Atmospheric rivers (ARs) are narrow regions responsible for the majority of the poleward water vapor transport across the midlatitudes. They are characterized by high water vapor content and strong low level winds, and form a part of the broader warm conveyor belt of extratropical cyclones. Although the meridional water vapor transport within ARs is critical for water resources, ARs can also cause disastrous floods especially when encountering mountainous terrain. They were labeled as atmospheric rivers in the 1990s, and have since become a well-studied feature of the midlatitude climate. We briefly review the conceptual model, the methods used to identify them, their main climatological characteristics, their impacts, the predictive ability of numerical weather prediction models, their relationship with large-scale ocean-atmosphere dynamics, possible changes under future climates, and some future challenges.

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The properties of the evolution of solar granulation have been studied using an 80 minute time series of high spatial resolution white-light images obtained with the Swedish Vacuum Solar Telescope at the Observatorio del Roque de los Muchachos, La Palma. An automatic tracking algorithm has been developed to follow the evolution of individual granules, and a sample of 2643 granules has been analyzed. To check the reliability of this automatic procedure, we have manually tracked a sample of 481 solar granules and compared the results of both procedures. An exponential law gives a good Ðt to the distribution of granular lifetimes, T . Our estimated mean lifetime is about 6 minutes, which is at the lower limit of the ample range of values reported in the literature. We note a linear increase in the timeaveraged granular sizes and intensities with the lifetime. T \ 12 minutes marks a sizeable change in the slopes of these linear trends. Regarding the location of granules with respect to the meso-and supergranular Ñow Ðeld, we Ðnd only a small excess of long-lived granules in the upÑows. Fragmentation, merging, and emergence from (or dissolution into) the background are the birth and death mechanisms detected, resulting in nine granular families from the combination of these six possibilities. A comparative study of these families leads to the following conclusions : (1) fragmentation is the most frequent birth mechanism, while merging is the most frequent death mechanism ; (2) spontaneous emergence from the background occurs very rarely, but dissolution into the background is much more frequent ; and (3) di †erent granular mean lifetimes are determined for each of these families ; the granules that are born and die by fragmentation have the longest mean lifetime (9.23 minutes). From a comparison of the evolution of granules belonging to the most populated families, two critical values appear for the initial area in a granular evolution : 0.8 Mm2 and 1.3 Mm2 These values mark limits charac-77). terizing the birth mechanism of a granule, and predict its evolution to some extent. The Ðndings of the present work complement the earlier results presented in this series of papers and reinforce with new inputs, as far as the evolutionary aspects are concerned, the conclusion stated there that granules can be classiÐed into two populations with di †erent underlying physics. The boundary between these two classes could be established at the scale of d g \ 1A .4.

A 90 minute time series of high spatial resolution white-light images of solar granulation, obtained at the Swedish Vacuum Solar Tower (Observatorio del Roque de los Muchachos, La Palma), was analyzed to study how the physical properties of the granules changed with size. The observational material was corrected for global motions and for the instrumental proÐle, and a subsonic Ðlter was applied. A deÐni-tion of granular border was adopted using the inÑection points of the intensity of the images, and the granular cells were deÐned as areas including, in addition to the granules, one-half of their surrounding intergranular lanes. Using time series to investigate the average behavior of solar granulation has three strong advantages : the Ðrst is the possibility of removing the acoustic waves ; second, the possibility of estimating the e †ect of the variability of seeing on our results ; and, third, the opportunity to attain high statistical signiÐcance in the analysis as a result of the large number of extracted granules (61,138).It is shown that the granules of the sample can be classiÐed according to their mean and maximum intensities and their fractal dimension into two regimes, with diameters smaller than and larger than 1A .4, respectively. A broad transition region in which both regimes coexist was found. The resolved internal brightness structure of both the granules and the intergranular lanes shows a linear increase of the number of substructures with the granular and intergranular areas. The diameters of these substructures range between our e †ective resolution limit and with preferential sizes at and (D0A .3) D1A .5, 0A .65 0A .55, respectively. Moreover, it seems that large and small granules are unevenly distributed with respect to the large-scale vertical Ñows. Thus smaller granules are more concentrated along downdrafts whereas larger ones preferentially occupy the updrafts. Finally, a physical scenario compatible with the existence of these two granular populations is discussed.

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Eurasian fall snow cover changes have been suggested as a driver for changes in the Arctic Oscillation and might provide a link between sea-ice decline in the Arctic during summer and atmospheric circulation in the following winter. However, the mechanism connecting snow cover in Eurasia to seaice decline in autumn is still under debate. Our analysis is based on snow observations from 820 Russian land stations, moisture transport using a Lagrangian approach derived from meteorological re-analyses. We show that declining sea-ice in the Barents and Kara Seas (BKS) acts as moisture source for the enhanced Western Siberian snow depth as a result of changed tropospheric moisture transport. Transient disturbances enter the continent from the BKS region related to anomalies in the planetary wave pattern and move southward along the Ural mountains where they merge into the extension of the Mediterranean storm track.

This paper presents the analysis of high-resolution Stokes observations of eight different umbral dots in a sunspot. The spectra were recorded with the La Palma Stokes Polarimeter, attached to the Swedish Vacuum Solar Telescope. The observed line profiles have been inverted to yield the height stratifications of temperature, magnetic field, and line-of-sight velocity, as well as their respective Wilson depressions. We report on systematic differences in the properties of umbral dots with respect to the nearby umbra, including small upflows ($100 m s À1 ), higher temperatures ($1 kK), and weaker fields ($500 G) with more horizontal orientations ($10 ). The field weakening is strongly correlated with the Wilson depression, suggesting that it may be due to an opacity effect (as one is looking at higher layers). The inclination excess, on the other hand, is real and cannot be ascribed to formation height issues. The results obtained from our semiempirical modeling are discussed within the context of the currently existing scenarios for the subsurface structure of sunspots. The observational signatures revealed by our analysis fit well within both the ''spaghetti'' and the monolithic models.

Hydrological processes play a key role in the Arctic, as well as being an important part of the response of this region to climate change. The origin of the moisture arriving (and then precipitating) in the Arctic is a crucial question in our understanding of the Arctic hydrological cycle. In an attempt to answer this, the present study uses the Lagrangian diagnosis model FLEXPART (FLEXible PARTicle dispersion model) to localize the main sources of moisture for the Arctic region, to analyze their variability and their contribution to precipitation, and to consider the implications of any changes in the transport of moisture from particular sources within the system. From this analysis, four major moisture sources appear as the most important moisture supplies into the system: the subtropical and southern extratropical Pacific and Atlantic Oceans, North America, and Siberia. Oceanic sources play an important role throughout the year, whereas continental ones only take effect in summer. The sink areas associated with each source have been shown to be moderately influenced by changes in atmospheric circulation, mainly associated with the East Atlantic pattern for the Atlantic source and related to West Pacific and Pacific/North American (PNA) teleconnection patterns for the Pacific one. On the other hand, the variability over the sinks does not seem to be significantly related to changes in evaporation at an interannual scale.