Combined use of MODIS, AVHRR and radiosonde data for the estimation of spatiotemporal distribution of precipitable water

In this paper, the atmospheric precipitable water (PW) was estimated by means of Advanced Very High Resolution Radiometer (AVHRR) thermal channels brightness temperature difference (ΔT), over the broader area of Greece. The AVHRR derived ΔT was calculated in a grid of 5 × 5 km cells; the corresponding PW value in each grid cell was extracted from Moderate Resolution Imaging Spectroradiometer (MODIS) Level 2 product (near infrared algorithm). MODIS derived PW values were adjusted to the AVHRR overpass time by using PW rates of change. These rates were estimated from time series of radiosonde measurements, provided by four synoptic meteorological stations located in Athens, Thessaloniki, Heraklion and Izmir. Next, to estimate the relationship between adjusted PW and ΔT, a robust linear regression algorithm was applied. Since regression coefficients corresponded to the broader area of Greece, the regression relationship was applied to AVHRR data for the period 2001-2005, to predict the annual and seasonal variability of PW over the study area. Radiosonde derived PW values at the above synoptic stations were used to validate the AVHRR derived PW spatiotemporal distribution. A very good agreement between radiosonde and AVHRR derived PW values was observed since a RMSE of 0.46 cm was calculated using a validation data set that covered a five years period.