Evaluating the Arctic System Reanalysis high Arctic summer using ASCOS field data

The Arctic has experienced large climate changes over recent decades, the largest for any region on Earth. The warming has been more than twice as large as the global average, and substantial changes in, for example, sea-ice cover and thickness and in permafrost and vegetation have been observed.

To understand the underlying reasons for this apparent climate sensitivity and the impact of these changes, reanalysis is an invaluable tool. Reanalysis takes all the available observations into account and uses a model to optimally interpolate in space and time to locations where observations are not available, thus generating a four-dimensional set of data. Using reanalysis products thereby provides a better description of the system and its development than can be gained from any single observation system.

However, the available global reanalyses do not have the spatial resolution needed to resolve the atmospheric dynamics at a sufficiently fine scale. The Arctic System Reanalysis (ASR) is a regional reanalysis using the polar version of the Weather and Research Forecast (Polar WRF) atmospheric model that, by the virtue of being regional, can be affordably run at higher resolution. The reanalysis is forced at the lateral boundaries by the European Centre for Medium-Range Weather Forecasts ERA-Interim global reanalysis. Moreover the outer domain of the reanalysis cover areas far enough south to include the mid-latitude data rich areas, while the inner domain can be run at a substantially higher resolution than in the global reanalyses. Still, the ASR reanalysis products need to be evaluated preferably using independent data; this is a problem in the Arctic where data are sparse and as much as possible of the available data is assimilated in the reanalysis.

In this study we use data from the Arctic Summer Cloud-Ocean Study (ASCOS, www.ascos.se) to evaluate ASR performance for the central Arctic during August and early September 2008. We use an experimental atmosphere-only reanalysis with a nominal resolution of ~30 km in the central Arctic. The ASCOS field experiment was deployed on the Swedish icebreaker Oden north of 87°N in the Atlantic sector of the Arctic; the data used in this study were collected both during the transits to and from Longyearbyen on Svalbard (2 August – 7 September) and during a three-week ice drift with the Oden moored to a drifting multi-year ice floe 12 August – 2 September, when intensive measurements were taken on the ice and onboard. These data have the advantage of being independent of ASR, i.e., they were not assimilated into the reanalysis, and being detailed enough to evaluate the process descriptions in the ASR. For the full time period we use regular weather observations and 6-hourly radiosoundings along with detailed cloud observations from the MMCR cloud radar. From the ice drift we additionally use surface radiation, surface and ice temperatures and eddy-correlation fluxes from two masts erected on the ice. We also use profiles from tethered soundings along with wind observations from a SODAR also deployed on the ice; a 449 MHz wind profiler deployed on board is also used. Together these observations provide detailed and continuous observations of boundary-layer structure, cloud macro- and microphysics and free tropospheric characteristics in a vertical column from the surface up through the troposphere.