Page 1© Crown copyright 2007 Physics for ‘High Resolution’ UM Configurations Peter Clark Met Office (Joint Centre for Mesoscale Meteorology, Reading)

© Crown copyright 2007 Page 1

Physics for

‘High Resolution’ UM Configurations

Peter Clark

Met Office (Joint Centre for Mesoscale Meteorology, Reading)


Talk Outline

1.Current status

2.Microphysics

3.Turbulence

4.Q&A

5.Urban surface exchange

6.Radiation


‘High Resolution’ Aims & objectives

Prediction of individual major storms over 1-3 h timescale. Fine scale DA – nowcasting.

Useful (statistical) prediction of storm characteristics over 24 h timescales.

Organisation and triggering better than parametrization can achieve.

Improvement of forecast characteristics particularly affected by surface forcing:

rainfall, visibility and fog, extreme wind, applicability to the urban environment

Intermediate scale model (4 km) operational since early 2006. UK coverage. 3h cycle 3DVAR+Latent Heat Nudging/Moisture Observation Processing

System.

Development of a new convective scale NWP model and data assimilation configuration with a grid length of about 1km.

1.5 km ‘on-demand’ 450x450 km Dec 2006. 1.5 km UK 2009. Ensembles 2011+


Current status – 1 & 1.5 high resolution models

Substantial experience running at 1 km. Now implemented 1.5 km. 76 levels (2x38), 50 s timestep. (Will be implementing 70 level set soon).

Enhanced microphysics (see later).

Standard BL scheme+del-4 horizontal diffusion, no convection scheme. (See later)

Standard MOSES II 9-tile surface exchange (ITE 25 m land-use over GB) + anthropogenic heat source. New ‘two-tile’ urban scheme under test.

Radiation called every 5 min (6 timesteps) + radiation on slopes.

Variable resolution working and likely to be adopted for 2009 implementation.


Microphysics


High Resolution Microphysics

Operational Unified Model

Wilson and Ballard (1999)

“Cloud Resolving” Models

Diagnostic


UM Physics Status for Convective Scale

Enhanced microphysics available from UM 6.0

Bulk, single moment formulation Switches enable choices:

Single ice prognostic with diagnostic split between snow/ice.

Prognostic/diagnostic rain.

Graupel/no graupel Most evaluation done with intermediate scheme

Tested in idealised model, especially GCSS LBA diurnal cycle (Grabowski et al. 2006).

Major benefits of prog. rain for ‘seeder/feeder’ orographic enhancement.

Cloud liquidwater

Water vapour

GraupelIce

crystalsSnow

aggregatesRain

Cloud liquidwater

Water vapour

Ice crystals +

Snow aggregates

Rain


Diurnal Cycle Case Study

Data from TRMM LBA observational campaign (Rondonia, Brazil)

Initialisation from representative single profile at sunrise (07:30 am local time). Diurnally varying surface fluxes. Bicyclic model domain.

Intercomparison of CRMs (GCSS Deep Convection WG Case 4, Grabowski et al. 2006).

Focus on development of convection in first 6 hours. Observed onset of precipitation is ~10:30 (3 hours after sunrise).

Plan view of model

surface rain rate 6 hours after sunrise (1.30pm local

time).

Average rainrates

through the diurnal cycle from TRMM-LBA radar.


UM Reference

GCSS TRMM-LBA Diurnal Cycle

UM with enhanced microphysics

Timeseries of vertical profiles of hydrometeor water contents

Comparison with CRM – possibly excessive glaciation


CSIP IOP 18 – 25/08/2006

Modis Terra 1125 UTCRadar 1130 UTC

Cloud streets from coast

Squall line

1146-1148 UTC

3GHz Radar


Unified Model 1.5km Domain

•360x288 gridpoints•76 Vertical Levels•Nested in UK 4km model•Initial and LBC operational 06 UTC 12 km ‘UK Mesoscale’ •No additional DA


1.5 km L76 UM Forecast 13 UTC

Cross Section


Microphysics sensitivity 11 UTC

Control

With graupel

No rain evaporation

Wind speed Potential Temp

White contours=Cloud fraction

Dashed line=Freezing level

HeterogeneousNucleation only at T<-40C


Future plans - microphysics

Improvements to vertical transport (especially graupel).

Improved timestep dependence – especially Bergeron-Findeisen.

Two moment single ice to replace single moment ice and snow.

Minor updates to process rates.


Turbulence


Turbulence at 1 km

Current forecasting capability of UM at 1 km horizontal resolution uses ‘standard’ non-local 1D BL plus fixed (del-4) horizontal diffusion.

Works well but not perfectly. Anticipate need for 3D scheme, but highly asymmetric grid. Starting point is Smagorinsky-Lilly approach: horizontal and vertical

diffusion function of Richardson no., shear and a mixing length that scales with grid length.

Tested robustness of the UM dynamics and implementation of scheme by comparing genuine large-eddy simulation with the Met office Large-eddy model (which has been thoroughly tested at this limit).

Dry CBL Cu-capped BL (BOMEX equilibrium trade cumulus case)

Tested appropriate choice of scheme at ~1 km using idealised diurnal cycle and real cases.


Problems with initiation and shallow cumulus

MSG High Res Visible 1 km Cloud-top temperature

1 km precipitation rate

CSIP IOP 12 28/07/2005

Cirrus

Cloud streets

Radar (5km)

We have a consistent problem of precipitation from explicit ‘shallow’ cumulus.


Subgrid turbulence scheme in UM

2mSf Ri 2

h hSf Ri

jiij

j i

uuS

x x

1/ 2

2

, ,1,3

1/ 2

2ij iji j

S S S

22 20 0

1 1 1

k z z

Smagorinsky-Lilly subgrid-turbulence scheme with Richardson number (Ri) based stability factor

where

Mixing length scale

Wind shear

Stability function (unstable)

1/ 2(1 16 )mf Ri 1/ 21(1 40 )

0.7hf Ri and

0 sC x where


Dry CBL idealised model set up

Met Office Unified model in idealised mode: bi-periodic domain prescribed forcings e.g. surface fluxes and geostrophic winds

Dry convective boundary layer case: prescribed surface heat flux of 300Wm-2

initial mixed layer up to 1km with overlying stratification Domain 5kmx5kmx5km resolution 50 m in horizontal for both. Refined vertical grid near the surface

for UM. Comparison with Met Office Large-eddy model in the same configuration.

Smagorinsky model: Mixing length = Cs D where D is the horizontal grid length- significantcontrol of sub-grid dissipation. Lilly ’69 derives a value of Cs=0.17 for a homogeneous inertial sub-range. In practical large-eddy simulation Cs is adjusted in the region of this value. A value of Cs=0.23 is used in the control UM and LEM simulations.


UM/LEM comparison at 50 m resolution

W at 1 km snapshotUM Cs=0.46 UM Cs=0.23

UM Cs=0.115 LEM

•UM works at 50 m resolution•Requires Cs smaller than LEM•Cu-capped BL acceptable

•More variability•Within range of other models


UM Simulations

Reference:1D vertical non-local boundary layer scheme.Constant horizontal diffusion.

3DSL “3D” Smagorinsky-Lilly local turbulent mixing

scheme with Cs=0.23.

Series of sensitivity simulations with variations to mixing length (Cs) and combinations of the above.


Sensitivity to grid resolution (Surface rainrate)

REFERENCE

Increasing delay of first rain and overshoot with decreasing resolution

“3D” Smagorinsky scheme

reduces overshoot significantly and reduces variation of delay with res.

200m “3D” Smagorinsky scheme is close to 200m CRM (within uncertainty)

1km reference run has the first rain at the same time as the 200m UM and CRM

3DSL Cs=0.23


Increasing delay of first rain and overshoot with decreasing resolution

“3D” Smagorinsky scheme reduces overshoot significantly and reduces variation of delay with res.

200m “3D” Smagorinsky scheme is close to 200m CRM (within uncertainty)

1km reference run has the first rain at the same time as the 200m UM and CRM

Sensitivity to grid resolution (Hydrometeor Content)

REFERENCE

3DSL Cs=0.23


Impact of vertical mixing

Increased vertical mixing in the boundary layer leads to earlier convective initiation

All UM runs have constant horizontal diffusion K=1430


Impact of vertical mixing

Increased vertical mixing in the boundary layer leads to earlier convective initiation

All UM runs have constant horizontal diffusion K=1430


Impact of horizontal mixing

Increased horizontal mixing in the boundary layer leads to later convective initiation

All UM runs have the non-local boundary layer scheme in the vertical


Impact of horizontal mixing

Increased horizontal mixing in the boundary layer leads to later convective initiation

All UM runs have the non-local boundary layer scheme in the vertical.

ConstDiff Coefficient: K=1430.

Max Diff for Cs runs: K=2086.


Implications for sub-grid turbulence param.

Results are a subtle balance of horizontal mixing (delays initiation) and vertical mixing (promotes initiation).

For 1km grid resolution, the results suggest: The non-local scheme is appropriate for vertical mixing in the boundary layer.

There is a need for increased mixing of convective updraughts in the free-troposphere to reduce the overshoot. A shear/stability dependent approach is more physical than constant coefficient diffusion.

For 200m grid resolution, the results suggest: The shear/stability dependent approach of the Smagorinsky-Lilly scheme is

more appropriate than the non-local scheme.

The model is close to convergence (from earlier comparison with 100m resolution simulations).


Impact of turbulence scheme on convective forecast (CSIP IOP18 - 25th Aug 2005)

Horiz Cs=0.075 Horiz Cs=0.10 Horiz Cs=0.15

Reference Satellite IR and RadarSatellite (Visible) MODIS


Convective cell statistics (CSIP IOP18)Sensitivity to turbulence scheme

Model data is area-averaged to 5km radar grid

Reference

Radar

Cell Area (>2 mm/h)

Cell Number (>2 mm/h)

Radar

Reference



Model data is area-averaged to 5km radar grid

Average convective cell size as a function of rainrate threshold Average number of convective cells

as a function of rainrate threshold

ReferenceRadar

Radar

Reference



Reference run has too many, too small convective cells compared to the observations, particularly at low rain rates.

Simulations with horizontal turbulence scheme have cell sizes closer to observed, particularly as the horizontal mixing is increased (higher Cs).

Simulations with the horizontal turbulence scheme have cell numbers closer to observed, particularly at lower rain rates (<4 mm/hr) but still have too many cells with higher rain rates (> 4mm/hr).

(Note, the 8mm/hr threshold is dominated by the main organised squall line in the radar and is not representative.)

The model still does not have enough stratiform rain around convective cores.


Summary

3D sub-grid turbulent mixing parametrization introduced into the UM (based on Smagorinsky-Lilly). UM works as LES (50 m).

At ~ 1 km use hybrid approach combining the 1D non-local boundary layer scheme with aspects of the 3D scheme.

Tested in idealised and real case studies and can have a very significant impact on convective initiation and evolution.

Reduces over-prediction of small convective cells at 1.5km. Reduces excessive rain rates in larger storms.


Future plans - turbulence

‘Blended’ BL and (moist) 3D turbulence.Mixed turbulence/large

eddy behaviour in BLSmagorinsky outside (?)

Stochastic backscatter. Initially based on

Weinbrecht/MasonExtensions for shallow

Cu?

Mix

ing

leng

th

∆x

Turbulence scale


Questions & Answers


Urban Surface Exchange


Urban surface exchange in the UM

The UM uses a ‘tile’ surface exchange scheme, including an ‘urban’ tile.

The urban tile is quite crude: Enhanced roughness. Enhanced drainage. Modified albedo. Urban ‘canopy’ to

represent thermal inertia of buildings.

Anthropogenic heat source

Nocturnal heat island in 1.5 km forecast– 05/07/2006 00 UTC


Impact of anthropogenic heat flux

23 Cases

London Weather Centre Remote Rural

With AHF

No AHF


Urban Canyons

Troof

Twall1 Twall2

Tfloor

Troof

Tcanyon

Tcanyon

Tcanyon

•Negligible roof<>canyon coupling.•Single canyon temperature.•Implies two-tile simplification.

•Resistance measurements (Barlow/Harman)•Resistance model (Harman)•Radiation model and two surface simplification (Harman)•Two-tile surface only (Best) UM•Two-tile with radiation model single column UM (Harman)•Further work, full UM (Porson)


Surface-only tests

Martin Best


Next Steps

Fully integrated two-tile model in UM. Parameter provision – different approaches.

Validate in surface only model Model intercomparison.

Impact on mesoscale flow. Boundary layer development through urban/rural/urban

transition.

Revisit momentum transport (and scalar) – move away from effective roughness treatment.


Radiation


Radiation

Edwards-Slingo radiation scheme has been modified to include slope aspect and angle in direct solar radiation part. (Dominant terms, based on Oliphant et al 2003).

Significant impacts on screen temperature but very difficult o demonstrate impact on forecast.


Scientific question. Spatial impact of model changes.

Single case

Change to vertical levels

Roberts, 2007, MWR (In press)


Scientific question. Spatial impact of model changes.

Single case

Modelling radiation on slopes


Questions & Answers

Page 1© Crown copyright 2007 Physics for ‘High Resolution’ UM Configurations Peter Clark Met Office (Joint Centre for Mesoscale Meteorology, Reading)

Documents

crown copyright

microphysics slide

radiation slide

models diagnostic slide

h cycle

idealised model

convection scheme

um physics status