itle: Windstorm Poly (2023): phenomenology, predictability and the role of the North Sea

Abstract

Windstorm Poly, which developed overnight from 4th to 5th, 2023, was a major summer windstorm in the Netherlands, characterized by rapid intensification and severe wind gusts. The Royal Netherlands Meteorological Institute (KNMI) issued severe weather warnings, including code red, as the storm’s windfield intensified. The storm resulted in wind gusts of up to 146 km/h in IJmuiden and significant damage from Zandvoort to Alkmaar. This study investigates the development and unique characteristics of Storm Poly, focusing on the influence of record-high sea surface temperatures and atmospheric dynamics. We use the Regional Atmospheric Climate Model (RACMO), Harmonie and ERA5 reanalysis data to analyze Poly’s formation and evolution. Our findings suggest that the interaction between an upper-level trough and convective showers was the primary driver of the storm’s rapid intensification. The RACMO simulations demonstrate that while Sea Surface Temperatures (SSTs) variations impacted the storm’s characteristics, including wind speed, precipitation, and storm scale, they are not the dominant factor in Poly’s rapid cyclogenesis. The study also explores the hypothesis of a sting-jet phenomenon contributing to Poly’s severe wind gusts. However, analyses from ERA5 and the new cycle of HARMONIE (CY46) datasets showed no evidence of typical sting-jet dynamics. Instead, the strongest winds are attributed to surface processes and warm-core dynamics, indicative of a hybrid cyclone undergoing subtropical transition. This research highlights the complexity of storm dynamics and underscores the need for improved high-resolution modeling to understand and predict similar events. Future investigations should focus on past severe summer storms in the Netherlands to determine if Storm Poly is unique in its hybrid characteristics and consider the implications of climate change on the future frequency and intensity of such storms.