Land-Atmosphere CouplingLand-Atmosphere Coupling

Land-atmosphere coupling strength: the degree to which the atmosphere responds to anomalies in land surface state

GLACEGLACE (Global Land-Atmosphere Coupling Experiment): An intercomparison of land-atmosphere coupling strength across a range of atmospheric general circulation models

A pilot study (Koster et al., 2002) shows a wide disparity in the land-atmosphere coupling strength between 4 models

W Simulations: Establish a time series of surface conditions

Step forward thecoupled AGCM-LSM

Write the valuesof the land surface prognostic variablesinto file W1_STATES

Step forward thecoupled AGCM-LSM

Write the valuesof the land surface prognostic variablesinto file W1_STATES

time step n time step n+1

(Repeat without writing to obtain simulations W2 – W16)

Experiment Design

R(S) Simulations: Run a 16-member ensemble, with each member forced tomaintain the same time series of surface(deeper) prognostic variables

Step forward thecoupled AGCM-LSM

Throw out updated values of land surfaceprognostic variables; replace with values for

time step n fromfile W1_STATES

Step forward thecoupled AGCM-LSM

time step n time step n+1

Throw out updated values of land surfaceprognostic variables; replace with values for

time step n+1 fromfile W1_STATES

All simulations are run from June through AugustCourtesy of Zhizhang Guo

All simulations in ensemble respond to the land surface boundary condition in the same way is high intra-ensemble variance is small

Simulations in ensemble have no coherent response to the land surface boundary condition is low intra-ensemble variance is large

Define diagnostic variables that describes the impact of the surface boundary on the generation of precipitation.

Diagnostic Analysis

Ω = _________________16σ(t) – σ(t,E)

15σ(t,E)

2 2

2

Courtesy of Zhizhang Guo

In principle, imposing land surface boundary states should decrease the intra-ensemble variance of the atmospheric fields.

pdf of precipitation ata given point, acrossensemble members

correspondingpdf when land boundaryis specified

We are examining this in GLACE by looking at the variance ratio: 2

P (S)

2P (W)

Courtesy of Zhizhang Guo

OSUOSU:: Experiment Design using Experiment Design using GFS/OSUGFS/OSU

AGCM used: GFS T62 L64LSM: OSU LSM: OSU Initial Conditions: CPC AMIP (provided by Jha)Three ensemble experiments: W, R, S16 members each ensembleEach member covers 1994/06/01 to 1994/08/31The land variables prescribed in ensemble

R-expSoil moisture at 2 layersSoil temp at 2 layersCanopy water contentSnow depth

S-expSoil moisture at 2nd layer

Noah:Noah: Experiment Design using Experiment Design using GFS/NoahGFS/Noah

AGCM used: GFS T62 L64LSM: NoahLSM: NoahInitial Conditions: CPC AMIP (provided by Jha)Three ensemble experiments: W, R, S16 members each ensembleEach member covers 1994/06/01 to 1994/08/31The land variables prescribed in ensemble

R-expTotal soil moisture at 4 layers changed from 2 to 4 layersLiquid soil moisture at 4 layers new fieldSoil temp at 4 layers changed from 2 to 4 layersCanopy water contentWater equivalent snow depthActual snow depth new field

S-expTotal soil moisture at 2nd-4th layers increased from 1 to 3 Liquid soil moisture at 2nd-4th layers new field

NoahX:NoahX: Experiment Design using Experiment Design using GFS/NoahGFS/Noah

AGCM used: GFS T62 L64LSM: NoahLSM: NoahInitial Conditions:

Atm: CPC AMIP (provided by Jha)Land: Noah cycled GDAS

Courtesy of Zhizhang Guo

Courtesy of Zhizhang Guo

The The impact of land surface impact of land surface conditions on atmospheric conditions on atmospheric processes is examinedprocesses is examined

SummarySummary

a. Results show a broad disparity a. Results show a broad disparity in the inherent coupling strengths in the inherent coupling strengths of the different models.of the different models.

b. Coupling strength is controlled b. Coupling strength is controlled by the model’s atmospheric by the model’s atmospheric formulation (boundary layer formulation (boundary layer processes and moist convection).processes and moist convection).

a. Results show a broad disparity a. Results show a broad disparity in the inherent coupling strengths in the inherent coupling strengths of the different models.of the different models.

b. Coupling strength is controlled b. Coupling strength is controlled by the model’s atmospheric by the model’s atmospheric formulation (boundary layer formulation (boundary layer processes and moist convection).processes and moist convection).