© crown copyright met office met office experiences with convection permitting models humphrey lean...

© Crown copyright Met Office

Met Office Experiences with Convection Permitting Models

Humphrey Lean MetOffice@Reading, Reading, UK

Nowcasting Workshop, Boulder, Oct 2011


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 2005.

• “On demand” 1.5km model (9 domains) from Dec 2006

• UKV 1.5km model from Nov 2009.

• Extended range UK 4km (global downscaler) from Dec 2010

• Nowcasting Demonstration system from June 2012

• Convective ensemble (2.2km) from June 2012


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


UKV Domain

744(622) x 928( 810) points

1.5x1.5

1.5x4

1.5x4

4x1.5 4x1.5

4x4

4x4

4x4

4x4

Variablezone

Inner 1.5km domain covers most of UK.Gridlength increases to 4km at edge.


UKV Model from 03UTC 19/11/2009


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


Forecast visibility at 12 UTC 10/12/2003 from 18 UTC 09/12/2003

12km 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

© Crown copyright Met OfficeUrban heat islands in UKV

Heatwave temperature (00 UTC 19th July 2006)

Why High resolution?Benefits from more detailed land use: Urban Heat Islands


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).


3rd May 2002Scattered convection case

History 1: No convection scheme

Nigel Roberts


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.


Example of rainfall forecastsSquall line southern England

14 UTC 1st July 2003T+7 forecast

12km 4km

1km Radar


Rainfall Accumulations 12-18 UTC 16th August 2004

12 km 4 km NIMROD radarForecasts from 03 UTC

Peak Accumulations >60mmOn 4 km gridPositional error and false alarm

Boscastle Flood

Peter Clark


Snow Showers penetrating inland

• Well known problem with parameterised convection is showers not penetrating far enough inland.

25th Nov 2010 Snow showers coming in on NE wind gave significant snow in NEEngland


Snow Showers penetrating inland24 hour precip accumulation (mm) 25th Nov 2010

1km radar UKV (1.5km) NAE (12km)Operational models


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


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.


Skill depends on the scale you look at

Nigel RobertsNigel RobertsRoberts and Lean MWR 2008


Summary FSS scoresUKV vs NAE

Forecast

Ranges

Percentage of times UKV better minus percentageof times its worse.Background colour gives indication of statistical significance(green >95%).

Marion Mittermaier and Matthew Trueman


Summary FSS scoresUKV vs UK4

Forecast

Ranges

Percentage of times UKV better minus percentageof times its worse.Background colour gives indication of statistical significance(Green >95%).

Marion Mittermaier and Matthew Trueman


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”)


Gridscale structure in 750 hPa w13UTC 12/05/2010

4km 1.5km 500mEmilie Carter


Compare gridlengths down to 100m

4km 1.5km

500m 100m Radar (1km)

Emilie Carter

Features continue to get smaller

12 UTC from 06 UTC run 7th Aug 2011

Areas shown are 80x80km(whole domain of 100m)


Need to find out how to do best wecan at 1.5km.

• Mixing (horizontal and vertical)

• Shallow convection scheme

• Standard convection scheme (as in UK4)?

• Stochastic backscatter

• Microphysics

• Other?


Effect of vertical mixing at 1.5km

Radar Control Smag+Vert mixing

• Vertical Mixing has big effect on no of cells


Need to constrain model set up with observations

• DYnamical and Microphysical Evolution of Convective Storms (DYMECS). Hogan et. al. U of ReadingTrack 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.


Rainfall (mm/hr)

Rainfall (mm/hr)

Reflectivity (dBZ)

Reflectivity (dBZ)

Chilbolton RadarRadar Composite PPI Composite

Grey: 5dBZ isosurfaceRed: 30dBZ isosurface

Thorwald Stein


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.


Thank you for listening.Any questions?

© crown copyright met office met office experiences with convection permitting models humphrey lean...

Documents

kmradar slide

ne england slide

matthew trueman slide

orographic rain slide

apps ukv

scale models

model orography

parameterised convection