# Presentation: 921235

## A Suitable Bias Correction for CFS Hindcast and Forecast

**A Suitable Bias Correction for CFS Hindcast and Forecast**

**1. Objectives**

- a. Have a suitable bias correction for CFS
-     hindcast and forecast;
- b. Give a more objective assessment for CFS
-     performance in seasonal forecast.

*P. Peng, A Kumar, W. Wang, Q. Zhang and B. Jha*

- CPC/NWS/NOAA

**2. Data**

-  CFS hindcast for the period of 1981-2004,
-  NCEP Reanalyses-2,
-  NCEP GODAS data,
-  OI SST.

**3. Findings**

- Figure 1   Temperature profiles per month used for GODAS
- in five consecutive latitude bands.  The third panel from the
- top shows that in the tropics (10S-10N) there was a big increase
-  of observational data from _[the Expendable Bathythermograph ](http://www.aoml.noaa.gov/goos/uot/xbt-what-is.php)_
- (XBT) and _[the Tropical Atmosphere Ocean Array of Moored Buoys](http://www.pmel.noaa.gov/tao/)_
-  (TAO) around 1990.

- Figure 2   The longitude-time plots of the tropical (10S-10N) SST anomalies.  From the left to the right panels are GODAS SST, _[Optimum Interpolation (OI) SST](http://www.emc.ncep.noaa.gov/research/cmb/sst_analysis/)_ and their difference, respectively.  The discontinuity of the GODAS SST bias around 1990 is evident as shown in the right panel.

- Figure 3  DJF SST anomaly averaged over the global tropics (10S-10N) for the period of 1982-2004.   The black, red and yellow lines represent the observations (OI SSTs), the ensemble mean of CFS 1-month lead forecasts and the individual members of the CFS forecasts, respectively. It is obvious that in the earlier period (1982-1990), the CFS forecasted SSTs are warmer than the observed, while in the later period the situation is reversed.

- Figure 4   Same as Fig. 3 except the forecasted anomalies are adjusted to be with respect to the climatology before and after 1990 respectively

- Figure 5  Anomaly correlation (AC) skill of surface air temperature with the *whole period based* bias correction (upper), sub-period based bias correction (middle) and their difference. The left is for one month lead forecast and the right is for six month lead forecast.

- Figure 6  Same as Fig. 5 except for 200hPa height.

- Figure 7  Same as Fig.5 except for precipitation.

- Figure 8  The absolute value of the difference between
- the climatology of the two sub-periods (upper),  estimated variability (standard deviation) of the 13-year climatology (middle), and the ratio of the two quantities (lower).  The ratio indicates the significance of the difference in climatology between the two sub-periods.  The left and right panels are for one month lead and two month lead forecasts respectively.

**4. Summary**

- The increase of observational data used for global oceanic data assimilation (GODAS) since 1990 caused the assimilated tropical and subtropical SST systematically lower (~0.5C) than previous time (see Figures 1 and 2). CFS hindcasts showed similar discontinuity because of the use of GODAS data as oceanic initial conditions (see Figure 3)
- Because of the discontinuity, the CFS hindcast skill has been underestimated by the conventional bias correction (i.e., calculating forecasted anomalies with respect to model climatology of the *whole* hindcast period (1981-2004)).
- A more suitable bias correction is proposed to minimize the impact of the discontinuity, that is, dividing the hindcast period into two sub-periods (i.e., 1981-1989 and 1990-2004) and defining model forecasted anomalies as the deviations from the sub-period climatology of the model (see Figures 5-7).

-  CFS hindcast skill has been significantly improved in many areas of the tropics and subtropics by the sub-period based bias correction.
- b. For CFS forecasts, climate anomalies
-   are suggested to be calculated with respect to the climatology since 1990, so as to mitigate the impact of the discontinuity in GODAS.