Research Interests

• Climate modelling
• Stratospheric dynamics
• Stratosphere-troposphere coupling
• Vortex dynamics
• Geophysical turbulence
• Other

Climate modelling

• General circulation modelling


The existence of relevant, easily computed benchmark solutions is essential for the continuing development of new, efficient dynamical cores of atmospheric general circulation models (CGMs). To be useful, these should involve dynamically interesting flows typical of those encountered in the atmosphere, and yet have the property that rigorous, model-independent numerical convergence can be easily demonstrated. See here for some recently developed initial value problem test cases and a discussion of the role of explicit diffusion.


The BOB pseudo-spectral GCM dynamical core has been developed to allow fast, efficient computation at high resolution. The latest versions of the shallow water and primitive equation models are available, together with model documentation, from the Computational Science section of NCAR's Scientific Computing division here.


• Effects of numerical resolution


Steep gradients of potential vorticity are known to arise in typical atmospheric flows as a result of mixing and erosion by wave breaking. Examples include the stratospheric polar vortex edge, the subtropical edge of the surf zone, and the tropopause. These gradients control Rossby wave propagation throughout the atmosphere, and yet are poorly resolved in climate models. Important questions include: What is the minimum horizontal resolution needed for an accurate representation of these gradients and the wave motions they support? In what sense is numerical convergence possible?

Stratospheric Dynamics

• Fundamentals of Rossby wave propagation


The traditional framework of linear Rossby wave propagation (e.g. Charney & Drazin, 1961) considers disturbances to a smooth background state, the simplest of which is a uniform potential vorticity distribution associated with uniform zonal velocity and a background planetary beta effect. However, observations of the stratospheric polar vortex edge, the subtropical edge of the surf zone, and the midlatitude tropopause all indicate that typical atmospheric potential vorticity distributions are more step-like than smooth. A more suitable framework for studying linear wave propagation is therefore that of edge-waves on a PV discontinuity. Such an approximation is not only more appropriate but also allows important simplifications (e.g. a reduction in dimensionality) and provides insight into wave propagation. A system of diagnostics based on this edge wave approximation is being developed that is expected to aid the interpretation of both modelling and observations.


• Internal stratospheric variability


Coherent modes of internal variability have been known for some time to exist in low order and low resolution models of the stratosphere. As with other dynamical systems, however, it has not been clear whether such coherent modes are true modes of the system or merely a result of excessive numerical truncation. Recent high resolution modelling indicates that coherent modes of internal variability do persist in stratosphere only models. Given the likely dynamical coupling of the stratosphere and troposphere, a better understanding of internal stratospheric variability is of potential importance to medium range tropospheric weather forecasting.

On longer timescales the stratospheric circulation can have an influence on details of the tropospheric climate. A better understanding of seasonal and interannual variability of the stratospheric polar vortex is therefore a subject of ongoing research.

Another area of importance to the thermal structure and chemical composition is the theory of downward control and the wave driven mean meridional circulation. Current high resolution modelling is examining the sensitivity of this circulation to the underresolution of potential vorticity gradients in current climate models.

Stratosphere Troposphere Coupling

• Upper troposphere/lower stratosphere

Vortex Dynamics

• Stability of ellipsiodal vortices


• Atmospheric vorticies

Atmospheric Turbulence

Other

• Tropical Meteorology


• Other