Drivers of Cold Frontal Hourly Extreme Precipitation: A Climatological Study

Understanding the processes driving extreme precipitation is paramount to socioeconomic interest. In the mid-latitudes extreme precipitation events are strongly associated with cold fronts. By exploring drivers across a wide range of scales, this study aims to improve our understanding of processes influencing frontal precipitation. Past research predominately focused on detailed studies of individual frontal extreme events. Here we present the first climatological study of frontal characteristics and their impact on precipitation.
Using hourly resolved ERA5 data, cold fronts are detected using the equivalent potential temperature gradient, and associated conditions from the synoptic to the meso-scale are identified. Further, seasonal and regional dependencies are explored. Quantile regression models are employed to find the strongest drivers of frontal precipitation and to quantify these relationships. Additionally, composite analysis are used to study the synoptic conditions and meso-scale structure of extreme events.
Findings reveal that humidity close to the frontal boundary, convergence of different scales and the low level jet speed contribute most to formation of extreme precipitation events. Interestingly, we discovered that stronger fronts, characterized by a significant change in humidity, do not always lead to a higher chance of extreme precipitation. This is evident in the weak correlation between the humidity gradient and frontal precipitation, in contrast with the relationship observed for the temperature gradient.
The findings of this study improve our understanding of cold frontal processes. Additionally, they provide the foundation to evaluate model performance and climate change projections.