Effects of Cloud/rain on Ka band altimeter J. Tournadre et al
Ifremer Brest Slide 2 Altika : altimeter in Ka band (see Noubel and
Verron presentations) Better performances but one major problem at
Ka band (35.75 GHz) rain and cloud can strongly attenuate the
signal and distort the echo waveform. Important to quantify :
Impact on echo waveforms and geophysical parameters estimate (use
of MLE4 algo) Percentage of data possibly lost due to rain and/or
clouds OSTS Hobart, March 2007 Slide 3 Previous studies (Tournadre
1999). In presence of rain larger than 1-2 mm/hr the distortion of
the waveform will inhibit the geophysical parameter retrieval. At
Ka band the attenuation by cloud droplet is about 1.1 dB/km par g/m
3 (10 time larger than Ku band), not negligible. Cloud at Ka = rain
at Ku. Cloud more frequent than rain. Necessity to analyze in
detail the effect of cloud on the signal Waveform modeling in
presence of cloud Estimation of attenuation, off-nadir angle,
leading edge slope at 1Hz and 20 Hz as a function of cloud
parameters (IWC, height, diameter,..) Estimation of the impact of
cloud on the geophysical parameters (ssh,s0, SWH) retrieval:
waveform modeling and MLE4 (Stenou et al). Determination of data
availibility Rain/cloud flag definition based on the signal
analysis (Altika is single-frequency) Slide 4 Analytical model of
waveform based on Brown model + an attenuation term A attenuation
by cloud or rain k attenuation coefficient in dB/km rain cloud R
rain rate, m v liquid water content (g/m 3 ) Waveform modeling
Slide 5 For clouds Attenuation depends on Integratedd cloud liquid
water (ICLW) Simpler than the rain case Slide 6 Cloud liquid water
Typical values for the properties of clouds. The values are merely
modal-means. The range of observed values is quite large. The
radius of cloud droplets is r (microns), the effective optical
radius optically is r', N is the number of droplets per cubic
centimetre, L is the liquid water content of the cloud (g/m 3 ).
For all clouds, the level of observation is just below the freezing
level, except for fog and cirrus. (Hess et al 1998) Slide 7 Cloud :
water droplet and ice cristals Ice has a much lower attenuation
coefficient and can be neglected in the computation. Slide 8 Slide
9 Cloud liquid water content data Several satellite sensors gives
estimates of the integrated cloud liquid water content such as
SSM/I, AIRS or MODIS on Aqua/Terra. However to have a good modeling
we need high resolution data. Slide 10 Waveform distortion by rain
and cloud Ka band : strong attenuation by rain and cloud But more
important: strong distortions of the waveform shape : modification
of leading edge and plateau slope 10 1 0.1 Altika WF over a 10 km
2.5 mm/hr rain cell Return powerAttenuation Slide 11 Altika
unavailability by rain : Use of JASON ENVISAT,TOPEX rain
climatology: same sampling as Altika; Probability of rain greater
than 1-1.5 mm/hr Slide 12 Slide 13 Waveform modeling: cloud with
gaussian distribution of ICLW ICLW. Cloud radius Distance
nadir/cloud Attenuation Off-nadir angle Stronger distortion for
small high ICLW cloud Slide 14 Comparison 1Hz 20Hz Small difference
on attenuation for cloud radius larger than the footprint (~8km)
For ICLW Deformation can be important even for low IWC (~0.2kg/m)
For IWC >0.6 kg/m2 rain most probable Below 0.2 kg/m few data
will be contaminated Distortion depends more strongly n the
variability of IWC within the footprint than mean value Slide 24 To
be done : Availability maps Test of the feasibility of an
operational rain/cloud flag based on and variations Modis06 level3
daily products clouds. Cloud fraction, cloud liquid water
histograms Slide 25 Conclusion Model shows that the mean impact of
cloud is weak for CLWC lower than 0.2 kg/m For higher CLWC it will
be necessary to flag the data Wavelet analysis of the sigma0 and
off- nadir angle variations can be used to flag the data affected
by rain and clouds.. Slide 26 Problem: Definition of a rain flag As
Altika certainly mono-frequency the rain flag can not be similar to
the JASON or ENVISAT one (dual frequency). Critical for light to
very light rain (
