Building long homogeneous temperature series across Europe: a new approach for the blending of neighboring series

Long and homogeneous series are a necessary requirement for reliable climate analysis. Relocation of measuring equipment from one station to another, such as from the city center to a rural area or a nearby airport, is one of the causes of discontinuities in these long series which may affect trend estimates. In this paper an updated procedure for the composition of long series, by combining data from nearby stations, is introduced. It couples an evolution of the blending procedure already implemented within the European Climate Assessment and Dataset (which combines data from stations no more than 12.5 km apart from each other) with a duplicate removal, alongside the quantile matching homogenization procedure. The ECA&D contains approximately 3000 homogenized series for each temperature variable prior to the blending procedure, around 820 of these are longer than 60 years; the process of blending increases the number of long series to more than 900. Three case studies illustrate the effects of the homogenization on single blended series, showing the effectiveness of separate adjustments on extreme and mean values (Geneva), on cases where blending is complex (Rheinstetten) and on series which are completed by adding relevant portions of GTS synoptic data (Siauliai). Finally, a trend assessment on the whole European continent reveals the removal of negative and very large trends, demonstrating a stronger spatial consistency. The new blended and homogenized data-set will allow a more reliable use of temperature series for indices calculation and for the calculation of gridded data-sets, and will be available for users on www.ecad.eu

Percentile indices for assessing changes in heavy precipitation events

Many climate studies assess trends and projections in heavy precipitation events using precipitation percentile (or quantile) indices. Here we investigate three different percentile indices that are commonly used. We demonstrate that these may produce very different results and thus require great care with interpretation. More specifically, consideration is given to two intensity-based indices and one frequency-based index, namely (a) all-day percentiles, (b) wet-day percentiles, and (c) frequency indices based on the exceedance of a percentile threshold. Wet-day percentiles are conditionally computed for the subset of wet events (with precipitation exceeding some threshold, e.g. 1 mm/d for daily precipitation). We present evidence that this commonly used methodology can lead to artifacts and misleading results if significant changes in the wet-day frequency are not accounted for. Percentile threshold indices measure the frequency of exceedance with respect to a percentile-based threshold. We show that these indices yield an assessment of changes in heavy precipitation events that is qualitatively consistent with all-day percentiles, but there are substantial differences in quantitative terms. We discuss the reasons for these effects, present a theoretical assessment, and provide a series of examples using global and regional climate models to quantify the effects in typical applications. Application to climate model output shows that these considerations are relevant to a wide range of typical climate-change applications. In particular, wet-day percentiles generally yield different results, and in most instances should not be used for the impact-oriented assessment of changes in heavy precipitation events

On tail trend detection: modeling relative risk

The climate change dispute is about changes over time of environmental characteristics (such as rainfall). Some people say that a possible change is not so much in the mean but rather in the extreme phenomena (that is, the average rainfall may not change much but heavy storms may become more or less frequent). The paper studies changes over time in the probability that some high threshold is exceeded. The model is such that the threshold does not need to be specified, the results hold for any high threshold. For simplicity a certain linear trend is studied depending on one real parameter. Estimation and testing procedures (is there a trend?) are developed. Simulation results are presented. The method is applied to trends in heavy rainfall at 18 gauging stations across Germany and The Netherlands. A tentative conclusion is that the trend seems to depend on whether or not a station is close to the sea.Comment: 38 page

Observations: Atmosphere and Surface

Executive SummaryThe evidence of climate change from observations of the atmosphere and surface has grown significantly during recent years. At the same time new improved ways of characterizing and quantifying uncertainty have highlighted the challenges that remain for developing long-term global and regional climate quality data records. Currently, the observations of the atmosphere and surface indicate the following changes:Atmospheric CompositionIt is certain that atmospheric burdens of the well-mixed greenhouse gases (GHGs) targeted by the Kyoto Protocol increased from 2005 to 2011. The atmospheric abundance of carbon dioxide (CO2) was 390.5 ppm (390.3 to 390.7) in 2011; this is 40% greater than in 1750. Atmospheric nitrous oxide (N2O) was 324.2 ppb (324.0 to 324.4) in 2011 and has increased by 20% since 1750. Average annual increases in CO2 and N2O from 2005 to 2011 are comparable to those observed from 1996 to 2005. Atmospheric methane (CH4) was 1803.2 ppb (1801.2 to 1805.2) in 2011; this is 150% greater than before 1750. CH4 began increasing in 2007 after remaining nearly constant from 1999 to 2006. Hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulphur hexafluoride (SF6) all continue to increase relatively rapidly, but their contributions to radiative forcing are less than 1% of the total by well-mixed GHGs

Indices for daily temperature and precipitation extremes in Europe analyzed for the period 1901-2000

The regional climate of the Greater Baltic Area is complex and varies at a multitude of scales in space and time. This thesis contributes to increased understanding of climate change and climate variability in this area focusing on four significant research topics. Droughts have a considerable ecological and socio-economic impact. The occurrence of rainfall is strongly controlled by large-scale atmospheric circulation. The observed summer North Atlantic Oscillation (SNAO) was correlated to a gridded dataset of the self-calibrating Palmer Drought Severity Index. A more positive circulation index is strongly linked to dry conditions over large parts of Southern Fennoscandia and northern Central Europe. Less distinct but still significant is the coupling to wetter conditions in the eastern Mediterranean. Using tree-ring based SNAO and precipitation reconstructions over 550 a, the relationship was investigated back in time in a multicentury perspective. Prior to the instrumental period the coupling is generally less pronounced but holds for distinct periods of drought. A database of up to 121 daily more than century-long instrumental records of precipitation and temperature over Europe was analyzed for trends in climate extremes. Over the 20th century a clear increase of warm extremes and a decreasing trend in cold extremes could be detected. Precipitation extremes became slightly more frequent and precipitation amounts increased, especially during winter. The ongoing warming resulted in a significantly extended thermal growing season in the Greater Baltic Area has extended significantly during the last century. An analysis of 48 long-term daily mean temperature records over this area revealed an overall lengthening of about one week between 1951-2000 mostly contributed by an earlier start in spring. The strongest change was observed at stations adjacent to the Baltic Sea in the South and the weakest in the North East. The 100-year records at Danish stations reveal a maximum shift in start (-22.8 d), end (12.6 d) and growing season length (33.5 d). The sub-daily precipitation characteristics in the region are not very well understood yet. By studying hourly observations for 1996-2008 from 93 stations all over Sweden, a distinct summer season diurnal cycle with an afternoon peak mainly contributed by convective activities during summer was identified for inland stations. Along the East coast the influence of the Baltic Sea is evident showing a weaker cycle peaking in the early morning. The observed diurnal cycle was compared to simulations from the Rossby Centre regional climate model (RCA3) run at 50, 25, 12 and 6 km grid resolution. In general the model tends to simulate too frequent convective precipitation events of light intensity. The simulated peak timing is about 2-4 hours too early and the amplitude too high. The model performance varies depending on the spatial resolution. The 6-km simulation most realistically captures the peak timing and the diversity in the spatial pattern. With increasing model resolution the fraction of large-scale (convective) precipitation is increasing (decreasing). The results indicate the need for improvement of the convection parameterization scheme

Changes in climate extremes and their impacts on the natural physical environment

This chapter addresses changes in weather and climate events relevant to extreme impacts and disasters. An extreme (weather or climate) event is generally defined as the occurrence of a value of a weather or climate variable above (or below) a threshold value near the upper (or lower) ends (‘tails’) of the range of observed values of the variable. Some climate extremes (e.g., droughts, floods) may be the result of an accumulation of weather or climate events that are, individually, not extreme themselves (though their accumulation is extreme). As well, weather or climate events, even if not extreme in a statistical sense, can still lead to extreme conditions or impacts, either by crossing a critical threshold in a social, ecological, or physical system, or by occurring simultaneously with other events. A weather system such as a tropical cyclone can have an extreme impact, depending on where and when it approaches landfall, even if the specific cyclone is not extreme relative to other tropical cyclones. Conversely, not all extremes necessarily lead to serious impacts. [3.1] Many weather and climate extremes are the result of natural climate variability (including phenomena such as El Niño), and natural decadal or multi-decadal variations in the climate provide the backdrop for anthropogenic climate changes. Even if there were no anthropogenic changes in climate, a wide variety of natural weather and climate extremes would still occur. [3.1] A changing climate leads to changes in the frequency, intensity, spatial extent, duration, and timing of weather and climate extremes, and can result in unprecedented extremes. Changes in extremes can also be directly related to changes in mean climate, because mean future conditions in some variables are projected to lie within the tails of present-day conditions. Nevertheless, changes in extremes of a climate or weather variable are not always related in a simple way to changes in the mean of the same variable, and in some cases can be of opposite sign to a change in the mean of the variable. Changes in phenomena such as the El Nino-Southern Oscillation or monsoons could affect the frequency and intensity of extremes in several regions simultaneously