ITCZ trend analysis via Geodesic P-spline smoothing of the AIRWAVE TCWV and cloud frequency datasets

The Inter Tropical Convergence Zone (ITCZ) is the region of the Earth's atmosphere where the trade winds converge. This region is characterized by rising air, strong convection, clouds and heavy precipitation and it is tightly related to changes in climate patterns on a global scale. For these reasons assessing the ITCZ migrations is of extreme importance for climate monitoring. This can be achieved through the use of satellite data of different kind. In the last decades several quantities have been used as proxies for this purpose, e.g. infrared radiance measured at the top of atmosphere (TOA), precipitation datasets or vertical and horizontal wind components. In this work the ITCZ position is determined and its time evolution is analysed using the Total Column Water Vapour (TCWV) data, retrieved using the Advanced Infra-Red Water Vapour Estimator algorithm (AIRWAVE). AIRWAVE was developed for the retrieval of the TCWV from the Along Track Scanning Radiometer (ATSR) instrument series, operational from 1991 to 2012. It allows the TCWV retrieval from infra-red channels at 11 and 12 μm exploiting the ATSR nadir and forward viewing geometries, for day/night and cloud-free sea surface scenarios. The information on cloud coverage from ATSRs is used as correlative information in order to expand the ITCZ analysis to land scenes. The TCWV and cloud frequency datasets are analysed with a Geodesic P-spline efficient spatial smoothing method specifically developed to extract information from large datasets. The posterior distribution of the model is considered for identification of the ITCZ both over sea and land with associated uncertainty quantification. The resulting AIRWAVE/cloud frequency monthly fields are analysed to detect trends in the ITCZ latitudinal displacement over the 20 years of the ATSR family lifetime. Results indicate that no significant trends can be detected in the 1991–2012 time period.