Scale-dependence of the predictability of precipitation from continental radar images. Part I: Description of the methodology

The lifetime of precipitation patterns in Eulerian and Lagrangian space derived from continental-scale radar images is used as a measure of predictability. A three-step procedure is proposed. First, the motion field of precipitation is determined by variational radar echo tracking. Second, radar reflectivity is advected by means of a modified semi-Lagrangian advection scheme assuming stationary motion. Third, the Eulerian and Lagrangian persistence forecasts are compared to observations to calculate the lifetime and other measures of predictability. The procedure is repeated with images that have been decomposed according to scales to describe the scale-dependence of predictability. The analysis has a threefold application: (i) determine the scale-dependence of predictability, (ii) set a standard against which the skill for quantitative precipitation forecasting by numerical modeling can be evaluated, and (iii) extend nowcasting by optimal extrapolation of radar precipitation patterns. The methodology can be applied to other field variables such as brightness temperatures of weather satellites imagery.