Geophysical and Astrophysical Fluid Dynamics

Abstract:

Abstract In this work, we study the development of the internal boundary layer (IBL) induced by a surface roughness discontinuity, where the downstream surface has a roughness length greater than that upstream. The work is carried out in the EnFlo meteorological wind tunnel, at the University of Surrey, in both thermally neutral and stable cases with varying degrees of stability. For the neutrally-stratified boundary layer, the IBL development in the log-law region shows good agreement with the diffusion model proposed by Panofsky and Dutton (Atmospheric turbulence, Wiley, New York, 1984) provided that a modified origin condition is introduced and its growth rate is dictated by a constant diffusion term. However, the model over-predicts the growth of the IBL in the outer layer, where the IBL depth grows slowly with fetch following a power function with exponent n being 0.61 (whereas the original model prescribes $$n\approx 0.8$$ n ≈ 0.8 ). For the stably-stratified boundary layers, n is found to further reduce as the bulk Richardson number, $$\textrm{Ri}_\textrm{b}$$ Ri b , increases. The analysis of the top region of the IBL shows that the slow growth rate is due to a combination of the decay of the diffusion term and a significantly negative mean wall-normal velocity, which transports fluid elements towards the wall. Considering these two effects, a modified diffusion model is proposed which well captures the growth of the IBL for both neutrally and stably-stratified boundary layers. Graphical abstract

Abstract:

Pattern-forming with externally imposed symmetry is ubiquitous in nature but little studied. We present experimental studies of pattern formation and selection by spatial periodic forcing in rapidly rotating convection. When periodic topographic structures are constructed on the heated boundary, they modulate the local temperature and velocity fields. Symmetric convection patterns in the form of regular vortex lattices are observed near the onset of convection, when the periodicity of the external forcing is set close to the intrinsic vortex spacing. We show that the new patterns arise as a dynamical process of imperfect bifurcation which is well described by a Ginzburg–Landau-like model. We explore the phase diagram of buoyancy strength and periodicity of external forcing to find the optimal experimental settings for which the vortex patterns best match that of the external forcing.

Abstract:

Recent Juno microwave observations revealed some puzzling features of the ammonia distribution. In particular, an ammonia-poor layer extends down to levels of tens of bars in Jupiter outside the equatorial region to at least ±40^° [Li et al. 2017]. Such a depletion has not yet emerged in general circulation models (GCMs). Guillot et al. [2020] showed that ammonia vapour can dissolve in water ice within violent storms, forming ammonia-rich hail, or "mushballs", that leads to an efficient transport of ammonia to the deeper atmosphere and hence its observed depletion. However, this mechanism has not been tested in numerical simulations in which convective events are self-consistently determined. 

We present a simple parameterization scheme for the mushball process. Our scheme determines the mushball concentration using the water-ammonia equilibrium phase diagram, and considers the transport of water and ammonia due to its associated downdraft. We implemented this scheme to a GCM based on the MITgcm [Young et al. 2019] that includes the following key parameterizations: a water moist convection scheme, a simple cloud microphysics model for water and ammonia, a dry convection scheme, and a two-stream radiative transfer scheme. We present our preliminary results using water and ammonia abundance according to Juno observations. Further, we discuss the ability of the "mushball" scheme to reproduce the Juno observations and explore which parameters are the most important to understand the ammonia distribution in the deep layers of Jupiter.