Precipitation variability and extreme weather events – Definition, observations

Definitions

Equivalent to the definition of climate variability (see above and IPCC (2013a)), precipitation variability can be defined as the variation in the mean state, standard deviation and/or occurrence of precipitation. Precipitation variability can refer to all spatial and temporal scales beyond that of individual rainfall events. Differentiation is usually made between interannual and intra-annual precipitation variability. Interannual variability refers to changes in annual precipitation amounts. Intra-annual variability refers to changes in precipitation pattern within a year/season while the annual precipitation amounts remain constant.

Although extreme weather events are easily recognised, no unique and universal definition of “extreme events” exists (Stephenson 2008). The relativeness of the concept

“extremeness” and the context dependence of the extreme weather events (availability or selection of climate record; climate history, location) are reasons for this lack of a common definition (Stephenson 2008; Smith 2011; IPCC 2012). Furthermore, an extreme weather event is a complex entity with several attributes such as rate of occurrence, magnitude, temporal duration, timing, and spatial scale that makes it difficult to completely described it with a single number (Stephenson 2008). Extremes can also result ‘from the interactions between two unrelated geophysical phenomena’ (IPCC 2012). However, accepted descriptions of single extreme weather events are events that have maximum values of certain meteorological variables or values which exceed above or below a pre-existing thresholds/critical levels on a continuous scale (Stephenson 2008; IPCC 2012). A generally accepted fact of an extreme weather event is its rarity at a particular place and time of year.

According to the definition in the fifth IPCC assessment report (IPCC 2013a) extreme weather events ‘would normally be as rare as or rarer than the 10^th or 90^th percentile of a probability density function estimated from observations’. Another definition of extreme weather events arises from a more ecological perspective which includes extremeness in the driver and the response; thus, an extreme weather event can be an occurrence of a statistically rare or unusual event that alters the ecosystem structure and/or function beyond the level of what is considered normal or typical (Smith 2011). However, such a definition based on the response of ecosystems or organisms are problematic due to the spatial and time context dependence of events.

In this thesis, I focused on changed precipitation variability by the experimentally manipulated occurrence of extreme drought and heavy rainfall events. Drought is commonly defined as an abnormal local precipitation deficit (Dai 2011; IPCC 2012). Heavy rainfall is generally a large precipitation event with an accumulation rate exceeding a specific and geographically dependent value (AMS 2012). For the severe weather warning, the German Weather Service defines heavy rainfall as a precipitation event with an amount greater than or equal to 25 mm within one hour (Deutscher Wetterdienst 2005). There are two types of approaches to identify or quantify drought or heavy rainfall events. One is the use of indices such as the Plamer drought severity index (PDSI) or the standardized precipitation–evapotranspiration index (SPEI) (Hartmann et al. 2013; Isbell et al. 2015). A second common approach is the extreme value theory to approximate the distribution of annual extremes of precipitation rates (Kharin et al. 2013). In this thesis, drought and heavy rainfall events were calculated according to the extreme value theory based on local

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precipitation data for the growing season of the years 1961-2000 (Kreyling et al. 2008a, Manuscript 4 & 5). Drought was defined as the number of consecutive days with less than 1 mm of daily precipitation and heavy rainfall as the prolonged rainfall period with largest precipitation amounts during the growing season. The Gumbel I distribution (Gumbel 1958) was fitted to the annual extremes. Thus, events with recurrences of 100 and 1000 years, respectively, were calculated. This approach was chosen in this thesis, because it considered the local weather history, the rate of occurrence with a defined magnitude, and because it is repeatable and transferable in other places or regions. However, using extreme value statistics for defining extreme weather events is restricted by the time scale of the available local data set. In choosing a prolonged rainfall period instead of maximum single day rainfall events, this thesis took a common Central European meteorological condition, the Vb-track of cyclones, into account. The Vb-track of cyclones, named by van Bebber in 1891, is a constellation of warm and moist air masses that can cause prolonged and abundant precipitation leading to flooding events in Europe (Malberg 2007; Klose 2008). For example, heavy rainfall caused by a Vb track of a cyclone led to severe flooding and destructions along the Elbe River in August 2002 (Malberg 2007). However, single day heavy rainfall events were also considered in Manuscripts 1, 2 & 3.

Observations

The changing climate leads to extreme weather events that are unprecedented in their frequency, intensity, spatial extent, duration and timing (Meehl et al. 2000; IPCC 2012).

Since the 1950s, changes in extreme weather events have been observed for many parts of the world. Although the high quality climate data sets with a daily resolution which are required for extreme values analysis are lacking for several parts of the world, alterations in the occurrence of several extreme events were detected with varying confidence (Seneviratne et al. 2012; Hartmann et al. 2013). Overall, in more regions the number of heavy rainfall events has likely increased than decreased (Easterling et al. 2000a; Groisman et al. 2005; Seneviratne et al. 2012). Especially in North America and Northern and Central Europe, the frequency and intensity for heavy rainfall events has increased during the last decades (Alexander et al. 2006; Kunkel et al. 2013a; Kovats et al. 2014). In Europe, this increase is most pronounced during winter though changes in summer are also observed with lower inconsistency due to regional and seasonal variations (Alexander et al. 2006;

Seneviratne et al. 2012; Zolina et al. 2013). Assessments of past changes in droughts are difficult and partly controversial. Dai (2011) found increases of drought events for wide areas of the world, especially for most of Europe, whereas findings of van der Schrier et al. (2006) and Sheffield & Wood (2008) showed that dryness trends for Northern and Southern Europe are contrasting. However, scientists agree that in particular Southern Europe has experienced more intense and longer droughts. West Africa also shows increases in intensity and duration of droughts, whereas the frequency, intensity and duration of droughts decreased in central North America and North-western Australia (Dai 2011; Seneviratne et al. 2012). In Germany, the precipitation pattern showed larger regional and seasonal variability during the last decades. An increase of the intensity of winter precipitation by 34%

(1971-2000, Table 1) was observed which was most pronounced in the northern part of Germany, while the summers demonstrated a trend to drying, with the exception of the southern part of Bavaria where summer rainfall became more intense (Schönwiese et al.

2005; Trömel & Schönwiese 2007, 2008; Zolina et al. 2009; Deutscher Wetterdienst 2015).

12 Projections

Theoretically, a precipitation event, which is extreme at the present climate, may become more common or rare in the future, it may also increase or decrease in magnitude. This depends on the alteration in the overall distribution of the precipitation variable and on the change at the end of its distribution curve due to climate change (IPCC 2012). For the projections of climate change a variety of models with a huge range of complexity and scales are used. Since the 4^th assessment report of the IPCC, climate models have been further developed and improved. However, large-scale patterns of precipitation are still less well simulated than temperature patterns (Flato et al. 2013). The model simulation of extreme weather events has also substantially progressed although the sensitivity of extreme weather events to temperature variability or trends is underestimated in the majority of models (Min et al. 2011; Flato et al. 2013). For the 21^st century, global climate models project not only an increase in total precipitation, but also an increasing probability of more frequent and intense precipitation events for most parts of the world (e.g. Northern America, Northern & Central Europe, Eastern Australia, Asia) due to global warming (Schär et al. 1996; Allen & Ingram 2002; Christensen & Christensen 2003; Karl & Trenberth 2003; Hegerl et al. 2004; Groisman et al. 2005; Alexander et al. 2006; Sillmann et al. 2013; Kunkel et al. 2013b; Fischer et al.

2013; Peterson et al. 2014). Droughts are expected to intensify and last longer during the next decades mainly due to increasing evapotranspiration and/or reduced rainfall amounts.

However, projections for droughts are not as strong and uniform as heavy rainfall projections and they show high seasonal and regional variations. Regions such as South and Central Europe, central North America, Mexico, Northeast Brazil and Southern Africa will be mostly affected by more intense and longer drought periods. Drought projections for the other regions are inconsistent and of lower confidence because of the lack of observational data for modelling and appropriate dryness definitions and indices (Sousa et al. 2011; Seneviratne et al. 2012; Hewitson et al. 2014; Kovats et al. 2014). Thus, the precipitation pattern across Europe will continue to change in this century with an increase in extremes: Central and Northern Europe will experience higher winter precipitation events, whereas heavy summer precipitation and the frequency of wet days becomes less over most of Europe, especially in the south (Räisänen et al. 2004; Giorgi et al. 2004; Beniston et al. 2007; May 2008; Wagner et al. 2013; Kovats et al. 2014). Particularly for the Mediterranean more intense and longer drought events are projected (Giorgi & Lionello 2008).

Projections for Germany in 2071-2100 show alterations in the precipitation variability with an increase in the intensity and frequency of extreme precipitation events in comparison to the time period 1961-1990, but the long-term annual precipitation amount is expected to remain the same. However, an increase of drought frequency is projected for the north-eastern, south-western, and southern parts of Germany (Schönwiese et al. 2005; Jacob 2009; Pfeifer et al. 2015). Schwarzak et al. (2015) projected also more persistent summer droughts for Central Eastern Germany until 2100. Furthermore, heavy rainfall events will intensify in Germany with high changes in southern and south-eastern parts during winter (KLIWA 2011;

Pfeifer et al. 2015; Schwarzak et al. 2015). Additionally, the probability of heavy rainfall events in summer was projected to increase for most parts of Bavaria, the region along the Rhine and Schleswig-Holstein (Schönwiese et al. 2005). However, recent projections show no robust increase of extreme summer precipitation in Germany (Pfeifer et al. 2015).

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