Frictional effects on cyclones

I am interested in the action of parameterised physical processes within numerical weather prediction models, especially their effects on the development of mid-latitude cyclones. A very useful variable to look at in this regard is the potential vorticity (PV). To within the accuracy of some large-scale balance condition, the PV essentially encodes all information about the large-scale flow. Moreover, it is conserved by advection, but may be generated and/or redistributed by parameterised processes. Therefore, if we can figure out how some particular process has modified the PV field then we also know how this process has affected the large-scale dynamics. I've developed a method for determining these modifications as diagnostics from the Met Office unified model. The method is a useful tool for studying the effect of friction on cyclone dynamics.

An interesting result to emerge recently from several groups (eg, Cooper et al, Stoelinga) is that boundary-layer friction can generate positive anomalies of PV, due to baroclinic effects. This suggests that the spin-down of cyclones may be a more complicated process than just the traditional mechanism found in the textbooks - the barotropic spin-down of a cyclonic vortex due to Ekman pumping in the boundary layer (it is straightforward to show that this barotropic mechanism destroys PV).

Through simulations of baroclinic waves we have found a baroclinic frictional mechanism which can inhibit cyclone development. Positive, baroclinically-generated PV is formed primarily near the cold front and is carried along the front and upwards through the boundary layer by a warm conveyor belt (WCB) flow. The WCB may terminate with a W2 flow, transporting the generated PV towards the low centre and forming a positive anomaly just above the boundary layer. It turns out (partly due to the Ekman circulation) that the positive anomaly formed is thin and so is associated with an anomaly of static stability. The increased stability immediately above the boundary layer reduces the coupling between upper and lower levels, thereby restricting the growth of the system.

The same mechanism has also been observed within "real" systems (ie, those simulated from real initial conditions with a full forecast model). However, the PV structures in the boundary layer are more complex in these cases, due to the action of moist processes.

Some links for this work:

Collaborators:

Ian Boutle also has some information about this research on his website, including some animations and some of his internal presentations and reports that are not directly available below.

Publications:

1. A paper on the interactions between the Ekman pumping and baroclinic mechanisms
2. A note on the Ekman pumping and baroclinic damping mechanisms (Explains the differences between Adamson et al (2006) and Beare (2007).)
3. A paper on boundary layer controls of baroclinic waves
4. A paper on frictional damping of baroclinic waves

Talks:

1. Abstract for a talk presented by Stephen Belcher at the AMS 18th Symposium on Boundary Layers and Turbulence, Stockholm in June 2008.
2. A talk presented by Ian Boutle on his first year PhD work in this area, at the RMetS Student Assembly 2007 in Edinburgh.
3. Abstract for a talk presented by Stephen Belcher at the AMS 15th Conference on Air-Sea Interaction, Portland in August 2007.
4. An internal group seminar on frictional damping of baroclinic waves
5. A talk on the baroclinic mechanism in idealized simulations presented by Stephen Belcher as a DAMTP seminar in May 2006.
6. A talk on the PV (and other Lagrangian) diagnostics, presented at a UWERN mesoscale modelling workshop in March 2006.
7. A talk on the baroclinic mechanism in idealized simulations, presented by Stephen Belcher as a Met Office seminar in August 2005.
8. A talk on boundary layer PV, presented at the 2003 EGS-AGU-EUG conference.
9. A departmental seminar on PV within the boundary layer (Also available as a powerpoint, with animations included.)
10. A talk on the PV diagnostics, presented at the 2002 UWERN meeting

Posters:

1. A poster on boundary layer processes in mid-latitude cyclones, presented by Ian Boutle at the Les Houches summer school on Atmospheric Boundary Layers: Concepts, Observations, and Numerical Simulations in June 2008.
2. A poster on boundary layer processes in mid-latitude cyclones, presented by Ian Boutle at the Royal Met Soc National Meeting on Extratropical Cyclones in January 2008.
3. A poster on the effects of friction on cyclones, presented at the 2002 EGS conference