In-situ Microwave Brightness Temperature Variability from Ground-based Radiometer Measurements at Dome C in Antarctica Induced by Wind-formed Features

Space-borne microwave radiometers are among the most useful tools to study snow and to collect information on the Antarctic climate. They have several advantages over other remote sensing techniques: high sensitivity to snow properties of interest (temperature, grain size, density), subdaily coverage in the polar regions, and their observations are independent of cloud conditions and solar illumination. Thus, microwave radiometers are widely used to retrieve information over snow-covered regions. For the Antarctic Plateau, many studies presenting retrieval algorithms or numerical simulations have assumed, explicitly or not, that the subpixel-scale heterogeneity is negligible and that the retrieved properties were representative of whole pixels. In this presentation, we investigate the spatial variations of brightness temperature over arange of a few kilometers in the Dome C area (Antarctic Plateau)

Simulation of the Microwave Emission of Multi-layered Snowpacks Using the Dense Media Radiative Transfer Theory: the DMRT-ML Model

DMRT-ML is a physically based numerical model designed to compute the thermal microwave emission of a given snowpack. Its main application is the simulation of brightness temperatures at frequencies in the range 1-200 GHz similar to those acquired routinely by spacebased microwave radiometers. The model is based on the Dense Media Radiative Transfer (DMRT) theory for the computation of the snow scattering and extinction coefficients and on the Discrete Ordinate Method (DISORT) to numerically solve the radiative transfer equation. The snowpack is modeled as a stack of multiple horizontal snow layers and an optional underlying interface representing the soil or the bottom ice. The model handles both dry and wet snow conditions. Such a general design allows the model to account for a wide range of snow conditions. Hitherto, the model has been used to simulate the thermal emission of the deep firn on ice sheets, shallow snowpacks overlying soil in Arctic and Alpine regions, and overlying ice on the large icesheet margins and glaciers. DMRT-ML has thus been validated in three very different conditions: Antarctica, Barnes Ice Cap (Canada) and Canadian tundra. It has been recently used in conjunction with inverse methods to retrieve snow grain size from remote sensing data. The model is written in Fortran90 and available to the snow remote sensing community as an open-source software. A convenient user interface is provided in Python

A circle swimmer at low Reynolds number

Swimming in circles occurs in a variety of situations at low Reynolds number. Here we propose a simple model for a swimmer that undergoes circular motion, generalising the model of a linear swimmer proposed by Najafi and Golestanian (Phys. Rev. E 69, 062901 (2004)). Our model consists of three solid spheres arranged in a triangular configuration, joined by two links of time-dependent length. For small strokes, we discuss the motion of the swimmer as a function of the separation angle between its links. We find that swimmers describe either clockwise or anticlockwise circular motion depending on the tilting angle in a non-trivial manner. The symmetry of the swimmer leads to a quadrupolar decay of the far flow field. We discuss the potential extensions and experimental realisation of our model.Comment: 9 pages, 9 Figure

Scalar <i>φ</i>⁴ field theory for active-particle phase separation

Recent theories predict phase separation among orientationally disordered active particles whose propulsion speed decreases rapidly enough with density. Coarse-grained models of this process show time-reversal symmetry (detailed balance) to be restored for uniform states, but broken by gradient terms; hence detailed-balance violation is strongly coupled to interfacial phenomena. To explore the subtle generic physics resulting from such coupling we here introduce `Active Model B'. This is a scalar $\phi^4$ field theory (or phase-field model) that minimally violates detailed balance via a leading-order square-gradient term. We find that this additional term has modest effects on coarsening dynamics, but alters the static phase diagram by creating a jump in (thermodynamic) pressure across flat interfaces. Both results are surprising, since interfacial phenomena are always strongly implicated in coarsening dynamics but are, in detailed-balance systems, irrelevant for phase equilibria.Comment: 15 pages, 7 figure

Killing by type VI secretion drives genetic phase separation and correlates with increased cooperation

By nature of their small size, dense growth and frequent need for extracellular metabolism, microbes face persistent public goods dilemmas. Genetic assortment is the only general solution stabilizing cooperation, but all known mechanisms structuring microbial populations depend on the availability of free space, an often unrealistic constraint. Here we describe a class of self-organization that operates within densely packed bacterial populations. Through mathematical modelling and experiments with Vibrio cholerae, we show how killing adjacent competitors via the Type VI secretion system (T6SS) precipitates phase separation via the ‘Model A' universality class of order-disorder transition mediated by killing. We mathematically demonstrate that T6SS-mediated killing should favour the evolution of public goods cooperation, and empirically support this prediction using a phylogenetic comparative analysis. This work illustrates the twin role played by the T6SS, dealing death to local competitors while simultaneously creating conditions potentially favouring the evolution of cooperation with kin

Surface melting observations in Antarctica by microwave radiometers: Correcting 26-year time series from changes in acquisition hours

International audienceSurface melting duration and extent of the Antarctic coasts and ice-shelves is a climatic indicator related to the summer temperature and radiative budget. Surface melting is easily detectable by remote sensing using passive microwave observations. The preliminary goal of this study is to extend to 26 years an existing data set of surface melting [Torinesi, O., Fily, M., Genthon, C. (2003), Interannual variability and trend of the Antarctic summer melting period from 20 years of spaceborne microwave data, J. Climate, 16(7), pp. 1047–1060] by including the most recent years of observation. These data come from 4 microwave sensors (the Scanning Multichannel Microwave Radiometer (SMMR) and three Special Sensor Microwave Imager (SSM/I)) observing the surface at different hours of the day. Since surface melting varies throughout the day as the air temperature or the radiation, the interannual melting extent and duration time series are biased by sensor changes. Using all the sensors simultaneously available since 2002, we were able to model the diurnal variations of melting and use this hourly model to correct the long-term time series. This results in an unbiased 26-year long time series better suited for climate analysis. The cooling trend found by Torinesi et al. using uncorrected time series for the 1980–1999 period is confirmed but the decreasing rate is weaker after correction. Furthermore, extending the series up to summer 2004–2005 reveals recent changes: the last 2 summers have been particularly warmer over all the East Antarctica compared to the 10 previous years, thus ending the cold period of the 1990s. The trend over 1980–2005 is no longer toward cooling but complex climatic variations appear

Réaction à un composé salivaire et tolérance à la coupe de génotypes de Dactylis glomerata L issus de populations ayant une expérience contrastée du pâturage

Une étude comparative est réalisée sur 8 génotypes de dactyle (Dactylis glomerata L) récoltés dans 2 parcelles des Pyrénées qui ne diffèrent que par l'intensité du pâturage qui s'y exerce depuis 30 ans. Nous montrons, à l'aide de mesures de biomasses effectuées sur des plantes cultivées en pots à partir d'une talle unique : - que certains génotypes tolèrent mieux que d'autres la coupe, simulant le pâturage; - que la thiamine, mimant la salive des bovins, n'a pas d'effet constant sur la repousse des plantes; - qu'il y a une cohérence entre la capacité de repousse des génotypes et l'expérience que les populations ont du pâturage depuis 30 ans. Des notations du stade phénologique (structure morphologique) lors de chaque coupe permettent d'expliquer le cas particulier d'un génotype présentant une photosynthèse nette particulièrement forte en fin d'expérience (15 semaines après l'installation) : les plantes correspondant à ce génotype avaient en effet épié avant la première coupe. On en conclut que le stade phénologique des plantes est très important à considérer pour l'examen de leur capacité de repousse et qu'il doit être pris en compte avant de conclure à des réactions photosynthétiques spécifiques à la défoliation.Examination of potential effect of a salivary chemical and defoliation tolerance of Dactylis glomerata L genotypes with diverse histories of grazing management. Eight genotypes of Dactylis glomerata L plants were obtained from 2 more or less heavily grazed grasslands (grazed for 30 yr). Yield of individual plants produced from a single tiller were compared (table I) and it was observed that : - some genotypes are more tolerant to clipping (simulated grazing) than others (figs 1, 2); - thiamine, simulating cattle saliva has no constant effect on regrowth (fig 1); - clipping tolerance of genotypes was consistent with the grazing experience of population over a 30-yr period (fig 1). Phenologic records for each cutting provided an explanation for the case of a genotype («D», fig 4) which exhibited high net photosynthesis at the end of the experiment (15 wk after transplantation). These plants produce fertile tillers before the first cut. We conclude that the phenologic stage of plants is very important to consider in the examination of regrowth capacity and must be taken into consideration before putting forward a conclusion relating to specific photosynthetic response to defoliation

Snow grain-size profiles deduced from microwave snow emissivities in Antarctica

International audienceSpaceborne microwave radiometers are an attractive tool for observing Antarctic climate because their measurements are related to the snow temperature. However, the conversion from microwave emission to snow temperature is not simple and strongly depends on the emissivity through snow properties. This difficulty in predicting the snow property profile for Antarctic conditions is the main bottleneck in the retrieval of accurate climate information from microwave radiometers. We attempt to explain the vertically polarized emissivity at 19.3 and 37 GHz derived from brightness temperatures acquired by the Special Sensor Microwave/Imager (SSM/I) and physical temperature from the ERA-40 re-analysis. In Antarctica the snow emissivities at 19.3 and 37 GHz are nearly equal, although a decrease with frequency is expected. To explain this, we consider various profiles of snow grain size and density and predict their emissivity using a dense-medium radiative transfer (DMRT) model. The results show that the emissivities cannot be explained by constant profiles of grain size and density. Heterogeneous snowpacks need to be considered. We first test random variations of snow density and grain radius with depth and then monotonic and continuous variations in the snow grain radius. In both cases, we show that an overall increase of the snow grain radius with depth is required to match the observed emissivity in Antarctica. In addition, two parameters characterizing the snow grain profiles are retrieved and compared with (1) in situ measurements of grain size at various locations in East Antarctica, (2) grain size estimated using visible spaceborne radiometers and (3) a semi-empirical relationship for grain growth