Nicolas Gaussiat, Anthony Illingworthand Robin Hogan
Beeskow, 12 Oct 2005
Liquid Water Path from radiometers and lidar.
m sky
m b
T - Tτ = log
T - T
• 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.
l v dτ = k LWP + k VWP + τ
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.
l v dτ = k LWP + k VWP + τ
SOLUTION: Add a calibration error, ‘C’ tothe equations.When lidar identifies no water cloud, set LWP = 0,use this to constrain ‘C.
Assuming calibrations errors :
28,
2828,28
22,
2222,22
v
d
v
d
k
C
k
C
2828,28,28,28
2222,22,22,22
CVWPkLWPk
CVWPkLWPk
dvl
dvl
228
222 CCJ
2822,
28,22min C
k
kCJ
v
v
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 C22 and C28.
28
228
22
222
CC
J
282822,
2228,22min C
k
kCJ
v
v
Principe of the lidar+radiometer technique:
Example :‘C’ factors reset each time no water cloud.
LWP forcedto zero whenno water cloud.
Another Another example:
LWP OFFSET +200 g m-2
- 60g m-2
Sensitivity to drift in T:old technique
Add 5K to Tb (28GHz) and thento Tb(22GHz)
(a) old technique (b) new method
One month’s data: apply 1 to 5K offsets.
Robustness of the new technique :
NEW METHOD:Tb error 5K:introduces only 2% error in LWP
1
5
1
5
LWP error as function of time betweenclear sky events
1hr6min 10hr
Error about 5-10 g m-2
Comparison of three methods
old remove mean lwp new before and after cloud