Nicolas Gaussiat, Anthony Illingworth and Robin Hogan Beeskow, 12 Oct 2005 Liquid Water Path from radiometers and lidar.

Radiometers measure brightness temperatures. T b, that are converted into optical depths, . Optical depths are linearly related LWP and VWP : k l and k v 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.

Assuming calibrations errors : In clear sky conditions LWP = 0: Radiometers have same perf : Radiometers have different perf : where  22 and  28 are the expected standard deviations of respectively C 22 and C 28. Principe of the lidar+radiometer technique:

Example : ‘C’ factors reset each time no water cloud. LWP forced to zero when no water cloud.

Another Another example:

LWP OFFSET +200 g m -2  - 60g m -2  Sensitivity to drift in T: old technique Add 5K to T b (28GHz)  and then to T b (22GHz) 

(a) old technique (b) new method One month’s data: apply 1 to 5K offsets. Robustness of the new technique : NEW METHOD: T b error 5K: introduces only 2% error in LWP

LWP error as function of time between clear sky events 1hr 6min 10hr Error about 5-10 g m -2

Comparison of three methods old remove mean lwp new before and after cloud