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Page 1© Crown copyright 2006 The Convective-scale UM Physics Developments Richard Forbes (MET OFFICE, Joint Centre for Mesoscale Meteorology, Reading)

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Presentation on theme: "Page 1© Crown copyright 2006 The Convective-scale UM Physics Developments Richard Forbes (MET OFFICE, Joint Centre for Mesoscale Meteorology, Reading)"— Presentation transcript:

1 Page 1© Crown copyright 2006 The Convective-scale UM Physics Developments Richard Forbes (MET OFFICE, Joint Centre for Mesoscale Meteorology, Reading) October 2006

2 Page 2© Crown copyright 2006 Talk Outline 1.Current status of convective-scale modelling at the Met Office. 2.Recent developments in sub-grid parametrization schemes. Convective-Scale Modelling (JCMM): Peter Clark, Rachel Capon, Richard Forbes, Carol Halliwell, Humphrey Lean, Andrew Macallan, Nigel Roberts Convective-Scale Data Assimilation (JCMM): Susan Ballard, Mark Dixon, Zhihong Li, Olaf Stiller, Sean Swarbrick

3 Page 3© Crown copyright 2006 Current status of convective-scale UM

4 Page 4© Crown copyright 2006 Developments in convective scale NWP  Development of version of model appropriate for convective scale since ~2000.  Now running routinely at ~1 km (and higher) in research mode.  Encouraging results so far, but many enhancements under development  Recent testing focussed on convective storm cases from the Convective Storms Initiation Project (CSIP)  Also assessing model for other extreme events (flooding/fog/wind….)  Emphasis increasingly on data assimilation (3DVAR+LHN, 4DVAR in future).  4 km ‘intermediate’ UK model quasi-operational.  1.5 km ‘on-demand’ small area model planned for early 2007  1.5 km UK model planned for 2009 (next supercomputer).

5 Page 5© Crown copyright 2006 Standard Domains Previous Current

6 Page 6© Crown copyright 2006 CSIP IOP 18 12km/4km/1.5km comparison Animation of surface rain rates for 12km, 4km, 1.5km and radar from 0800 UTC to 2000 UTC on 25/08/2005 UM 12km UM 4km UM 1.5kmRadar

7 Page 7© Crown copyright 2006 CSIP IOP 18 – 25/08/ :30Z Radar 1130 UTC Modis Terra Visible Image1.5 km Model 6hr Forecast

8 Page 8© Crown copyright 2006 NWP Model Orography 12 km4 km 1 km Height of model orography (m)

9 Page 9© Crown copyright 2006 Fog Forecasting: Case Study 24 h loop 18 UTC 09/12/2003 1km L76 Forecast Log(Visibility) RMS Error 12 km 4 km 1 km 12 km

10 Page 10© Crown copyright 2006 Convective-scale UM verification Rainfall accumulation fraction skill score for different horizontal length scales

11 Page 11© Crown copyright 2006 Convective-scale UM verification Fraction skill score for hourly rainfall accumulations (for a 50km length scale and relative threshold of the 90 th percentile) for convective case studies in 2004/2005 (12 cases, 48 f/c). Dashed lines (spinup) Solid lines (assimilation) 4km spin-up significantly longer than 1km spin-up. Assimilation better than spin-up at all forecast times. After initial period, 1km better than 4km better than 12km.

12 Page 12© Crown copyright 2006 Convective-scale UM Issues  Initiation of convection is of prime importance – if the model does not correctly initiate, the subsequent forecast will be in error.  Need to understand the inherent predictability of different mechanisms (e.g. surface forced sea-breeze convergence, orography, gravity waves, secondary initiation) -> CSIP  The subsequent evolution of the convective cells is particularly dependent on the sub-grid turbulent mixing and then the microphysics parametrization once condensation/precipitation begins.  Turbulence, microphysics and surface exchange parametrizations are all areas of active development.

13 Page 13© Crown copyright 2006 Sub-grid parametrization developments  Sub-grid turbulence/boundary layer:  3D Smagorinsky-Lilly first order turbulent mixing scheme (stochastic backscatter ?)  Blending with non-local 1D scheme for intermediate resolutions.  Microphysics:  Graupel, representation of ice/snow hydrometeors, numerics, warm rain processes.  Impact of latent heat terms on the dynamics (cold pools).  Surface Exchange:  Soil moisture, soil properties, LAI, urban areas, lakes, snow….  Radiation:  Included slope aspect and angle into the incoming direct short-wave radiation scheme.  Parametrized Convection:  At 4km, CAPE dependent CAPE closure timescale to limit convective parametrization when high CAPE.  At ~1km, shallow convection mass-flux scheme being tested.

14 Page 14© Crown copyright 2006 Sub-grid Turbulent Mixing

15 Page 15© Crown copyright 2006 Parametrization of sub-grid mixing in the UM  Existing parametrizations in UM:  In the vertical  Deep/mid-level/shallow convection parametrization scheme  1D non-local boundary layer scheme (Lock et al. 2000)  In the horizontal  Conservative operator with constant diffusion coefficient  For high resolution, require a 3D turbulence parametrization  First order scheme may be sufficient (do higher order schemes provide any benefit ?)  We have implemented a variant of Smagorinsky-Lilly subgrid model.  Eddy-viscosity and eddy-diffusivity computed from resolved strain-rate, scalar gradients and certain prescribed length scales.

16 Page 16© Crown copyright 2006 Sub-grid turbulence scheme  Questions:  What are the resolution convergence properties ?  At what resolution does it become important to use a 3D local-mixing based approach ?  Can we improve on the intermediate resolutions ?  Do we need to treat the boundary layer differently to the free troposphere ?  Idealised simulations  Dry convective boundary layer  Shallow cumulus  Diurnal cycle of deep convection  Squall line  Real convective case studies

17 Page 17© Crown copyright 2006 Sub-grid turbulence: Dry CBL  Dry convective boundary layer  Initial neutral 1km deep boundary layer  300 Wm -2 surface heat flux  Boundary layer deepens with time and entrains air at top  Can look at properties as the horizontal grid resolution varies

18 Page 18© Crown copyright 2006 Sub-grid turbulence: Diurnal Cycle  Diurnal cycle of deep convection (GCSS Deep Convection WG Case 4).  UM simulations 100m to 4km resolution. Comparison with other CRMs.  Increasing onset delay and overshoot with decreasing resolution.  3D Smagorinsky scheme reduces delay and overshoot. UM with 1D BL scheme UM with 1D BL scheme + const. horiz diffusion UM with 3D Smagorinsky

19 Page 19© Crown copyright 2006 Sub-grid turbulence: 16/06/05 Case study  Impact of 3D Smagorinsky turbulence scheme is to reduce intensity of over-active convective cells. 1km UM with 1D boundary layer scheme 1km UM with 3D Smagorinsky scheme Radar (5km res.)

20 Page 20© Crown copyright 2006 Microphysics

21 Page 21© Crown copyright 2006 Microphysics and cold pools  The microphysics parametrization has an impact on cold pool generation through evaporative cooling, which affects the evolution of the convection and secondary initiation.  Many uncertainties and approximations in microphysical schemes which can affect the location and intensity of latent heating/cooling. Primary Initiation (Coastal convergence /orography) Cold Pool Secondary Initiation

22 Page 22© Crown copyright 2006 CSIP IOP 18 – 25/08/ :30Z Radar 1130 UTC Modis Terra Visible Image1.5 km Model 6hr Forecast

23 Page 23© Crown copyright 2006 CSIP IOP 18 – 25 th August 2005 – 12 UTC 4 km 12 km Screen Temperature

24 Page 24© Crown copyright 2006 CSIP IOP 18 – 25 th August 2005 Chilbolton Timeseries: Near-surface temperature

25 Page 25© Crown copyright 2006 Sensitivity to Microphysics: Case study Surface rainfall rate (mm/hr) at 13:00 UTC on 04/07/2005 from the 1km UM and radar. UM 1kmUM 1km on 5km radar grid Radar 5km 300 km

26 Page 26© Crown copyright 2006 Quantifying Microphysical Impacts  Some changes affect the mean precipitation, others have more of a dynamical impact (through influencing the cold pool generation) leading to shorter de-correlation times.

27 Page 27© Crown copyright 2006 Surface Exchange

28 Page 28© Crown copyright 2006 Surface exchange:  Soil moisture PDM (Probability Distribution Model)  What percentage of the rainfall remains in the soil and what percentage is runoff into the rivers ?  Urban representation  Street canyon/roof tops, anthropogenic heat source  Soil properties  Van Genuchten  Seasonally varying vegetation (Leaf Area Index)  JULES Joint UK Land Environment Simulator  Collaborative land surface model development (Met Office, UK Universities, Research Institutes)  Stand alone single-point / regional / global  Part of the UM system (used for NWP and Climate)

29 Page 29© Crown copyright 2006 Urban Impact on 20 m Temperature T+12 00Z 11/05/2001 Point 2 Point 3 Point 1 Point 4 Point 1: Upstream Point 2: Central London Point 3: Downstream Suburbs Point 4: Downstream Rural

30 Page 30© Crown copyright 2006 Summary

31 Page 31© Crown copyright 2006 Summary  4km UM operational for the UK (since May 2005). 1.5km on-demand UM operational km UK domain operational in  Current UM dynamics/physics giving broadly successful results. Verification methods show benefit of ~1km model over lower resolution models (with assimilation). (Need an appropriate method of verification for precipitation in high res. models).  However, there are still many improvements to be made and physics changes to investigate. For convection…..  Convective Initiation: Surface characteristics (can give predictability).  Early stages of convective development Turbulence scheme is a key factor.  Convective evolution and secondary initiation: Microphysics and cold pools.  Use of a hierarchy of idealised studies for understanding the implementation of sub-grid parametrizations can be very informative.

32 Page 32© Crown copyright 2006 The End


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