Exciting standing Rossby waves in the large rotating annulus

Abstract

Laboratory experiments with rotating tanks remain the premier physical analogue for atmospheric dynamics. Often, the equipment involved is engineered to be sufficiently versatile and modular so as to be able to accommodate experiments that explore a wide range of atmospheric processes. The exercise of initially configuring the apparatus then involves running experiments that sweep through parameter space to identify the specific dynamical regimes of interest. This process is typically considered part of the development and testing phase of a given project, and these initial experiments are generally left unreported; while many of these test experiments may not be directly relevant to the project at hand, they may be useful for other applications or scientific communities. Here we report on a series of 93 different laboratory experiments run with the intention of identifying suitable experimental configurations that excite and sustain standing Rossby waves (i.e. Rossby waves that are stationary with respect to topography). We use the Large Rotating Annulus at the Australian National University; this unique apparatus has independent control over the annulus sidewall temperatures and background rotation rate, as well as the rotation rate of a differentially-rotated topographic bump. These three conditions are systematically varied through an extensive range of experimental parameter space. Configurations are found in which standing Rossby waves are present; unsurprisingly, these are experiments with eastward zonal flow and a sufficiently strong background gradient of potential vorticity. The behaviour of these standing Rossby waves follows that predicted by established theory; for example, the wavelength of the standing waves scales with the square root of the background flow speed divided by the gradient of potential vorticity. The standing and transient variability fields are quantified, and the partitioning of variability between them depends on the existence of standing Rossby waves; the variability in experiments with and without standing Rossby waves is predominantly standing and transient, respectively. Subsequent efforts will focus on specific experiments with standing Rossby waves, with the aim to provide insight into how wave behaviour may respond to climate change. Show more