The effect of flow and orography on the spatial distribution of the predictability of rainfall

Introduction Conclusions References Tables Figures Back Close Full Screen / Esc Abstract Introduction Conclusions References Tables Figures Back Close Full Screen / Esc Abstract The spatial distribution and scale-dependence of the very-short term predictability of precipitation by Lagrangian persistence is studied under different flow regimes in connection with the presence of orographic features. Data from the weather radar composite of eastern Victoria, Australia, a 500 km × 500 km domain at 10 min temporal and 5 2 km × 2 km spatial resolutions, covering the period from February 2011 to October 2012, were used for the analyses. The scale-dependence of the predictability of precipitation is considered by decomposing the radar rainfall field into an 8-levels multi-plicative cascade using a Fast Fourier Transform. The rate of temporal development of precipitation in Lagrangian coordinates is estimated at each level of the cascade under 10 different flow regimes, which are stratified by applying a k-means clustering algorithm on the diagnosed velocity fields. The predictability of precipitation is measured by its lifetime, which is derived by integrating the Lagrangian auto-correlation function. The lifetimes were found to depend on the scale of the feature as a power law, which is known as dynamic scaling, and to vary as a function of flow regime. The lifetimes also 15 exhibit significant spatial variability and are approximately a factor two longer on the upwind compared with the downwind slopes of terrain features. The scaling exponent of the spatial power spectrum also shows interesting geographical differences. These findings provide opportunities to perform spatially inhomogeneous stochastic simulations of space-time precipitation to account for the presence of orography, which may 20 be integrated into design storm simulations and stochastic precipitation nowcasting systems.