Scale-dependent analyses of precipitation forecasts and cloud properties using the Dynamic State Index

Abstract

The objective of this study is a scale-dependent analysis of precipitation forecasts of the German Weather Service's (DWD) non-hydrostatic Lokal-Modell (LM, COSMO-DE) with regard to dynamical-statistical parameters and cloud properties. We propose a newly designed Dynamic State Index (DSI) to evaluate precipitation processes. The DSI is presented in the context of a case study in the synoptic scale and in a statistical approach. The DSI quantitatively describes the deviation from a stationary, adiabatic and reversible solution of the primitive equations. As demonstrated by the example of the winter storm "Kyrill", the analysis of the vertical structure of the DSI gives a relation to the IPV-Thinking, introduced by Hoskins et al. (1985). Furthermore, the DSI-pattern features the characteristic filament-like structure of rainbands with embedded convective cells. In a next step the DSI is not only correlated with modelled precipitation but also with observed precipitation as well as cloud types. The absolute value of the DSI shows moderate correlations with hourly LM and high correlations with hourly COSMO-DE forecast data, based on 24 hour predictions. The statistical analysis of clouds with the index reveals a DSI-threshold, which is used to introduce a novel precipitation activity index of different cloud classes. In conclusion, the results highlight the importance of dynamical processes for the generation of rainfall.