3D simulations of gas puff effects on edge density and ICRF coupling in ASDEX Upgrade

In recent experiments, a local gas puff was found to be an effective way to tailor the scrape-off layer (SOL) density and improve the ion cyclotron range of frequency (ICRF) power coupling in tokamaks. In order to quantitatively reproduce these experiments, to understand the corresponding physics and to optimize the gas valve positions and rates, simulations were carried out with the 3D edge plasma transport code EMC3-EIRENE in ASDEX Upgrade. An inter-ELM phase of an H-mode discharge with a moderate gas puff rate (1.2 x 10(22) electrons s(-1)) is used in our simulations. We simulated cases with gas puff in the lower divertor, the outer mid-plane and the top of the machine while keeping other conditions the same. Compared with the lower divertor gas puff, the outer mid-plane gas puff can increase the local density in front of the antennas most effectively, while a toroidally uniform but significantly smaller enhancement is found for the top gas puff. Good agreement between our simulations and experiments is obtained. With further simulations, the mechanisms of SOL density tailoring via local gas puffing and the strategies of gas puff optimization are discussed in the paper

Parameter estimation in an atmospheric GCM using the Ensemble Kalman Filter

International audienceWe demonstrate the application of an efficient multivariate probabilistic parameter estimation method to a spectral primitive equation atmospheric GCM. The method, which is based on the Ensemble Kalman Filter, is effective at tuning the surface air temperature climatology of the model to both identical twin data and reanalysis data. When 5 parameters were simultaneously tuned to fit the model to reanalysis data, the model errors were reduced by around 35% compared to those given by the default parameter values. However, the precipitation field proved to be insensitive to these parameters and remains rather poor. The model is computationally cheap but chaotic and otherwise realistic, and the success of these experiments suggests that this method should be capable of tuning more sophisticated models, in particular for the purposes of climate hindcasting and prediction. Furthermore, the method is shown to be useful in determining structural deficiencies in the model which can not be improved by tuning, and so can be a useful tool to guide model development. The work presented here is for a limited set of parameters and data, but the scalability of the method is such that it could easily be extended to a more comprehensive parameter set given sufficient observational data to constrain them

Quantification of the Greenland ice sheet contribution to Last Interglacial sea level rise

During the Last Interglacial period (~ 130–115 thousand years ago) the Arctic climate was warmer than today, and global mean sea level was probably more than 6.6 m higher. However, there are large discrepancies in the estimated contributions to this sea level change from various sources (the Greenland and Antarctic ice sheets and smaller ice caps). Here, we determine probabilistically the likely contribution of Greenland ice sheet melt to Last Interglacial sea level rise, taking into account ice sheet model parametric uncertainty. We perform an ensemble of 500 Glimmer ice sheet model simulations forced with climatologies from the climate model HadCM3, and constrain the results with palaeodata from Greenland ice cores. Our results suggest a 90% probability that Greenland ice melt contributed at least 0.6 m, but less than 10% probability that it exceeded 3.5 m, a value which is lower than several recent estimates. Many of these previous estimates, however, did not include a full general circulation climate model that can capture atmospheric circulation and precipitation changes in response to changes in insolation forcing and orographic height. Our combined modelling and palaeodata approach suggests that the Greenland ice sheet is less sensitive to orbital forcing than previously thought, and it implicates Antarctic melt as providing a substantial contribution to Last Interglacial sea level rise. Future work should assess additional uncertainty due to inclusion of basal sliding and the direct effect of insolation on surface melt. In addition, the effect of uncertainty arising from climate model structural design should be taken into account by performing a multi-climate-model comparison

Ionospheric response to the corotating interaction region-driven geomagnetic storm of October 2002

Unlike the geomagnetic storms produced by coronal mass ejections (CMEs), the storms generated by corotating interaction regions (CIRs) are not manifested by dramatic enhancements of the ring current. The CIR-driven storms are however capable of producing other phenomena typical for the magnetic storms such as relativistic particle acceleration, enhanced magnetospheric convection and ionospheric heating. This paper examines ionospheric plasma anomalies produced by a CIR-driven storm in the middle- and high-latitude ionosphere with a specific focus on the polar cap region. The moderate magnetic storm which took place on 14–17 October 2002 has been used as an example of the CIR-driven event. Four-dimensional tomographic reconstructions of the ionospheric plasma density using measurements of the total electron content along ray paths of GPS signals allow us to reveal the large-scale structure of storm-induced ionospheric anomalies. The tomographic reconstructions are compared with the data obtained by digital ionosonde located at Eureka station near the geomagnetic north pole. The morphology and dynamics of the observed ionospheric anomalies is compared qualitatively to the ionospheric anomalies produced by major CME-driven storms. It is demonstrated that the CIR-driven storm of October 2002 was able to produce ionospheric anomalies comparable to those produced by CME-driven storms of much greater Dst magnitude. This study represents an important step in linking the tomographic GPS reconstructions with the data from ground-based network of digital ionosondes

Detecting Fauna Habitat in Semi-Arid Grasslands Using Satellite Imagery

Managing grasslands for biodiversity conservation is a relatively recent phenomenon and there is uncertainty over the most effective strategy. Past research has found that intermediate levels of disturbance (e.g. burning or grazing) may be required to maintain the natural mosaic of small-scale patterning required for a diverse range of flora and fauna species. For sustainable grassland management, appropriate methods of spatial assessment and temporal monitoring are required, to facilitate understanding of how past and present climate, land management and landscape features influence vegetation structure. Due to the expense and time-consuming nature of conventional ground-based monitoring, satellite remote-sensing techniques offer a feasible approach

3D simulations of gas puff effects on edge plasma and ICRF coupling in JET

Recent JET (ITER-Like Wall) experiments have shown that the fueling gas puffed from different locations of the vessel can result in different scrape-off layer (SOL) density profiles and therefore different radio frequency (RF) coupling. To reproduce the experimental observations, to understand the associated physics and to optimize the gas puff methods, we have carried out three-dimensional (3D) simulations with the EMC3-EIRENE code in JET-ILW including a realistic description of the vessel geometry and the gas injection modules (GIMs) configuration. Various gas puffing methods have been investigated, in which the location of gas fueling is the only variable parameter. The simulation results are in quantitative agreement with the experimental measurements. They confirm that compared to divertor gas fueling, mid-plane gas puffing increases the SOL density most significantly but locally, while top gas puffing increases it uniformly in toroidal direction but to a lower degree. Moreover, the present analysis corroborates the experimental findings that combined gas puff scenarios-based on distributed main chamber gas puffing-can be effective in increasing the RF coupling for multiple antennas simultaneously. The results indicate that the spreading of the gas, the local ionization and the transport of the ionized gas along the magnetic field lines connecting the local gas cloud in front of the GIMs to the antennas are responsible for the enhanced SOL density and thus the larger RF coupling