The Mellor and Yamada second-moment turbulence-closure models (MY-74) have been widely used in atmospheric and oceanographic models. We will review the derivation of the basic equations and discuss the procedures used t establish the hierarchy of closure models. A three-deimensioanl mesorcale model HOTMAC, which is based on the MY-74 turbulence closure equations, has been appleid extensively to simulate airflows over complex terrain. Some features of HOTMAC include twoj-way nested grids and nonhydrostatic pressure computation. An attempt was made to consider building and terrain effects simultanceously in HOTMAC. Terrain effects typically extend more than 10 km, so a computational domain should be at least on the order of tens of kiolmeters. Consequently, a typicla horizontal grid spacing of 1-10 m is needed to resolve building effects in an urban area. To accomplish this goal, we nested a building area (400 mx400 m) in a large area (230 km x 266 km) to include mountains and ocean. Terrain effects were successfuly simulated in the inner grid, which was impossible when the nesting methods were not empolyed.
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