Climatological Thresholds of Daily Rainfall

This technical note summarizes the results from daily rainfall data analysis with relation to annual rainfall amounts. By investigating the behavior of correlation coefficients between the annual rainfall amounts and the within-the-year cumulative frequency distribution functions of wet days with various threshold rainfall depths, it was found that (1) the annual rainfall amount is weakly correlated with the number of either dry or wet days—thus, the zero rainfall depth is not a proper threshold for the explanation of dry or wet years; and (2) there exist certain thresholds of daily rainfall depth (the climatological thresholds) with which the cumulative frequency of wet days can be well related with the annual rainfall amount. Also with this threshold, the wet and dry years can be seen divided clearly into separate groups of the cumulative frequency distribution functions, without any overlaps, which were common when using the zero rainfall depth as a threshold. Because it is found that there exist certain thresholds to be used for explaining the annual rainfall amount changes, these might also be used effectively for the analysis of possible changes in flood or drought patterns caused by global warming.