Surface snow accumulation is the primary mass input to the Antarctic ice sheets. As the dominant term among various components of surface snow accumulation (precipitation, sublimation/deposition, and snow drift), precipitation is of particular importance in helping to assess the mass balance of the Antarctic ice sheets and their contribution to global sea level change. The Polar MM5, a mesoscale atmospheric model based on the fifth-generation Pennsylvania State University-NCAR Mesoscale Model, has been run for the period of July 1996 through June 1999 to evaluate the spatial and temporal variability of Antarctic precipitation. Drift snow effects on the redistribution of surface snow over Antarctica are also assessed with surface wind fields from Polar MM5 in this study. It is found that areas with large drift snow transport convergence and divergence are located around escarpment areas where there is considerable katabatic wind acceleration. It is also found that the drift snow transport generally diverges over most areas of East and West Antarctica with relatively small values. The use of the dynamic retrieval method (DRM) to calculate precipitation has been developed and verified for the Greenland ice sheet. The DRM is also applied to retrieve the precipitation over Antarctica from 1979 to 1999 in this study. Most major features in the spatial distribution of Antarctic accumulation are well captured by the DRM results. In comparison with predicted precipitation amounts from atmospheric analyses and reanalyses, DRM calculations capture more mesoscale features of the precipitation distribution over Antarctica. A significant upward trend of ϩ1.3 to ϩ1.7 mm yr Ϫ2 for 1979-99 is found from DRM and forecast precipitation amounts for Antarctica that is consistent with results reported by other investigators and indicates that an additional 0.05 mm yr Ϫ1 is being extracted from the global ocean and locked up in the Antarctic ice sheets. While there is good agreement in this trend among all of the datasets, the interannual variability about the trend on the continental scale is not well captured. However, on the subcontinental scale, the interannual variability about the trend is well resolved for sectors in West Antarctica and the South Atlantic. It is also noted that the precipitation trend is weakly downward over much of the continent.

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Abstract. The retrieval of an accurate spatial and temporal record of contemporary Greenland precipitation is a uniquely challenging task because of the extreme variability in both atmospheric processes and the resulting precipitation distribution over relatively small spatial scales. A comparison of precipitation data sets composed of monthly mean values from recent studies shows a convergence on the general features of the long-term spatial patterns but substantial disagreement on the temporal variability both regionally and for all of Greenland. There is general agreement on a long-term Greenland average of about 35 cm yr -1 and on long-term values for regional scales, although differences for outlying data sets exceed 50% of the observed glaciological estimate for particular regions. A fundamental problem is the inadequate topographic representation of Greenland in the numerical analyses. Nearly all of the data sets are overly dry for high-elevation areas, as seen from comparisons with glaciological observations from Summit. The east-central region of Greenland is found to be particularly susceptible to the temporal discontinuities in data sets which employ operational analyses. In contrast, there is strong agreement among all methods on the temporal variability for the west-central region over a 15-year period. From the comparison it is concluded that none of the data sets is able to capture all of the regional-scale features. In general, however, the deficiencies of each data set are readily identifiable from comparison and evaluation in the context of circulation features. Agreement among the methods on particular regions and timescales gives increased confidence in drawing conclusions related to aspects of Greenland's precipitation climatology. In particular, an enhanced precipitation retrieval method is found to be less susceptible to data artifacts than other methods using operational analyses. In the north, anomalously high precipitation is associated with cyclonic development near the Fram Strait. For west-central Greenland the close agreement among methods is related to the dominant contribution of the mean circulation.

We show that fore-aft asymmetry, a generic feature of living organisms and some active matter systems, can have a strong influence on the collective properties of even the simplest flocking models. Specifically, an arbitrarily weak asymmetry favoring front neighbors changes qualitatively the phase diagram of the Vicsek model. A region where many sharp traveling band solutions coexist is present at low noise strength, below the Toner-Tu liquid, at odds with the phase-separation scenario well describing the usual isotropic model. Inside this region, a 'banded liquid' phase with algebraic density distribution coexists with band solutions. Linear stability analysis at the hydrodynamic level suggests that these results are generic and not specific to the Vicsek model.Non-reciprocal (effective) interactions are interesting but rather rare in physical systems [1]. They are, however, likely to be more common in active matter. A nice example of action-reaction symmetry breaking was given recently by Soto and Golestanian for catalytically active colloids [2]. A strong case is that of self-propelled objects interacting solely via volume exclusion: their shape governs their effective interaction (e.g. aligning or not) and thus their collective behavior [3]. In the context of animal and human collective motion, asymmetric interactions are quite generic, and this asymmetry lies mostly in the relative position and weight of neighbors: the importance and quality of the information perceived by living organisms usually varies with its origin: In animal groups one often -but not always, cf. the cannibalistic behavior of locusts in [4,5]-expects that frontal stimuli such as neighbor positions matter more to an individual than events taking place in its back. Somewhat surprisingly, this generic fore-aft asymmetry has not been much investigated per se. It is explicitly mentioned in some works [6], and implicitly present in a number of models, see, e.g. [7], and the rather complicated escape-pursuit mechanisms introduced in [8,9] to describe marching locusts, or the 'motion guided attention' of [10]. It can even be found in variants of simple flocking models such as the Vicsek model, where local alignment of constantspeed particles competes with noise [22][23][24]. In [15][16][17][18][19], the introduction of a limited angle of vision was shown to have an influence on the shape of cohesive moving groups, on the degree of ordering, etc. Asymmetric interactions are also present in 'metric-free' models introduced in the context of bird flocks [11][12][13][14].In all cases mentioned above, it was not shown that fore-aft asymmetry alone can lead to qualitatively new collective phenomena. Recently, though, the influence of fore-aft asymmetric neighbors was investigated in the context of flocking models incorporating fast 'inertial spin' variables [20,21]. Both these works argue that the combination of asymmetric neighbors and fast variables induces new collective behavior.In this Letter, we show that fore-aft asymmetry alone has a strong i...

SUMMARYWe examine four parametrizations of the unit sphere in the context of material stability analysis by means of the singularity of the acoustic tensor. We then propose a Cartesian parametrization for vectors that lie a cube of side length two and use these vectors in lieu of unit normals to test for the loss of the ellipticity condition. This parametrization is then used to construct a tensor akin to the acoustic tensor. It is shown that both of these tensors become singular at the same time and in the same planes in the presence of a material instability. The performance of the Cartesian parametrization is compared against the other parametrizations, with the results of these comparisons showing that in general, the Cartesian parametrization is more robust and more numerically efficient than the others.