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Cycling data assimilation and forecast experiments in August 2016 together with a case study of an intense Arctic cyclone (AC16) are performed. Initial conditions from newly developed Multi-Resolution Incremental Four-Dimensional Variational (MRI-4DVAR) and Three-Dimensional Variational (3DVAR) data assimilation along with forecasts from the polar version of the Weather Research and Forecasting (Polar WRF) model, mimicking operational configurations, are applied. The tasks are to evaluate MRI-4DVAR performance during a 20-day cycling run, to investigate the impacts of initial conditions on the forecast skill of AC16, and to identify the factors impacting AC16's predictability. The results from the 20-day cycling period demonstrate the robustness and reliability of MRI-4DVAR for data assimilation and subsequent forecast skill. Multiple processes, including mergers of Arctic cyclones, mergers of vortices, vertical coupling between low-level and upper-level circulations, baroclinic processes and jet stream forcing, contributed to the generation and development of AC16. Compared to the initial conditions from 4DVAR, 3DVAR produced amplified polar vortices, stronger baroclinic instability, intensified upper-level jet streams and a stronger low-level frontal zone, causing the overdevelopment of AC16 in 3DVAR-based forecasts. For MRI-4DVAR, the accurate prediction of AC16 5-7 days ahead is likely due primarily to the more accurate representation of upper-level atmospheric fields, that was facilitated by better satellite radiance assimilation with MRI-4DVAR that also produced a balanced initial model state. It is concluded that the high-resolution Polar WRF which is optimized for Arctic conditions combined with 4DVAR facilitated the improved prediction of AC16 compared to the Global Forecast System (GFS) operational deterministic global forecast. [ABSTRACT FROM AUTHOR]

The Arctic System Reanalysis version 1 (ASRv1), a high-resolution regional assimilation of model output, observations and satellite data across the mid- and high latitudes of the Northern Hemisphere, and the global European Centre for Medium Range Forecasting Interim Reanalysis (ERAI) are compared with atmospheric observations for the period December 2006 to November 2007. Results throughout the troposphere show observations to be well assimilated in the ASRv1, as monthly and annual near-surface (upper-level) temperature, dew-point (relative humidity), pressure (geopotential height) and wind-speed biases compared with surface stations and radiosondes are very small. These results are similar to the ERAI, although wind-speed biases are significantly smaller in the ASRv1. Despite the ASRv1's use of a 3D-variational (Var) assimilation compared with the ERAI's 4D-Var, similar results suggest that a regional approachwith higher-resolution terrain and a detailed land-surface description forced by a global reanalysismay improve the assimilation of observations and help offset temporal information lost by the 3D-Var compared with the 4D-Var. However, the ASRv1 forecast field results compared with the ERAI are mixed. The ASRv1 and ERAI show negative precipitation biases during cool months compared with gauge observations, and too much precipitation falls in the ASRv1 during summer in the midlatitudes. Stations north of 60°N demonstrate smaller precipitation biases in the ASRv1 than the ERAI except during the summer, when the ASRv1 is very dry. Short-wave radiation compared with observations is much too large in the ASRv1, and both reanalyses show long-wave radiation deficits during most months. These results point to inadequacies in model physics in the ASRv1 (e.g. convective and radiation schemes) that will continue to be refined in subsequent versions of the ASR. [ABSTRACT FROM AUTHOR]

Foehn events over the McMurdo Dry Valleys (MDVs), the largest ice-free region of Antarctica, promote glacial melt that supports biological activity in the lakes, streams, rocks and soils. Although MDVs foehn events are known to depend upon the synoptic-scale circulation, the physical processes responsible for foehn events are unknown. A polar-optimized version of the Weather Research and Forecasting model (Polar WRF) is used for a case study of a representative summer foehn event from 29 December 2006 to 1 January 2007 in order to identify and explain the MDVs foehn mechanism. Pressure differences across an elevated mountain gap upstream of the MDVs provide forcing for southerly flow into the western, upvalley entrance of the MDVs. Complex terrain over the elevated gap and the MDVs leads to mountain wave effects such as leeside acceleration, hydraulic jumps, wave breaking and critical layers. These mountain wave effects depend on the ambient (geostrophic) wind direction. Pressure-driven channelling then brings the warm, dry foehn air downvalley to eastern MDV sites. Brief easterly intrusions of maritime air into the eastern MDVs during foehn events previously have been attributed to either a sea-breeze effect in summer or local cold-pooling effects in winter. In this particular case, the easterly intrusions result from blocking effects of nearby Ross Island and the adjacent Antarctic coast. Temperature variability during the summer foehn event, which is important for meltwater production and biological activity when it exceeds 0°C, primarily depends on the source airmass rather than differences in foehn dynamics. [ABSTRACT FROM AUTHOR]