# Presentation: 102252

## A Review of Derechos and the Role of Upper-Level Wind Shear on the Maintenance of the Associated Convective Systems

- Michael C. Coniglio
- National Research Council/NOAA/NSSL

## Outline

- Derecho Overview
- Definition, climatology, environments, forecasting issues
- Theories for strength and longevity of strong, linear MCSs
- Some issues with “RKW Theory” (Rotunno et al. 1988 & 2004 JAS)
- Alternative Idea
- Numerical simulations
- connection to observations

## What is a Derecho?

- In general, a long swath (> 400 km) of widespread severe wind gusts (> 26 m s-1) produced by an extratropical Mesoscale Convective System (MCS)
- Johns and Hirt (1987 WAF)
- Need occasional stronger wind gusts
- Bentley and Mote (1998 BAMS)
- Include other convective structures
- Coniglio and Stensrud (2004 WAF)
- Compromise between above two definitions

## Derecho Characteristics

- Derecho-producing MCSs often contain “bow echoes”
- Occur on wide range of scales (10-400 km and 20 minutes to several hours)
- Generally reflect forward advancement of convectively cooled outflow (larger scale outflow termed “cold pool”)

- Fujita (1978)

## Derecho Characteristics

- Some derecho-producing MCSs contain embedded supercells and/or cyclonic circulations
- Most severe wind damage often observed with these circulations NORTH of the bow-echo apex

- Atkins et al. (2004) AMS 22nd Conference on Severe Local Storms

## Example

- Progressive Derecho:  27-28 May 2001

- see Miller et al. (2002) AMS 21st Conference on Severe Local Storms

## Example

- Serial Derecho: 13 May 2002

## Why do we care?

-  Propensity for extreme winds

- Bob Johns, SPC Norman, OK

## Why do we care?

- “Boundary-Waters” Derecho July 4, 1999
- 477,000 acres blown down
- Sustained winds > 58 mph for 30-40 minutes
- Peak gusts > 100 mph

- 100 km

- USDA Forest Service, Superior National Forest

## Why do we care?

- “Boundary-Waters” Derecho July 4, 1999

- Minnesota Department of Natural Resources

## Why do we care?

- 1986-2003 fatalities due to:
- Derechos – 154
- Hurricanes – 254
- F0 & F1 Tornadoes – 71
- F0, F1 & F2 Tornadoes – 229
- Insured Losses:
- July 16, 1980 event - $1.3 billion
- (Isabel ~ $1.6 billion)
- May 31, 1998 event - $431 million
- (Bonnie ~ $394 million)

- Ashley and Mote (2004) AMS 22nd Conference on Severe Local Storms

## When do derechos occur?

-  270 events from 1980-2001

- ~70% occur in
- May-Aug (“Warm
- Season”)

- Coniglio and Stensrud (2004) WAF

## Where do derechos occur?

-  171 events, _all months_, 1986-2001

- Coniglio and Stensrud (2004) WAF

## Where do derechos occur?

-  113 events, _May-Aug_, 1986-2001

- Coniglio and Stensrud (2004) WAF

## Where do derechos occur?

-  58 events, _Sep-Apr_, 1986-2001

- Coniglio and Stensrud (2004) WAF

## Can we forecast them?

- Short term (~0-4 h) forecasts:
- Aided greatly by WSR-88D Radar Network
- Bow echoes and persistent embedded circulations
- Longer term forecasts (6-24 h)?
- Not there yet
- Continued poor performance of operational models in predicting warm-season precip (see June 2004 BAMS)
- Timing and location of convective initiation
- ETA model with parameterized convection has difficulty with realistic MCS propagation*

*******Bukovsky et al. (2004) AMS 22nd Conference on Severe Local Storms**

## Can we forecast them?

- Cloud-resolving forecast models?
- Develops convective motions on the grid scale
- Weather Research and Forecasting (WRF) model holds promise
- 4-km resolution 36-h WRF simulations initialized at 00Z run during BAMEX project
- often develops bowed-MCS structures and does reasonably well at 2-6 h lead times (www.joss.ucar.edu/bamex/meetings/weisman_wrf/), but...
- Predictability and reliability at longer lead times (> 6 h) is poor
- Ensemble approach?

## Pattern recognition of synoptic-scale environments – Warm season vs. dynamic

Zonal Pattern and many hybrids exist – near continuum of flow-pattern types

- Zonal Pattern and many hybrids exist – near continuum of flow-pattern types

- Can we forecast them?

- Johns (1993) WAF

*“**Warm Season” Pattern*

*“**Dynamic” Pattern*

- Coniglio et al. (2004) WAF

## Can we forecast them?

- Ingredients-based techniques
- Used for predicting convective mode and intensity
- Broad similarity between derecho and supercell environments
- Morphology of initiating process and the distribution of initial convective cells may be important
- Discrete vs. linear propagation
- Growth of the convectively generated cold pool
- Renders pre-convective forecasts of derechos difficult

- Doswell and Evans (2003) Atms. Res.

## Can we forecast them?

- Ingredients-based techniques lead to parameter clues for MCS strength and _longevity_
- Persistence of external forcing
- Depth and stability of low-level inflow
- Strength and distribution of vertical wind shear
- Strength and distribution of cold thunderstorm outflow
**What are the physical connections between the environments and MCS strength and longevity**
**Idealized numerical simulations useful**

- Evans and Doswell (2001) WAF

- Coniglio et al. (2004) WAF

## Model Environments

- Organized bow echoes like moderate to strong low-level shear in idealized numerical simulations
- Likelihood of bow echoes     as depth of shear-layer
- Organized bow echoes do not occur if the shear layer extends above 5 km (Weisman and Rotunno 2004 JAS)

- X

- X

- Modified from Weisman (1993) JAS

## Physical Connection?

- 	Based on vorticity balance between cold pool and low-level vertical wind shear (“RKW Theory”)*
- Upright convection (_deepest lifting_) favored when circulations balance
- RIJ forms between sloping updraft and cold pool
- Vorticity associated with elevated RIJs help to “prop-up” the updrafts to promote further vigorous convection and mesoconvective organization
- Process is greatly favored with strong low-level shear and weak shear above the cold pool

- Weisman (1993) JAS

- *Weisman and Rotunno (2004) JAS

## What about Observations?

- _91 derechos sampled during initial or early mature stages_

- Coniglio et al. (2004) WAF

## What about Observations?

- _91 derechos sampled during initial or early mature stages_

- Coniglio et al. (2004) WAF

**  ****X**
- mean

## What about Observations?

- see Miller et al. (2002) AMS 21st Conference on Severe Local Storms

**0000 UTC**
**28 MAY 2001**
**OUN**

- CAPE ~ 3800-4800 J/kg
- CIN ~ 0 J/kg
- LI ~ -10
- 0-2.5-km shear ~ 11 m/s
- 5-10-km shear ~30 m/s

## Models vs. Reality

- Cooler-season events often do have strong low-level wind shear and weaker shear aloft, but
- Conundrum:  Idealized numerical models probably most relevant to warm-season environments
- Many warm-season cases have relatively weak low-level shear and significant shear above the cold pool
- RKW mechanism (cold pool/low-level shear balance) may not be the primary controlling mechanism
- Alternative Ideas?
**Is there some basic dynamical importance to the existence of deep unidirectional vertical wind shear above the cold pool?**

- Coniglio et al. (2004) WAF

## Alternative Ideas?

- Guided by the observations, what is the role of the upper-level shear in idealized simulations on
- the basic 2D lifting of environmental air above a cold pool
- the 3D evolution of simulated convective systems?

## 2D Simulations

- Use NCOMMAS (Wicker and Wilhelmson 1995) to produce a set of dry 2D density current runs in neutral stability
- Usex=z=250 m, x=240 km, z=16 km
- Introduce a “cold pool” through a cooling function
- Run out to 1.5 h
- Use trajectories to calculate vertical parcel displacements after 0.5 h
- 7 runs: 0 to 30 m s-1 of 5-10 km shear in 5 m s-1 increments

- 12 m s-1 over 0-2.5 km

- 20 m s-1 over 0-5 km

- Cold pool motions

## Results: Maximum vertical velocity

- Strongest upward motion
- occurs for case with no
- upper-level shear

## Results: Maximum low-level (0-2 km) parcel displacements

- Lifting is enhanced for
- weak to moderate
- upper-level shear

## Results: 2D, no upper-level shear

## Results: 2D, 10 m s-1 upper-level shear

## Conclusion: 2D simulations

- Upper-level shear (above moderate low-level shear) _increases vertical parcel displacements_ _through an overturning updraft _(Moncrieff 1981), despite lower w along interface (Shapiro 1992, Moncrieff and Liu 1999)
- Shear entirely above cold pool can accomplish this
- What happens in 3-D?

## Results: 3D simulations, 2 - 4 hSurface precipitation mixing ratio, gust front, winds

- 0 m s-1 5-10 km shear

- 15 m s-1 5-10 km shear

## Results: 3D simulationsstorm-relative wind profiles at 3.5 h

## Results: 3-D simulationsTrajectories 3- 4 h

- 0 m s-1 shear:

- 15 m s-1 shear:

- 30 m s-1 shear:

## Observations:108 derechos sampled in weak/moderate forcing 

- Observations:108 derechos sampled in weak/moderate forcing

- Large differences in 5-10 km shear
- between beg./mature & decay

- _Note absence of critical _
- _Level for decay soundings_

**29**

**35**

**44**

- Coniglio et al. (2004) WAF

## Applications

- Can provide nowcast for demise of strong, linear MCSs – lifting of critical layer
- Requires forecast of cold pool motion (Corfidi 2003 WAF) & storm-relative wind profile
- Probabilistic forecasts of linear MCSs structure/longevity at longer lead times?
- Forecast “experiment” to take place in Summer 2005 at Storm Prediction Center
- May help explain how strong MCSs can persist after dark in situations not forced by a LLJ
- Overturning of residual-layer parcels may not care about nocturnal inversion (as long as cold pool is replenished)

## Final Thoughts

- While low-level shear is important, upper-level shear also is important - existence and location of critical layer with upper-level shear important component of MCS structure and maintenance
- Systems are maintained for longer periods in 3D and are substantially larger (not shown)
- _Observations suggests importance of upper-level shear_
- _Role of 3D convective structures on maintenance unresolved_

## THANKS FOR COMING!

***THANKS FOR COMING!***