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© Crown copyright Met Office Met Office Experiences with Convection Permitting Models Humphrey Lean Reading, UK Nowcasting Workshop,

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Presentation on theme: "© Crown copyright Met Office Met Office Experiences with Convection Permitting Models Humphrey Lean Reading, UK Nowcasting Workshop,"— Presentation transcript:

1 © Crown copyright Met Office Met Office Experiences with Convection Permitting Models Humphrey Lean Reading, UK Nowcasting Workshop, Boulder, Oct 2011

2 © Crown copyright Met Office Timeline of MO convection permitting models Met Office has been experimenting with convection permitting versions of UM since 2001 (NH UM version). UK 4km model in operational suite since April On demand 1.5km model (9 domains) from Dec 2006 UKV 1.5km model from Nov Extended range UK 4km (global downscaler) from Dec 2010 Nowcasting Demonstration system from June 2012 Convective ensemble (2.2km) from June 2012

3 © Crown copyright Met Office UKV Model Runs out to T+36 4 times a day. 3 hour 3dvar assimilation cycle with nudging of radar reflectivity. 1.5km over most of UK, variable res to 4km at edge of domain Similar configuration to low res models except: No convection scheme Smagorinsky turbulence Prognostic rain

4 © Crown copyright Met Office UKV Domain 744(622) x 928( 810) points 1.5x x4 4x1.5 4x4 Variable zone Inner 1.5km domain covers most of UK. Gridlength increases to 4km at edge.

5 © Crown copyright Met Office UKV Model from 03UTC 19/11/2009

6 © Crown copyright Met Office Carlisle Flood - Observed and Forecast Accumulations Roberts, Forbes + EA 12 km 4 km 1 km Hand analysis of gauges and radar 12 km 1 km Model Orography Why High resolution? Benefits from more detailed orography 1: Orographic Rain

7 © Crown copyright Met Office Forecast visibility at 12 UTC 10/12/2003 from 18 UTC 09/12/ km L38 (part domain) 1km L76 Visibility (m) at station height, synoptic observations (km) Rachel Capon and Peter Clark Why High resolution? Benefits from more detailed orography 2: Fog in valleys

8 © Crown copyright Met Office Urban heat islands in UKV Heatwave temperature (00 UTC 19 th July 2006) Why High resolution? Benefits from more detailed land use: Urban Heat Islands

9 © Crown copyright Met Office However biggest benefit is expected to be in representation of Convection. Explicit convection means losing problems associated with parameterisation at these gridlengths. Also represent related features which are important (convergence lines etc).

10 © Crown copyright Met Office 3 rd May 2002 Scattered convection case History 1: No convection scheme Nigel Roberts

11 © Crown copyright Met Office History 2: Smagorinsky turbulence At early stage in research into km scale models found that horizontal diffusion was needed to reduce gridscale structure. Also discovered that applying too much uniform horizontal diffusion had detrimental effect on convective initiation (delay). This was motivation for using Smagorinsky turbulence (only apply once shear built up). Currently UKV uses 2D Smagorinsky with BL mixing in vertical.

12 © Crown copyright Met Office Example of rainfall forecasts Squall line southern England 14 UTC 1 st July 2003 T+7 forecast 12km4km 1kmRadar

13 © Crown copyright Met Office Rainfall Accumulations UTC 16 th August km 4 km NIMROD radar Forecasts from 03 UTC Peak Accumulations >60mm On 4 km grid Positional error and false alarm Boscastle Flood Peter Clark

14 © Crown copyright Met Office Snow Showers penetrating inland Well known problem with parameterised convection is showers not penetrating far enough inland. 25 th Nov 2010 Snow showers coming in on NE wind gave significant snow in NE England

15 © Crown copyright Met Office Snow Showers penetrating inland 24 hour precip accumulation (mm) 25 th Nov km radarUKV (1.5km) NAE (12km) Operational models

16 © Crown copyright Met Office How about objective verification? Need to take care with standard gridpoint scores! April to Oct 2010 Equitable Threat Score (ETS) Using gauges M Mittermaier, N Roberts & S Thompson submitted to Met Apps UKV 1.5 km UK 4 km NAE 12 km Global ~25 km

17 © Crown copyright Met Office Predictability Issues Use of 1.5km model does NOT automatically mean that we can issue forecasts with 1.5km accuracy. Small scales less predictable so individual showers not predictable more than a few hours ahead (unless driven by larger scale feature such as orography or convergence line). Consequences for: 1. Sensitivity testing of convective scale systems except in extreme cases one case is meaningless 2. Verification of models scale selective techniques 3. Interpretation/presentation of forecasts Avoid presenting unpredicable information. Move to probabalistic presentation.

18 © Crown copyright Met Office Skill depends on the scale you look at Nigel Roberts Roberts and Lean MWR 2008

19 © Crown copyright Met Office Summary FSS scores UKV vs NAE Forecast Ranges Percentage of times UKV better minus percentage of times its worse. Background colour gives indication of statistical significance (green >95%). Marion Mittermaier and Matthew Trueman

20 © Crown copyright Met Office Summary FSS scores UKV vs UK4 Forecast Ranges Percentage of times UKV better minus percentage of times its worse. Background colour gives indication of statistical significance (Green >95%). Marion Mittermaier and Matthew Trueman

21 © Crown copyright Met Office Problems with representation of convection UKV does improve on representation of convection in lower resolution models with convective parameterisations. However problems remain: Peak rain rates often too great. w too large (up to 15m/s in UK) Cell properties very dependent on mixing settings. Too much gridscale structure (esp in w). Know in principle that deep convection very under-resolved at 1.5km. Evidence that behaviour at 1.5km still dominated by gridlength (convection permitting rather than resolving)

22 © Crown copyright Met Office Gridscale structure in 750 hPa w 13UTC 12/05/2010 4km 1.5km 500m Emilie Carter

23 © Crown copyright Met Office Compare gridlengths down to 100m 4km1.5km 500m 100mRadar (1km) Emilie Carter Features continue to get smaller 12 UTC from 06 UTC run 7 th Aug 2011 Areas shown are 80x80km (whole domain of 100m)

24 © Crown copyright Met Office Need to find out how to do best we can at 1.5km. Mixing (horizontal and vertical) Shallow convection scheme Standard convection scheme (as in UK4)? Stochastic backscatter Microphysics Other?

25 © Crown copyright Met Office Effect of vertical mixing at 1.5km Radar ControlSmag+Vert mixing Vertical Mixing has big effect on no of cells

26 © Crown copyright Met Office Need to constrain model set up with observations DYnamical and Microphysical Evolution of Convective Storms (DYMECS). Hogan et. al. U of Reading Track cells with Chilbolton research radar and build up statistics of properties of convection. (runs from now through summer 2012). COnvective Precipiatation Experiment (COPE). Blythe et. al. with Met Office. Development of convection in SW England. (Peninsular convergence lines) (field programme summer 2013). For both of these will compare to UM at gridlengths between 100m and 4km.

27 © Crown copyright Met Office Rainfall (mm/hr) Reflectivity (dBZ) Chilbolton Radar Radar CompositePPI Composite Grey: 5dBZ isosurface Red: 30dBZ isosurface Thorwald Stein

28 © Crown copyright Met Office Conclusions Met Office has gathered much experience with convection permitting versions of the UM. Current models are used for longer time ranges than nowcasting. Many benefits seen for representation of convection and other phenomena Problems still remain with representation of convection which are being addressed Need to consider predictability issues for verification and interpretation of forecasts.

29 © Crown copyright Met Office Thank you for listening. Any questions?


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