Atmospheric water vapor content over land surfaces derived from the AVHRR data: application to the Iberian Peninsula

A study has been carried out using simulated NOAA/advanced very high resolution radiometer (AVHRR) data at 11 and 12 μm (with LOWTRAN-7, MODTRAN 2.0, and the TIGR database), AVHRR images of the Iberian Peninsula and the Palma de Mallorca Island, radiosonde observations at seven meteorological stations, and the AVISO database provided by Meteo France to describe, compare, and analyze two different approaches for estimating the total atmospheric water vapor content (W) over land surfaces from AVHRR data. These two techniques are: 1) the split-window covariance-variance ratio (SWCVR), based on a quadratic relationship between W and the ratio of the spatial covariance and variance of brightness temperatures measured in channels 4 (T₄) and 5 (T₅) of AVHRR in subsets of N neighboring pixels and 2) the linear split-window relationship (LSWR), based on a linear regression between W and the difference of brightness temperatures measured in the same channels (ΔT=T₄-T₅). The results demonstrate the advantage of the SWCVR technique for regions with a certain level of thermal heterogeneity (standard deviation of T₄ in the subset >0.5 K), which is capable of estimating W from NOAA-14 afternoon and night passes over the Iberian Peninsula with a standard deviation of 0.5 (g cm^-2), whereas the LSWR technique predicts the atmospheric water vapor with a standard deviation from 1.3-1.5 (g cm^-2). Finally a water vapor image of the entire Iberian Peninsula constructed by applying the SWCVR to NOAA-14 data is presented