Difficulties Associated with the Application of Covariance–Variance Techniques to Retrieval of Atmospheric Water Vapor from Satellite Imagery

Arecent article by Ottlé et al. (1997) has discussed the use of covariance–variance techniques for the determination of atmospheric water vapor content from split-window radiance measurements. We have applied the recommended technique to several sets of data from both AVHRR and ATSR and find no correlation with associated water vapor measurements for oceanic data and for AVHRR data over the land. However, some weak correlation is found with ATSR-2 data over land, but our analysis shows that any such relation is fragile and, at the moment, cannot be used operationally.