Haraldur Ólafsson

The Effect of Rotation and Surface Friction on Orographic Drag

Abstract A numerical, hydrostatic model is used to investigate the form and magnitude of the pressure drag created by 3D elliptical mountains of various heights (h) and aspect ratios (R) in flows characterized by uniform upstream velocity (U) and stability (N). Three series of simulations, corresponding to increasing degrees of realism, are performed: (i) without rotation and surface friction; (ii) with rotation, but no surface friction; (iii) with rotation and surface friction. For the simulations with rotation, the Coriolis parameter has a typical midlatitude value and the upstream flow is geostrophically balanced. The surface friction is introduced by the use of a typical roughness length. For low values of the nondimensional height (Nh/U), the pressure drag is reduced by the effect of rotation, in agreement with well-known results of linear theory. This seems to be valid until Nh/U ∼ 1.4, that is, in the high drag regime. On the other hand, for large values of Nh/U, that is, in the blocked flow regime...

Nonlinear flow past an elliptic mountain ridge

Abstract The hydrostatic flow over an elliptical mountain of aspect ratio 5 is explored by numerical experiments. The upstream profiles of wind and stability are constant, the Coriolis effect is ignored, and there is free slip at the lower boundary. In these conditions, the, flow characteristics depend mainly on the nondimensional mountain height, Nh/U. The authors have conducted experiments with Nh/U varying from 0.500 to 6.818. For low values of Nh/U, the results confirm the linear theory of Smith, which predicts stagnation aloft, leading to wave breaking and, on the upstream slope, leading to flow splitting. For higher values of Nh/U, the authors find that wave breaking ceases on the axis of symmetry but continues on each side of this axis. Even for the highest value of Nh/U used (6.818), significant areas of wave breaking and wave activity aloft are found. For all values of Nh/U, a substantial part of the flow is diverted vertically above the mountain. The detailed study of the kinematic pattern withi...

THE GREENLAND FLOW DISTORTION EXPERIMENT

Greenland has a major influence on the atmospheric circulation of the North Atlantic-western European region, dictating the location and strength of mesoscale weather systems around the coastal seas of Greenland and directly influencing synoptic-scale weather systems both locally and downstream over Europe. High winds associated with the local weather systems can induce large air-sea fluxes of heat, moisture, and momentum in a region that is critical to the overturning of the thermohaline circulation, and thus play a key role in controlling the coupled atmosphere-ocean climate system. The Greenland Flow Distortion Experiment (GFDex) is investigating the role of Greenland in defining the structure and predictability of both local and downstream weather systems through a program of aircraft-based observation and numerical modeling. The GFDex observational program is centered upon an aircraft-based field campaign in February and March 2007, at the dawn of the International Polar Year. Twelve missions were fl...

AN EARTH-SYSTEM PREDICTION INITIATIVE FOR THE TWENTY-FIRST CENTURY

Some scientists have proposed the Earth-System Prediction Initiative (EPI) at the 2007 GEO Summit in Cape Town, South Africa. EPI will draw upon coordination between international programs for Earth system observations, prediction, and warning, such as the WCRP, WWRP, GCOS, and hence contribute to GEO and the GEOSS. It will link with international organizations, such as the International Council for Science (ICSU), Intergovernmental Oceanographic Commission (IOC), UNEP, WMO, and World Health Organization (WHO). The proposed initiative will provide high-resolution climate models that capture the properties of regional high-impact weather events, such as tropical cyclones, heat wave, and sand and dust storms associated within multi-decadal climate projections of climate variability and change. Unprecedented international collaboration and goodwill are necessary for the success of EPI.

Flow in the Lee of Idealized Mountains and Greenland

Abstract A series of idealized simulations of flow impinging on large mountains is conducted to investigate the impact of the mountain on the flow far downstream and to shed some light on the effects that Greenland may have on airflow over the North Atlantic. The upstream profiles of wind and stability are kept constant, there is no surface friction, the Rossby number is 0.4, and the nondimensional mountain height (ĥ = Nh/U) is varied from 1 to 6. The maximum sea level pressure deficit, the maximum geopotential height deficit, and the orographically generated potential vorticity all increase with increased ĥ, showing no signs of abrupt change as the flow enters the regime of upstream blocking. The potential vorticity produced at the mountain is accumulated in vortices that are advected downstream. The vortices are associated with a larger pressure gradient to the south of the wake, giving rise to stronger westerlies at the surface as well as at upper levels. This process can explain how Greenland may affe...

The Small Unmanned Meteorological Observer SUMO: Recent developments and applications of a micro-UAS for atmospheric boundary layer research

During the last 5 years, the Small Unmanned Meteorological Observer SUMO has been developed as a flexible tool for atmospheric boundary layer (ABL) research to be operated as sounding system for the lowest 4 km of the atmosphere. Recently two main technical improvements have been accomplished. The integration of an inertial measurement unit (IMU) into the Paparazzi autopilot system has expanded the environmental conditions for SUMO operation. The implementation of a 5-hole probe for determining the 3D flow vector with 100 Hz resolution and a faster temperature sensor has enhanced the measurement capabilities.Results from two recent field campaigns are presented. During the first one, in Denmark, the potential of the system to study the effects of wind turbines on ABL turbulence was shown. During the second one, the BLLAST field campaign at the foothills of the Pyrenees, SUMO data proved to be highly valuable for studying the processes of the afternoon transition of the convective boundary layer.

Sources of aerosol nitrate and non-sea-salt sulfate in the Iceland region

Daily aerosol filter samples were collected on Heimaey, Iceland (63.40° N, 20.30° W), beginning in July 1991. Samples were analyzed for NO3−, non-sea-salt (nss) SO42−, and methanesulfonate (MSA). Along with SO2 and nss-SO42−, MSA is a product of the atmospheric oxidation of dimethyl sulfide (DMS) that is produced by marine organisms. For much of the time, concentrations of aerosol nss-SO42− and NO3− were relatively low. Occasionally, however, concentrations increased sharply, by an order of magnitude or more, often for periods of several days. These concentration peaks were usually associated with the presence of a high-pressure field over western Europe; the large-scale wind fields associated with the high pressure subsequently transported pollutants to the Iceland region. The 2-year mean NO3− aerosol concentration was 0.239 μg/m3, while that for nss-SO42− was 0.642 μg/m3; the median values were, respectively, 0.113 μg/m3 and 0.367 μg/m3. Excluding the high-aerosol events (i.e. about 10% of the samples), the NO3− average was 0.131 μg/m3 and that for nss SO42− was 0.385 μg/m3; these values are similar to those measured in the pristine South Pacific. Thus, although pollution events were relatively infrequent, they had a substantial impact on atmospheric chemistry in this region, in effect doubling the annual mean concentrations. There was a very strong seasonal cycle in MSA concentrations, with a summer maximum of about 500 ng/m3, which decreased to a few ng/m3 in December. The seasonal cycle of MSA matches that ofPhaeocystis pouchetii andEmiliania huxleyi, both of which are strong DMS producers; intense and widespread blooms of these organisms are often found around Iceland in the late spring and summer. During the summer, the nss-SO42−MSA ratio was very low much of the time, suggesting that biogenic DMS was the dominant source of aerosol nss-SO42− in this region in this season.

The Norwegian IPY–THORPEX: Polar Lows and Arctic Fronts during the 2008 Andøya Campaign

From a weather forecasting perspective, the Arctic poses particular challenges for mainly two reasons: 1) The observational data are sparse and 2) the weather phenomena responsible for severe weather, such as polar lows, Arctic fronts, and orographic influences on airflow, are poorly resolved and described by the operational numerical weather prediction (NWP) models. The Norwegian International Polar Year (IPY)– The Observing System Research and Predictability Experiment (THORPEX) project (2007–10) sought to significantly improve weather forecasts of these phenomena through a combined modeling and observational effort. The crux of the observational effort was a 3-week international field campaign out of northern Norway in early 2008, combining airborne and surface-based observations. The main platform of the field campaign was the Deutsches Zentrum fur Luft- und Raumfahrt (DLR) research aircraft Falcon, equipped with lidar systems for profiling of aerosols, humidity, and wind, in addition to in situ measu...

Modelling of spatial variability and topographic effects over Arctic fjords in Svalbard

The spatial variability of near-surface variables and turbulent surface fluxes was investigated in three Arctic fjords in Svalbard applying the Weather Research and Forecasting (WRF) mesoscale model. Ten real cases from winter and spring 2008, representing the most common large-scale flow directions, were simulated at 9, 3 and 1 km resolutions for 36 h each. Validation against tower observations and radiosoundings showed fairly good agreement, although a systematic warm and moist bias and slightly overestimated wind speeds were found close to the surface. The spatial variability within a fjord was large and it often reached levels comparable to the temporal variability. The spatial variability of the surface fluxes of sensible and latent heat was mostly controlled by the air and sea surface temperatures instead of wind speed. The same cases were also simulated without any topography over Svalbard. The topography increased the spatial variability but the influence on the mean values was not systematic, except that a clear warming effect was seen in all the fjords studied. The role of surface type increased with increasing air–sea temperature difference and was dominating over topographic effects for the air temperature, specific humidity and turbulent heat fluxes.

Mean gust factors in complex terrain

Gust factors are analyzed for wind speeds greater than 10 m/s in a large set of data from automatic weather stations in the complex terrain of Iceland. The mean gust factors appear to be independent of the static and dynamic stability of the atmosphere, suggesting that gravity wave dynamics may compensate for the damping of turbulence in stably stratified flows. The mean gust factor depends however on wind speed and nearby topography and decreases regularly with increasing wind speed and station altitude. High mountains close to the weather stations give strong downstream wind gusts. In a subset of the data, containing only statically stable flows, mean gust factors of 1.6 or more may be obtained if an upstream mountain rises at least 200 m above the weather station and the distance to the mountain is less than 10 times its height above the station.