Demonstrating the effect that climate change is having on regional weather is a subject which occupies climate scientists, government policy makers and the media. After an extreme weather event occurs, the question is often posed, 'Was the event caused by anthropogenic climate change?' Recently, a new branch of climate science (known as attribution) has sought to quantify how much the risk of extreme events occurring has increased or decreased due to climate change. One method of attribution uses very large ensembles of climate models computed via volunteer distributed computing. A recent advancement is the ability to run both a global climate model and a higher resolution regional climate model on a volunteer's home computer. Such a set-up allows the simulation of weather on a scale that is of most use to studies of the attribution of extreme events. This article introduces a global climate model that has been developed to simulate the climatology of all major land regions with reasonable accuracy. This then provides the boundary conditions to a regional climate model (which uses the same formulation but at higher resolution) to ensure that it can produce realistic climate and weather over any region of choice. The development process is documented and a comparison to previous coupled climate models and atmosphere-only climate models is made. The system (known as weather@home) by which the global model is coupled to a regional climate model and run on volunteers' home computers is then detailed. Finally, a validation of the whole system is performed, with a particular emphasis on how accurately the distributions of daily mean temperature and daily mean precipitation are modelled in a particular application over Europe. This builds confidence in the applicability of the weather@home system for event attribution studies.

A 1200-year multiproxy record of tree growth and summer temperature at the northern pine forest limit Published by: http://www.sagepublications.com can be found at: The Holocene Additional services and information for AbstractCombining nine tree growth proxies from four sites, from the west coast of Norway to the Kola Peninsula of NW Russia, provides a well replicated (> 100 annual measurements per year) mean index of tree growth over the last 1200 years that represents the growth of much of the northern pine timberline forests of northern Fennoscandia. The simple mean of the nine series, z-scored over their common period, correlates strongly with mean June to August temperature averaged over this region (r = 0.81), allowing reconstructions of summer temperature based on regression and variance scaling. The reconstructions correlate significantly with gridded summer temperatures across the whole of Fennoscandia, extending north across Svalbard and south into Denmark. Uncertainty in the reconstructions is estimated by combining the uncertainty in mean tree growth with the uncertainty in the regression models. Over the last seven centuries the uncertainty is < 4.5% higher than in the 20th century, and reaches a maximum of 12% above recent levels during the 10th century. The results suggest that the 20th century was the warmest of the last 1200 years, but that it was not significantly different from the 11th century. The coldest century was the 17th. The impact of volcanic eruptions is clear, and a delayed recovery from pairs or multiple eruptions suggests the presence of some positive feedback mechanism. There is no clear and consistent link between northern Fennoscandian summer temperatures and solar forcing.

SUMMARYMaps of Ertel potential vorticity on isentropic surfaces (IPV) and/or quasi-geostrophic potential vorticity (QGPV) are well established in dynamical meteorology as powerful sources of insight into dynamical processes involving 'balanced' flow. In the present study, we derive maps of zonal-mean IPV and QGPV in Jupiter's upper troposphere and lower stratosphere by making use of a combination of velocity measurements, derived from the tracking of cloud features in images from the Voyager 1 and 2 and Cassini missions, and thermal measurements from the Voyager 1 IRIS and Cassini CIRS instruments. IPV and QGPV are mapped and compared for the entire globe between latitudes ±55 • . Profiles of zonally averaged PV show some evidence for a step-like pattern suggestive of local PV homogenization, separated by strong PV gradients in association with eastward jets, though on differing scales in the northern and southern hemispheres. The northward gradient of PV (IPV or QGPV) is found to change sign in several places in each hemisphere, even when baroclinic contributions are taken into account. The relationship of lateral gradients of IPV and QGPV with the corresponding mean zonal flows indicate that the northern hemisphere may be closer to marginal stability with respect to Arnol'd's second stability theorem than the southern hemisphere.