Turbulence processes

Turbulence is defined by irregular fluctuations in wind, temperature and humidity caused by the combined effect of many 3-dimensional eddies of different sizes in the flow. In the atmospheric boundary layer it is generated by wind shear and heating or cooling at the surface. Turbulence is important as it causes mixing in the atmosphere and is distributing energy, gases and particles horizontally and vertically. This mixing is very efficient in turbulent flows in comparison to laminar flows, flows without turbulent eddies. In most natural flows a large range of both small and large vortices or eddies can be observed. This is in contrast to many human built constructions such as fountains and flow in pipes (depending on the velocity of the flow) where neighboring fluid elements tend to move together in a similar direction. Turbulence in the atmospheric boundary layer varies with the conditions in the atmosphere and the amount of turbulence strongly varies with the strength of the wind speed and the heat flux from the surface. During a windy day the turbulence in the atmosphere can quickly mix any gas or particles being released at surface level, whereas reduced atmospheric turbulence levels (or complete lack of turbulence) close to the ground can in cold climates on cold winter days be important both for extreme weather situations in terms of extreme cold events and increased concentrations of air pollutants.

The research by the Meteorology group in Uppsala focuses on various aspects of the basic understanding of turbulence, its intensity and its effects applied to relevant atmospheric and oceanographic conditions. These include also applications in renewable energy, dispersion modeling, sound propagation and turbulent exchange processes. In marine environments we study the impact of surface gravity waves on turbulence features, it has been shown that the properties of long ocean waves can alter the turbulence structure and the air-sea exchange mechanisms (see Figure 1, showing an example from a large-eddy simulation of convective turbulence influenced by wind shear in the presence of surface waves). Wind storms and tropical hurricanes are powerful atmospheric motion systems that need to be studied using a variety of tools such as high-frequency turbulence instruments located in masts, on wave buoys and ships. Numerical models with high spatio-temporal resolution are also needed to study the turbulent processes that occurs and affects society and human safety.

Through national and international collaborations, we also study parts of the diurnal cycle and how it affects turbulence in for instance afternoons and transitions between the turbulence dominated boundary layer in the afternoon, and the much less turbulent boundary layer during the night. Diurnal and yearly cycles in the amount of turbulence as well as the sizes and energy content of turbulent eddies are highly relevant for the establishment of wind power, especially over forests where loads due to turbulence is expected to be higher. In these studies, often a combination of field measurements, numerical simulation using large-eddy simulation and/or mesoscale numerical weather prediction models as well as development of statistical methods and models are used. An example is shown in Figure 2, where the wind speeds from different atmospheric sensors placed over a forested hill in southern Sweden is show over the course of the diurnal cycle.

Figure 1: The resolved vertical wind fluctuations are visualized for an LES case of strong swell waves with alternating pattern of wave correlated updrafts (in red) and downdrafts (in blue) seen near an idealized ocean surface. Formation of larger-scale boundary layer roll vortices elongated in the geostrophic wind direction are also indicated by visualizing iso-surfaces of positive updrafts of 1 m/s (in red) for this case influenced by wind shear and buoyancy effects.

Figure 2: The diurnal cycle of wind speeds measured over a forested hill. Red and orange arrows indicate measurements with lidar (LIght Detection and RAnging), blue with sodar (Sound Detection and RAnging) and purple with Sonic anemometers in a 180 m tall measurement tower.

Persons: Johan Arnqvist, Erik Nilsson, Anna Rutgersson