1. DYMECS The evolution of thunderstorms in the Met Office Unified Model Kirsty Hanley Robin Hogan John Nicol Robert Plant Thorwald Stein Emilie Carter Carol Halliwell Humphrey Lean Andrew Macallan Mal Clarke Alan Doo Darcy Ladd Thorwald Stein (t.h.m.stein@reading.ac.uk)

2. Convection-permitting models (e.g. UKV) struggle with timing and characteristics of convective storms NimrodUKV Original slide from Kirsty Hanley Model storms too regular (circular and smooth) Not enough small storms (smaller than 40 km 2 ) Model storms have typical evolution (not enough variability)

3. Track rainfall features in Nimrod data and UKV surface precipitation Analyse bulk storm statistics (area, mean rainfall) Use tracking information for real-time tracking with Chilbolton Study storm height evolution Derive vertical velocities from RHI scans through convective cores How to evaluate thunderstorms

4. Tracking storms

5. At T+1, compare image with previous time step Use TITAN overlap method to check for storm movement: K1(T) L1(T+1) U B= (K1,L1) 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)

6. Tracking storms 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” K1 L1 L2 L3 L4 K2 K3 L5 K4

7. Tracking storms What if K4 were fast-moving? – Use velocity information… Taking velocity as displacement of centroid brings trouble for breaking/merging events. Use FFT method to track displacement between rainfall images at larger scale K1 L1 L2 L3 L4 K2 K3 L5 K4

8. Storm statistics – 07-20 UTC Maximum rain-rateStorm area Original slide from Kirsty Hanley Too few weakly precipitating storms Too low maximum rain rate? Hail? Errors in Nimrod? Too few small storms (less than 40 km 2 )

9. Bin storms by area – 10 storms per bin Compare bin-averaged storm area with area-averaged rain- rate of each bin. NimrodUKV Storm statistics – 04-20 UTC Original slide from Kirsty Hanley Small but intense stormsSmall storms with weak precip

10. Sensitivity studies – total precipitation 3D SmagorinskyPrognostic graupel KK autoconversion Rhcrit = 0.99 Original slide from Kirsty Hanley

11. Observations (Nimrod) Normalised time Normalised area … and mean rainfall Model (UKV 3Z) Obtain mean evolution of area

12. Combine area and rainfall for mean storm life cycle Observations (Nimrod) Normalised mean rainfall Normalised area Model (UKV 3Z) Modelled storms stay too long at peak area Modelled storms all have peak rainfall at the same time Normalised area

13. Tracking storms … with Chilbolton

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

15. Too near: Miss tops of storm or takes too long to finish RHI Too far: Miss low-level precipitation and coarser resolution Height Distance from radar Radial winds Reflectivity Rainfall Prioritizing Storms Sweet spot for RHI scans

16. 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 50 100 150 200 Prioritizing Storms

17. Tracking storm “1504” Storm tracked in Nimrod data over 3-hour period shows growth of surface rainfall area as well as intensification in mean rainfall. Area Mean rainfall

18. Tracking storm “1504” Analysis of Chilbolton volume scans shows increase in height as area remains constant. Occurrence of 40dBZ coincides with higher mean rainfall in Nimrod data. Area Mean rainfall Height

19. Tracking storm “1504” Analysis of Chilbolton volume scans shows increase in height as area remains constant. Occurrence of 40dBZ coincides with higher mean rainfall in Nimrod data. Height

20. Tracking storm “1504” Analysis of Chilbolton volume scans shows increase in height as area remains constant. Occurrence of 40dBZ coincides with higher mean rainfall in Nimrod data. Height

21. Tracking storm “1504” Storm properties can be linked to different stages in life cycle. Attempt similar approach in hourly model cloud fields by forward modelling reflectivities. Area UM Storm for same case Growth Stable Intensification

22. 20 dBZ height statistics UKV (model) Chilbolton (obs) Morning Afternoon

23. Vertical velocities  Red towards radarBlue away from radar  Original slide from Robin Hogan

24. Estimation of vertical velocities from continuity Vertical cross-sections (RHIs) are typically made at low elevations (e.g. < 10 °) Radial velocities provide accurate estimate of the horizontal winds Assume vertical winds are zero at the surface Working upwards, changes in horizontal winds at a given level increment the vertical wind up to that point Must account for density change with height Original slide from John Nicol

25. Reflectivity (dBZ)Radial velocity (m/s) Vertical velocity (m/s)Horizontal velocity (m/s) 2D wind field (m/s) 10:55 UTC 26/08/2011 Sets of four vertical scans through a convective core can be used to track radial velocity features to retrieve vertical velocities. Original slide from John Nicol

26. 23/08 07/0304/04 07/0826/0827/0803/11 04/1113/12 Strong convection  w ≈1.7m/s Moderate convection  w ≈1.3m/s Weak convection  w ≈0.5m/s 14/1226/0103/03 10/0411/04 Moderate convection  w ≈0.9m/s Simple categorisation by std. dev. of vertical velocities (dBZ>15) Original slide from John Nicol

27. Assume that convergence at the lowest detectable level extends down to the surface No divergence into plane from cross-radial winds Likely to be true for linear structures (e.g. fronts) orientated perpendicular to the radar scan but not for circular structures. Vertical cross-section viewed from above Radar Are vertical velocities underestimated in cases such as this? By a factor two? Assumptions Original slide from John Nicol

28. Discussion Small storms: Need to go to higher resolution? Detecting convergence in dopplerized network