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# Liquid Water Path from radiometers and lidar. - PowerPoint PPT Presentation

Liquid Water Path from radiometers and lidar. Nicolas Gaussiat, Anthony Illingworth and Robin Hogan. Beeskow, 12 Oct 2005. Radiometers measure brightness temperatures. T b , that are converted into optical depths,  . Optical depths are linearly related LWP and VWP :

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Presentation Transcript

Nicolas Gaussiat, Anthony Illingworth

and Robin Hogan

Beeskow, 12 Oct 2005

Tb, that are converted into optical depths, .

Optical depths are linearly related LWP and VWP :

kl and kv are path averaged coefficients.

d is the ‘dry’ optical depth

Two frequencies, two equations, two unknowns – find LWP and VWP.

PROBLEM: Calibration errors, uncertainty over ‘k’ coefficients

Cause errors in lwp – it can even go negative.

Some values negative.

SOLUTION:

Add a calibration error, ‘C’ to

the  equations.

When lidar identifies no water cloud, set LWP = 0,

use this to constrain ‘C.

Assuming calibrations errors : retrieved.

In clear sky conditions LWP = 0:

where s22 and s28 are the expected standard deviations of respectively C22 and C28.

Example : retrieved.‘C’ factors reset each time no water cloud.LWP forcedto zero whenno water cloud.

Another retrieved.Another example:

Sensitivity to drift in T: retrieved.old technique

to Tb (28GHz) 

and then

to Tb(22GHz) 

LWP OFFSET

+200 g m-2

- 60g m-2

Robustness of the new technique : retrieved.

One month’s data:

apply 1 to 5K offsets.

5

5

NEW METHOD:

Tb error 5K:

introduces only

2% error in LWP

1

1

(a) old technique (b) new method

LWP error as function of time between retrieved.

clear sky events

5-10 g m-2

10hr

1hr

6min

Comparison of three methods retrieved.

old remove mean lwp new

before and after cloud