Page 1© Crown copyright 2006SRNWP 9-12 October 2006, Zurich Variable resolution or lateral boundary conditions Terry Davies Dynamics Research Yongming.

© Crown copyright 2006 SRNWP 9-12 October 2006, Zurich Page 1

Variable resolution or

lateral boundary conditions

Terry Davies Dynamics Research

Yongming Tang, Junichi Ishida


UM Dynamical Core

Fully compressible, non-hydrostatic deep atmosphere

Semi-Lagrangian advection Semi-implicit time integration

Horizontal C grid Vertical – Charney-Phillips Scheme Hybrid height coordinate

(1 )t t t t t td d

DuF

Dtu u F F

Davies, Cullen, Malcolm, Mawson, Staniforth, White, Wood Quart. Journal Roy. Met. Soc 2005


Vertical and Horizontal Grid Staggering

Horizontal staggering - Awakawa C-grid

No grid decoupling

Better geostrophic adjustment for wavelengths of gridsize less than Rossby radius of deformation

Vertical staggering - Charney-Phillips

No computational modes

More consistent with thermal wind balance

Can have complications in coupling with boundary layer parametrisation

W,

U,ρ

W,

Ui-1/2 ,j

Vi,j+1/2

Vi,j-1/2

Ui+1/2 ,j∏i,j


Physical Parametrizations

Edwards-Slingo Radiation (Edwards & Slingo 1996)

Mixed phase precipitation (Wilson & Ballard 1999)

New Boundary Layer + 38L (Lock et al 2000)

New GWD scheme + GLOBE orography

smoothed (Raymond filter)

Modern spectral database for gaseous absorption in the atmosphere + new H2O continuum -

flexible configuration Multiple scattering included Better optical properties for

clouds inc. non-spherical ice parametrisation








Physically based transitions between vapour, liquid, ice and rain

Ice content now a prognostic variable rather than diagnosed from cloud scheme








Allows for non-local mixing in unstable regimes

Scheme diagnoses 6 different mixing regimes in order to represent stable, well mixed and cumulus processes

Scheme includes boundary layer top entrainment parametrisation

Improved interaction with the convection scheme








Simplified scheme Expression for linear 2D flow

used to calculate total surface pressure drag

Gravity wave amplitudes proportional to depth of sub-grid mountains above blocked layer

Remainder of drag is attributed to flow blocking


Future Developments

Dynamical core improvements

More consistent treatment of moisture

Conserving semi-Lagrangian advection scheme

Variable resolution grid

Resolution increases - (70 levels, 40km)

New physical parametrizations

New prognostic cloud scheme

New convection scheme


UM Operational Configurations

Global 40 kmN320L50

North Atlantic & European 12 km

UK 12 km (retired)

New UK 4 km


Convective scale NWP

Variable Resolution / Nested models

Case Studies with one-way nested UM NWP with variable resolution UM


Variable Resolution

Grid StructureGrid varies from coarse resolution at the outer boundaries smoothly to a uniform fine resolution in the interior of the domain

UniformHigh Res

zoneVar-Res 2Var-Res 1

UniformCoarse Res 1

UniformCoarse Res 2

Typically, there are 3 regions, and inflation ratio R1 = R2 = 5~10%e.g. = 1 km, R1 = R2 = 10 % N_vr = 34 / 24 / 15 points = 25 / 10 / 4 km

R2R1


Variable Resolution

Grid StructureGrid varies from coarse resolution at the outer boundaries smoothly to a uniform fine resolution in the interior of the domain

UniformHigh Res

zoneVar-Res 2Var-Res 1

UniformCoarse Res 1

UniformCoarse Res 2

Typically, there are 3 regions, and inflation ratio R1 = R2 = 5~10%

e.g. = 1 km, R1 = R2 = 10 % N_vr = 34 / 24 / 15 points = 25 / 10 / 4 km

R2R1


Costs (relative to 4km)

Area/ 2 km 1.85km 1.5 km 1 km

100% 8 10 19 64

25% 5 - 10 25

10% - - - 10


Convective scale NWP

Forecasting precipitation from severe convection

Parametrized convection – limited success Very high resolution models (over a small domain), with

detailed controlling factors, such as surface forcing and orography – promising

Nesting -- typically 3 - 5:1 Requires a smooth transition Mismatch of grids and model physics (e.g. coarse

resolution model does not explicitly represent convection). Possible solution: variable resolution ?


UM NWP Model Domains

1-Way Nested Limited Area Configuration

HORIZONTAL VERTICAL (lowest km)


NWP Model Orography

12 km 4 km 1 km

Height of model orography (m)


Case Study

3rd May 2002 caseMay 3 2002 case is a scattered convection case.

To compare 1 km to 4 km variable resolution to a 1 km model nested inside a 4 km model .

First, the conventional nested model.


May 3 2002 Case ----- Nested model

1 km high resolution nested model and radar rainfall at 14 UTC

1 km4 km12 km Radar


May 3 2002 Case ----- Nested model

1 km high resolution nested model and radar rainfall at 15 UTC

1 km4 km12 km Radar


Summary of nested model result

3rd May 2002 case Nested models suffered two major problems:

Spin up problem: at the inflow boundaries (northern) the nested model is too slow to produce convection.

Transition problem: at the end of the run when finally the large convection cells are being advected in from the 4 km model, they remain as large cells in the north.

How well will variable resolution model do ?


May 3 2002 Case ----- variable resolution model

Rainfall at 14 UTC. The three regions of the variable resolution domain are also shown

Radar1km


May 3 2002 Case ----- variable resolution model

Rainfall at 15 UTC. The three regions of the variable resolution domain are also shown

Radar1km


Summary

In the variable resolution model, when the ratio of the minimum and maximum grid is the same as a conventional nesting ratio of 1 : 4, it performs better in resolving convective scale storms. In particular it has overcome the problems of spin up and transition, highlighted in the nested model.

Further study is needed on the physical parametrization schemes if ratio > 4.

We are currently working on a grid-scale dependent convection scheme.


Lateral boundaries

To run variable resolution LAM will still need lbcs.

Current lbcs use standard merging technique

Semi-Lagrangian advection applies lbcs naturally

Apply appropriate lbcs to Helmholtz equation

Need to filter small-scale outflow information

For mpp, lateral boundary files do not need external halos


The End

Page 1© Crown copyright 2006SRNWP 9-12 October 2006, Zurich Variable resolution or lateral boundary conditions Terry Davies Dynamics Research Yongming.

Documents

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