3 km qr, V θ’ z=250 m 50 km locational bias 50 km

⁄

Weisman and Trapp (2003)

Weisman and Trapp (2003)

Weisman and Trapp (2003)

Weisman and Trapp (2003)

Weisman and Trapp (2003)

Trapp and Weisman (2003)

Trapp and Weisman (2003)

Atkins and Laurent, MWR, 2009

Atkins and Laurent, MWR, 2009

Thompson et al., WAF 2012 0-1 km BWD (bulk wind difference)

Example of a “Serial” MCV/MCS Case 0915 UTC 27 May 1998 0015 UTC 28 May 1998 0715 UTC 28 May 1998 2315 UTC 28 May 1998 0515 UTC 29 May 1998 1215 UTC 29 May 1998

IOP 1 200 km

Widespread Instability IOP 8 Mean Wind Profile 900 hPa 1730 UTC 11 June dBz 70 60 50 X 40 30 20 10 Widespread Instability m/s

Equivalent Potential Vorticity: = 0 for isentropic motions Equivalent Potential Vorticity:

Potential Vorticity: = 0 for isentropic motions Vorticity:

Long-time Behavior of MCSs Cool H Warm L H Cool (twice)

Raymond and Jiang (JAS 1990) Conceptual Model of Isentropic Lifting within a Steady Balanced Vortex (e.g., MCV)

MCV Induced Lifting and Destabilization Fritcsh et al. 1994, MWR

Low-Level Jet Scenario

Flash-Flood-Producing Convective Systems Associated with Mesoscale Convective Vortices Russ Schumacher and Richard Johnson WAF (2008)

…integrate along a parcel’s path: Vorticity Equation: Vertical Vorticity: …integrate along a parcel’s path: tilting stretching