NUMERICAL SIMULATION OF CONVECTIVE SCALING
Abstract
The purpose of the research was to study convective scaling in the atmosphere using a two-dimensional, nonlinear spectral model, and to investigate the difference between the aspect ratios for mesoscale cellular convection (MCC) and laboratory cells. To investigate this difference, previous workers have pursued one of two approaches. Firstly, physical processes such as latent heat release or eddy anisotropy may act to increase the aspect ratio of the convection from cumulus scale to mesoscale (i.e. MCC). The alternate approach, adopted here, suggests that there is a mesoscale organization of small-scale convective elements into a system with a collectively broad horizontal scale, consistent with radar observations of convective organization in clear air and data collected by aircraft within MCC. Such a process is likely attributable to the nonlinearity inherent in the shallow convection problem. Two new features of the model were a large domain aspect ratio (L/d = 28.28) and many degrees of freedom. No extra physics were included. The Rayleigh number Ra was varied over the range 2 (LESSTHEQ) Ra/Ra(,c) (LESSTHEQ) 800. Beyond Ra/Ra(,c) = 4 the model predicted the cell wavelength (lamda) to increase with Ra, in accord with laboratory observations but contrary to theory and previous two-dimensional model results. The largest wavelengths ((lamda)(TURNEQ)9.43 at Ra/Ra(,c) = 600, 800) were at the lower limit of those observed for MCC and dimensionally within the range of observed diameters. The convective development proceeded, first, as a series of rapid mergers then, secondly, as a series of slower mergers between adjacent cells. The convective re-organization was reflected by characteristic changes in heat flux (Nu). Steady-state velocity and temperature fields showed similarities to laminar and turbulent laboratory convection, due to the similarity between the governing equations even though free boundaries were used. For Ra/Ra(,c) (LESSTHEQ) 175 Nu varied proportionally to Ra('0.338). Steady-state kinetic energy and available potential energy exhibited roughly linear behavior as functions of Ra, the latter reflecting the formation of the thermal boundary layers. Transitions in the slope of the heat flux H (= RaNu) curve were diagnosed at Ra/Ra(,c) = 4.8, 13.6, 41.5 and 95.2, corresponding to experimental findings.
Degree
Ph.D.
Subject Area
Atmospheric sciences
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