Conditional symmetric instability and the development of sting jets Oscar Martinez-Alvarado Sue Gray Laura Baker Department of Meteorology University of Reading European Meteorological Society Annual Meeting 29 September – 03 October 2008 Amsterdam, The Netherlands

Extratropical cyclones: Shapiro-Keyser model 2 CSI and Sting Jets – Contact: WCB CCB IIIIIIIV 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 3 CSI and Sting Jets – Contact: 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 4 CSI and Sting Jets – Contact:

Conditional symmetric instability A combination of gravitational and inertial instabilities in a moist atmosphere Leads to slantwise convective circulations Diagnosed by (Schultz and Schumacher, 1999) –M g – * e relationship Negative geostrophic moist potential vorticity (MPV g ) –Slantwise convective available potential energy (SCAPE) 5 CSI and Sting Jets – Contact: M g increasing e * increasing

Objective To determine the importance of conditional symmetric instability as a mechanism leading to sting jets 6 CSI and Sting Jets – Contact:

Case study: A storm in February Source: Met Office website Case Identified from climatology study over 7 years of MST radar data (Parton, 2007) 7 CSI and Sting Jets – Contact:

Numerical simulation UK Met Office Unified Model version ˚ (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 8 CSI and Sting Jets – Contact:

Searching for sting jets Wind strength > 35 m/s Relative humidity > 80 % Vertical velocity < m/s Lagrangian backward trajectories 9 CSI and Sting Jets – Contact:

A sting jet in the storm in February 2002 Wind strength at the top of the boundary layer 10 CSI and Sting Jets – Contact: m/s

A sting jet in the storm in February 2002 Model-derived surface wind gusts 11 CSI and Sting Jets – Contact: m/s

Sting Jets and CSI: Conceptual picture Browning (2004) Ascending Descending 12 CSI and Sting Jets – Contact:

Sting Jets and CSI PVU Pressure (hPa) Sting jet Ascending branch Moist PV along trajectories 13 CSI and Sting Jets – Contact:

Sting Jets and CSI PVU Pressure (hPa) Moist PV along trajectories 14 CSI and Sting Jets – Contact:

Sting Jets and CSI J/kg Cloud contour Frontal zone CAPE contours SCAP E Ascending air 15 CSI and Sting Jets – Contact:

Sting Jets and CSI J/kg SCAP E 16 CSI and Sting Jets – Contact:

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 17 CSI and Sting Jets – Contact:

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, 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, CSI and Sting Jets – Contact: