An experimental study on an instability of a baroclinic flow caused by a surface cooling in a rotating fluid

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An axisymmetric cool island circulation that is generated when a circular region of the bottom of a semi-infinite fluid is cooled in a rotating frame and associated baroclinic instability waves are studied by means of a laboratory experiment on the large turntable of the Meteorological Research Institute, a theoretical consideration and a numerical experiment.In the laboratory experiment, a liquid crystal sheet at the bottom of the water tank is used to visualize the temperature distribution near the bottom and PIV(Particle Image Velocimeter) is used to measure the horizontal distributions of the horizontal velocity. The following results are found : When the basic rotation is either absent or small, the cool island circulation remains axisymmetric. When it becomes larger than a critical value, disturbances with circumferential wavenumber 2-3 appear. As the basic rotation is increased further, the wavenumber of the disturbances increase and the flow field becomes chaotic. It is also found that lycopodium is the most suitable tracer particle for PIV measurement.Additional theoretical considerations reveal that the cool island circulation has three different regimes depending on the non-dimensional angular velocity and non-dimensional stratification : Diffusion, gravity current and gravity wave regimes. The only non-linear regime is the gravity current regime, and is always unstable for a slab-symmetric case. For an axisymmetric case, however, the flow is stable when the basic rotation is small and the radius of deformation is larger than the diameter of the cool island.