The Climatology of Air-Mass and Frontal Extreme Precipitation

Extreme climatic and weather events are imperishable subjects of climatological studies. Although the understanding of these rare and unexpected phenomena has improved over the past decades, the factors governing their occurrence and magnitude still need to be studied. This chapter starts with the executive summary including the most important conclusions on origin-based extreme precipitation types (air-mass and frontal precipitation) and their relationship to atmospheric circulation which are discussed in this book. It also reviews a vast body of literature on synoptic contributions to the development of extreme precipitation. It presents sources of meteorological and synoptic data used throughout this study and discusses the quality of weather data and the criteria used for sectioning precipitation series. The climatology of frontal precipitation and air-mass precipitation presented in this volume has been developed using daily totals recorded at 513 European weather stations during the period spanning December 1950 to February 2008. A large set of synoptic charts covering the same period has also been used. The majority (64 %) of the individual precipitation series has been found to be complete, that is, either free from gaps or where any gaps account for no more than 1 % of days in the study period. This chapter also includes the executive summary, which brings together the most important conclusions from the comprehensive synoptic/climatic analysis of extreme precipitation taking into account both circulation types and atmospheric fronts in Europe. The analysis has demonstrated the existence of regional groups with seasonal patterns of occurrence of extreme precipitation types. A regional and seasonal variability in the relationship between the occurrence of origin-based types of extreme precipitation and atmospheric circulation was also discovered.

Definitions of extreme climatological and meteorological events may depend on the objective of the study, the type of data, or the researcher’s subjective opinion. This chapter discusses definitions of extreme climatic events and presents indices of precipitation extremes found in the literature. The spatial and seasonal variability in a range of these indices were analysed, and the results were used to select criteria for identifying extreme precipitation in Europe adopted in this study. These criteria were then applied in subsequent chapters. During the study period, the maximum daily precipitation totals in Europe ranged from 37.6 to 520 mm. An overwhelming majority of stations (89 %) recorded daily precipitation greater than 50 mm less than once per year. Because there is great variability between precipitation regimens in Europe, this study adopted a statistical definition of extreme precipitation events. The events were identified separately for each of the weather station and in each of the months using an empirical distribution of the daily precipitation totals. Precipitation totals exceeding or equal to the 95th percentiles of daily precipitation were selected as extreme. The 95th percentile (95P) was calculated from days with daily totals ≥1 mm during the period 1961–1990. The resulting spatial and seasonal variability of the threshold values corresponding to the 95P daily precipitation across Europe is similar to the spatial and seasons variability of the monthly totals.

The occurrence as well as the daily and annual patterns of precipitation are inseparably linked with cloud formation processes. Upward air movement, alongside its sufficient humidity, is a precondition of precipitation. This chapter uses existing studies to discuss mechanisms leading to the formation of clouds. These mechanisms are then used to identify seven origin-based extreme precipitation types, that is, air-mass precipitation and a breakdown of frontal precipitation depending on the front. The chapter discusses the spatial extent of extreme air-mass and frontal precipitation in general and then focusses on the spatial and seasonal variabilities in all the origin-based types of precipitation in Europe.

The spatial extent of frontal precipitation events (defined as the number of stations involved on a single day) is much greater than that of air-mass events. The frequency of occurrence of various origin-based types of precipitation in Europe follows discernible spatial and seasonal variabilities, which are driven by the varying pace of cyclones life and by ground relief. This latter factor also affects cyclogenesis, thus having an influence on the spatial variability of origin-based types of extreme precipitation. A clear majority of extreme precipitation in Europe is linked to weather fronts. In each season, the average proportion of frontal precipitation in the overall number of days with extreme precipitation was several times greater than that of air-mass precipitation.

Both the strength and the spatial extent of these relationships between NAO and extreme precipitation display seasonality, with a winter peak, and deppend on origin-based precipitation type. In autumn, the influence of NAO is weaker than in winter, but clearly stronger than in summer and spring. In winter, the positive NAO phase accompanies a majority of extreme precipitation in an overwhelming majority of the Northern and Western Europe, as moist air masses move over this part of Europe. A majority of precipitation occurring at that time is associated with either a cold front (Fc) or with an occluded front (Fo), depending on the region. In southern Europe advection from the Atlantic Ocean and the occurrence of extreme precipitation is associated mostly with the negative NAO phase. Extreme precipitation occurring then is associated with the passage of different fronts (Ff) or with an occluded front (Fo).

The frequency of origin-based types of extreme precipitation in Europe displays a regionally varied relationship with the direction of the advection and the type of the pressure system. This chapter discusses relationships between origin-based types of precipitation and mesoscale circulation. For the purpose of this chapter, circulation types were identified for each grid point between 30°N and 80°N and 30°W and 70°E at the interval of 2.5° of latitude and longitude. The direction of air advection was identified with the geostrophic wind direction. The type of the pressure system was identified using the vorticity formula. The study also identified the conditional probability of the occurrence of each origin-based type of extreme precipitation in each of these circulation types.

All origin-based types of extreme precipitation occur in both cyclonic and anticyclonic situations, but the occurrence frequency of the former is higher. Dependencies between the occurrence of frontal precipitation and atmospheric circulation are characterised by a greater degree of regional variability than these between air-mass precipitation and atmospheric circulation. Extreme air-mass precipitation has the strongest association with the circulation in Southern Europe, in mountainous areas, on the southeastern North Sea coast, and on the western slopes of the Scandinavian Mountains. Extreme precipitation of the frontal type tends to be linked with both the direction of air advection and the type of the pressure system. In cyclones these relationships display strong seasonality, whereas in anticyclones they remain stable throughout the year. The breakdown of frontal precipitation into the types of associated fronts helps better define the relationship between its occurrence and atmospheric circulation, especially in cyclonic synoptic situations.