First results of turbulence measurements in a wind park with the Small Unmanned Meteorological Observer SUMO

The Small Unmanned Meteorological Observer (SUMO), equipped with a miniaturized 5-hole probe for turbulent flow measurements with 100 Hz temporal resolution, has for the first time been operated in and around a wind farm. The 5 day campaign from May 9-13 took place at a small wind park of 21 1 MW-turbines close to Vindeby on Lolland, Denmark and was dedicated to the investigation of the effects of wind turbines on boundary layer turbulence. A total of 20 SUMO flight missions carrying the turbulence system have been performed during the campaign. In spite of a few pitfalls related to the fine-tuning of the autopilot system and in the configuration and synchronization of the corresponding data logging systems, this campaign provided promising results indicating the capability and future potential of small UAS for turbulence characterization in and around wind farms. Subsequent flights upwind and downwind of the park revealed qualitatively a distinct enhancement in the turbulence level behind the wind farm.publishedVersio

Unravelling the March 1972 northwest Greenland windstorm with high-resolution numerical simulations

Thule Air Base in northwest Greenland experienced an extreme windstorm during the night of 8/9 March 1972. The event is not among official WMO records because the anemometer broke after recording the highest gust of 93 m s−1. A recent study re‐examined the event based on coarse‐resolution reanalyses and observations which, however, did not fully resolve the proposed storm processes. This is the first study that uses high‐resolution numerical simulations to investigate the processes associated with this windstorm. A cold‐frontal inversion and strong flow across the mountain ridge upstream of Thule Air Base (associated with a passing low pressure system) are shown to be key factors for the severe downslope windstorm in the lee of the ridge. It is shown that trapped lee waves occurred during the initial phase of the storm, but did not contribute to the highest wind speeds as proposed in the previous study. It is confirmed that rotor circulations occurred which possibly contributed to the large wind variability and, hence, large differences between individual observation sites. However, no rotors were present at the time of the highest simulated wind speed. Instead, wave breaking above the lee slope is found to indirectly contribute to the wind maxima by facilitating Kelvin–Helmholtz instability at the top of the shooting flow that caused intense wind speed pulsations near the surface. In agreement with the previous study, a corner jet was simulated, however it was not responsible for the strong winds in the vicinity of the air base. Sensitivity experiments showed that the flow field was considerably influenced by the high topography downstream of the air base and that simulations with very thin or no sea ice cover over Baffin Bay resulted in a weaker frontal inversion and up to about 30% lower maximum wind speeds.publishedVersio

Evaluation of three numerical weather prediction models for the Weddell Sea region for the Austral winter 2013

It is widely recognized that numerical weather prediction (NWP) results for the Antarctic are relatively poor compared to the mid-latitudes. In this study, we evaluate output from three operational NWP systems: the ECMWF, Global Forecast System (GFS) and Antarctic Mesoscale Prediction System (AMPS), for the Austral winter (June-August) of 2013 for the Weddell Sea region, paying special attention to regional patterns of error statistics. This is the first evaluation of NWP systems over the Southern Ocean that also addresses the accuracy of forecasted vertical profiles. In the evaluation, we use data from land- and ship-based automatic weather stations (AWS) and radiosoundings. While the ECMWF and AMPS forecasts are on average biased cold and dry near the surface, the GFS forecasts are on average biased warm and moist. The near-surface wind speed is on average overestimated by the AMPS forecasts, whereas it is slightly underestimated by the forecasts of the other two NWP systems. Among the variables investigated, all three NWP systems forecast the near-surface specific humidity most accurately, followed by the temperature and then the wind speed. The forecast quality for the near-surface and upper-air wind speed degrades the most rapidly with increasing lead time, compared to the other variables. ECMWF is the overall best NWP system when compared against both the near-surface and upper-air observations, followed by AMPS and then GFS. The generally poorest model performance is found in locations with complex terrain along the coast of the Antarctic continent, and the best over the ocean.publishedVersio

The impact of assimilating data from a remotely piloted aircraft on simulations of weak-wind orographic flow

Orographic winds near a 914 m high mountain in Southwest-Iceland are explored using unique observations made aloft with a small remotely piloted aircraft, as well as with traditional observations and high-resolution atmospheric simulations. There was an inversion well above mountain top level at about 2 km with weak winds below. Observed winds in the lee of the mountain were indicative of flow locally enhanced by wave activity aloft. Winds descended along the lee slope with a prevailing direction away from the mountain. They were relatively strong and gusty at the surface close to the mountain, with a maximum at low levels, and weakening and becoming more diffuse a short distance further downstream. The winds weakened further aloft, with a minimum on average near mountain top level. This situation is reproduced in a high-resolution atmospheric simulation forced with atmospheric analysis as well as with the observed lee-side profiles of wind and temperature below 1.4 km. Without the additional observations consisting of the lee-side profiles, the model fails to reproduce the winds aloft as well as at the surface in a region in the lee of the mountain, as was also the case for the operational numerical models at that time. A sensitivity simulation indicates that this poor performance is a result of the poorly captured strength and sharpness of the inversion aloft. The study illustrates, firstly, that even at very low wind speed, in a close to neutral low-level flow, gravity waves may still be a dominating feature of the flow. Secondly, the study presents an example of the usefulness of lee-side atmospheric profiles, retrieved by simple model aircraft, for improving numerical simulations and short-term weather forecasting in the vicinity of mountains. Thirdly, the study confirms the sensitivity of downslope flow to only moderate change in the sharpness of an upstream inversion.publishedVersio

Impact of Assimilation of Radiosonde and UAV Observations from the Southern Ocean in the Polar WRF Model

Weather forecasting in the Southern Ocean and Antarctica is a challenge above all due to the rarity of observations to be assimilated in numerical weather prediction (NWP) models. As observations are expensive and logistically challenging, it is important to evaluate the benefit that additional observations could bring to NWP. Atmospheric soundings applying unmanned aerial vehicles (UAVs) have a large potential to supplement conventional radiosonde sounding observations. Here, we applied UAV and radiosonde sounding observations from an RV Polarstern cruise in the ice-covered Weddell Sea in austral winter 2013 to evaluate the impact of their assimilation in the Polar version of the Weather Research and Forecasting (Polar WRF) model. Our experiments revealed small to moderate impacts of radiosonde and UAV data assimilation. In any case, the assimilation of sounding data from both radiosondes and UAVs improved the analyses of air temperature, wind speed, and humidity at the observation site for most of the time. Further, the impact on the results of 5-day-long Polar WRF experiments was often felt over distances of at least 300 km from the observation site. All experiments succeeded in capturing the main features of the evolution of near-surface variables, but the effects of data assimilation varied between different cases. Due to the limited vertical extent of the UAV observations, the impact of their assimilation was limited to the lowermost 1–2-km layer, and assimilation of radiosonde data was more beneficial for modeled sea level pressure and near-surface wind speed.publishedVersio

Present Temperature, Precipitation, and Rain‐on‐Snow Climate in Svalbard

The Svalbard Archipelago has undergone rapid warming in the recent decades leading to warmer and wetter winter conditions. This study relates the present (2013–2018) 2 m temperature, precipitation, and rain-on-snow (ROS) climate in Svalbard to different atmospheric circulation (AC) types utilizing the high-resolution numerical weather prediction model Application of Research to Operations at Mesoscale (AROME)-Arctic. We find that the 2 m median temperatures vary most across AC types in winter and spring and in summer they vary the least. In all seasons the 10th percentile 2 m temperatures are above 0°C with southwesterly AC types over Svalbard. In comparison, the relationship between AC type and precipitation varies more spatially, with most accumulated precipitation and highest median precipitation intensities with onshore flow over open water. Our results suggest that sea ice explains a large part of the local variability in both 2 m temperature and precipitation. In the studied period ROS is a frequent phenomenon up to 150 m above sea level (ASL) on land, with most events in the southwestern parts of the archipelago (57 cases during five winter seasons). ROS events in winter occur predominantly with AC types from the southerly sector or during a low-pressure center/trough passage. The southwesterly cyclonic AC type, with a low-pressure center west of Svalbard, is the most frequent AC type for ROS events. In addition to being the most frequent, the southwesterly AC has the largest spatial coverage of ROS.publishedVersio

Assessment of Atmospheric Reanalyses with Independent Observations in the Weddell Sea, the Antarctic

Surface layer and upper-air in situ observations from two research vessel cruises and an ice station in the Weddell Sea from 1992 and 1996 are used to validate four current atmospheric reanalysis products: ERA-Interim, CFSR, JRA-55, and MERRA-2. Three of the observation data sets were not available for assimilation, providing a rare opportunity to validate the reanalyses in the otherwise datasparse region of the Antarctic against independent data. All four reanalyses produce 2 m temperatures warmer than the observations, and the biases vary from +2.0 K in CFSR to +2.8 K in MERRA-2. All four reanalyses are generally too warm also higher up in the atmospheric boundary layer (ABL), with biases up to +1.4 K (ERA-Interim). Cloud fractions are relatively poorly reproduced by the reanalyses, MERRA-2 and JRA-55 having the strongest positive and negative biases of about +30 % and -17 %, respectively. Skill scores of the error statistics reveal that ERA-Interim compares generally the most favorably against both the surface layer and the upper-air observations. CFSR compares the second best and JRA-55 and MERRA-2 have the least favorable scores. The ABL warm bias is consistent with previous evaluation studies in high latitudes, where more recent observations have been applied. As the amount of observations has varied depending on the decade, season, and region, the consistency of the warm bias suggests a need to improve the modeling systems, including data assimilation as well as ABL and surface parameterizations.Peer reviewe

Simulations of the Bergen orographic wind shelter

Even though the coast of western Norway is very windy, the centre of Bergen is rather calm. To gain further understanding of this wind shelter, we study the flow in the complex topography of Bergen during two south-westerly windstorms using surface observations and high-resolution numerical simulations. The results reveal large spatial variability in the local wind field. In some areas, there are periods of sustained winds of more than 25m s−1, while at nearby locations the winds are typically less than 5m s−1. The centre of Bergen is among the calmest areas. To investigate the effect of the individual mountains upstream (Løvstakken) and downstream (Fløyen) on the wind field in the city centre of Bergen, they have been removed stepwise from the model topography. Areas with relatively large wind speed reductions are found immediately downstream of Løvstakken and immediately upstream of Fløyen. At Florida, situated close to the city centre, both a wake effect of Løvstakken and a blocking effect of Fløyen are evident, but the latter is more prominent. The total impact of both mountains on the winds in the city is close to the sum of each of them. A spillover effect of Løvstakken acts to substantially increase the local precipitation in the centre of Bergen. The spillover effect is presumably less pronounced for cases with weaker winds.publishedVersio

IWIN: the Isfjorden Weather Information Network

In an effort led by the University Centre in Svalbard (UNIS), with support from the Norwegian Meteorological Institute (MET Norway), the Isfjorden Weather Information Network (IWIN) is under development in the Isfjorden region in central Svalbard. The network substantially expands upon the relatively sparse existing operational network of weather stations and consists of compact and cost-efficient all-in-one weather stations permanently installed at lighthouses around Isfjorden and on board small tourist cruise ships trafficking the fjord from spring to autumn. All data from the network until June 2023, as presented in this paper, can be found at https://doi.org/10.5281/zenodo.8137588 (Frank et al., 2023b). New data become freely available in near-real time via MET Norway's data portals (https://doi.org/10.21343/ebrw-w846, Frank et al., 2023a). The IWIN data are highly valuable for scientific purposes such as atmospheric boundary layer research, the validation and development of numerical weather prediction models, and assimilation in these, as well as the planning and safe conduction of outdoor activities in the region.publishedVersio