Liquid Water Path from radiometers and lidar.
description
Transcript of Liquid Water Path from radiometers and lidar.
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