543
MW A
<23.8 GHZ)
forward swam
<371 ciano »rock
poet 1.5x2 ton
reiatuUOO)
nocir »warn
(555 rvadr poets
i km resotution)
Figure 1 : ATSR-M viewing geometry
32 . ATSR-MW
The ATSR-MW radiometer has been designed to correct the altimeter path delay due to tropospheric humidity.
It is a nadir viewing sensor and it has two channels at 23.8 and 36.5 GHz. These frequencies have been chosen
to provide the precipitable water and cloud liquid water over oceans along the satellite track. The instantaneous
field of view (IFOV) which is shown on Figure 1, is a circle of about 20 km. The time sampling corresponds to
an horizontal spacing of 1 km but the signal is integrated over 8 scans to obtain a sampling rate close to the
altimeter sampling rate ( Is). The final resolution is an ellipse of 28x20 km 2 . A complete description of the
radiometer and of its performances is given in Bernard et al.(1994).
4- ATSR-MW ALGORITHM
Algorithms have been proposed by Eymard et al. (1992) to retrieve the columnar water vapour content (Wv),
the cloud liquid water (L), and the altimeter wet path correction (dh), using a radiative transfer model
developed by Lojou et al.(1994). These relationships have been obtained from multilinear regression analysis
between the modeled brightness temperatures and the geophysical parameters Wv, L, dh and the surface wind
speed U, using the following form :
coefficients. Since the launch of ERS1, these algorithms have been corrected and the retrieved geophysical
parameters have been validated by comparison with coincident routine radiosonde measurements (Eymard et
al., 1994). These data are 1200 coincident radiosounding data over global oceans, extracted from the
European Centre for Medium range Weather Forecasts (ECMWF) archives between August 1991 and
September 1992.
The comparison for the retrieved water vapour is plotted on Figure 2 and shows a very good
agreement between the local measurements and the radiometer estimations. The mean bias is zero and the
standard deviation is 0.36 g/cm^ .
(3)
channels
Toi is a reference atmospheric temperature for the frequency considered and aq, ai and b are constants
