conditional symmetric instability and the development of sting jets oscar martinez-alvarado...
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Conditional symmetric instability and the development of sting jets
Oscar Martinez-Alvarado ([email protected])
Sue GrayLaura Baker
Department of MeteorologyUniversity of Reading
European Meteorological Society Annual Meeting29 September – 03 October 2008Amsterdam, The Netherlands
Extratropical cyclones:Shapiro-Keyser model
2CSI and Sting Jets – Contact: [email protected]
WCB WCBWCB
WCB
CCB CCB CCB CCB
I II III IV
• Extratropical cyclones carry with them the potential of
causing catastrophic damage to both life and property.
• Regions of strong winds at low levels
― Warm conveyor belt (WCB)
― Cold conveyor belt (CCB)
Sting Jets• Able to generate highly
damaging winds of more than 100 km/h
• Mesoscale (~100 km) region of strong surface winds occurring in rapidly deepening extratropical cyclones
• Transient (~ few hours), possibly composed of multiple circulations
• Occur at the tip of the hooked cloud head
3CSI and Sting Jets – Contact: [email protected]
CCB
WCB
III
Sting Jets
• Mechanisms hypothesized to be important:
– Evaporative cooling of cloudy air, and
– The release of conditional symmetric instability
(CSI)
• Vertical transport of mass and momentum
through boundary layer needed to yield surface
wind gusts
4CSI and Sting Jets – Contact: [email protected]
Conditionalsymmetric instability
• A combination of gravitational and
inertial instabilities in a moist
atmosphere
• Leads to slantwise convective
circulations
• Diagnosed by (Schultz and
Schumacher, 1999)
– Mg – *e relationship
• Negative geostrophic moist potential
vorticity (MPVg)
– Slantwise convective available potential
energy (SCAPE)
5CSI and Sting Jets – Contact: [email protected]
*1g g eMPV
Mg incr
easi
ng
e* increasing
Objective
• To determine the importance of conditional symmetric instability as a mechanism leading to sting jets
6CSI and Sting Jets – Contact: [email protected]
Case study: A storm in February 2002
0300
0500
0700
0200
0700
0400
08000400
Source: Met Office website
• Case Identified from climatology study over 7 years of MST radar data (Parton, 2007)
7CSI and Sting Jets – Contact: [email protected]
Numerical simulation
• UK Met Office Unified Model version 6.1
• 0.11˚ (equivalent to 12 km) horizontal resolution
• Enhanced vertical resolution (76 levels)
• Initial time: 1200 UT on 25/02/2002
• Initial conditions from ECMWF operational analysis data interpolated to the model resolution
8CSI and Sting Jets – Contact: [email protected]
Searching for sting jets
• Wind strength > 35 m/s
• Relative humidity > 80 %
• Vertical velocity < -0.05 m/s
• Lagrangian backward trajectories
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A sting jet in thestorm in February 2002
Wind strength at the top of the boundary layer
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m/s
A sting jet in the storm in February 2002
Model-derived surface wind gusts
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m/s
Sting Jets and CSI:Conceptual picture
Browning (2004)
Ascending
Descending
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Sting Jets and CSI
PVU
Pre
ssure
(h
Pa)
Sting jet
Ascending branch
Moist PV along trajectories
13CSI and Sting Jets – Contact: [email protected]
Sting Jets and CSI
PVU
Pre
ssure
(h
Pa)
Moist PV along trajectories
14CSI and Sting Jets – Contact: [email protected]
Sting Jets and CSI
J/kg
Cloud contour
Frontal zone
CAPE contours
SCAPE
Ascending air
15CSI and Sting Jets – Contact: [email protected]
Conclusions• A sting jet was identified in the case study
• It is likely this was responsible for part of the strong gusts recorded at the surface
• The presence of CSI around the region of occurrence has been verified using two alternative methods
– Positive SCAPE
– Regions of negative moist PV
• Non-zero CAPE is also present in the same regions
• Circulations are possibly due to the release of a combination of moist gravitational and moist symmetric instabilities
17CSI and Sting Jets – Contact: [email protected]
References
1. Browning, K. A., 2004: The sting at the end of the tail: Damaging winds associated with extratropical cyclones. Q. J. R. Meteorol. Soc., 130, 375-399.
2. Parton, G. A., 2007: Observation and interpretation of strong winds in the mid-troposphere. PhD thesis, University of Manchester.
3. Schultz, D. A. and P. N. Schumacher, 1999: The use and misuse of conditional symmetric instability. Mon. Wea. Rev., 127, 2709-2732.
18CSI and Sting Jets – Contact: [email protected]