AM Seminar: On the Global-mean Vertical Tracer Mixing in Planetary Atmospheres

Speaker Name: 
Xi Zhang
Speaker Title: 
Assistant Professor
Speaker Organization: 
University of California, Santa Cruz
Start Time: 
Monday, January 14, 2019 - 4:00pm
End Time: 
Monday, January 14, 2019 - 5:00pm
Location: 
Baskin Engineering 372
Organizer: 
Abhishek Halder

Abstract:

Most numerical models of chemistry and cloud formation on planetary atmospheres adopt a one-dimensional (1D) chemical-diffusion approach to approximate the global-mean vertical tracer transport. The physical underpinning of the key parameter in this framework, eddy diffusivity, is usually obscure. In this presentation I will present a theory to formulate the 1D effective eddy diffusivity for a 3D stratified atmosphere. I will show that the eddy diffusivity depends on the large-scale circulation strength, horizontal mixing due to eddies and waves and local tracer sources and sinks. There are three regimes of diffusivity in planetary atmospheres. In the first regime where the tracer lifetime is short compared with the transport timescale and horizontal tracer distribution under chemical or microphysical equilibrium is uniformly distributed across the globe, global-mean vertical tracer mixing behaves diffusively. But the traditional assumption in current 1D models that all chemical species are transported via the same eddy diffusivity generally breaks down. Different chemical species in a single atmosphere should in principle have different eddy diffusion profiles. In the second regime where tracer is short-lived but horizontal tracer distribution under chemical equilibrium is non-uniformly distributed, a significant non-diffusive component might lead to a negative diffusivity under the diffusive assumption. In the third regime where the tracer is long-lived, global-mean vertical tracer transport is also largely influenced by non-diffusive effects. I will show numerical simulations of 2D tracer transport on fast-rotating zonally symmetric planets (e.g., Jupiter) and 3D tracer transport on tidally locked exoplanets with a strong day-night contrast (e.g., hot Jupiters). The simulation results validate our theory of 1D effective eddy diffusivity over a wide parameter space. 

Bio:

Dr. Xi Zhang is an assistant professor in Earth & Planetary Sciences at UCSC. Xi got his Bachelor degree in space physics at Peking University in China in 2007 and PhD of planetary science at California Institute of Technology in 2013. Prior joining UCSC in 2015, he was the Bisgrove Postdoctoral Fellow at University of Arizona. Xi’s research expertise is planetary climate. He studies the dynamics, chemistry, cloud formation, radiative transfer and evolution of atmospheres on planets and brown dwarfs. At UCSC, Xi teaches an introductory course “planetary discovery” and an upper-division class “planetary atmospheres”. Additional information can be found at his website: https://websites.pmc.ucsc.edu/~xiz/