SWING, The Score-Weighted Improved NowcastinG Algorithm: Description and Application

Abstract

Because of the ongoing climate change, the frequency of extreme rainfall events at the global scale is expected to increase, resulting in higher social and economic impacts. Thus, improving the forecast accuracy and the risk communication is a fundamental goal to limit social and economic damages. Both Numerical Weather Prediction (NWP) and radar-based nowcasting systems still have open issues, mainly in terms of precipitation correct time/space localization predictability and rapid forecast accuracy decay, respectively. Trying to overcome these issues, this work aims to present a nowcasting system combining an NWP model (WRF), using a 3 h rapid update cycling 3DVAR assimilation of radar reflectivity data, with the radar-based nowcasting system PhaSt through a blending technique. Moreover, an innovative post-processing algorithm named SWING (Score-Weighted Improved NowcastinG) has been developed in order to take into account the timely and spatial uncertainty in the convective field simulation. The overarching goal is to pave the way for an easy and automatic communication of the heavy rainfall warning derived by the nowcasting procedure. The results obtained applying the SWING algorithm over a case study of 22 days in the fall 2019 season suggest that the algorithm could improve the predictive capability of a traditional deterministic nowcasting forecast system, keeping a useful forecast timing and thus integrating the current forecast procedures. Eventually, the main advantage of the SWING algorithm is also its very high versatility, since it could be used with any meteorological model also in a multi-model forecast approach.

Is an NWP-Based Nowcasting System Suitable for Aviation Operations?

“…Although the results of this study are encouraging, the severe convective cells forecast at high temporal and spatial resolution remain a challenge. A further step forward in improving the predictive capability of a traditional nowcasting forecast system could be the application of a combined approach between WRF simulations with a 3D-Var cycling assimilation and a radar-based nowcasting system through a blending technique [54].…”

“…In the vertical, 50 terrains following sigma levels are adopted, and the model top is set to 50 hPa. The physical parametrizations set for this study were already successfully tested in several convective events with similar peculiarities and adopted for the WRF-CIMA operational chain [17,[54][55][56][57] that supports the DPC activities. For the long-wave and shortwave radiation process, the Rapid Radiative Transfer Model for Global Climate Models [58] is applied, while a WRF single-moment six-class scheme [59] with six different types of hydrometeors is adopted for the microphysics.…”