1. © Crown copyright Met Office Convective Scale Data Assimilation needs and experiences from Nowcast Demponstration Project real-time trial in 2012 Susan Ballard, Zhihong Li, Dingmin Li, David Simonin, Lee Hawkness-Smith, Cristina Charlton-Perez

2. © Crown copyright Met Office Introduction In March 2012 the Met Office implemented an expereimental hourly 4D-Var assimilation cycle with 6-hour forecasts in the Nowcasting Demonstration Project (NDP) with 1.5km model for southern England and Wales Able to study skill in forecasting floods in southern England and Wales in summer 2012 In July 2012 the NDP was also ported to run on the P7 supercomputer and is due to run to March 2013 Aim to get hourly 4D-Var for whole UK on next supercomputer Aim in next few years for assimilation of GPS data, radar refractivity and radar reflectivity and ceilometer backscatter

3. © Crown copyright Met Office Configuration of the NDP ModelResolutionDA method DA time window CyclingForecast Length UKV (UK) 1.5km3D-Var (3km)3 hr T+36 (every 6h) NDP (Southern UK) 1.5 km4D-Var (3km)1 hr T+6h(P6)12h(P7) Nested in UKV. LBCs updated every 30mins; refreshed every 6hours(P6)/3hours(P7)

4. © Crown copyright Met Office Use of Observations in NDP 4D-Var assimilation of sub-hourly observations: Doppler radial winds from 5 radars, every 10 mins winds from 4 wind profilers every 15 mins MSG SEVIRI satellite radiances: channel 5, channel 6 & window channels (sea only) every 15 mins hourly 3D moisture from satellite + surface cloud obs hourly MSG cloud- and humidity-tracked winds hourly aircraft temperature & wind hourly surface temperature, relative humidity, wind & pressure Latent Head Nudging of radar-derived surface rain rates every 15 minutes Sadly we didn’t have time to test real-time 15min GPS datastream enough and it had a negative impact on a good case – may need improved data assimilation scheme to get benefit for convective cases

5. © Crown copyright Met Office Observation cut off T+45mins Observation Processing~1-2mins T-60minsT+0 T+60mins Nudging Radar-derived rain rates (every 15 mins) 1 hour fcst from T-90mins for background (first guess) 1hour to 2hour fcst from T-90mins with output every 10mins for T-30mins to T+30mins for Simulating observations In 4D-VAR Next analysis time Previous analysis time Latent Heat Nudging T+30minsT-30mins 4D-Var Observation window Current analysis Observations 3D-Var ~ 1 - 2mins 4D-Var ~ 3 - 10mins takes longer time if raining as more Doppler radial wind data 12hour fcst from T-30mins Takes ~10mins Available T+60mins NDP CYCLE

6. © Crown copyright Met Office Spreading observations in space: assimilating a 2K temperature increment with 1K error at 850hPa UKV length scales derived from 24 & 12 hour forecast differences at radiosonde locations: 180 km for streamfunction, 130 km for velocity potential / unbalanced pressure & 90 km for humidity NDP length scales derived directly from 6 & 3 hour forecast differences and vary with vertical mode: 60 - 10 km for velocity potential / stream function & between 30 km - 2 km for unbalanced pressure / humidity © Crown copyright Met Office

7. Vertical spreading of information For integrated column observations such as GPS column water the maximum increments will go in where the background error variance of specific humidity is greatest Spreading of single level variables such as refractivity will depend on vertical correlations not shown

8. © Crown copyright Met Office Line Convection: 1500UTC 28 th May 2012 Thunderstorms not present at analysis times STEPS failed to predict storms: neither extrapolation nor UK4 had them UKV developed isolated storms too far east NDP has a good representation of the thunderstorms NDP T+5 STEPS T+5 UKV T+12 radar Sferics obs

9. © Crown copyright Met Office Line Convection: 1500UTC 28 th May 2012 Thunderstorms not present at analysis times NDP has a good representation of the thunderstorms UKV developed isolated storms too far east NDP with GPS data also develops spurious storm too far east NDP T+5 UKV T+12 radar Sferics obs NDP with GPS T+5

10. © Crown copyright Met Office Thunderstorms & flooding June 28 th 2012: NDP forecasts for 09UTC NDP T+3 NDP T+1 NDP T+2 radar 8UTC NDP first to really capture Southern storms starting in S Wales

11. © Crown copyright Met Office Thunderstorms & flooding June 28 th 2012: NDP forecasts for 12UTC NDP T+4 NDP T+1 NDP T+2 radar

12. © Crown copyright Met Office Flash Flooding, July 12 th 2012, 13UTC UKV T+10 UKV T+4 NDP T+4 NDP T+1 radar UKV T+10 is best match to reality Later UKV & NDP appear to break up the rain band Need improved assimilation scheme - Probably background errors and control variables/balance relationships for this situation

13. © Crown copyright Met Office NDP experiences - subjective Some very good forecasts in centre of domain of thunderstorms with improvement on current extrapolation/forecast merging nowcast (UK4PP), UKV etc Location of convection very much tied to high ground in many cases – really useful to have orography in map background to see this Some poor forecasts where erroneous UKV lbc fc was made worse in NDP – need larger domain, better lbc Flooding events near boundaries – static – very similar to UKV – need larger domain eg Aberystwyth –better location of max than UK4 5Aug Pembrokeshire and Bristol – UKV had very slight displacement of PV anomaly and not enough convection around edge – NDP very similar Need to improve simulation of precipitation, cloud and convection in UM so that less patchy/blobby – will improve forecasts and DA – need new UM parametrizations or higher resolution? Loss of convergence line - flooding along S coast and Boscastle – in 10hour UKV fc, lost in 4hour UKV fc - hourly NDP assimilation also lost convergence line – need to improve B?, flow dependent, more convergence, w? Is there a problem with use of doppler winds? UK4 most often missed these events, it was poor on summer convection so UK4PP similarly poor For NDP size domain T+7 onwards just same as UKV lbc – need a larger domain to have more impact from NDP DA Need better treatment of GPS data in variational analysis – possibly problems with vertical spreading in convective situations

14. © Crown copyright Met Office P6 August 2012 0.2 mm/h - 40 km square UKVPP P6 June and July 2012 0.2 mm/h - 40 km square UK4PP Same LBC and Same validity time – available at same time to forecasters UKV is a longer range forecast with earlier data time than NDP forecast eg NDP 6Z to 10Z uses UKV 3Z lbc and is compared to 3Z UKV forecasts Of course UKPP is verified against itself at T+0/T+1 etc as radar rain rate is used for extrapolation and verification - we need an independent measure of skill and correct rain rates radar/guage merged analysis or impact on hydrology? UKPP ie UKVPP and UK4PP are Current extrapolation forecast Ie STEPS including merging With latest available UK4 or UKV NWP forecasts

15. © Crown copyright Met Office Future Work - Novel Obs Radar Refractivity – improved humidity in pre-convective environment Doppler winds – improved use - observation operator and error Wind profiler - treat as radar – new obs operator and better definition of obs error - Direct use of Reflectivity Direct use of Ceilometer backscatter for cloud and aerosol COPE field campaign summer 2013– flooding, improve convection model and DA, NDP Flood – NERC programme of collaboration with EA and Met Office to use COPE Mode-s aircraft winds High resolution AMVs Improved background errors Trial and improve impact of GPS data for convective regimes Cloud top pressure derviation in OPS – closer to model Insect winds DIAMET – use of cases to improve convective scale DA, B, reflectivity assimilation for frontal situations

16. © Crown copyright Met Office Needs from COPE Validation data for DA of: Reflectivity – cloud and precip in vertical and horizontal Refractivity – humidity profiles and variation in horizontal on level heights Doppler winds – DA methods to get correct vertical structure and horizontal convergence Ceilometer – cloud and aerosol Improved UM simulation of convection Plus?

17. © Crown copyright Met Office See http://www.metoffice.gov.uk/research/news Questions?