Variograms of radar reflectivity to describe the spatial continuity of Alpine precipitation

The authors use variograms of radar reflectivity as a summary statistic to describe the spatial continuity of Alpine precipitation on mesogamma scales . First, how to obtain such variograms is discussed. Second, a set of typic al variograms of Alpine precipitation is found. Third, some examples are gi ven on how these variograms can be used to tackle several questions such as , What spatial variation of precipitation rate can be found in Alpine catch ments? What difference can be expected between the measurements at two poin ts separated by a given distance? To what accuracy can areal precipitation be estimated from point observations? Are there preferred regions for conve ction in Alpine precipitation? Variograms are obtained using a method-of-mo ments estimator together with high-resolution polar reflectivity data of we ll-visible regions. Depending on the application, the variogram was determi ned in terms of linear precipitation rate, logarithmic reflectivity, or lin ear reflectivity. Spatial continuity was found to vary significantly both i n time and space in the various types of Alpine precipitation analyzed so f ar. At a separation distance of 10 km, the expected difference of reflectiv ity ranges from 4 dBZ (factor of 2.5 in stratiform rain or snow) to about 1 3 dBZ (factor of 20 in a mesoscale convective system). In a 96-h period of heavy rain in the southern European Alps, maximum variation occurred in ups lope regions (frequent convection), while close to the crest of the Alps th e variation was relatively weak (persistent stratiform rain). The represent ativeness of a point observation, which can be quantified given the variogr am, therefore depends on both the time and the location within the Alps and also on the integration time (integrated rainfall maps being less variable than instantaneous ones). For a 576-km(2) basin and 40-min average rain, t he fractional error of the basin precipitation estimated by a gauge measure ment ranges from 11% (variogram of stratiform autumn rain) to 65% (variogra m of a mesoscale convective system). Next steps will extend the variogram a nalyses to a larger space-time domain toward a climatic description of spat ial continuity of Alpine precipitation.