Assessing CFS Model Bias Associated Assessing CFS Model Bias Associated with the Marine Stratus over with the Marine Stratus over

Southeastern PacificSoutheastern Pacific

P. XieP. Xie1)1), W. Wang, W. Wang1)1), , W. HigginsW. Higgins1)1), M. Cronin, M. Cronin2)2), P.A. Arkin, P.A. Arkin3), 3), and R. Wellerand R. Weller4)4)

1) NOAA Climate Prediction Center1) NOAA Climate Prediction Center2) NOAA Pacific Marine Environmental Laboratory2) NOAA Pacific Marine Environmental Laboratory

3) ESSIC, Univ. of Maryland3) ESSIC, Univ. of Maryland4) Woods Hole Oceanographic Institution4) Woods Hole Oceanographic Institution

Acknowledgements:Acknowledgements:Mingyue Chen, Ake Johansson, and Suranjana SahaMingyue Chen, Ake Johansson, and Suranjana Saha

Warm SST Bias over SE PacificWarm SST Bias over SE Pacific

in CFS in CFS CMIPCMIP Simulations Simulations

Questions for This Year:Questions for This Year:

How long does it take for the warm bias to develop in the CFS forecasts from its initial condition to what is observed in CFS/CMIP?

examining CFS hindcasts

To what extent the warm bias is caused by the insufficiently simulated stratus clouds over SE Pacific?

performing CMIP simulations with radiation over SE Pacific corrected

Examining the Evolution Examining the Evolution of Warm SST Biasof Warm SST Bias

Looking at CFS hindcasts with different forecast leading times and comparing them the with initial (GODAS/R2) and balanced conditions (CMIP)

Daily and monthly climatology of CFS

hindcasts for a 23-year period from 1981 to 2003

Initiating date / month

Leading time from 1-45 days and 0 – 8 months for the daily and monthly hindcast climatologies, respectively

Evolution of the Evolution of the Warm SST BiasWarm SST Bias[Monthly Climatology [Monthly Climatology of CFS Hindcasts]of CFS Hindcasts] Averaged with respect to

different leading months using hindcasts with all different initiating months;

Warm bias starts from coastal areas and spreads steadily toward NW;

After 8 months, warm bias reaches very close to its balanced condition (CMIP)

Evolution of the Evolution of the Warm SST BiasWarm SST Bias[Daily Climatology[Daily Climatologyof CFS Hindcasts]of CFS Hindcasts]

Warm bias grows quite fast along the coast during the first month

Mean SST Bias Mean SST Bias over SE Pacificover SE Pacific

Evolution of mean warm SST bias over the SE Pacific as seen from the daily (top) and monthly (bottom) CFS hindcasts

Warm SST bias reaches 80% of that in the CFS/CMIP after 8 months

Bias Evolution in Bias Evolution in the Hindcasts with the Hindcasts with Different Initial Different Initial MonthsMonths

Warm SST bias starts at different coastal regions and evolves through different processes

SEP Mean Bias SEP Mean Bias Evolution for Evolution for Hindcasts with Hindcasts with Different Initiating Different Initiating TimesTimes Warm bias develops relatively

quickly in the first month

Warm bias grows very fast during boreal spring

Warm bias reaches close to the balanced condition from the spring of the second year

Bias evolution seasonally dependent

Bias Evolution Bias Evolution Near the Coast Near the Coast [76[76ooW;13W;13ooS]S]

Warm bias reaches close to its balanced condition in the first month

Warm bias may grow at a rate of more than 0.1oK/day during the first 30 days of hindcasts (e.g. in September)

Bias Evolution Bias Evolution Over Open Ocean Over Open Ocean [85[85ooW;20W;20ooS]S]

Warm bias develops relatively fast in the first 30 days (but not as much as that over the coastal region)

Bias evolution regionally dependent

Evolution of Evolution of Cloudiness Cloudiness in the in the CFS ModelsCFS Models

Averaged with respect to different leading times using hindcasts with all different initiating months

Cloudiness reached very close to its balanced condition in the first month

Evolution of Net Evolution of Net RadiationRadiation in the in the CFS ModelCFS Model

Net downward radiation biased in the initial condition

Adjusted to its balanced condition in a month

Positive radiation bias over a narrow band along the coast

RadiationRadiation and and SSTSST Biases for Biases for Different MonthsDifferent Months Monthly hindcasts climatology

for the first targeting month

Seasonal variations in location and distribution for both the SST and the radiation biases during early months of forecasts

The SST and the radiation biases are co-located through the seasonal cycle

Excessive incoming radiation caused by insufficiently simulated stratus clouds is a major contributor to the warm SST bias

RadiationRadiation and and SSTSST Biases in the Biases in the Hindcasts of Diff. Hindcasts of Diff. Leading MonthsLeading Months

Positive radiation bias confined within coastal areas throughout the forecast periods

Warm SST bias spreads out from the coastal region toward NW during the forecast period

Other mechanisms responsible for the spreading the warm bias

Examining the Impacts Examining the Impacts of the Insufficiently Simulated of the Insufficiently Simulated Stratus Clouds over SE PacificStratus Clouds over SE Pacific

Quantifying the stratus clouds’ contributions to the warm SST bias under balanced condition (CMIP)

Performing CFS CMIP simulations with radiation budget over SE Pacific corrected with daily climatology of observed radiation (SRB)

Comparing radiation-corrected CMIP simulations with the standard CMIP runs

Annual Mean BiasAnnual Mean Bias

The warm SST bias over the SEP is reduced by about half

Interannual VariabilityInterannual Variability

Interannual standard deviation of monthly SST

Tropical interannual variability is weakened with the reduction of the warm SST bias over SE Pacific

ENSOENSO

NINO3.4 index

Intensity of ENSO reduced compared to the standard CFS CMIP simulations and observations

Answers (?) Answers (?) How Long Does it take to develop the warm bias?

Seasonal and regionally dependent Faster during spring and reaches close to balance spring of the 2nd year More rapidly over coastal region than over open ocean On average, the warm bias reaches 80% of its balanced condition after 8

months

To what extent the insufficiently simulated stratus clouds play in forming and maintaining the warm bias

Triggering the warm bias over the narrow band along the coast Providing excessive energy to the model ocean over the coastal region Other mechanisms (oceanic and ar-sea interactions) responsible for re-

distributing the energy and spreading the warm SST bias Warm bias reduced by ~half if the excessive incoming radiation is

corrected Insufficiently simulated stratus clouds not the only factor contributing to

the warm SST bias