Abstract
MEMO is a fully vectorized nonhydrostatic mesoscale model using terrain-following coordinates. The numerical solution is based on second-order discretization applied on a staggered grid which is allowed to be non-equidistant in all directions. Special care is taken that conservative propeprties are preserved within the discrete model equations. The discrete pressure equation is solved with a direct elliptic solver in conjuction with a generalized conjugate gradient method. Advective terms are treated with an explicit, monotonicity-preserving discretization scheme with only small implicit diffusion. Turbulent diffusion is described using an one-equation turbulence model, while at roughness height similarity theory is applied. An efficient scheme is applied to calculate radiative transfer. The algebraic surface heat budget equation and an one dimensional heat conduction equation are solved to obtain the surface temperature over land and the soil temperature. In the frame of the APSIS exercise A the model MEMO was used to simulate the mesoscale flow in the Athens basin on May 25, 1990. Being in satisfactory agreement with observations, the results indicate that weak pressure gradients accompanied by warm advection aloft may lead to stagnant conditions and thus to severe air pollution episodes in Athens.
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