Rba. Koelemeijer et al., A fast method for retrieval of cloud parameters using oxygen A band measurements from the Global Ozone Monitoring Experiment, J GEO RES-A, 106(D4), 2001, pp. 3475-3490
The Global Ozone Monitoring Experiment (GOME) on board the ERS-2 is-designe
d to measure trace gas column densities in the Earth's atmosphere. Such ret
rievals are hindered by the presence of clouds. The most important cloud pa
rameters that are needed to correct trace gas column density retrievals for
the disturbing effects of clouds are the (effective) cloud fraction and cl
oud top pressure. At present, in the operational COME data processor an eff
ective cloud fraction is derived for each pixel, but cloud top pressure is
assumed a priori and is deduced from a climatological database. Here we rep
ort an improved cloud retrieval scheme, which simultaneously retrieves the
effective cloud fraction and cloud top pressure from COME data. This algori
thm, called Fast Retrieval Scheme for Clouds from the Oxygen A band (FRESCO
), makes use of reflectivities as measured by GOME inside and outside the o
xygen A band (758-778 nm). For validation, the results of FRESCO are compar
ed to effective cloud fractions and cloud top pressures derived with standa
rd methods from colocated measurements made by the Along Track Scanning Rad
iometer-2 (ATSR-2). The brightness temperatures of the cloudy pixels as mea
sured by ATSR-2 are related to cloud top pressures using temperature profil
es from the European Center for Medium-range Weather Forecasts model. Gener
ally, the results from FRESCO and ATSR-2 agree reasonably well. For the eff
ective cloud fractions the average difference (based on a comparison of 322
points) is 0.04; the standard deviation is 0.09. For the cloud top pressur
es, only points with an effective cloud fraction larger than 0.1 have been
compared. For these 236 points the average difference in cloud top pressure
is 65 hPa, and the standard deviation is 92 hPa.