A Subgrid Orography Scheme:Ready for Prime Time

Steven Ghan and Tim ShippertPacific Northwest National Lab

Ghan, S. J., X. Bian, A. G. Hunt, and A. Coleman, 2002: The thermodynamic influence of subgrid orography in a global climate model, Climate Dynamics, 20, 31-44.

Subgrid scheme

Implementation

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• Applies to all column physics– clouds– radiative transfer– vertical mixing– surface physics

Elevation ClassificationElevation Classifi cation

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Progress

• Scheme applied to CAM and CLM – Euler dycore– Finite-volume dycore

• Developmental branch updated to cam3.2.41• Bit-for-bit agreement between SP, SMP, SPMD• Restarts bit-for-bit agreement• Bit-for-bit agreement with dev trunk if subgrid scheme turned

off• Energy conservation demonstrated to within 0.01 Wm-2

• Load balancing within nodes for both dycores• Load balancing between nodes for both dycores (IJHPCA, 2005)• Runoff distributed according to elevation of river transport

model surface elevation• AMIP simulations at T42 and 2˚x2.5º resolution• Detailed evaluation in eight regions (J. Climate, 2006a)• IPCC A1B simulation 1977-2100 at 2˚x2.5˚ resolution (J.

Climate, 2006b)

Experiments

• AMIP SST – Initialize September 1977– Run through 1989– T42

• TOPOG 11 elevation classes• No TOPOG

– 2x2.5• TOPOG 11 elevation classes• No TOPOG

– 1x1.25• No TOPOG

• IPCC A1B– 2x2.5– TOPOG 11 elevation classes– 1977-2100

Evaluation

temperature

March Snow

Precipitation in other regions

Does the Scheme Impact the Grid Cell Mean Climate?

Precipitation at 2x2.5 resolution

Surface Air Temperature 2x2.5

Zonal and Annual Mean

Planetary Energy Balance

Global Annual Mean

T42TOPOG

T42 NOTOP

T42 2X2.5 TOPOG

2X2.5 NOTOP

2X2.5

FSNT 234.9 234.0 0.8 233.4 232.2 1.2

FSNS 160.3 159.1 1.2 18.8 157.3 1.4

FLNT 233.6 233.5 0.1 233.6 233.2 0.4

FLNS 57.4 56.6 0.8 58.9 57.9 0.9

RAD -101.51

-101.95 0.4 -100.1 -100.4 0.3

SHFLX 20.4 19.7 0.7 18.1 17.8 0.2

LHFLX 81.1 82.2 -1.2 82.1 82.6 -0.5

TFLX 101.5 101.9 -0.4 100.2 100.4 -0.2

RAD+TFLX

0.01 0.00 0.01 0.01 -0.01 0.02

PREC 2.779 2.821 -0.042 2.810 2.829 -0.020

QFLX 2.778 2.820 -0.042 2.810 2.829 -0.020

Computational Burden

Zonal Mean Burden

Code Changes

• Unlike any other parameterization– All column physics applied to each elevation class

• Mostly manifest at higher levels– phys_grid

phys_grid_initcreate_chunksscatter_field_to_chunkgather_chunk_to_field

– phys_types– dp_coupling– history

Summary

• The subgrid scheme provides valuable regional detail at a modest computational cost.

• It produces some biases, largely due to neglect of rainshadow.

• Its biases can be ameliorated by reducing grid size to 50-100 km.

• It produces little impact on grid cell means of most fields, except for snow water.

• It can be treated as an option with minimal retuning.

Future Work

• Update to support CCSM and hi-res CLM:– pass elevation class fields through the coupler– optional online mapping of atmosphere fields to CLM grid

• Present to LMWG for approval.• Submit proposal to SSC.• Merge on to developmental trunk.• Continue to maintain as CCSM changes.