Elsevier

Atmospheric Research

Volume 248, 15 January 2021, 105182
Atmospheric Research

Moisture fluxes conducive to central European extreme precipitation events

https://doi.org/10.1016/j.atmosres.2020.105182Get rights and content

Highlights

  • Central European precipitation extremes are often due to extra strong moisture fluxes.

  • Even better correspondence is reached when combining fluxes with vertical velocity.

  • Main variants of precipitation extremes are caused by northern and western fluxes.

  • Division into moisture flux variants perfectly matches spatial clustering of events.

  • Each variant mainly occurs in the season when strong fluxes from that direction occur.

Abstract

For the emergence of central European extreme precipitation events (EPEs), moisture supply mainly from the Atlantic or Mediterranean and an ascent of moist air are the most important ingredients. In the present study, we aim to define groups of EPEs with characteristic moisture flux conditions in the area of extra high upward vertical velocity, as well as to find the connection between the variants of moisture flux conditions and the spatio-seasonal distribution of extreme precipitation. Using ERA5 reanalysis data, we assigned one of three moisture flux variants to the set of 43 EPEs from 1979 to 2013 and thus specified (i) events with dominating moisture flux from the north (Nf), (ii) events with dominating moisture flux from the west (Wf), and (iii) other low-flux events (Ot). The classification of EPEs corresponds perfectly with seasonal and spatial patterns of extreme precipitation: while Nf events were concentrated only in the warm half-year and affected mainly eastern central Europe, Wf events occurred in the west and exclusively from September to March. Mainly in the case of Nf events, a direct link existing between anomalies in moisture flux and precipitation extremes can be beneficial for improving skill in forecasting EPEs.

Introduction

Large-scale extreme precipitation events in central Europe are usually associated with anomalies of atmospheric conditions at the synoptic scale. Meteorological variables, including potential vorticity, vertical velocity, and moisture flux, are studied in connection with the occurrence of hydrometeorological extremes (Martius et al., 2006; Kašpar and Müller, 2014) when the interaction of several anomalous variables allows the emergence of heavy rainfall or flooding in central Europe (Müller et al., 2009).

In elevated parts of the study area of central Europe, intense integrated vapour transport seems to be directly connected to extreme floods, as was demonstrated by Froidevaux and Martius (2016) in the Swiss Alps. Similar results were found in the southeastern Alps by Müller and Kašpar (2011), who revealed a close relationship between extreme flood events and moisture flux at the 850 hPa level in the respective area.

Both the Atlantic and the Mediterranean are important sources of moisture for precipitation in central Europe (Sodemann and Zubler, 2010). Warm-conveyor-belt advection and shallow convection in mid-latitude cyclones help move moisture into the free troposphere and transport it over long distances (Boutle et al., 2011). Apart from the large-scale moisture transport from distant water bodies, local land evaporation may play a crucial role in the emergence of heavy precipitation. This effect was shown in the case of extreme precipitation in August 2002 (Sodemann et al., 2009; Stohl and James, 2004) as well as during other mostly summer precipitation events (Sodemann and Zubler, 2010). In this study, we focus only on synoptic-scale processes of moisture transport that are major contributors to heavy precipitation throughout the year.

Cyclones moving along the Vb track (Van Bebber, 1891) are one of the main synoptic patterns controlling large-scale summer precipitation extremes in central Europe (Messmer et al., 2015). During the Vb events, cyclones move from the region of Genoa to the northeast and bring Mediterranean air into central Europe. The warm and moist air turns around the centre of the cyclone, so we actually experience moisture flux from the north or the northeast in the lower troposphere (Müller and Kašpar, 2010). Orographically enhanced precipitation induced by the Vb cyclone often results in extensive flooding on the northern slopes of the Alps and other west-east oriented mountain ranges in central Europe. Flooding then also affects the lowlands north of them, especially the Elbe and Oder basins and parts of the Danube basin (Nissen et al., 2014). Compared to the flood of August 2002 (Řezáčová et al., 2005), the more recent flood of May/June 2013 did not have the classic Vb character, but multiple cyclones tracked rather westward into central Europe, affecting mainly the Elbe and Danube basins (Grams et al., 2014). Nevertheless, a persistent northern moisture flux was a common feature for the development of heavy precipitation in 2013.

Another type of synoptic pattern brings moisture from the western sector with Atlantic frontal systems. Their influence is the largest in northwestern central Europe, and it decreases towards the southeast. The pattern is mostly related to winter precipitation extremes (Beurton and Thieken, 2009; Hofstätter et al., 2017). An intense moisture flux during these situations is induced by associated mid-latitude cyclones with a generally strong flow from the western sector. In winter, extreme winds tend to be connected to precipitation extremes mainly in northwestern central Europe (Martius et al., 2016), where the simultaneous occurrence of extreme events is often associated with low-pressure systems over the North Sea (Pfahl, 2014).

Mean moisture fluxes at the 850 hPa level have mostly western directions in central Europe (Fig. 1) in accordance with the prevailing winds from the west to the east in the mid-latitudes. The same is true for the mean vertically integrated water vapour flux (IWVF; Wypych et al., 2018), which confirms the significance of western zonal fluxes. The meridional component of the mean moisture flux is much less intense, especially in winter; however, July shows a slightly negative (northern) IWVF in southeastern central Europe, when the northwestern part tends to have positive values in July and October (Wypych et al., 2018). Differences in the significance of zonal and meridional moisture fluxes are the reason for concentrating on moisture flux anomalies that deviate from expected values and allow us to compare moisture fluxes from different directions. However, the question remains to what extent the occurrence of precipitation extremes can be explained by the presence of moisture flux extremes.

For the emergence of extreme precipitation, the moisture supply is not sufficient because only sustained ascent of moist air provides suitable conditions (Doswell III et al., 1998; Froidevaux and Martius, 2016). In the present study, selected extreme precipitation events (EPEs) from 1979 to 2013 are classified with respect to moisture flux magnitudes and direction in the area of extra high upward vertical velocity, which helps to distinguish various circulation types of the EPEs. The resulting variants of moisture flux conditions are assumed to correspond with spatial clusters of EPEs (Gvoždíková et al., 2019). By connecting EPEs to extremes of moisture flux, we want to explain the spatial occurrence and distribution of these events during the year, as there are great differences in the origin of “summer” and “winter” EPEs. At the spatial level of central Europe, we show the correspondence between the seasonality of moisture flux extremes from different directions and the seasonal variability in extreme precipitation events in the study area.

Section snippets

Reference precipitation events

Reference events are represented by a set of extreme precipitation events (EPEs), which were selected in our previous study (Gvoždíková et al., 2019) focusing on the spatial and temporal characteristics of central European EPEs. In the area of five main river basins (Rhine, Weser/Ems, Elbe, Danube, and Oder; see Fig. 1a), 53 maximum precipitation events were selected in 53 years between 1961 and 2013 by the value of the weather extremity index (WEI; Müller et al., 2015), combining the rarity of

Climatology of moisture fluxes in central Europe

In the lower troposphere, western directions of moisture flux dominate over central Europe, as demonstrated in Fig. 1b, presenting average moisture fluxes at the 850 hPa level. The Alpine region is characterized by some deformation due to higher altitudes, but the values of moisture flux could be biased considering that the 850 hPa level is sometimes below the model terrain. The dominance of western fluxes is again evident in Fig. 2a, showing the direction and magnitude of mean daily moisture

Variants of EPEs with respect to moisture flux conditions

There are 43 selected EPEs between 1979 and 2013 (Table 1) ranked by precipitation extremeness (WEI; Gvoždíková et al., 2019) when we are able to assign one of five variants of moisture flux conditions to each of them (Section 2.5). In fact, all EPEs could be classified into only three variants, namely, Nf, Wf or Ot (Table 3), which means that western and northern moisture fluxes are especially important for extreme precipitation in central Europe. No Sf or Ef cases were detected, although the

Seasonality of EPEs and moisture fluxes

Groups of extreme precipitation events, differentiated according to moisture flux conditions, also substantially differ in terms of their seasonal distribution (Fig. 8a): Wf events occurred exclusively from September to March, while Nf and Ot events occurred mostly in the warm part of the year (from April to September). This difference can be interpreted in terms of the seasonal distribution of moisture fluxes under extra high upward vertical velocity conditions. In general, the period from May

The role of the vertical velocity

In central Europe, mainly western and northern moisture fluxes are connected to precipitation extremes. However, the supply of moisture itself needs to be supported by the upward motion of air. These are two basic components of heavy precipitation that are often studied together (Giannakaki and Martius, 2015; Doswell III et al., 1998).

In the present study, we decided to overlap the fields of moisture flux and vertical velocity to explain the occurrence of extreme precipitation events in central

Concluding remarks

  • -

    We used the ERA5 reanalysis data to divide a set of 43 central European extreme precipitation events from 1979 to 2013 into three groups (Table 3) representing (i) events with dominating moisture flux from the northern sector (Nf), (ii) events with dominating moisture flux from the western sector (Wf), and (iii) low flux events (Ot). The classification was based on the moisture flux magnitudes and direction in the area of extra high upward vertical velocity, which are both important ingredients

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the Czech Science Foundation (Grant No. 19-05011S). We would also like to acknowledge ECMWF for providing ERA5 reanalysis.

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