Future work on explicit convection

Transcription

1 Future work on explicit convection Humphrey Lean et al Reading, UK Cascade Downstream meeting, Oct 2014 Thanks to many colleagues

2 Introduction Convection permitting models have provided a step change in our ability to forecast convection. Here will talk about future developments in NWP models with explicit convection. Boscastle Flood 2004 one of the earliest demonstrations of the benefits of convection permitting models

3 Talk Plan Where are we with convection permitting models? Current work to further improve convection permitting models: Blending/Shallow parameterisations Higher resolution Mixing Microphysics

4 UKV Model looks very realistic The Cockermouth Flood, 19/11/ km L70 From 3 UTC 19/11 12 UTC analysis

5 UKV 1.5km UK Model convection 1.5km model Radar 15 UTC 12 th April 2012 Convective cells too large and too intense. Not enough light rain.

6 Average cell diameter (km) (averaged over 22 convective cases) Threshold Radar UKV (mm/hr) UKV Under-resolved Emilie Carter 1. Look at higher res to understand models and for future. 2. What to do about 1.5km? 6

7 Gridscale structure in 750 hpa w 13UTC 12/05/2010 case of light scattered showers 4km 1.5km 500m Emilie Carter

8 Poorly resolved convection What should a 1.5 km model resolve? For scales < 6 km, showers ought to be parametrized Theoretically justified in UKV? This issue has recently become more pressing in UKV due to blended boundary layer formulation which greatly improves StCu but diffuses out small showers.

9 Grey zone BL parametrization 20 th April 2012 (DYMECS) UKV (1.5km grid) Radar UKV control Seamless BL only Some large, some small showers Suppresses small showers

10 Shallow cumulus parametrization Traditional parametrization gives more or less uniform light convective rain (have experimented with modulating by cloud depth). Main problem is balancing parameterised and explicit convection in a satisfactory way in all cases. Blending + Shallow Cu param UKV A Lock

11 Grey zone shallow cumulus parametrization Main problem is balancing parameterised and explicit convection in a satisfactory way in all cases. Radar UKV control Some large, some small showers P2 blending Some resolved, some paramd showers

12 Increasing resolution beyond 1.5km Helps to better understand model in general. Get a feel for dependence of various things on model resolution. Helps future discussions about how to use increases in computer power (resolution vs domain size vs no ensemble members vs forecast length) Whilst UK 100m model will be a long way off may well want to run order 100m smaller models over selected areas (already are!).

13 4km 2.2km 1.5km 500m 200m 100m 1km RADAR Emilie Carter

14 COPE IOP 2 July sea breeze convergence Vertical velocity at 500m amsl Different scale! Below 500m starting to resolve BL turbulence K Hanley Kirsty Hanley

15 3D visualisation of data 25 Aug 2012 Cutaway: reflectivity Surface: rainrate Shading: extent of cloud Robin Hogan

16 John Nicol

17 DYMECS Conclusions UKV clearly under-resolves many small showers in UK High res models (~100m) improve some aspects but also some problems. Current configurations have a tendency to collapse clouds too much in horizontal when going to highest resolutions. Representation very sensitive to mixing. Also sensitivity to microphysics (fall speed). Problem is likely to be balance between explicit and parameterised turbulence (turbulence grey zone). Ongoing work: Measure turbulence in clouds (Matt Feist PhD).

18 Varying the mixing length Can we make the 200m model produce larger storms on 25 th Aug 2012 by changing the mixing length? Need to reduce the timestep to 3s to avoid hitting stability limit. Can produce larger storms but at the expense of the smaller storms. Increasing the mixing with height may be a solution i.e. make Smagorinsky scheme dependent on Δz. 200m l=300m 2491 storms l=100m 3583 storms l=40m 4359 storms Kirsty Hanley

19 Change in mixing length with height. 200m model Carol Halliwell

20 Subgrid Mixing scheme (P Clark) Does the Smagorinsky-Lilly scheme work as intended in buoyancy-dominated conditions? Are the stability functions optimal for saturated conditions? Effect of ENDGAME? 3D prognostic sub-filter TKE scheme as extension to existing TKE-based BL scheme (MY L2.5, 3).. (Cf, Cuxart, Bougeault and Redelsberger, 2000 ) Alternative schemes (Wyngaard (2004) retain tilting terms, prognostic stress and scalar fluxes..) Compare with other schemes in other models? 20

21 More detailed assessment of 3S Smagorinsky Peter Clark and Peter Hill (Met Office). Compute sub-filter fluxes from high-resolution model at different filter scales. Compare with Smagorinsky formulation. Compare with Smagorinsky in lower-resolution runs. Test case currently LBA diurnal cycle. SBL->CBL- >Cu->Cb. 21

22 Example TKE Comparison 22 Peter Hill (work in progress)

23 3D microphysics scheme Improvements since CASCADE (Boutle, Field, Furtado, Wilkinson): New snow PSD fallspeed representation Prognostic graupel and lightning New warm rain changes (DSD, KKautoconversion, Scale aware rates) China simulations running with all latest 3d improvements (Kalli Furtado et al.)

24 CASIM:Cloud-AeroSol Interacting Microphysics Ben Shipway 1, Adrian Hill 1, Dan Grosvenor 2, Amy Hodgson 2, Mohit Dalvi 1, Graham Mann 2, Annette Miltenberger 2, Paul Field 1,2 1: MetOffice, 2: Leeds New bulk microphysics scheme Initially developed for high resolution (km-scale) model Represents droplets, rain, pristine ice, snow and graupel Up to 3 moments can be selected to represent species Droplet/ice number prognosed from aerosol population Mechanical aerosol processing (e.g. Through droplet coalescence) New scheme common to UM and LEM

25 Cloud scheme implemented in CASIM Dan Grosvenor (Leeds) Small domain test of scheme with changing aerosol loadings. Cloud amount linked to aerosol loading in non-linear manner

26 Research using CASIM Warm cloud research fellow (Dan Grosvenor) ICE research fellow ci (Annette Miltenberger) Volcanic aerosol-cloud interactions PhD (Amy Hodgson) BACCHUS modelling of cloud aerosol-ice sheet interactions in the arctic (just appointed start Jan 2015) CLARIFY Namibian stratocumulus sheet+bba DACCIWA SWAfrica stratus, shallow Cu + megacity pollution and BBA ICE-D, ice and dust [RUBICON, conv invig/suppression submitted to NERC] [H2020 primavera, Global convection permitting, europe wide km-scale;...submitted to H2020 EU]

27 Conclusions Convection permitting, order 1.5km, models are providing benefits for forecasting and research models. Explicit representation of convection at 1.5km has problems particularly for smaller showers as often seen in UK. Grey zone convection parameterisation needed for km scale models. Higher resolution models improve some aspects but also have problems. Current research into subgrid mixing schemes, microphysics.

28 Thank you for listening. Any questions?