Xin Qu1, Alex Hall1, Steve Klein2, Peter Caldwell2

1Dep’t of Atmospheric and Oceanic Sciences University of California,Los Angeles

2Program for Climate Model Diagnosis and Intercomparison, LawrenceLivermore National Laboratory

June 10, 2013

On the spread of changes in marine lowcloud cover in climate model simulations

of the 21st century

Goal 1: Understand the persistent spreadin simulated changes in low cloud cover

Model

Heuristic model

!

"LCC =#LCC

#EIS"EIS +

#LCC

#SST"SST + R

Sensitivity ofLCC to SST

Climatechangein SST

Residualterm

Climatechangein EIS

Climatechangein LCC

Sensitivity ofLCC to EIS

Sensitivities of LCC to EIS and SST are estimated based onsimulated 20th century climate variability.

Simulated changes in EIS and SST

There is asystematicincrease in EIS ina warmingclimate.

SST increase ismuch larger inmagnitude thanEIS increase.

EIS and SST slopes

• EIS slope is generally positive, while SST slope is generally negative.• The magnitudes of EIS and SST slopes vary significantly from modelto model.

SST contribution is the major source ofthe spread

• The SST contribution accounts for nearly two-thirds of theintermodel spread variance of LCC and about half of it in CMIP5models.• Either EIS contribution or the residual is not significantlycorrelated with LCC changes.

EIS contribution SST contribution Residual

Role of SST slope to the spread

LCC changes are positively correlated with the SST slope.

Goal 2: Link LCC changes to PBL andcloud parameterizations

• Models with a largeLCC decreases aremostly those with the“K(Ri)” PBL schemeand “Diag(PDF)”cloud scheme.

• Models with a largeLCC increase aremostly those with the“Diag(Slingo)” cloudscheme (denoted bystars).

K(Ri) K(Ri)

PDF PDF

Slingo Slingo

Link SST slope to PBL and cloudparameterizations

• Models with a largeSST slope are mostlythose with the“K(Ri)” PBL schemeand “Diag(PDF)”cloud scheme.

• Models with asmall or even apositive SST aremostly those with the“Diag(Slingo)” cloudscheme.

K(Ri) K(Ri)

PDF PDF

Slingo Slingo

Goal 3: An observed constrain on SSTslope

Observed constrain on SST slope suggests LCC will likely decrease.

ISCCP/ERA-Interim ISCCP/ERA-Interim

K(Ri)+PDF K(Ri)+PDF

Slingo Slingo

An argument for a future decrease in LCC

!

"LCC =#LCC

#EIS"EIS +

#LCC

#SST"SST + R

Sensitivity ofLCC to SST

Climatechangein SST

Residualterm

Climatechangein EIS

Climatechangein LCC

Sensitivity ofLCC to EIS

(1) SST change is much larger than EIS change(2) SST and EIS slopes are similar in magnitude.

Summary

1. The linear contributions of the estimated inversionstrength (EIS) and sea surface temperature (SST) tosimulated changes in low-cloud cover (LCC) arequantified.

2. The SST contribution accounts for a big portion of theintermodel spread in LCC changes.

3. The sensitivity of LCC to SST perturbations drives thespread in LCC changes. This sensitivity is stronglyinfluenced by cloud and boundary layer parameterizationsin the global models.

4. Observed constrain on the sensitivity suggests LCC willlikely decrease.

Extra slides

Simulated changes in EIS and SST

Warming in tropical warming pools is greater than warmingin low cloud regions.

Simulated changes in EIS and SST

Warming over nearby land is generally greater thanwarming in the low cloud regions.

EIS slope is NOT linked to PBL and cloudparameterizations

There is nosystematicrelationshipbetween the EISslope andparameterizations.