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[1] Atmospheric blocking can influence near-surface temperature via circulation and radiative forcing. This study investigates the relevance of blocking for co-located (sub-)daily temperature extremes and the spatial variability of this relationship in the Northern Hemisphere. It is shown that over large parts of the high-latitude continents warm temperature extremes often occur simultaneously with atmospheric blocking at the same location. Taking also weak blocks into account, more than 80% of the six-hourly warm extremes are associated with blocking, e.g., in eastern Canada, Scandinavia and parts of Siberia. On the contrary, cold extremes typically are not related to co-located atmospheric blocking. This difference between warm and cold extremes points to differences also in the physical driving mechanisms of the extremes. The strong linkage of warm temperature extremes and blocking should be considered when investigating changes of temperature extremes with global warming. Citation: Pfahl, S., and H. Wernli (2012), Quantifying the relevance of atmospheric blocking for co-located temperature extremes in the Northern Hemisphere on (sub-)daily time scales, Geophys. Res. Lett., 39,

Owing to the huge potential impact of precipitation extremes on society, it is important to better understand the mechanisms causing these events, and their variations with respect to a changing climate. In this study, the importance of a particular category of weather systems, namely cyclones, for the occurrence of regional-scale precipitation extremes is quantified globally using the ECMWF Interim reanalysis (ERA-Interim) dataset. Such an event-based climatological approach complements previous case studies, which established the physical relationship between cyclones and heavy precipitation. A high percentage of precipitation extremes is found to be directly related to cyclones. Regional hot spots are identified where this percentage of cycloneinduced precipitation extremes exceeds 80% (e.g., in the Mediterranean region, Newfoundland, near Japan, and over the South China Sea). The results suggest that in these regions changes of heavy precipitation with global warming are specifically sensitive to variations in the dynamical forcing, for example, related to shifts of the storm tracks. Furthermore, properties of cyclones causing extreme precipitation are investigated. In the exit regions of the Northern Hemisphere storm tracks, these cyclones are on average slightly more intense than low pressure systems not associated with precipitation extremes, but no differences with respect to minimum core pressure are found in most other parts of the midlatitudes. The fundamental linkage between cyclones and precipitation extremes may thus provide guidance to forecasters involved in flood prediction, but it is unlikely that forecasting rules based on simple cyclone properties can be established.

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[1] With the help of a Lagrangian moisture source diagnostic, linkages between stable isotope measurements in water vapor in Rehovot (Israel), with typical sampling times of 8 hours, and the meteorological conditions in the evaporation regions are established. These linkages can be formulated in quantitative terms, and are also quantitatively comparable with other data from isotope measurements over the ocean and with simple theoretical calculations. On the one hand, a strong negative correlation (r = À0.82) between relative humidity with respect to sea surface temperature in the source regions and measured deuterium excess (d) is found, corroborating results from isotope global circulation model simulations. This relationship can also be applied to model d in a larger region, as shown for a sample case. On the other hand, sea surface temperature in the evaporation regions does not correlate well (r = À0.21) with measured d. This finding contradicts results from other models. Although requiring confirmation by isotope data from different regions, this weak correlation is potentially of major importance for the interpretation of deuterium excess measured in ice cores, which has been used to reconstruct moisture source temperatures for past climates.Citation: Pfahl, S., and H. Wernli (2008), Air parcel trajectory analysis of stable isotopes in water vapor in the eastern Mediterranean,

The time that water takes to travel through the terrestrial hydrological cycle and the critical zone is of great interest in Earth system sciences with broad implications for water quality and quantity. Most water age studies to date have focused on individual compartments (or subdisciplines) of the hydrological cycle such as the unsaturated or saturated zone, vegetation, atmosphere, or rivers. However, recent studies have shown that processes at the interfaces between the hydrological compartments (e.g., soil‐atmosphere or soil‐groundwater) govern the age distribution of the water fluxes between these compartments and thus can greatly affect water travel times. The broad variation from complete to nearly absent mixing of water at these interfaces affects the water ages in the compartments. This is especially the case for the highly heterogeneous critical zone between the top of the vegetation and the bottom of the groundwater storage. Here, we review a wide variety of studies about water ages in the critical zone and provide (1) an overview of new prospects and challenges in the use of hydrological tracers to study water ages, (2) a discussion of the limiting assumptions linked to our lack of process understanding and methodological transfer of water age estimations to individual disciplines or compartments, and (3) a vision for how to improve future interdisciplinary efforts to better understand the feedbacks between the atmosphere, vegetation, soil, groundwater, and surface water that control water ages in the critical zone.

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Abstract. Variations of stable water isotopes in water vapourhave become measurable at a measurement frequency of about 1 Hz in recent years using novel laser spectroscopic techniques. This enables us to perform continuous measurements for process-based investigations of the atmospheric water cycle at the time scales relevant for synoptic and mesoscale meteorology. An important prerequisite for the interpretation of data from automated field measurements lasting for several weeks or months is a detailed knowledge about instrument properties and the sources of measurement uncertainty. We present here a comprehensive characterisation and comparison study of two commercial laser spectroscopic systems based on cavity ring-down spectroscopy (Picarro) and off-axis integrated cavity output spectroscopy (Los Gatos Research). The uncertainty components of the measurements were first assessed in laboratory experiments, focussing on the effects of (i) water vapour mixing ratio, (ii) measurement stability, (iii) uncertainties due to calibration and (iv) response times of the isotope measurements due to adsorption-desorption processes on the tubing and measurement cavity walls. Based on the experience from our laboratory experiments, we set up a one-week field campaign for comparing measurements of the ambient isotope signals from the two laser spectroscopic systems. The optimal calibration strategy determined for both instruments was applied as well as the correction functions for water vapour mixing ratio effects. The root mean square difference between the isotope signals from the two instruments during the field deployment was 2.3 ‰ for δ 2 H, 0.5 ‰ for δ 18 O and 3.1 ‰ for deuterium excess. These uncertainty estimates from field measurements compare well to those found in the laboratory experiments. The present quality of measurements from laser spectroscopic instruments combined with a calibration system opens new possibilities for investigating the atmospheric water cycle and the land-atmosphere moisture fluxes.