[1] We present a new Lagrangian diagnostic for identifying the sources of water vapor for precipitation. Unlike previous studies, the method allows for a quantitative demarcation of evaporative moisture sources. This is achieved by taking into account the temporal sequence of evaporation into and precipitation from an air parcel during transport, as well as information on its proximity to the boundary layer. The moisture source region diagnostic was applied to trace the origin of water vapor for winter precipitation over the Greenland ice sheet for 30 selected months with pronounced positive, negative, and neutral North Atlantic Oscillation (NAO) index, using the European Centre for Medium-Range Weather Forecasts' ERA-40 reanalysis data. The North Atlantic and the Nordic seas proved to be the by far dominant moisture sources for Greenland. The location of the identified moisture sources in the North Atlantic basin strongly varied with the NAO phase. More specifically, the method diagnosed a shift from sources north of Iceland during NAO positive months to a maximum in the southeastern North Atlantic for NAO negative months, qualitatively consistent with changes in the concurrent large-scale mean flow. More long-range moisture transport was identified during the NAO negative phase, leading to the advection of moisture from more southerly locations. Different regions of the Greenland ice sheet experience differing changes in the average moisture source locations; variability was largest in the north and west of Greenland. The strong moisture source variability for Greenland winter precipitation with the NAO found here can have a large impact on the stable isotope composition of Greenland precipitation and hence can be important for the interpretation of stable isotope data from ice cores. In a companion paper, the implications of the present results are further explored in that respect.

An intercomparison experiment involving 15 commonly used detection and tracking algorithms for extratropical cyclones reveals those cyclone characteristics that are robust between different schemes and those that differ markedly.

[1] This paper provides a review of stratosphere-troposphere exchange (STE), with a focus on processes in the extratropics. It also addresses the relevance of STE for tropospheric chemistry, particularly its influence on the oxidative capacity of the troposphere. After summarizing the current state of knowledge, the objectives of the project Influence of Stratosphere-Troposphere Exchange in a Changing Climate on Atmospheric Transport and Oxidation Capacity (STACCATO), recently funded by the European Union, are outlined. Several papers in this Journal of Geophysical ResearchAtmospheres special section present the results of this project, of which this paper gives an overview. STACCATO developed a new concept of STE in the extratropics, explored the capacities of different types of methods and models to diagnose STE, and identified their various strengths and shortcomings. Extensive measurements were made in central Europe, including the first monitoring over an extended period of time of beryllium-10 ( 10 Be), to provide a suitable database for case studies of stratospheric intrusions and for model validation. Photochemical models were used to examine the impact of STE on tropospheric ozone and the oxidizing capacity of the troposphere. Studies of the present interannual variability of STE and projections into the future were made using reanalysis data and climate models.

A global climatology of warm conveyor belts (WCBs) is presented for the years 1979-2010, based on trajectories calculated with Interim ECMWF Re-Analysis (ERA-Interim) data. WCB trajectories are identified as strongly ascending air parcels (600 hPa in 2 days) near extratropical cyclones. Corroborating earlier studies, WCBs are more frequent during winter than summer and they ascend preferentially in the western ocean basins between 258 and 508 latitude. Before ascending, WCB trajectories typically approach from the subtropics in summer and from more midlatitude regions in winter. Considering humidity, cloud water, and potential temperature along WCBs confirms that they experience strong condensation and integrated latent heating during the ascent (typically .20 K). Liquid and ice water contents along WCBs peak at about 700 and 550 hPa, respectively. The mean potential vorticity (PV) evolution shows typical tropospheric values near 900 hPa, followed by an increase to almost 1 potential vorticity unit (PVU) at 700 hPa, and a decrease to less than 0.5 PVU at 300 hPa. These low PV values in the upper troposphere constitute significant negative anomalies with amplitudes of 1-3 PVU, which can strongly influence the downstream flow. Considering the low-level diabatic PV production, (i) WCBs starting at low latitudes (,408) are unlikely to attain high PV (due to weak planetary vorticity) although they exhibit the strongest latent heating, and (ii) for those ascending at higher latitudes, a strong vertical heating gradient and high absolute vorticity are both important. This study therefore provides climatological insight into the cloud diabatic formation of significant positive and negative PV anomalies in the extratropical lower and upper troposphere, respectively.

SummaryAs wind and solar power provide a growing share of Europe’s electricity1, understanding and accommodating their variability on multiple timescales remains a critical problem. On weekly timescales, variability is related to long-lasting weather conditions, called weather regimes2–5, which can cause lulls with a loss of wind power across neighbouring countries6. Here we show that weather regimes provide a meteorological explanation for multi-day fluctuations in Europe’s wind power and can help guide new deployment pathways which minimise this variability. Mean generation during different regimes currently ranges from 22 GW to 44 GW and is expected to triple by 2030 with current planning strategies. However, balancing future wind capacity across regions with contrasting inter-regime behaviour – specifically deploying in the Balkans instead of the North Sea – would almost eliminate these output variations, maintain mean generation, and increase fleet-wide minimum output. Solar photovoltaics could balance low-wind regimes locally, but only by expanding current capacity tenfold. New deployment strategies based on an understanding of continent-scale wind patterns and pan-European collaboration could enable a high share of wind energy whilst minimising the negative impacts of output variability.

A novel method is introduced to generate climatological frequency distributions of meteorological features from gridded datasets. The method is used here to derive a climatology of extratropical cyclones from sea level pressure (SLP) fields. A simple and classical conception of cyclones is adopted where a cyclone is identified as the finite area that surrounds a local SLP minimum and is enclosed by the outermost closed SLP contour. This cyclone identification procedure can be applied to individual time instants, and climatologies of cyclone frequency, fc, are obtained by simple time averaging. Therefore, unlike most other climatologies, the method is not based on the application of a tracking algorithm and considers the size of cyclones. In combination with a conventional cyclone center tracking algorithm that allows the determination of cyclone life times and the location of cyclogenesis and cyclolysis, additional frequency fields can be obtained for special categories of cyclones that are generated in, move through, or decay in a specified geographical area. The method is applied to the global SLP dataset for the time period 1958–2001 from the latest 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40). In the Northern Hemisphere and during winter, the cyclone frequency field has three maxima in the Pacific storm track (with fc up to 35%), the Atlantic storm track (with fc up to 32%), and the Mediterranean (with fc up to 15%). During the other seasons the fc values are generally reduced in midlatitudes and the subtropical monsoon areas appear as regions with enhanced fc. In the Southern Hemisphere, the seasonal variations are smaller with year-round maxima of fc in the belt from 50° to 70°S (along the coast of Antarctica, with maximum values of almost 40%) and to the east of the Andes (with fc up to 35% during summer). Application of a lifetime threshold value significantly reduces fc, in particular over and close to the continents. Subsets of cyclone frequency fields are calculated for several subjectively chosen regions of cyclone genesis, passage, and lysis. They show some interesting aspects of the behavior of extratropical cyclones; cyclones that decay along the U.S. West Coast, for instance, have a short lifetime and originate almost exclusively from the eastern North Pacific, whereas long-lived and long-distance Pacific cyclones terminate farther north in the Gulf of Alaska. The approach to calculate frequency distributions of atmospheric flow structures as introduced in this study can be easily applied to gridded data from global atmospheric models and assimilation systems. It combines the counts of atmospheric features with their area of influence, and hence provides a robust and easily interpretable measure of key meteorological structures when comparing and evaluating different analysis datasets and climate model integrations. Further work is required to comprehensively exploit the presented global ERA-40 cyclone climatology, in particular, aspects of its interannual variability.

A novel object-based quality measure, which contains three distinct components that consider aspects of the structure (S), amplitude (A), and location (L) of the precipitation field in a prespecified domain (e.g., a river catchment) is introduced for the verification of quantitative precipitation forecasts (QPF). This quality measure is referred to as SAL. The amplitude component A measures the relative deviation of the domain-averaged QPF from observations. Positive values of A indicate an overestimation of total precipitation; negative values indicate an underestimation. For the components S and L, coherent precipitation objects are separately identified in the forecast and observations; however, no matching is performed of the objects in the two datasets. The location component L combines information about the displacement of the predicted (compared to the observed) precipitation field's center of mass and about the error in the weighted-average distance of the precipitation objects from the total field's center of mass. The structure component S is constructed in such a way that positive values occur if precipitation objects are too large and/or too flat, and negative values if the objects are too small and/or too peaked. Perfect QPFs are characterized by zero values for all components of SAL. Examples with both synthetic precipitation fields and real data are shown to illustrate the concept and characteristics of SAL. SAL is applied to 4 yr of daily accumulated QPFs from a global and finer-scale regional model for a German river catchment, and the SAL diagram is introduced as a compact means of visualizing the results. SAL reveals meaningful information about the systematic differences in the performance of the two models. While the median of the S component is close to zero for the regional model, it is strongly positive for the coarser-scale global model. Consideration is given to the strengths and limitations of the novel quality measure and to possible future applications, in particular, for the verification of QPFs from convection-resolving weather prediction models on short time scales.

This study presents the first climatology of so-called warm conveyor belts (WCBs), strongly ascending moist airstreams in extratropical cyclones that, on the time scale of 2 days, rise from the boundary layer to the upper troposphere. The climatology was constructed by using 15 yr (1979-93) of reanalysis data and calculating 355 million trajectories starting daily from a 1Њ ϫ 1Њ global grid at 500 m above ground level (AGL). WCBs were defined as those trajectories that, during a period of 2 days, traveled northeastward and ascended by at least 60% of the zonally and climatologically averaged tropopause height. The mean specific humidity at WCB starting points in different regions varies from 7 to 12 g kg Ϫ1. This moisture is almost entirely precipitated out, leading to an increase of potential temperature of 15-22 K along a WCB trajectory. Over the course of 3 days, a WCB trajectory produces, on average, about four (six) times as much precipitation as a global (extratropical) average trajectory starting from 500 m AGL. WCB starting points are most frequently located between approximately 25Њ and 45ЊN and between about 20Њ and 45ЊS. In the Northern Hemisphere (NH), there are two distinct frequency maxima east of North America and east of Asia, whereas there is much less zonal variability in the Southern Hemisphere (SH). In the NH, WCBs are almost an order of magnitude more frequent in January than in July, whereas in the SH the seasonal variation is much weaker. In order to study the relationship between WCBs and cyclones, an independent cyclone climatology was used. Most of the WCBs were found in the vicinity of a cyclone center, whereas the reverse comparison revealed that cyclones are normally accompanied by a strong WCB only in the NH winter. In the SH, this is not the case throughout the year. Particularly around Antarctica, where cyclones are globally most frequent, practically no strong WCBs are found. These cyclones are less influenced by diabatic processes and, thus, they are associated with fewer high clouds and less precipitation than cyclones in other regions. In winter, there is a highly significant correlation between the North Atlantic Oscillation (NAO) and the WCB distribution in the North Atlantic: In months with a high NAO index, WCBs are about 12% more frequent and their outflow occurs about 10Њ latitude farther north and 20Њ longitude farther east than in months with a low NAO index. The differences in the WCB inflow regions are relatively small between the two NAO phases. During high phases of the Southern Oscillation, WCBs occur more (less) frequent around Australia (in the South Atlantic).