PROJECT SUMMARY REPORT

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Project Title	Rotating Stratified Flow Over Rough Orography
Name of Group Leader	Prof Julian Hunt
Home Laboratory	Department of Space & Climate Physics, University College London
E-mail address	jcrh@cpom.ucl.ac.uk
Telephone	00 44 (0) 207 678 7743

1. Project objectives (no more than 10 lines)

Hunt and colleagues showed using a shallow-layer perturbation model that fine scale features (such as coastal wind jets) are caused by Coriolis effects, stable stratification, and changes in surface conditions, such as surface roughness or elevation (e.g. a coastline, a cape, or a ridge of hills). They showed the main flow variations occur over a horizontal scale of order the Rossby deformation radius (30-300km) and at scales much smaller (1-2km) near the edge-line separating the surface change. Other rapidly changing meteorological variables are inversion layer height, deflection of the wind, wake flows, etc. Our objective was to further understand these types of flows by visualising the flow, and measure local density fluctuations, as it is peturbed by a bottom feature (e.g. a roughness strip, a cape, and a elongated barrier) under continuous stratifrication. The experiments were repeated under two-layer stratification in order to assess how applicable the shallow-layer model is to realistic flows.

2. Main achievements and difficulties encountered (no more than 20 lines)

Under continuous stratification, velocity fields of perturbed flows generated by a thin roughness strip, a thick roughness strip, a elongated barrier, and a cape, were resolved using high resolution Correlation Imaging Velocimetry (CIV). Preliminary results show distinct low-level positive and negative jets either parallel to the interface (in the case of the wide roughness strip) or in the wake (in the case of the elongated barrier, the thin roughness strip, and the cape), producing very large velocity gradients. The jets decrease over a transverse length scale of approximately the Rossby deformation radius. Upstream effects of surface resistance were confined to within a distance of order the Rossby deformation radius. There was also some evidence that the density field was perturbed either side of the bottom feature due to fluid rising/falling on the left/right hand side. These features are in accordance with the shallow-layer perturbation model of Hunt and colleagues and meteorological observations. Speed-up jets were also evident under two-layer stratification, demonstrating that the shallow-layer perturbation model is applicable to realistic flows. For flow around a cape the upstream circulation split when blocked, with some recirculating upwind and some flowing around the cape and forming a jet extending downwind and a circulation cell in the lee of the cape. The proportion of the flow recirculating upwind appeared to depend on the mean flow speed, with more circulating at higher upstream speeds, providing a potential mechanism to link large-scale circulation changes around Antarctica with enhanced warming over the western Antarctic Peninsula. Main difficulties encountered were i) reliably visualising the velocity fields under two-layer stratification, ii) mixing of the two-layer stratification interface, iii) malfunctioning conductivity probes and iv) the range in density expected being close to the accuracy of the probes.