Methods of substructuring in lake circulation dynamics

A semi-implicit semi-spectral hydrodynamic primitive equation model is used in combination with a substructuring technique to study wind-induced motions and tracer di€usion in the homogeneous and strati®ed Lake Constance. An impulsively applied spatially uniform wind is applied in the long direction of the lake (305° from True North) lasting inde®nitely. Tracer mass is released at various locations of the free surface over a ®nite area for 24 h starting together with the imposed wind. We compute for a given wind and tracer scenario the ̄ow and concentration ®elds for the entire lake using a coarse grid resolution. Within three di€erent subregions, where improvements of the obtained results are sought, computations are repeated by using a ®ner grid and employing the results of the global computations at the open boundaries of the subregion. We demonstrate that this substructuring technique can be used: (i) to improve results where subgrid processes are signi®cant as e.g., near shore, at early times in the neighbourhood of tracer sources and in complex geometric areas (bays); (ii) to better resolve and graphically display velocity and tracer concentration distributions on larger scales. We employ the technique for the homogeneous and the strati®ed Lake Constance. The technique is seen to be an economically ecient procedure in improving computational results when implementation of ®ne resolutions is not feasible. Ó 2000 Elsevier Science Ltd. All rights reserved. AMS: 65M06; 65M12; 65M70; 76B15; 86A05