A simulation of an undular Bore over the Gulf of Mexico

Der „undular Bore“ der am 27. April 2011 über dem Golf von Mexiko beobachtet wurde ist nicht der einzige bekannte Fall dieser Art. Vielmehr kommt dieses Phänomen über dem Golf mit einer gewissen Regelmäßigkeit im Frühjahr vor. Im Archiv des CIMSS Satelliten Blogs finden sich fünf verschiedene Beobachtungen aus den Jahren 1997, 1998, 2007, 2008 und 2011, mit Boren die im März oder April auftraten. In anderen Gegenden treten solche Wellen ebenfalls auf, wobei die Morning Glory Cloud über dem australischen Golf von Carpentaria wohl am bekanntesten ist. Im Prinzip kann ein undular Bore als ein schwacher hydraulischer Sprung betrachtet werden, wobei im Gegensatz zu einem starken kein Wellenbrechen mit einhergehender ausgeprägter Dissipation entsteht, sondern ein stabiler Wellenzug generiert wird. Bei den dabei entstehenden Wellen handelt es sich um solitäre Wellen, einem Typ nichtlinearer Schwerewellen, die sich mit konstanter Geschwindigkeit ausbreiten ohne ihre Gestalt zu verändern. Die theoretische Beschreibung von solitären Wellen geht auf die Korteweg- und de Vries (KdV) Gleichung zurück, die zunächst für Flachwasserwellen hergeleitet wurde. Eine so- litäre Welle ist hierbei eine Welle die durch ein Gleichgewicht des nichtlinearen und des dispersiven Terms stabil bleibt und sich mit konstanter Geschwindigkeit ausbreitet. Auf Brooke Benjamin, Russ Davis, Andreas Acrivos und Hiroaki Ono geht hierbei eine Erwei- terung auf interne Schwerewellen in einem Kanal, die BDO-Gleichung, zurück. Dabei dient eine starke Inversion als Wellenführung und solitäre Wellen können im unteren Teil der Atmosphäre entstehen. Als Auslösemechanismus dient in vielen Fällen der kalte Ausfluss von Gewittern der sich als „density current“, also Dichteströmung, am Boden ausbreitet und bei Kollision mit einem Seewind oben beschriebenen Wellen ausbildet. Alternativ zu einzelnen konvektiven Zellen lösen auch Kaltfronten Boren aus und auch die Kollision von zwei Seewindsystemen wurde als Auslösemechanismus bereits beobachtet. Die Simulation des vorliegenden Falls mit dem Wheater Reseach and Forecasting Modell hat zum Ziel die Evolution der Bore über dem Meer zu simulieren und die Wellen mit Theorie und Satellitenbildern zu vergleichen. Darüber hinaus kann der Entstehungspro- zess, von dem keine Satellitenbilder vorliegen, untersucht werden. Die geringe Wellenlänge macht einen horizontalen Gitterpunktsabstand von 1 km notwendig, was bei der beträcht- lichen räumlichen Ausdehnung der Bore (etwa 800 km lang, 70 km breit) zu relativ großen Domänen führt. Ein Schlüsselaspekt ist die Ausbildung der Inversion an denen sich die Wellen ausbreiten, und so ist eine erhöhte vertikale Auflösung in den unteren 1,5 km von Vorteil. Zum Schluss der Arbeit werden die Ergebnisse der Modellsimulation mit den theoretischen Vorhersagen der BDO Theorie verglichen.The undular bore observed on 27th of April 2012 is not the only known case like this. In fact, this phenomenon develops over the Gulf of Mexico in the spring quite regularly. The CIMMS Satellite Blogs archive holds at least five different observations of the years 1997, 1998, 2007, 2008 and 2011, where the bores appeared either in March or April. There are undular bores in other areas of the world as well, among them the well known Morning Glory Cloud which appears over the australian Gulf of Carpentaria. In principle is an undular is bore a weak hydraulic jump which causes a train of sta- ble waves, in contrast to a strong hydraulic jump where wave breaking and intensive dissipation occurs. These waves are so called solitary waves, a type of nonlinear gravity waves that propagate at constant speed without change of shape. The theory of solitary waves has its roots in the Korteweg- and de Vries (KdV) equa- tion, originally derived for shallow water waves in a narrow channel. Such a wave keeps its shape since the nonlinear term is in perfect balance with the dispersive term of the KdV and it propagates with constant phase velocity. Brooke Benjamin, Russ Davies, Andreas Acrivos and Hiroaki Ono derived an equivalent of the KdV, the BDO equation, valid for internal gravity waves traveling in a wave guide. For the atmosphere, a strong low level inversion acts as a wave guide, and solitary waves may travel inside. In many cases, the trigger for the bore is the cold outflow of a thunderstorm, a density current, ad- vancing at the ground. When colliding with a sea wind, the described waves are excited. Alternatively, cold fronts or the collision of two sea wind systems can trigger bores. The simulation of the present case using the Wheater Reseach and Forecasting model aims to get a better understanding of the evolution of the bore over the ocean and to compare the waves with theory and satellite images. Beyond that, the process responsi- ble for the creation of the bore, for which no satellite images are available, can be studied and information of the evolution of the bore when entering the sea area is obtained. The small wavelength of the bore requires a horizontal resolution of 1 km and the combination with the quite large spatial dimensions of the bore (800 km long, 70 km wide) leads to rel- atively big domains. A key aspect is the correct simulation of the inversion which guides the waves, wherefore a higher vertical resolution in the lowest 1.5 km is an advantage. Finally, a comparison of model results with present BDO theory will be carried out

Microscale CFD Simulations of a Wind Energy Test Site in the Swabian Alps with Mesoscale Based Inflow Data

The current study describes analyses of the WINSENT wind energy test site located in complex terrain in Southern Germany by highly resolved numerical simulations. The resolved atmospheric turbulence is simulated with Delayed Detached Eddy Simulations by the flow solver FLOWer without consideration of the research wind turbines. The mean inflow and wind direction of the analysed time period is provided by precursor simulations of project partners. The simulation model chain consists of three codes with different time scales and resolutions. The model chain provides a data transfer from mesoscale WRF simulations to OpenFOAM. As a next step OpenFOAM provides inflow data in the valley of the terrain site for the present FLOWer simulations, the code with the highest resolution in space and time. The mean velocity field provided by OpenFOAM is superimposed with fluctuations that are based on measurements to obtain the small turbulent scales within the FLOWer simulations, which the previous tools of the model chain can not resolve. Comparisons with the two already installed met masts clarify that the current FLOWer simulations provide an adequate agreement with measured data. The results are verified with the application of a second simulation, in which a homogeneous velocity profile is superimposed with turbulence. Thus, comparisons with measured data showed that the benefit of using the inflow data of this model chain is especially evident near the ground

Urban Atmospheric Boundary-Layer Structure in Complex Topography: An Empirical 3D Case Study for Stuttgart, Germany

Investigation of the atmospheric boundary-layer structure in urban areas can be challenged by landscape complexity and the heterogenous conditions this instills. Stuttgart, Germany, is a city situated in a bowl-shaped basin and troubled by the accumulation of pollutants during weak-wind conditions. The center of Stuttgart is surrounded by steep slopes up to 250m above the basin floor, except for an opening to the northeast that allows runoff towards the Neckar river. Urban planning and regulation of air quality require advanced monitoring and forecasting skills, which in turn require knowledge about the structure of the atmospheric boundary layer (ABL), down to the surface. Three dimensional observations of the ABL were collected in the City Centre of Stuttgart in 2017. A laser ceilometer and a concerted network of Doppler lidar systems were deployed on roof-tops, providing continuous observations of the cloud base, the mixing-layer height and the three-dimensional wind field. The impact of weak-wind conditions, the presence of shear layers, properties of convective cells and the impact of nocturnal low-levels jets were studied for representative days in winter and summer. The observations revealed the development of distinctive layers with high directional deviation from the flow aloft, reoccurring as a dominant diurnal pattern. Our findings highlight the influence of topography and surface heterogeneity on the structure of the ABL and development of flow regimes near the surface that are relevant for the transport of heat and pollutants

Urban atmospheric boundary-layer structure in complex topography: an empirical 3D case study for Stuttgart, Germany

Investigation of the atmospheric boundary-layer structure in urban areas can be challenged by landscape complexity and the heterogenous conditions this instills. Stuttgart, Germany, is a city situated in a bowl-shaped basin and troubled by the accumulation of pollutants during weak-wind conditions. The center of Stuttgart is surrounded by steep slopes up to 250 m above the basin floor, except for an opening to the northeast that allows runoff towards the Neckar river. Urban planning and regulation of air quality require advanced monitoring and forecasting skills, which in turn require knowledge about the structure of the atmospheric boundary layer (ABL), down to the surface. Three-dimensional observations of the ABL were collected in the City Centre of Stuttgart in 2017. A laser ceilometer and a concerted network of Doppler lidar systems were deployed on roof-tops, providing continuous observations of the cloud base, the mixing-layer height and the three-dimensional wind field. The impact of weak-wind conditions, the presence of shear layers, properties of convective cells and the impact of nocturnal low-levels jets were studied for representative days in winter and summer. The observations revealed the development of distinctive layers with high directional deviation from the flow aloft, reoccurring as a dominant diurnal pattern. Our findings highlight the influence of topography and surface heterogeneity on the structure of the ABL and development of flow regimes near the surface that are relevant for the transport of heat and pollutants

A Two-Day Case Study: Comparison of Turbulence Data from an Unmanned Aircraft System with a Model Chain for Complex Terrain

The airborne measurement platform MASC-3 (Multi-Purpose Airborne Sensor Carrier) is used for measurements over a forested escarpment in the Swabian Alps to evaluate the wind field. Data from flight legs between 20 and 200 m above the ground on two consecutive days with uphill (westerly) flow in September 2018 are analyzed. In the lowest 140 m above the ground a speed-up is found with increased turbulence and changes in wind direction directly over the escarpment, whereas in the lowest 20 to 50 m above the ground a deceleration of the flow is measured. Additionally, simulation results from a numerical model chain based on the Weather Research and Forecasting (WRF) model and an OpenFOAM (Open Source Field Operation and Manipulation) model, developed for complex terrain, are compared to the data captured by MASC-3. The models and measurements compare well for the mean wind speed and inclination angle