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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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Liquid water path from radiometers and lidar
Liquid Water Path from radiometers and lidar.

Nicolas Gaussiat, Anthony Illingworth

and Robin Hogan

Beeskow, 12 Oct 2005


Radiometers measure brightness temperatures.

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.


In clear sky conditions non-zero values of LWP are retrieved.

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.


Principe of the lidar radiometer technique

Assuming calibrations errors : retrieved.

Principe of the lidar+radiometer technique:

In clear sky conditions LWP = 0:

Radiometers have same perf :

Radiometers have different perf :

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


Example c factors reset each time no water cloud lwp forced to zero when no water cloud
Example : retrieved.‘C’ factors reset each time no water cloud.LWP forcedto zero whenno water cloud.


Another another example
Another retrieved.Another example:


Sensitivity to drift in t old technique
Sensitivity to drift in T: retrieved.old technique

Add 5K

to Tb (28GHz) 

and then

to Tb(22GHz) 

LWP OFFSET

+200 g m-2

- 60g m-2


Robustness of the new technique
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

Error about

5-10 g m-2

10hr

1hr

6min


Comparison of three methods
Comparison of three methods retrieved.

old remove mean lwp new

before and after cloud


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