SIMULATION OF INTEGRAL CONVECTION CHARACTERISTICS BY TWO- AND THREE-DIMENSIONAL MODELS

L. I. Alekseeva, N. F. Vel'tishchev, and T. A. Myasnikova

Representativeness of two-dimensional convection models is studied to obtain data on vertical convective heat and momentum fluxes. A number of numerical experiments with two- and three-dimensional versions of the same convection model is performed for the following external parameter values: 5 ∙ 103 ≤ Ra ≤ 3 ∙ 104, 0 < R ≤ 100 (Ra is the Rayleigh number and R is the Reynolds number). Based on the analysis of modeling results, the following conclusions are made. In the absence of wind, heat fluxes in the two-dimensional model differ slightly from three-dimensional model fluxes. Therefore, two-dimensional models can be used to compute convective flows and heat fluxes without a loss of quality. In the case of a shear flow, two- and three-dimensional model computations differ substantially from each other and the difference grows with increasing R. At R = 100, heat fluxes in the two-dimensional model are approximately 1.5 times smaller than those in the three-dimensional model. Momentum fluxes differ not only quantitatively but qualitatively as well. Convective momentum fluxes in the three-dimensional model have the same sign as background fluxes, i.e., convection amplifies the vertical momentum flux. In the two-dimensional model, the convective momentum flux has the sign opposite to that in the background state, i.e., convection weakens the background vertical momentum flux. Results of three-dimensional model computations satisfy the conditions of conservation of total heat and momentum fluxes required in the problem solved with an accuracy of a few percent. These conditions are not fulfilled in the two-dimensional model at R >> 0. Based on the above, a conclusion is made that two-dimensional atmo¬spheric convection models fail give reliable information on the flow structure and on heat and momentum fluxes in the presence of the airflow with a strong vertical wind shear.