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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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.
Add a calibration error, ‘C’ to
When lidar identifies no water cloud, set LWP = 0,
use this to constrain ‘C.
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.
to Tb (28GHz)
+200 g m-2
- 60g m-2
One month’s data:
apply 1 to 5K offsets.
Tb error 5K:
2% error in LWP
(a) old technique (b) new method
LWP error as function of time between retrieved.
clear sky events
5-10 g m-2
old remove mean lwp new
before and after cloud