Comparison of total water vapor columns retrieved from satellite measurements: microwave radiances from AMSU-B and visible spectra from GOME/SCIAMACHY

We compare total water vapor (TWV) data retrieved from two different kinds of satellite data: From the microwave radiances of AMSU-B (Advanced Microwave Sounding Unit B) and from visible spectra of GOME (Global Ozone Monitoring Experiment) and SCIAMACHY (Scanning Imaging Absorption spectroMeter for Atmospheric CHartographY). The retrieval from AMSU-B data only works for dry polar atmosphere but independent of daylight, while retrieval from GOME/SCIAMACHY works globally but depends on daylight. The correlation between the two data sets is moderate to high (0.5 to 0.9) with the exception of late summer and autumn, when it is below 0.4. The most likely reason for the low correlation is that the microwave algorithm is least reliable in autumn because the frequency dependence of the sea ice emissivity (which enters into the algorithm) is modified by the summer surface melt and refreeze in a yet unknown way. The TWV data retrieved from GOME/SCIAMACHY have a slight dry bias with respect to TWV data from AMSU-B, as fully cloudy footprints are excluded from the retrieval with GOME/SCIAMACHY. As the two retrieval methods have different strengths and weaknesses but have only little mutual bias, TWV data from both methods can complement each other.