Dependence of precipitation on temperature at Florence and Livorno (Italy)

Abstract

Papers dealing with precipitation scenarios for a potentially warmer climate have often ignored the relationship between precipitation and temperature. Although the atmosphere has a larger capacity to hold water vapour at higher temperatures, plots of the mean wet-day precipitation amounts versus temperature may not reveal a clear positive relationship. This phenomenon is studied on detail for Florence and Livorno (Italy). Vorticity and the difference between the land and sea surface temperatures are 2 important factors that determine precipitation at these sites. The wet-day precipitation amount is explored as a function of these 2 variables and temperature using an iterative smoothing technique. The estimated functions show a marked increase in the mean wet-day precipitation amount with increasing temperature (~6% °C-1). The differences between the smoothed relations for Florence and Livorno are small and the smooths for the winter and summer halves of the year are also quite similar. Besides the mean wet-day precipitation amounts, the mean wet-hour amounts (mean rainfall intensities) of each day with rain are also analysed. These show a much stronger increase with increasing temperature (~11% °C-1), which is partly due to a decrease of rainfall duration with increasing temperature.