Progress toward a HIM-based IPCC-class

Coupled Climate Model

Robert HallbergNOAA Geophysical Fluid Dynamics Laboratory

With contributions from A. Gnanadesikan, B. Samuels, and M. Winton

Layered Ocean Model Users’ Workshop 2005

Progress toward a viable HIM-based coupled model

1)Code conversion to F90/FMS

2)Coupling C-grid ocean to B-grid Sea ice model

3)Full nonlinear equation of state used everywhere except as the vertical coordinate• Pressure gradients• Mixed layer dynamics• Shear Richardson Numbers

4)Parameter tuning conflicts & challenges• Shear Richardson number – EUC and Overflows• Excessive Buffer Layer Mixing

5)Comparisons between HIM & MOM-based coupled model solutions.

HIM Code conversion to F90/FMS

• F90-HIM code exactly reproduces C-HIM code answers.

• F90 code uses either static or dynamic memory allocation.• Model with static allocation twice as fast as with dynamic on O3k• Uses macros so this does not require prevasive code duplication

• C code is a few percent faster than static F90 code on 1 CPU.

• Probably from reuse of limited scope temporary variables.

• F90/FMS code scales slightly better than C code.• Less synchronization (8-way messages vs. 2x2-way messages)• Buffered, multithreaded I/O

• C code has a much smaller memory footprint than FMS/F90 code.

• Identical interface to MOM4• HIM interface accommodates multiple staggerings of wind stresses.• Same run-time specification of diagnostics as MOM4.

The GFDL Sea Ice Model (SIS)

• State-of-the-art dynamical/thermodynamical sea-ice model 5 ice thickness categories + leads 3 vertical levels Elastic-viscous-plastic ice internal forces (rheology)

• B-Grid Sea Ice / C-Grid Ocean Coupling Issues: Stress

• Ice-ocean stress calculated on a B-grid, then averaged to a C-grid

• Open-ocean stress goes directly from A-grid to C-grid No ice flow through narrow channels

• May also occur because of proper ice dynamics

Gnanadesikan et al., subm. J. Climate

MOM4/SIS Sea Ice Mass

Using Fully Nonlinear Equation of State

1)Vertical coordinate – 45 2000 layers plus a bulk mixed layer and buffer layer• Horizontal mixing is still along coordinate surfaces.

2)Pressure Gradients:• In-situ density with thermobaric compensation

described by Hallberg, Ocean Modelling, 2005 & LOM Workshop 2004

3)Mixed Layer Dynamics• Integral Potential Energy Budgets based on local fits

of and

4)Shear Richardson Numbers• Based on in-situ values of and

Sp,

,pS

pSp

0,

pS p

0,

Annual Maximum Mixed Layer Depth

Log o

f m

axim

um

mix

ed layer

depth

in m

Convective adjustment based on 0

Convective adjustment basedon 0 or 2000

Spurious Mixing Due toConvective Adjustment Based on

Annual Mean Coupled Model SST Anomalies

Model still requires substantial tuning to reduce biases.

Seasonal Cycle of Coupled Model SST Errors

JFM AMJ

ONDJAS

Excessive Mixing between Buffer and Mixed Layers

• The combination of seasonal and higher frequency (diurnal or storm/calm cycle) leads to excessive mixing in the buffer layer.

• Adding resolution to the sub-mixed-layer region should help substantially.• Not an issue if the resolution does not track the mixed layer base (e.g. in

HYCOM).

Buff

er

Layer

Mix

ed L

ayer

Late WinterBuoyancy Profile

SeasonalRestratification

6 AMProfile

1 PMProfile

6 AMProfile

Mix

ed L

ayer

5-year mean June Equatorial Undercurrent

Shear Richardson Number Mixing and the Equatorial Under Current

Without parameterized mixing, the EUC is much stronger and shallower.

June, Year 6 June, Year 6

Impact of Richardson Number Dependent Mixing on the Equatorial Undercurrent at 140°W

June, Year 6 June, Year 6

Comparison between HIM- and MOM-based Coupled Models

MOM HIM

Topography for HIM and MOM based 1° coupled models.

CM2.0/HIM & CM2.0/MOM Annual Mean SST Errors

• Similar error patterns – errors possibly due to atmospheric forcing.

• Differences in errors – oceanographic impact on the climate simulation.

• Coupled models with 2 different atmospheres confirms this CM2.0: B-grid dynamical core atmosphere CM2.1: S.J. Lin’s finite volume dynamical core atmosphere (FVCore)

CM2.0/HIM (untuned) CM2.0/MOM4 (tuned)

SST Error in 3RD PentadMOM HIM

CM2.0 CM2.0

CM2.1CM2.1

Surface Air Temperature Differences

in two pairs of coupled models

CM2.1 (FVCore atmos)CM2.0 (B-grid atmos)

HIM SAT-MOM SAT in 3rd Pentad

CM2.1MOM SST on Equator

ENSO in MOM and HIM Based Coupled Models

CM2.1HIM SST on Equator

Summary• GFDL has complementary 1° HIM- and MOM- based coupled

climate models. Multiple models help identify source of biases. Should provide invaluable estimates of the robustness of results.

• Significant recent HIM developments to achieve a coupled model: Conversion of code to FMS/F90. Nonlinear equation of state used everywhere in physical

equations. Multiple fully coupled models assembled and run for decades.

• Much work remains to tune HIM-based coupled model. Multiple buffer layers to avoid excessive mixing. Reconciliation of conflicts between overflow and EUC entrainment

parameterizations. Mixed layer and eddy parameter tuning.

TOA RADIATION IN 3RD PENTAD

W/m2 SW OLR NET

CM2.0MOM 236.6 235.0 1.6

CM2.0HIM 238.0 236.3 1.6

W/m2 SW OLR NET

CM2.1MOM 238.8 237.9 0.9

CM2.1HIM 240.8 239.3 1.5

Sea Ice Concentration Error 3RD PentadMOM HIM

CM2.0

CM2.1

CM2.0

CM2.1

Levitus Salinity CM2.0MOM CM2.0HIM

Year 17; 2 km Depth Salinity and Velocity