How intense daily precipitation depends on temperature and the occurrence of specific weather systems – an investigation with ERA5 reanalyses in the extratropical Northern Hemisphere

Abstract

Abstract. Precipitation and surface temperature are two of the most important variables that describe our weather and climate. Several previous studies investigated aspects of their relationship, for instance the climatological dependence of daily precipitation on daily mean temperature, P(T). However, the role of specific weather systems in shaping this relationship has not been analysed yet. This study therefore identifies the weather systems (WSs) that are associated with intense precipitation days as a function of T, focusing on the question of how this relationship, symbolically expressed as P(T, WS), varies regionally across the Northern Hemisphere and between seasons. To this end, we first quantify if intense precipitation occurs on climatologically warmer or on colder days. In winter, over most continental and ocean regions, intense precipitation falls on warmer days apart from the Mediterranean area and regions in the lee of the Rocky Mountains, where intense precipitation is favoured on colder days. In summer, only at high latitudes is intense precipitation favoured on warmer days, whereas continental areas experience intense precipitation on colder days. For selected regions in Europe and North America, we then identify the weather systems that occur preferentially on days with intense precipitation (referred to as wet days). In winter, cyclones are slightly dominant on colder wet days, whereas warm conveyor belts and atmospheric rivers occur preferentially on warmer wet days. In summer, the overall influence of atmospheric rivers increases, and the occurrence of weather systems depends less on wet day temperature. Wet days in the lee of the Rocky Mountains are influenced by most likely convective systems in anticyclones. Finally, we investigate P(T, WS) during the wettest and driest season in central Europe and the central United States (US). In qualitative agreement with the results from the first part of this study, the wettest winter is warmer than normal in central Europe but colder in the central US, and the wettest summer is colder in both regions. The opposite holds for the driest winter and summer, respectively. During these anomalous seasons, both the frequency and the precipitation efficiency of weather systems change in central Europe, while the wettest and driest seasons in central US mainly arise from a modified precipitation efficiency. Our results show that the precipitation–temperature–weather system relationship strongly depends on the region and that (extreme) seasonal precipitation is influenced by the frequency and precipitation efficiency of the different weather systems. This regional variability is reflected in the relative importance of weather system frequency and efficiency anomalies for the formation of anomalously wet and dry seasons. Interestingly, in some regions and seasons, the precipitation efficiency of weather systems is increased during anomalously cold seasons.