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Rognvaldsson O.,Institute for Meteorological Research | Rognvaldsson O.,University of Bergen | Bao J.-W.,National Oceanic and Atmospheric Administration | Agustsson H.,Institute for Meteorological Research | And 3 more authors.
Atmospheric Chemistry and Physics | Year: 2011

A severe windstorm downstream of Mt. Öraekull in Southeast Iceland is simulated on a grid of 1 km horizontal resolution by using the PSU/NCAR MM5 model and the Advanced Research WRF model. Both models are run with a new, two equation planetary boundary layer (PBL) scheme as well as the ETA/MYJ PBL schemes. The storm is also simulated using six different micro-physics schemes in combination with the MYJ PBL scheme in WRF, as well as one "dry" run. Output from a 3 km MM5 domain simulation is used to initialise and drive both the 1 km MM5 and WRF simulations. Both models capture gravity-wave breaking over Mt. Öraekull, while the vertical structure of the lee wave differs between the two models and the PBL schemes. The WRF simulated downslope winds, using both the MYJ and 2EQ PBL schemes, are in good agreement with the strength of the observed downslope windstorm. The MM5 simulated surface winds, with the new two equation model, are in better agreement to observations than when using the ETA scheme. Micro-physics processes are shown to play an important role in the formation of downslope windstorms and a correctly simulated moisture distribution is decisive for a successful windstorm prediction. Of the micro-physics schemes tested, only the Thompson scheme captures the downslope windstorm. © Author(s) 2011.

Arason T.,University of Iceland | Rognvaldsson O.,Institute for Meteorological Research | Rognvaldsson O.,University of Bergen | Olafsson H.,University of Iceland
Hydrology Research | Year: 2010

Atmospheric flow over Iceland has been simulated for the period January 1961 to July 2006, using the mesoscale MM5 model driven by initial and boundary data from the ECMWF. A systematic comparison of results to observed precipitation has been carried out. Undercatchment of solid precipitation is dealt with by looking only at days when precipitation is presumably liquid or by considering the occurrence and non-occurrence of precipitation. Away from non-resolved orography, the long term means (months, years) of observed and simulated precipitation are often in reasonable agreement. This is partly due to a compensation of the errors on a shorter timescale (days). The probability of false alarms (the model predicts precipitation, but none is observed) is highest in N iceland, particularly during winter. The probability of missing precipitation events (precipitation observed but none is predicted by the model) is highest in the summer and on the lee side of Iceland in southerly flows. © WA Publishing 2010.

Rognvaldsson O.,Institute for Meteorological Research | Rognvaldsson O.,University of Bergen | Jonsdottir J.F.,Formerly at the Hydrological Service | Olafsson H.,University of Bergen | Olafsson H.,University of Iceland
Hydrology Research | Year: 2010

Atmospheric flow over Iceland has been simulated for the period January 1961 to July 2006, using the mesoscale MM5 model driven by initial and boundary data from the European Centre for Medium Range Weather Forecasts (ECMWF). Firstly, the simulated precipitation is compared to estimates derived from mass balance measurements on the Icelandic ice caps. It is found that the simulated precipitation compares favourably with the observed winter balance, in particular for Hofsj̈kull, where corrections to take liquid precipitation and/or winter ablation into account have been made, and for the outlet glaciers DyngjujOkull and Br(Jarj̈kull. Secondly, the model output is used as input to the WaSIM hydrological model to calculate and compare the runoff with observed runoff from six watersheds in Iceland. It is found that model results compare favourably with observations. Overall, the MM5 V3-7 is somewhat better than the MM5 V3-5. The V3-7 is drier than V3-5 on upstream slopes. © IWA Publishing 2010.

Jonassen M.O.,University of Bergen | Olafsson H.,University of Bergen | Olafsson H.,University of Iceland | Agustsson H.,University of Iceland | And 3 more authors.
Monthly Weather Review | Year: 2012

In this study, it is demonstrated how temperature, humidity, and wind profile data from the lower troposphere obtained with a lightweight unmanned aerial system (UAS) can be used to improve high-resolution numerical weather simulations by four-dimensional data assimilation (FDDA). The combined UAS and FDDA system is applied to two case studies of northeasterly flow situations in southwestIceland from the international Moso field campaign on 19 and 20 July 2009. Both situations were characterized by high diurnal boundary layer temperature variation leading to thermally driven flow, predominantly in the form of seabreeze circulation along the coast. The data assimilation leads to an improvement in the simulation of the horizontal and vertical extension of the sea breeze as wellasof the local background flow. Erroneously simulated fog over the Reykjanes peninsula on 19 July, which leads to a local temperature underestimation of 8 K, is also corrected by the data assimilation. Sensitivity experiments show that both the assimilation of wind data and temperature and humidity data are important for the assimilation results. UAS represents a novel instrument platform witha large potential within the atmospheric sciences. The presented method of using UAS data for assimilation into high-resolution numerical weather simulations is likely to have a wide range of future applications such as wind energy and improvements of targeted weather forecasts for search and rescue missions. ©2012 American Meteorological Society.

Reuder J.,University of Bergen | Ablinger M.,Ludwig Maximilians University of Munich | Agustsson H.,University of Iceland | Agustsson H.,Institute for Meteorological Research | And 20 more authors.
Meteorology and Atmospheric Physics | Year: 2012

The FLOHOF field campaign took place in the period July 21 to August 24, 2007 on and in the surroundings of Hofsjökull glacier in Central Iceland. During the campaign, 18 automatic weather stations (AWS) recording temperature, humidity, wind speed, wind direction, pressure, and precipitation were deployed on and around the glacier. In addition, atmospheric soundings were performed N and S of Hofsjökull by a tethered balloon, pilot balloons, and two unmanned aerial systems (UAS). An energy balance station, consisting of a net radiometer and an eddy correlation flux measurement station, has also been installed. This paper describes the experimental setup of the campaign and presents first results of the data analysis with respect to transience of mountain-induced gravity waves, the extension of katabatic winds into the surrounding of the glacier, the occurrence of katabatic microfronts, and report on novel approaches to probe the vertical structure of the atmospheric boundary layer by UAS. The observed pressure perturbations related to transient gravity wave activity due to changing inflow conditions were between -2 and 2 hPa in general, with positive values upstream and negative values downstream. Differential heating of the glacier and its surrounding is triggering daytime katabatic flow from the glacier into its surrounding. During the campaign, those katabatic winds typically reached out 4-7 km from the edge of the glacier. During late night in clear sky conditions, frontal-like microstructures have been observed frequently with typical repetition times in the order of 30-60 min indicating the interaction of large-scale synoptic and nighttime katabatic density flows close to the ground. The first research application of the newly developed small unmanned meteorological observer proved the applicability of the system for atmospheric boundary layer research by successfully profiling the atmosphere up to 3.5 km above ground. © 2011 The Author(s).

Agustsson H.,Institute for Meteorological Research | Agustsson H.,University of Iceland | Olafsson H.,University of Iceland | Olafsson H.,University of Bergen
Meteorology and Atmospheric Physics | Year: 2012

Downslope windstorms at Kvísker in Southeast Iceland are explored using a mesoscale model, observations and numerical analysis of the atmosphere. Two different types of gravity-wave induced windstorms are identified. At the surface, their main difference is in the horizontal extent of the lee-side accelerated flow. Type S (Short) is a westerly windstorm, which is confined to the lee-slopes of Mount Öræfajökull, while a Type E (Extended) windstorm occurs in the northerly flow and is not confined to the lee-slopes but continues some distance downstream of the mountain. The Type S windstorm may be characterized as a more pure gravity-wave generated windstorm than the Type E windstorm which bears a greater resemblance to local flow acceleration described by hydraulic theory. The low-level flow in the Type E windstorm is of arctic origin and close to neutral with an inversion well above the mountain top level. At middle tropospheric levels there is a reverse vertical windshear. The Type S windstorm occurs in airmasses of southerly origin. It also has a well-mixed, but a shallower boundary-layer than the Type E windstorms. Aloft, the winds increase with height and there is an amplified gravity wave. Climate projections indicate a possible decrease in windstorm frequency up to the year 2050. © 2010 Springer-Verlag.

Hannesdottir H.,University of Iceland | Aoalgeirsdottir G.,University of Iceland | Johannesson T.,Icelandic Meteorological Office IMO | Guomundsson S.,University of Iceland | And 7 more authors.
Journal of Glaciology | Year: 2015

Simulations of the post-Little Ice Age evolution of three outlet glaciers of southeast Vatnajökull, Iceland - Skàlafellsjökull, Heinabergsjökull and Flàajökull - are presented. A coupled shallow-ice-approximation ice-flow and degree-day mass-balance model is applied that is calibrated with a 14 year record of in situ mass-balance measurements. The measured mass balance cannot be realistically represented by constant horizontal and vertical precipitation gradients. High-resolution (1 km) precipitation fields, derived from downscaled orographic atmospheric circulation models of precipitation, are required to capture the spatial variation of the winter mass balance. The observed ice volume around 1890 (15-30% larger than in 2000) can be simulated with 1°C lower temperatures and a 20% reduction in the annual precipitation, relative to the reference climate period, 1980-2000. The sensitivity of each glacier's annual balance to a change in temperature is -1.51 to -0.97mw.e. a-1 °C-1 and +0.16 to +0.65mw.e. a-1 for a 10% increase in precipitation. A steady-state experiment applying a step increase in temperature of 2°C (3°C), and precipitation increase of 10%, results in a >50% (80- 90%) decrease in ice volume.

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