1. Plus: Patrick Bunn, Richard Jones & Rob Wilbraham (UG students)
CASE STUDIES LEEDS
Tim Baker, Alan Blyth, Matt Clarke (MO), Peter Knippertz, John Marsham, Doug Parker & Phil Rosenberg
Plus: Patrick Bunn, Richard Jones & Rob Wilbraham (UG students)

2. Overview 1) Tornadic cold front, 28/29 November 2011 (IOP5a&b)
Paper 1 (lead Matt Clarke): Convective fine structure Midlands/northern England on 29 Nov 2011 using AWS & radar
Paper 2 (lead Phil Rosenberg): Control of the large scale on frontal evolution Nov 2011 using BAe146 & NAE
Addition: MEnv thesis Richard Jones, WRF modelling, convective structures, might contribute to Paper 1 or new publication
2) Cyclones Zafer & Friedhelm, 1 & 8 December 2011 (IOP6&8)
Paper 3 (lead Tim Baker): WRF modelling, influence of diabatic/microphysical processes on storm evolution & sfc winds
Addition: BSc thesis Rob Wilbraham, pressure tendency equation analysis, could contribute to Paper 3
3) Heavy precip / vortices 30 April / 1 May 2012 (IOP10)
BSc thesis Patrick Bunn, using AWS, Chilbolton & radiosondes
Collaboration with Jeff???

3. 28/29 November 2011 (IOP5a&b)
Tornadic cold front

4. Case Overview and Motivation
Cold front crossing UK
Front bulges in Midlands/Northern England and frontal precip splits into segments
Tornadoes reported in regions of segmentation
Possible link to fronts merging or interaction between two fronts
Available data
1 minute resolution AWS data (UK)
Possible 1 minute resolution Irish AWS data
Flight B655 in Bristol Channel and S. England

5. AWS ‘Time-to-space’ analysis
System motion vector
1-minute resolution surface data used to investigate the structure of the front at the surface, and it’s variation in the along-front direction
Time-to-space analysis assumes that time variations in parameters may be translated to spatial variations, using some representative system velocity, or surface winds
Line convection segment velocity, obtained from a sequence of radar data, was used as the system speed in this case
N.B. The method assumes that fluctuations in the parameter(s) of interest are associated mainly with the system, travel with the system, and do not evolve significantly over the chosen analysis period
Station ‘A’
Tt-40
Tt-0
Tt+40
Temperature at analysis time
Temperature at analysis time minus 40 minutes, translated using system motion vector
Temperature at analysis time plus 40 minutes, translated using system motion vector

6. AWS ‘Time-to-space’ analysis – 29 Nov 1330 UTC
Generated from individual minute observation using the observed segment velocity
Cold front from Plymouth to Aberdeen
Sharp decrease in Ɵe over North Wales / northern England
Narrow plume of high Ɵe ahead of front (warm advection within pre-frontal LLJ) K

7. AWS ‘Time-to-space’ analysis
Time-to-space shifted data was mapped onto a regular grid (at approx. 2 – 3 km grid-spacing) and interpolated to produce fields of surface parameters
Analysis shows sharp wind veer, pressure rise and temperature decrease were coincident over northern England, where the line convection was intense and bulged eastwards
Front was more diffuse in the south, where the line convection was highly segmented: some evidence that the main baroclinic zone lagged to the west of the surface trough and wind shift line

8. AWS ‘Time-to-space’ analysis – 29 Nov 1500 UTC
AWS data gridded.
Overlay of line-convection segments from radar.
Convection intense over northern England.
Front more diffuse in the south; main baroclinic zone lagged to the west of the surface trough?!
Temperature field shows substructures well.
Wind and pressure fields measured at fewer stations, but still give reliable detail on the mesoscale.
Line convection segment leading edges at analysis time are shown in white

9. AWS ‘Time-to-space’ analysis
Comparison of time-to-space analysis (right) with analysis using a single observation from each site (left)
Time-to-space analysis better-resolves the sharp gradients in temperature, pressure and winds along the front, particularly across the intense part of the front which crossed northern England

10. Evolution of radar-observed reflectivity lines along/near cold front
Multiple lines observed in early hours over Ireland (perhaps a remnant of the double frontal structure which was evident the previous day, when the front was still in the Atlantic). Merger of two (perhaps three) lines between 0600 and 0900 over Ireland, and subsequent merger with a prefrontal line (green) over Irish Sea
Line became intense following merger and accelerated eastwards, eventually forming an eastward-bulging segment over N England (this produced the tornadoes)
Frontal structure extremely complex in south – dominant (but fragmented) leading line, associated with much of surface wind veer and temperature decrease. Numerous (mostly weak) lines behind the leading line, (rearmost line approx. 300 km behind)

11. Evolution of radar-observed reflectivity lines along/near cold front
Line merger associated with intensification and eastward acceleration, creating bulge.
Segments tend to align with low-level shear.
Where the angle between the front and segments was small, lengthy, intense segments occurred (e.g. over southern parts of northern England). Where angle was larger, shorter segments were observed
Since the segments also moved along the front in the direction of the flow in the pre-frontal air, the orientation of the front within which a given segment is situated could change over time
This resulted in segments ‘fracturing’ away from the northern end of the long, intense line segment over northern England (near the apex of the bulge)
The tornadoes mostly occurred at or close to the location of this ‘fracturing’ process
Line convection segment leading edges in white, analysed at 30-min intervals

12. Publication Questions – Paper 1
What causes the front to bulge in the Midlands/North England region? Driven by large scale frontal motion or local squall line type mechanism?
What causes break up of precip into sections? Is this related to frontal merger, split front structure or preferential band alignment?
How does this relate to reported tornadoes?
What is the role of the gap between the hills in Wales and the Lake District?

13. Front evolution from NAE
Surface θe gradient sharpens, between 0600 and 0900, then surface front rapidly propagates across UK
Corresponds with frontal “catch up” seen in Radar
Moist band, penetrating to 500 hPa at frontal surface “breaks up” at 1500

14. Along front variation
Sharp double surface front seen in the south, more diffuse as latitude increases.
Some relationship between surface front and dry/moist boundary aloft at -10 to 0 degrees longitude. Maybe dry intrusion being mixed down or penetrated by convection

15. Along front variation By midday the front has sharpened at 52/56˚
Double front still evident in the South

16. Zooming to aircraft scales
Two dry slots associated with front.
U wind shows these are overrunning the low level airmass.
Similarities to split front, or perhaps two merging fronts.
NAE shows similar structure, but eastern dry slot is not as deep and western one does not have the same separation from the surface

17. Publication Questions – Paper 2
How does the double front merge?
What is the interaction between the fronts?
Are the fronts acting as two fronts or a split front or some intermediate?
How it this impacting on interaction with the surface?
How is the situation to the South different to that in the North (link to Paper 1)?
How well are these processes represented in MO model forecasts?

18. Cyclonic windstorms Zafer & Friedhelm
01/08 December 2011 (IOP6&8)
Cyclonic windstorms Zafer & Friedhelm

19. Zafer

20. Zafer – precip and winds

21. Friedhelm

22. WRF control runs Model Setup Hi-Res Setup
3 nested domains with 27km, 9km & 3km horizontal resolution.
100 vertical levels.
Microphysics: Morrison 2-moment scheme (with Chris D’s diagnostics).
Convective Parameterisations on in outer 2 domains.
Initiated at /12/11 with ECMWF analysis.

23. WRF control run

24. Sting Jet??

25. Control Run
No Latent Cooling

26. Latent Cooling effect on high winds.
Control Run
No Latent Cooling

27. Control Run 700 hPa
Control Run 900 hPa

28. With Diabatic heating
Without Diabatic heating
Difference

29. Idealised lifecyle experiments – the model
Heckley 1980.
Constant PV in the interior – entire dynamics controlled by upper and lower surface temperature patterns.
No boundary layer friction. No diabatic heating.
Periodic in x (3600 km wavelength) – rigid walls in y, at +/- 3600km.
Normal mode grown from small amplitude.
Some technical glitches still, to do with transformation from geostrophic space ...

30. Left: surface temperature colours, surface pressure (thick contours) and tropopause pressure (thin contours). Right: surface geostrophic wind speed (colours), surface pressiire (thick) and upper level geostrophic windspeed (thin).
Day +2

31. Publication Questions – Paper 3
Role of latent heat release for storm deepening?
Role of latent cooling to enhance subsidence and increase surface windspeed (sting jet)?
Role of orientation of upper-level jet and trough in wrapping around the cold conveyor belt jet?
Why does the wind maximum in the SW quadrant develop so rapidly?
Role of diabatically generated mid-level PV strip?
We might well split the results into two papers depending on the details of the outcomes.
More links to obs needed!

32. Heavy precipitation & vorticies
30 April / 01 May 2012 (IOP10)
Heavy precipitation & vorticies