Diplomanden-Doktoranden-Seminar Bonn, now Cologne – 8 Januar 2007

CMSAF water vapour in comparison to radiosondes and

CHAMP

Ralf Lindau

Diplomanden-Doktoranden-Seminar Bonn, now Cologne – 8 Januar 2007

Water vapour from ATOVS

Global daily fields of Layered Precipitable Water LPW are calculated by kriging method on 90 km resolution.

Advantage of kriging : An error map is available for each field.

Example: 1.10.2004

850 – 700 hPa

Diplomanden-Doktoranden-Seminar Bonn, now Cologne – 8 Januar 2007

Radiosondes

173 GUAN (GCOS Upper-Air Network)stations well distributed over the globe.

Diplomanden-Doktoranden-Seminar Bonn, now Cologne – 8 Januar 2007

It‘s the resolution, stupid

For any comparison of two data sets resolution is crucial.

If resolutions are not equal, the often actually seriously presented conclusion would be:

The low resolved observations underestimate high values and overestimate low values.

ATOVS fields: Daily , 90 kmRadiosondes: 4 times a day, point measurements

Thus, average the radiosondes over 1 day.But still Radiosonde data include additionally the spatial variance of daily

means within 90 x 90 km2

Diplomanden-Doktoranden-Seminar Bonn, now Cologne – 8 Januar 2007

Variability of Water Vapour

Famous paper:

Lindau, R. and E. Ruprecht, 2000: SSM/I-derived total vapour content over the Baltic Sea compared to independent data, Met.Zeitschrift, 9, No.2, 117-123.

1 day include 8 mm2 of variance

90 by 90 km2

include 3 mm2 of variance

The spatial variance of temporal means must be smaller than that of individuals (3 mm2).

Diplomanden-Doktoranden-Seminar Bonn, now Cologne – 8 Januar 2007

October 2004

+ 3.82 mm + 17 %

High correlation: r = 0.95

But wet bias of 3.82 mm2

Diplomanden-Doktoranden-Seminar Bonn, now Cologne – 8 Januar 2007

Happy with correlation ?

Correlation reduction due errors is:

1 - r = e2 / (2 + e2 )= 7.5 mm2 / 225 mm2

= 0.03

More than half of the scatter is explainedby random errors in ATOVS and RS.

But there is still the bias....

The error of ATOVS is explicitelycalculated within kriging.

The mean error variance for October 2004 and for those 173 gridboxes, where RS data is available is:

eAT = 6.19 mm2

The error variance of daily means from RS is calculated by: Internal variance / observ number:

eRS = 7.65 mm2

Diplomanden-Doktoranden-Seminar Bonn, now Cologne – 8 Januar 2007

Different layers

+ 2.35 mm + 21 % + 0.18 mm

+ 5 %

1000 – 850 hPa 700 – 500 hPa

Diplomanden-Doktoranden-Seminar Bonn, now Cologne – 8 Januar 2007

Time series of the bias

The bias is not confined to October 2004.

It persists through 10 month

Diplomanden-Doktoranden-Seminar Bonn, now Cologne – 8 Januar 2007

Discriminating GUAM data

Ocean / Land Height above sea

or is it the ice surface in the Antarctica?

Diplomanden-Doktoranden-Seminar Bonn, now Cologne – 8 Januar 2007

Ocean, Height, Ice ?

+ 2.85 mm + 16 %

+ 1.68 mm + 9 %

- 0.18 mm - 7 %

Diplomanden-Doktoranden-Seminar Bonn, now Cologne – 8 Januar 2007

Different First-guesses

No

GME

NCEP

Different first-guess (taken asbasis for the retrieval) result in strong differences.

Example: 1.9.2006, 700-500 hPa

Diplomanden-Doktoranden-Seminar Bonn, now Cologne – 8 Januar 2007

ATOVS vs Radiosondes

No

GME

NCEP

- 0.82 mm - 3 %

+ 3.67 mm + 14 %

+ 7.08 mm + 27 %

The corresponding bias vanishes for „No first guess“

and is doubled, if NCEP is used.

Diplomanden-Doktoranden-Seminar Bonn, now Cologne – 8 Januar 2007

... and what about Champ ?

Example for 1 monthof Champ data.

4239 obs/month1 obs/10 min

Irregularly spreadedover the globe

Diplomanden-Doktoranden-Seminar Bonn, now Cologne – 8 Januar 2007

ATOVS vs Champ850 – 700 hP

a700 – 500 hP

a

- 3.20 mm- 15 %

- 0.15 mm- 5 %

+ 0.02 mm+ 2 %

Diplomanden-Doktoranden-Seminar Bonn, now Cologne – 8 Januar 2007

Conclusion

ATOVS vs radiosondes- ATOVS has a wet bias compared to radiosondes.- Bias is decreasing with height.- It persists through all discrimination experiments.- First guesses play an important role for the retrieval.

Champ vs ATOVS- Champ has dry bias compared to ATOVS- (thus, seems to agree with RS, but no direct comparison possible)- Bias is decresing with height.