Trials of a 1km Version of the Unified Model for Short Range Forecasting of Convective Events

1. Humphrey Lean, Susan Ballard, Peter Clark, Mark Dixon, Zhihong Li and Nigel Roberts. Trials of a 1km Version of the Unified Model for Short Range Forecasting of Convective Events Met Office Joint Centre for Mesoscale Meteorology Reading, UK WSN05, Toulouse, Sept 2005

2. Background • Requirement to improve short range forecasts of hazardous weather. Severe convective events, fog etc. • Move towards short range NWP for nowcasting applications. • Moves towards higher resolution models • Enabled by improved computing resources and non-hydrostatic UM. • Until April 2005 highest resolution operational model 12km. • 4km UK model currently being implemented operationally (Bornemann et. al. paper 6.08). • Expect 1km model to provide further improvement.

3. High Resolution Trial Model (HRTM) • Run 12km, 4km and 1km models with fixed configurations (domains etc) and gather verification statistics to determine benefits/problems of high resolution (e.g. compared to 12km model). • Mostly concentrated on 1km model. • Main emphasis - convection out to T+6-9 • Also other significant weather e.g. Fog (Rachel Capon), Temperatures, Winds etc.

4. HRTM Domains Note that operational UK 4km model uses larger (whole UK) domain

5. Current Model Configurations

6. Assimilation Configuration • 12km 3d-Var, MOPS/LHN • 4km 3d-Var (scale selective), MOPS/LHN • 1km 4km increments, MOPS/LHN • 3 hour cycles all models • For more detail see Ballard et. al paper 3.02

7. Example of Rainfall forecasts 14 UTC 1st July 2003 T+7 forecast

8. Example of Rainfall forecasts (2) 15 UTC 13th June 2003 T+6 forecast

9. Summer 2004 Trial • Run seven cases from 2004 period (mostly • convective) • For each case run 4 forecasts at 3 hour intervals • Run suite with 4km, 1km assimilation and also • second with initialising 4km, 1km from 12km analyses. • Forecasts out to T+7 for 1km model • Aggregate statistics over forecasts and cases.

10. Summer 2004 Trial

11. Precipitation Verification Scores • Precipitation verification carried out on accumulations • Data interpolated/aggregated onto 5km grid. • Scale selective technique – filter out errors on scales less • than sampling radius. Scores as function of sampling radius • Aggregate data over all forecasts of all cases (i.e. 28 f/cs) • Either absolute or relative accumulation thresholds • (relative to look at spatial distribution and not bias). • More details in Roberts and Lean paper 8.27

12. Scores for 6 hour accums 16mm threshold Blue 12km Green 4km Red 1km Solid Assim Dotted spinup

13. Scores for 6 hour accums 1% threshold Blue 12km Green 4km Red 1km Solid Assim Dotted spinup

14. Scores for 1 hour accums 50k radius 4mm threshold Blue 12km Green 4km Red 1km Solid Assim Dotted spinup

15. Scores for 1 hour accums 50k radius 10% threshold Blue 12km Green 4km Red 1km Solid Assim Dotted spinup

16. Conclusions • Subjective analysis of cases often implies that 1km model • better than 12km and 4km. • Benefit over 4km that 1km is often better able to represent • convection explicitly. • Have aggregated results from 7 cases from summer 2004 • Currently tend to over predict rainfall. • If spatial distribution only is taken into account scores for 6 • hour accumulations show 1km model best for most spatial • scales.

17. Conclusions (continued) • For 1 hour accumulations scores again show benefits of • 1km model which improve with increasing forecast time. • Overall conclusion is that, although development still • required, 1km model clearly has potential to provide • improved short range forecasts of convective storms.