Does mesoscale instability control sting jet variability? Neil Hart, Suzanne Gray and Peter Clark Martínez-Alvarado et al 2014, MWR

Instability and Predictability Convective Instability Symmetric Instability Baroclinic Instability ~100km ~6hours ~1000km ~day ~1km ~30mins

Courtesy: ECMWF St Jude Forecast:Global Ensemble

Courtesy: MetOffice St Jude Forecast: MetOffice 4km

IR satellite image at 0600 UTC Courtesy: MetOffice & EUMETSAT St Jude Forecast: MetOffice UKV

Fig. 14 Browning 2004, QJRMS Why mesoscale instability? Browning 2004 hypothesized that Conditional Symmetric Instability (CSI) in the cloud head cloud is an explanation for the fingering seen in satellite imagery at the tip of some cloud heads The resulting slantwise circulation would see ascent into the cloud head with descent near the cloud head tip This hypothesizes a mechanism for sting jet descents, as seen in 1987 Great Storm

Moist Baroclinic LC1 experiment Fig. 7 Baker et al 2013, QJRMS Why mesoscale instability? Shading is number of pressure levels between 800hPa and 600hPa, that have CSI (MPVS*<0) Blue circle indicates position of air parcels manually identified as part of the sting jet descent T -10hrs

Why mesoscale instability? T -6hrs Moist Baroclinic LC1 experiment Fig. 7 Baker et al 2013, QJRMS Shading is number of pressure levels between 800hPa and 600hPa, that have CSI (MPVS*<0) Blue circle indicates position of air parcels manually identified as part of the sting jet descent

Why mesoscale instability? T -2hrs Moist Baroclinic LC1 experiment Fig. 7 Baker et al 2013, QJRMS Shading is number of pressure levels between 800hPa and 600hPa, that have CSI (MPVS*<0) Blue circle indicates position of air parcels manually identified as part of the sting jet descent

Friedhelm, Robert and Ulli Martínez-Alvarado et al 2014, MWR Smart & Browning 2013 Friedhelm 8 Dec ‘11Robert 27 Dec ’11Ulli 3 Jan ‘12 Identified with DSCAPE diagnostic applied to ERA-Interim (after Martínez-Alvarado 2012)

Courtesy: EUMETSAT, Sat24.com Cyclone Robert

Methodology Friedhelm 8 Dec ‘11Robert 27 Dec ’11Ulli 3 Jan ‘12 1. Produce 24 member ensemble simulations of each storm 2. Compute back trajectories from low-level jet region of each member 3. Cluster analysis to classify trajs. to identify descending airstreams 4. Explore link between these descents and CSI across ensemble

MetOffice Unified Model vn8.2 MOGREPS-Global ETKF 24 Init. Pert. Members (Bowler et al, 2008) MOGREPS-Regional N. Atl. & Europe Domain 12km, 70 Levels All storms initialised at 18 UTC the day before maximum intensity Results analysed further are T+10 to T+24 forecasts Model Setup

Synoptic Overview

Small spread in synoptic scale evolution between ensemble members: Good, since can now focus on mesoscale differences

Compute Back Trajectories Control Run from Cyclone Robert ensemble

Compute Back Trajectories Cloud Top Temperature Control Run from Cyclone Robert ensemble 850hPa 45m/s Isotach

Compute Back Trajectories Control Run from Cyclone Robert ensemble Trajectories Computed with Lagranto (Wernli & Davies, 1997)

Classification of Airstreams Ward’s Hierarchical Clustering Algorthim Use Relative Humidity to remove descents that started outside cloud head Identify class means that descend Cluster Class Mean Trajectories: Each trajectory described by x,y, P, θ w for 5 hours preceding arrival in low-level jet

Classification of Airstreams Ward’s Hierarchical Clustering Algorthim Use Relative Humidity to remove descents that started outside cloud head Identify class means that descend

Classification of Airstreams Use Relative Humidity to remove descents that started outside cloud head Identify class means that descend

Classification of Airstreams Use Relative Humidity to remove descents that started outside cloud head

Classification of Airstreams

Each Class contains a population of individual trajectories that arrive at given time. Next slide Size of these populations are gathered for all descent classes at all times for each ensemble member

# of Traj. Arriving in LLJ 1600

# of Traj. Arriving in LLJ 1600 Majority of of ensemble members have peak in # trajs at 12UTC

Ensemble Sensitivity Control run cloud head Control run 281K θ w 850hpa Interpret as change in # trajs for 1 s.d change in CSI metric X

Ensemble Sensitivity Methodology after Torn & Hakim 2008 X

Ensemble Sensitivity Methodology after Torn & Hakim 2008 X

Ensemble Sensitivity Methodology after Torn & Hakim 2008 X

Ensemble Sensitivity Methodology after Torn & Hakim 2008 X

 Consistent synoptic development across ensemble  Considerable variability in mesoscale wind features  Demonstrated method to classify descending airstreams  Large variability in number of descending trajectories across ensemble  Does mesoscale instability control sting jet variability? Strength of sting jet descent is associated with CSI in the cloud head (in Robert as simulated with MetUM) Conclusions

Cyclone Friedhelm

Cyclone Friedhelm Comparison to Martinez- Alvarado et al 2014 manual classification

Ensemble Sensitivity CSI across ensemble # trajs If correlation > threshold (0.5 used here), good!

Ensemble Sensitivity CSI across ensemble # trajs ∆x ∆y Calculate Gradient

Ensemble Sensitivity CSI across ensemble # trajs ∆x ∆y Ens. Sensitivity = ∆y (∆x = 1 s.d.)