Validation of the NCEP CFS forecasts

Suranjana Saha

Environmental Modeling Center

NCEP/NWS/NOAA/DOC

Chosen Topics:

• Validation of Extremes, especially in 500 hPa geopotential, temperature and precipitation

• QBO validation

• SST validation by ocean basins

About extremes:

• What is an extreme?

Large departure from normal, for example in temperature and/or precipitation, we have heat waves, cold spells, droughts, floods, etc.

• Given how important the effect of extremes is on society (life, property and the economy), did the CFS predict these events ?

About extremes:

• We evaluate skill in CFS predictions only on occasions when an extreme occurred in observations.

• Using monthly mean data, we define an extreme = |value| of anomaly of variable

>= 2 or 1.5 times local standard deviation.

EXTREME EVENTS IN PNA and NAO

In 500 hPa Geopotential

(Reanalysis 2 used for validation)

Conclusions (for PNA and NAO):

1. Modest capability to predict extreme values in PNA and NAO.

2. PNA extremes mainly related to ENSO (Feb 1983, Jan 1989)

3. NAO extremes appear equally predictable (Nov 1993, Feb 1986). Causes less clear

4. Skill for extremes, in terms of anomaly correlation, is an amplified version of skill in general.

EXTREME EVENTS IN TEMPERATURE

(Reanalysis-2 used for validation)

Conclusions (for Temperature)

1. US : Modest skill mainly in late spring

2. Europe : No skill

3. India : Modest skill mainly in winter

4. Africa : Modest skill mainly Northern winter

5. South America: Moderate skill throughout the year.

EXTREME EVENTS IN PRECIPITATION

(Xie-Arkin Precip used for validation)

Conclusions (for Precipitation)

1. US : Modest skill mainly in winter

2. Europe : No skill at all

3. India : Modest skill mainly Feb-May

4. Africa : Modest skill mainly Aug-Jan

5. South America: Modest skill throughout the year only for lead-1.

(Keep in mind there are complications when precipitation is skewed, or standard deviation is small (like deserts).

QBO

MONTHLY MEAN ZONAL MEAN

ZONAL WIND ANOMALY AT THE

EQUATOR

(Reanalysis-2 used for validation)

Conclusions (for QBO)

Baseline experiment regarding the QBO.

(No skill was expected)

1. The phenomenon disappears slowly in CFS, but is clearly identifiable in forecasts out to 6 months. This is better than anybody expected.

2. CFS has 64 levels in the vertical, of which 20 are above 100 hPa. The top of the model is at 0.2 hPa. It is possible that high density of levels near 50 hPa has been helpful in sustaining zonal winds.

VALIDATION OF SST FOR DIFFERENT OCEAN BASINS

(NCEP GODAS used for validation)

Conclusions (for SST)1. Spring Barrier clearly evident in prediction of SST in Nino 3.4

and Northern Pacific Regions

2. Moderate skill is present in the Indian Ocean only during spring.

3. Skill highest during September for both Southern Atlantic and Pacific Oceans.

4. Moderate skill is present in the equatorial Atlantic Ocean including the hurricane season (ASO)

5. Least skill present in the Northern Atlantic Ocean, mainly during the winter.