Non-stationarity in intermittent rainfall: the "dry drift"

Intermittency (i.e., the dry or rainy occurrences) is an important feature of rainfall. In the present contribution, it is shown that the intermittency results in non-stationarities in the rainfall field because the average rain rate appears to be a function of the distance to the closest dry area. This "dry drift" must be estimated and removed from the observations before conducting any structural analysis. Using the radar rain-rate product from MeteoSwiss, we show that the dry drift explains between 30 and 40% of the total variability of the rain rate field (in log scale). The experimental variograms characterizing the spatial (or temporal) structure of rainfall are better defined when the dry drift is removed. Using disdrometer data, it is shown that this dry drift also exist in DSD fields (e.g., for the total drop concentration and for the mean drop diameter). The importance of the dry drift for the understanding of rainfall is illustrated by three applications: the proportion of variability explained by the dry drift, the structural analysis of rainfall fields, and the influence of the dry drift on the Z-R relationship.