(MERI) Atmospheric Turbulence

Turbulence is one of the great unsolved problems of atmospheric science. It is present on a very wide range of scales, but the key areas of uncertainty lie at small scales – those which cannot be resolved by numerical models. Turbulence dominates the dynamics of the convective boundary layer over land, when sunlight heats the Earth’s surface creating buoyant plumes which mix with the air above. Buoyancy-driven turbulence is also found in clouds – most visibly in deep thunderstorms which are a seething mass of turbulent air, but also in layer clouds like stratocumulus or altocumulus. A completely different source of turbulence dominates in the upper troposphere and stratosphere, where powerful winds circulate the Earth in the form of jet streams. Wind shear around these jets gives rise to clear-air turbulence which can be a hazard for aircraft if sufficiently strong.

Because atmospheric turbulence is very hard to model, reliable and extensive measurements of it are a priority. However, most turbulence measurements so far have been made either in the surface layer (on towers or masts) or on platforms like aircraft or balloons which measure above the surface but have very limited coverage. There is a need for a long-term dataset of turbulent intensity at altitudes away from the Earth’s surface. This project will use radar remote sensing to examine the distribution of turbulence and its relation to the underlying meteorology. Radars are sensitive to turbulence in the inertial subrange, and therefore provide a measure of the eddy dissipation rate in the volume of atmosphere being probed.

The project will provide the student with a thorough grounding in radar remote sensing and small-scale atmospheric dynamics. The focus is mainly on data analysis and interpretation, although there is a possibility of targeted fieldwork using the radar if particular experiments are needed to advance the project.

The project will be based on measurements from the UK mesosphere-stratosphere-troposphere (MST) radar, a NERC national facility based near Aberystwyth in Wales (http://mst.nerc.ac.uk). This VHF radar (wavelength 6 m) has been measuring wind profiles above the site almost continuously since 1997, giving 20 years’ worth of data. Turbulence information is contained in the spectra of the radar echoes from the atmosphere. The start point of the project is to determine the optimum way to derive the spectral width from the radar measurements, and use this to construct a climatology of eddy dissipation rate over the site. There is a large body of literature concerned with such methods but very few have been taken as far as a climatology.

The next stage of the project will be to relate the turbulence measurements to particular atmospheric features, e.g. clouds, gravity waves and jet streams. This stage will depend on the direction the student wants to take but could examine questions such as:
- Can all instances of turbulence be categorised according to particular atmospheric features?
- How common is turbulence around jet streams? What is the distribution of intensity?
- Is there a relation between turbulence and breaking gravity waves?

Lastly, the project will examine the relation between the measured turbulence and that parameterised in a meteorological model. There are a number of ways of doing this – e.g. direct comparisons for particular case studies, collation of statistics for particular kinds of event using archived model fields, or a mixture of the two. In this way the results of the project will inform the development of improved turbulent parameterisations.