DYnamical and Microphysical Evolution of Convective Storms

1. DYMECS DYnamical and MicrophysicalEvolution of Convective Storms Thorwald Stein, Robin Hogan, John Nicol

2. Overview • Analysis of storm regions in Nimrod and UKV 3Z forecast • Tracking storm regions in rainfall data • Prioritizing storms to scan with the Chilbolton radar • A few interesting wet summer days

3. Nimrod data • 16 UK C-band radars (5.6-5.65 GHz, 5cm wavelength) • 1x1 km grid, 5min intervals • Study 400x400 km region centered on Chilbolton

4. Rainfall region analysis • Set rainfall threshold (1mm/hr) and minimum area (4km2) for regions to be labelled • Study daily statistics from hourly snapshots for model (UKV 3Z forecast) and observations

5. Rainfall region analysis: Area Model underestimatestotal number… Mostly missing smallstorms < 30km2 Comparable to 16 gridboxes at 1.5km

6. Rainfall region analysis: Max Rain Missing storms withlow maximumrain rates… Combined with area:Small storms with lowprecipitation rates

7. Rainfall region analysis: Mean Rain Divide region’stotal rainfallby storm area…Poor range of values.

8. Tracking storms • At T+1, compare image with previous time step • Use TITAN overlap method to check for storm movement: L1(T+1) K1(T) If OV(K1,L1) = A(B)/A(K1) + A(B)/A(L1) > threshold (e.g. 0.6) Then L1(T+1) is same storm as K1(T) B= (K1,L1) U

9. Tracking storms K4 L1 • L1 gets a new label (no overlap) • L2 gets a new label (OV(K1,L2) < threshold) • L3 gets label of K1 (OV(K1,L3) > OV(K1,L4)) and defined as “parent” • L4 gets a new label, but defined as “child” • L5 gets label of K2 (OV(K2,L5) > OV(K3,L5)) and property “accreted K2, K3” L2 K1 L3 L4 K2 L5 K3

10. Tracking storms K4 L1 • What if K4 were fast-moving? • Use velocity information… • Single overlap storms get velocity as distance between centroids • New storms (e.g. L2) get zero velocity • What about complex merging and breaking events? L2 K1 L3 L4 K2 L5 K3

11. Tracking storms Initiated from first image or from edge Initiated from another cell (break up) Initiated out of nothing (new cell) Nimrod radar observations UKV 3Z forecast

12. Tracking storms Track rainfall area and storm mean rainfall over storm life time Composite average storm life cycle Ellipses indicate 2D standard deviation Colours indicate time (blue = 0; red = 1) Nimrod radar observations UKV 3Z forecast

13. Chilbolton data • S-band radar (3 GHz, 10cm wavelength) • 2deg/s azimuth rate • 0.4 deg/s elevation rate • 300m range gate • 200km range

14. Prioritizing Storms 200 • Per storm, store: • Area • Azimuth • Range • [u,v] • Centroid • Bounding box • Et cetera... • Local rainfall maxima within storm (core, cell) work in progress... 150 100 50

15. Prioritizing storms Too near: Miss tops of storm ortakes too long to finish RHI Get rainfall maxima tostudy convectionand convergence Too far: Miss low-level precipitationand coarser resolution

16. Prioritizing Storms 200 • Scan scheduler: • Read nimrod scene • Prioritize storms • Issue radar commands • Scan strategy: • 4 RHI scans through each core in (clockwise-most) storm 1 • PPI volume scan (10 PPIs) through storm 1 • Repeat for next storm (anti-clockwise) • Finish with low-level PPI back to 1 150 100 50

17. PPI Composite Chilbolton Radar Nimrod Composite 160955 Rainfall (mm/hr) Reflectivity (dBZ) 161948 Rainfall (mm/hr) Reflectivity (dBZ) Grey: 5dBZ isosurface Red: 30dBZ isosurface

18. 7th August 2011 (Heavy thunderstorms)

21. 18th August 2011 (Bournemouth flooded)

22. 162247 163159

23. 23rd August 2011 (London floods predicted)

25. 062625 073245

26. 26th August 2011 (Floods in South Wales)

27. 093248 095442

28. 105917 112904

29. 27th August 2011 (Convective showers)

30. 151815

31. 100m model • Nested suite of models- 4km (UK4), 1.5km, 500m and 100m • Driven by the UK4 • 100m model • centred directly over Chilbolton • 80km x 80km • 70 vertical levels • 5 second time step • 3D Smagorinsky • Fixed domain

32. 100m model • Still developing and testing the model • Have a long list of experiments to try, including changing the time step, vertical resolution, BL scheme, Smagorinsky, … • So far identified 8 cases to investigate, mainly looking at several, small showers and convective initiation.

33. 9th March 2011 Lots of very weak showers over England from around 12Z, picked up a bit in the UKV, not much in the UK4 and not at all in the NAE. 4km 1.5km 500m 100m 1km

34. 9th March 2011 • Max w too high, tends to be at the top of the domain by the east boundary • 500m resolution structure can be seen in about 1/3 of the domain by the west boundary. • Cells are too small, numerous and intense, slightly worse than the 500m.

35. 12th May 2011 Rash of showers over much of UK in WNW flow. Better reproduced in UKV. Broken line of showers at 18z from Mersey to Wash reproduced in UKV. 4km 1.5km 500m 100m 1km

36. 12th May 2011 • “Explosion” of very small cells at the start • Generally too many cells • Lines of cloud/ precipitation appear evenly spaced just past the west

37. 26th May 2011 Rash of heavy showers over much of England and Wales in W flow. Better in UKV than UK4. • Structure of 500m model can be clearly seen in this case too along the west boundary • The lines of evenly spaced cloud and precipitation also appear 4km 1.5km 500m 100m 1km

38. Concerns from the first runs • Too many very small cells • Occasionally get an "explosion" of very small cells of precipitation • Spin-up issues • Can clearly see 500m resolution structure in some cases around the boundaries, up to 1/3 of the way into the domain • Lines of cloud and precipitation occur, evenly spaced and start about 1/4 of the way in from the boundary in more than one case • Max w seems too high in some cases • However, still in very early stage!