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Extended Subadiabatic Layer in Simulations of Overshooting Convection
Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Colorado, USA; Laboratory for Atmospheric and Space Physics, USA.
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Number of Authors: 8
2017 (English)In: Astrophysical Journal Letters, ISSN 2041-8205, E-ISSN 2041-8213, Vol. 845, no 2, L23Article in journal (Refereed) Published
Abstract [en]

We present numerical simulations of hydrodynamic overshooting convection in local Cartesian domains. We find that a substantial fraction of the lower part of the convection zone (CZ) is stably stratified according to the Schwarzschild criterion while the enthalpy flux is outward directed. This occurs when the heat conduction profile at the bottom of the CZ is smoothly varying, based either on a Kramers-like opacity prescription as a function of temperature and density or a static profile of a similar shape. We show that the subadiabatic layer arises due to nonlocal energy transport by buoyantly driven downflows in the upper parts of the CZ. Analysis of the force balance of the upflows and downflows confirms that convection is driven by cooling at the surface. We find that the commonly used prescription for the convective enthalpy flux being proportional to the negative entropy gradient does not hold in the stably stratified layers where the flux is positive. We demonstrate the existence of a non-gradient contribution to the enthalpy flux, which is estimated to be important throughout the convective layer. A quantitative analysis of downflows indicates a transition from a tree-like structure where smaller downdrafts merge into larger ones in the upper parts to a structure in the deeper parts where a height-independent number of strong downdrafts persist. This change of flow topology occurs when a substantial subadiabatic layer is present in the lower part of the CZ.

Place, publisher, year, edition, pages
2017. Vol. 845, no 2, L23
Keyword [en]
convection, hydrodynamics, turbulence
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:su:diva-147075DOI: 10.3847/2041-8213/aa83abISI: 000408251700003OAI: oai:DiVA.org:su-147075DiVA: diva2:1142142
Available from: 2017-09-18 Created: 2017-09-18 Last updated: 2017-09-18Bibliographically approved

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