EFFECTS OF TEMPERATURE-DEPENDENT VISCOSITY ON THERMAL-CONVECTION IN ASPHERICAL-SHELL

A numerical investigation is presented of the effects of a strongly te mperature-dependent viscosity on the radial structure, horizontal plan form, and heat transfer characteristics of a thermally convecting, inf inite Prandtl number, Boussinesq fluid in a spherical shell. Basal and mixed-mode heating are considered wherein viscosity varies with tempe rature according to a linearized Arrhenius theology. Three-dimensional , fully nonlinear calculations are performed and compared to the predi ctions of linear theory. For basal heating, three convective regimes ( mobile lid, sluggish lid, and stagnant lid) are observed in accord wit h theoretical and numerical studies in cartesian geometry. Solutions s tarred with similar initial conditions and comparable convective vigor result in distinct horizontal planforms which correspond to these thr ee regimes. In the mobile-lid regime, planforms with moderate horizont al length scales are characterized by linear downwelling sheets and qu asi-cylindrical upwelling plumes. Sluggish-lid flow assumes a much lon ger wavelength with linear bands of upwelling plumes and large cylindr ical downflows. Stagnant-lid flow is characterized by very small-scale flow patterns with upwelling plumes embedded in a network of cold, st iff fluid. Mobile-lid and sluggish-lid regimes are also observed in th e mixed-mode heating calculations with changes in horizontal length sc ales similar to those in the basally-heated cases.