Lead Author: Jennifer Israelsson, PhD Student Maths of Planet Earth CDT Reading
Published online in the Journal of Hydrology, Regional Studies, the paper is available here
Rainfall over west Africa has received a lot of interest the past decades due to the limited possibility of irrigation for the many farmers depending on rain fed crops. A major limitation for understanding the water cycle and changes in it, is the generally sparse rain gauge network. To work around this problem, satellites are instead being used to collect cloud measurements which can be used to estimate rainfall amounts. These estimates must be evaluated, and for some products calibrated, against ground measurements to provide accurate results. In order to do this, we need a good understanding of the rainfall behaviour between the rain gauges, i.e how far away from a rain gauge will it likely rain. This is called the decorrelation range for rainfall. This has up until now been a very difficult question to answer due to the long distances between the rain gauges.
Thanks to a new, dense daily rain gauge data set over Ghana, the spatial structure of rainfall for the different phases of the monsoon has been investigated. Previous studies have only considered a general decorrelation range whereas in this study a novel approach of estimating the decorrelation rate depending on the intensity of the rainfall event has been implemented. The anisotropic, i.e correlation changing with direction, pattern at the subweekly and local scale was also modelled for several aggregation periods. A rigorous rainfall climatology for all of Ghana is also presented to get a robust understanding of the current behaviour to better understand future scenarios. The spatial correlation structure of rainfall is found to vary greatly with the intensity of the rainfall event and the phase of the monsoon. At the very local scale (~10km), there is a much larger variation over the year at the lower intensities compared to the heavier intensities. The westward propagation of convective systems that are usually seen at the weekly-monthly scale can be seen even at short aggregation periods.