Research interests

Atmospheric sciences

I am interested in stratospheric dynamics. The large scale meridional overturning circulation known as the Brewer-Dobson Circulation (BDC) is regulating many observed variables, such as temperature, zonal wind, or trace gas distributions.

The upwelling branch cools the tropics diabatically, creating the tropical cold point, the coldest point in the lower stratosphere. This is where water vapor enters the stratosphere, and the resulting freeze drying is responsible for the extremely dry stratosphere. In the downwelling branch, the opposite is true, and descent warms the high latitudes and polar regions to temperatures above radiative equilibrium.

Tracer transport differs from streamlines defined by the meridional circulation, as mixing along constant potential temperature can be important. The most important tracers for my interest are ozone and water vapor. Both are transported by the circulation, but also influence the circulation by acting on the radiative equilibrium temperature (ozone absorbs incoming solar radiation, water vapor is a potent greenhouse gas). Thus, there is intrinsic nonlinearity between dynamics, trace gases, and radiation.

The BDC is mainly driven by planetary scale Rossby waves, which are generated in the troposphere. If the background wind is zonal and not too strong all the way into the stratosphere, these waves can propagate and finally break high in the stratosphere. The resulting Eliassen-Palm flux allows air parcels to cross surfaces of constant angular momentum, and propagate polewards. Stationary planetary waves of sufficient scale (wave-one and two) are generated by surface topography, and the Tibetan plateau and to somewhat lower degree the Rockies are the most important generators on Earth's surface. There is therefore a north-south asymmetry in wave drive, and in the BDC.

My focus is on the triangle radiation-tracers-dynamics, and I make uses of an idealized general circulation model to investigate sensitivities and cause-and-effect relationships in the stratosphere. An idealized model allows to concentrate on a few parameters, keeping everything else constant. The result is a basic understanding of leading-order processes, which are of importance in assessing the effects of the ozone hole and its recovery, and climate change.

Plasma Physics (PhD)

For my PhD was looking at climate change from a different point of view, namely the side of the quest for technological solutions. Doing computational plasma physics, my work was part of the research in thermonuclear fusion devices, such as tokamaks and stellarators. I wrote a particle-in-cell Monte Carlo code, and coupled it to a electromagnetic wave propagation and a magnetohydrodynamic equilibrium code. With this, I was the first to be able to perform self-consistent ion cyclotron heating simulations in fully three-dimensional plasma devices. The results could directly be applied to stability experiments and the proposition of a tool for increasing fusion performance.