Simon Vosper, Met Office

Very high resolution dynamical downscaling: 

Stable boundary layer flows in complex terrain 

Simon Vosper, Met Office 

Peter Clark, Emilie Carter, John Hughes, Adrian Lock, Aurore Porson, Stuart 

Webster, Peter Sheridan, Charlie Field, Jeremy Price, Andy Brown, Bradley Jemmet- 

Smith, Andrew Ross, John Edwards,…. 

© Crown copyright Met Office

Background 

• Can modern NWP be used to reliably forecast variations on 

kilometre-scales or smaller? 

• How well can we expect to predict the local detail of wind, 

temperatures, or fog? 

• In complex terrain, the stable boundary layer can exhibit rapid 

spatial changes in wind, temperature and fog. Cold pools can 

form in valleys during clear-sky calm nights. 

• These are important for applications such as forecasting along 

a route (e.g. for gritting), pollution dispersion, crop exposure… 


IR imagery of the Clun Valley, Shropshire 

22nd September 2011, Duffryn, ~2000 UTC 

© Crown copyright Met Office 

Thanks to Jeremy Price

Background 

• Even the finest resolution operational NWP models cannot 

resolve processes across ~1km hills and valleys. 

• Post-processing techniques are applied to correct raw model 

output. These rely on: 

• Statistical methods e.g. correct using nearby observations 

• Physically based techniques e.g. adjust air temperatures to 

account for errors in model orography height and unresolved 

processes 

• In the future, “dynamical downscaling” might become feasible 

for selected locations. This involves nesting very high-resolution 

grids within the NWP model to realistically predict the local 

detail of the flow. 


Questions 

Will focus on some high-resolution forecasts of flow and 

temperatures across complex terrain. 

Questions: 

• How do the model predictions improve as we go to higher 

resolution? 

• How does dynamical downscaling compare with simpler 

post-processing techniques? 


COLPEX Field Campaign 

COLd air Pooling EXperiment 

• Concerned with the formation of cold pools in 

valleys during stable night-time conditions. 

• Collaboration between Met Office and NCAS, 

involving the Univ. of Leeds, Salford and Surrey. 

• Clun Valley, Shropshire: 

• Rolling terrain ~200 m valley-summit, ~1 km 

wide 

• Ground network combined with upper-air and 

remote sensing measurements 


Location 

Clun valley 


Peter Clark

Instrument sites 

• 3 main mast 

sites 

• D=Duffryn 

• B=Burfield 

• S=Springhill 

• Instrumented 

transects 

• Radiosondes 

• Lidar 

• Instrumented 

vehicle 

2 km 


© Crown copyright Met Office Jeremy Price

Model configuration 

• Very high resolution simulations 

using the Met Office Unified Model 

• Nested from 4 km resolution domain 

to 1.5 km, and then 100 m model 

via a horizontally stretched grid 

• Enhanced vertical resolution: 12 

levels below 112 m vs 5 levels in 

operational model 

D=1.5km 

Stretching zone 

30 km 

Clun valley 

30 km 

D=100m 


(Peter Clark)

What does 100 m resolution give? 

Dx=100 m 

Dx=1.5 km 


Emilie Carter

9th September 2009 (IOP 4) 

Initial focus on the clear-sky COLPEX IOP 

Simulation from 15UTC 09 to 15 UTC 13 September 2009 

00 UTC 10/09/2009 


Potential temperature at 2m 

2009/09/09 1800 UTC 2009/09/09 1900 UTC 



2009/09/09 2000 1800 UTC 2009/09/09 2100 1900 UTC 



2009/09/09 2200 UTC 2009/09/09 2300 UTC 


North-South section through Upper Dyffryn, Clun Valley 

q ( o C) 

Dx=1.5 km 

Clearly © Crown 1.5km copyright resolution Met Office is inadequate!

Model screen temperature: D=100m L140 vs D=1.5km L70 

Duffryn 

Temperature 

minima well 

represented 

Daytime 

temperatures too 

cold 

Clear benefit 

of 100m 

resolution 

over 1.5km 


140-level 100 m model results 

Duffryn (Clun valley) Springhill (hill top) Burfield (valley) 

T( o C) T( o C) 

T( o C) 

-model 

-obs 

5 

Wind speed (ms -1 ) 8 Wind speed (ms -1 ) 8 Wind speed (ms -1 ) 

0 © Crown copyright Met Office 

0 

0

Cold pool strength 

• Repeatable nighttime 

DT of 

approx. -4 K 

• 100m L140 model 

gives good 

prediction of DT 

amplitude 

• Coarser vertical 

resolution (L70) 

results in weaker 

cold pools 


NWP temperature post-processing 

• A cheaper alternative to dynamical downscaling is to 

post-process the coarser (4 km) resolution predictions to 

account for the unresolved orography 

• Two-stage process: 

1. Perform correction for “smoothed” model height, based on local 

screen temperatures in UK4 (Sheridan et al. 2010) to determine 

an effective lapse rate. 

2. Additional “valley parametrization” accounts for cold pools in 

hollows (following Vosper & Brown 2008) 

• For each point, diagnose a local “valley depth”, H 

• Correction to temperature is a function of NH/U, where N and 

U are the buoyancy frequency and wind speed 


Post-processing 

10-11 Sep 

Observed 

4km model 

4km model + height correction 

4km model + height correction + valley parametrization 

•Need a hill parametrization? 


Peter Sheridan

Instrumented vehicle measurements 

10 Sept 2009 0520-0606 

Measured 

Post-processed 


Peter Sheridan



100m model vs 

post-processing 

• Post-processing fails 

to reproduce size of 

spatial variations: 

100m model 

100m model 

• 100m model hill 

tops are warmer 

• 100m model 

predicts interand 

intra- valley 

variability 


Peter Sheridan

Foggy case 10-11 Dec. 2009 

• Model predicts very 

detailed fog 

distribution 

• Fog forms at 

different times in 

different valleys 

• Correctly predicting 

the fog formation is 

challenging 

– Perhaps dynamical 

downscaling 

represents the best 

hope 

Model 

Obs 


Aurore Porson

Conclusions 

• Nesting of very high-resolution models (“dynamical 

downscaling”) can produce accurate and reliable forecasts 

of wind and temperature for stable boundary layer flows in 

complex terrain. 

• The character of cold pools in valleys is captured well, at 

least for “simple” clear-sky cases. 

• Observed drainage currents are reproduced 

• The available (reliable) detail is much greater than can be 

obtained using simple post-processing techniques applied 

to coarser resolution NWP data. 


Conclusions II 

• 100m-resolution simulations are expensive, requiring 

large computer resource. 

• For anything other than specific important small areas, 

they will remain out of reach for operational forecasting 

for the time being. 

• However, having established their physical realism, we 

can use such simulations to guide the development of 

improvements to the current post-processing techniques. 


Thank you for listening. 

Questions ? 


Vertical grid and timestep 

Operational grid: 70 levels up to 40 km 

5 q-levels below 112m 

Enhanced resolution: 140 levels up to 

40km 

12 q-levels below 112 m 

100m model uses a 5 second timestep 

For 20 ms -1 winds aloft, this implies a 

horizontal Courant number of 1. 

Made possible via SISL formulation of 

model 



Cold pool strength (IOP16)

Simon Vosper, Met Office

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?