Lars Robert Hole

Observation-based evaluation of surface wave effects on currents and trajectory forecasts

Knowledge of upper ocean currents is needed for trajectory forecasts and is essential for search and rescue operations and oil spill mitigation. This paper addresses effects of surface waves on ocean currents and drifter trajectories using in situ observations. The data set includes colocated measurements of directional wave spectra from a wave rider buoy, ocean currents measured by acoustic Doppler current profilers (ADCPs), as well as data from two types of tracking buoys that sample the currents at two different depths. The ADCP measures the Eulerian current at one point, as modelled by an ocean general circulation model, while the tracking buoys are advected by the Lagrangian current that includes the wave-induced Stokes drift. Based on our observations, we assess the importance of two different wave effects: (a) forcing of the ocean current by wave-induced surface fluxes and the Coriolis–Stokes force, and (b) advection of surface drifters by wave motion, that is the Stokes drift. Recent theoretical developments provide a framework for including these wave effects in ocean model systems. The order of magnitude of the Stokes drift is the same as the Eulerian current judging from the available data. The wave-induced momentum and turbulent kinetic energy fluxes are estimated and shown to be significant. Similarly, the wave-induced Coriolis–Stokes force is significant over time scales related to the inertial period. Surface drifter trajectories were analysed and could be reproduced using the observations of currents, waves and wind. Waves were found to have a significant contribution to the trajectories, and we conclude that adding wave effects in ocean model systems is likely to increase predictability of surface drifter trajectories. The relative importance of the Stokes drift was twice as large as the direct wind drag for the used surface drifter.

Biomass burning in eastern Europe during spring 2006 caused high deposition of ammonium in northern Fennoscandia

High air concentrations of ammonium were detected at low and high altitude sites in Sweden, Finland and Norway during the spring 2006, coinciding with polluted air from biomass burning in eastern Europe passing over central and northern Fennoscandia. Unusually high values for throughfall deposition of ammonium were detected at one low altitude site and several high altitude sites in north Sweden. The occurrence of the high ammonium in throughfall differed between the summer months 2006, most likely related to the timing of precipitation events. The ammonia dry deposition may have contributed to unusual visible injuries on the tree vegetation in northern Fennoscandia that occurred during 2006, in combination with high ozone concentrations. It is concluded that long-range transport of ammonium from large-scale biomass burning may contribute substantially to the nitrogen load at northern latitudes.

Modeling of sound propagation in the atmospheric boundary layer: Application of the MIUU mesoscale model

The MIUU mesoscale model was further developed, in order to include information on large-scale atmospheric fields from global or regional atmospheric climate- and weather-prediction models. For this purpose, a new lateral boundary condition was developed and implemented into the model. The new lateral boundary condition is a combination of two existing conditions, namely the flow relaxation scheme and the tendency modification scheme.Tests indicated that an optimum lateral boundary configuration would be obtained with moderate to strong flow relaxation at higher levels, small flow relaxation at lower levels (within the atmospheric boundary layer), upstream advection at the outermost 4 grid points, and 6% horizontal grid stretching starting at a substantial distance from the lateral boundaries. The flow relaxation coefficients should be specified carefully, in order to minimize the reflection of all kinds of waves at the lateral boundaries.The summer thermal low in the mean-sea-level pressure field over North America is traditionally analyzed over the northern end of the Gulf of California. The position of this low is influenced by the application of the so-called plateau correction in obtaining mean-sea-level pressure values from highly elevated stations in North America. A model study indicated that the low should be located approximately 450 km to the north and somewhat to the east of the above location. A statistical comparison of model results from two mesoscale models against upper-air and surface measurements from several sites was carried out. Statistical methods, however, give only an insufficient picture of overall model performance. A comparison between predicted and measured tracer concentrations could be used to better evaluate the overall performance of different models.Sound propagation in the atmosphere was predicted in a mountain valley using a mesoscale atmospheric model together with a sound propagation model. This suggests that forecasts of sound propagation should be possible in future.

Continuous In-Situ Soundings in the Arctic Boundary Layer: A New Atmospheric Measurement Technique Using Controlled Meteorological Balloons

Controlled Meteorological (CMET) balloons are small airborne platforms that use reversible lift-gas compression to regulate altitude. These balloons have approximately the same payload mass as standard weather balloons but can float for many days, change altitude on command, and transmit meteorological and system data in near-real time via satellite. Since 2004, more than 50 CMET balloons have been flown in nearly all of the earth’s major climate zones, from the Amazon to Antarctica. This paper describes one notable flight in 2011 in which a CMET balloon performed continuous soundings in the Arctic marine boundary layer off the coast of Svalbard. It is likely that this is the first time such a feat has been accomplished by a free balloon or any other flight platform. The series 18 consecutive profiles show the time evolution of the boundary layer as it is advected northward over a 10-h period. The paper focuses on the balloon design, control algorithm, and in-flight performance. Analysis of the unique atmospheric dataset will be the subject of a subsequent publication.

Comparison of Operational Oil Spill Trajectory Forecasts with Surface Drifter Trajectories in the Barents Sea

Six iSPHERE oil spill and current tracking buoys were deployed over the continental shelf of northern Norway (Nordkapp region) during spring-summer 2011. These drifters provide real-time GPS position location information to aid in locating marine oil spills and other leeward drifting objects. In this study we assess the differences between the spreading of the surface drifters and the trajectories forecast by the operational Lagrangian oil drift model at the Norwegian Meteorological Institute (met.no). The study investigates the reason for these differences, and we use a recently established new skill score as a measure of the model accuracy. The differences observed in this study are the consequence of the combined impact of the modeled wind, ocean and current constituents that force the oil drift model. Each numerical model is run on a grid of 4 km resolution, which means that many mesoscale features are either not represented well enough, or not represented at all. A problem with the ocean model, since eddies in the ocean are typically of a much smaller scale than in the atmosphere (40-50 km), and there are few observations to assimilate into the model. Studies such as this, comparing modeled trajectories with observed drifter trajectories are an important way to indirectly validate and improve ocean models, as well as improving the trajectory model itself.

The influence of Nunataks on atmospheric boundary layer convection during summer in Dronning Maud Land, Antarctica

The effects of nunataks on temperature profiles and wind patterns are studied using simulations from the Weather Research and Forecasting model. Simulations are compared to hourly observations from an automatic weather station located at the Troll Research Station in Dronning Maud Land. Areas of bare ground have been implemented in the model, and the simulations correspond well with meteorological measurements acquired during the 4 day simulation period. The nunataks are radiatively heated during daytime, and free convection occurs in the overlying atmospheric boundary layer. The inflow below the updraft forces strong horizontal convergence at the surface, whereas weaker divergence appears aloft. In a control run with a completely ice-covered surface, the convection is absent. In situ observations carried out by a remotely controlled balloon and a small model airplane compare well with model temperature profiles, but these are only available over the ice field upwind to the nunatak.

Controllable Meteorological Balloons for Arctic Research

While the Arctic is among the most isolated and inaccessible regions on earth, its weather and climate can strongly affect mid-latitude agricultural regions and population centers. Improving observational capabilities in the Arctic therefore has broad relevance. Over the past four years, Controlled Meteorological (CMET) balloons have been developed specifically for studying transport processes in the Arctic troposphere and boundary layer. These balloons, which are now approximately the same size as standard rawinsondes, can float for many days while performing vertical soundings on command via satellite. This sounding capability is particularly advantageous in the highly stratified polar atmosphere where shallow layers of air with differing character and origins are ubiquitous. The challenges of operating small battery-powered balloons in the Arctic are formidable; they include electrical and mechanical failures due to the extreme cold, fallout due to ice accumulation on the balloon envelope, and complex airspace issues. Results of this development work will be discussed and placed into context of planned and future studies.

Current status of deepwater oil spill modelling in the Faroe-Shetland Channel, Northeast Atlantic, and future challenges

As oil reserves in established basins become depleted, exploration and production moves towards relatively unexploited areas, such as deep waters off the continental shelf. The Faroe-Shetland Channel (FSC, NE Atlantic) and adjacent areas have been subject to increased focus by the oil industry. In addition to extreme depths, metocean conditions in this region characterise an environment with high waves and strong winds, strong currents, complex circulation patterns, sharp density gradients, and large small- and mesoscale variability. These conditions pose operational challenges to oil spill response and question the suitability of current oil spill modelling frameworks (oil spill models and their forcing data) to adequately simulate the behaviour of a potential oil spill in the area. This article reviews the state of knowledge relevant to deepwater oil spill modelling for the FSC area and identifies knowledge gaps and research priorities. Our analysis should be relevant to other areas of complex oceanography.

Influence of River‐Induced Fronts on Hydrocarbon Transport: A Multiplatform Observational Study

The Taylor Energy Site is located in the vicinity of the Mississippi Delta region over the Northern Gulf of Mexico (NGoM). Surface oil patches have been persistently observed within this site since 2004, when an oil rig was destroyed by Hurricane Ivan. A multiplatform observational experiment was conducted in April 2017 to investigate, for the first time, the main hydrocarbon pathways from the Taylor Energy Site toward the NGoM continental shelves, and the Gulf interior, under the influence of local and regional physical processes. Results indicate that the Mississippi River (MR)-induced fronts over the Taylor Energy Site, in combination with local circulation, prevailing winds and broader regional dynamics determine the hydrocarbon transport. The drifters deployed during the field experiment, in tandem with satellite data, drone imagery, wind measurements, and radar-derived data, efficiently described three major hydrocarbon pathways, associated with MR plume dynamics (downstream/upstream coastal currents) and basin-wide circulation (offshore pathway). Two different types of drifters, drogued and undrogued, showed clearly different pathways, which suggest potential differences in the expected advection of oil, depending on whether it forms a surface slick or whether it is partially mixed below the surface. The existence of multiple river fronts influenced the fate of oiled waters, preventing the hydrocarbons from reaching the Delta, like a natural boom barrier, trapping and directing the oil either westward or eastward. Thermohaline measurements showed that the MR plume near Taylor was 5–10 m deep, while the clearer ocean was characterized by a 40 m upper ocean homogenous layer.

Measurements by controlled meteorological balloons in coastal areas of Antarctica

Abstract An experiment applying controlled meteorological (CMET) balloons near the coast of Dronning Maud Land, Antarctica, in January 2013 is described. Two balloons were airborne for 60 and 106 hours with trajectory lengths of 885.8 km and 2367.4 km, respectively. The balloons carried out multiple controlled soundings on the atmospheric pressure, temperature and humidity up to 3.3 km. Wind speed and direction were derived from the balloon drift. Observations were compared with radiosonde sounding profiles from the Halley Research Station, and applied in evaluating simulations carried out with the weather research and forecasting (WRF) mesoscale atmospheric model. The most interesting feature detected by the CMET balloons was a mesoscale anticyclone over the Weddell Sea and the coastal zone, which was reproduced by the WRF model with reduced intensity. The modelled wind speed was up to 10 m s-1 slower and the relative humidity was 20–40% higher than the observed values. However, over the study period the WRF results generally agreed with the observations. The results suggest that CMET balloons could be an interesting supplement to Antarctic atmospheric observations, particularly in the free troposphere.