We converted a mesoscale atmospheric model HOTMAC (Yamada and Bunker, 1988) to simulate explicitly airflows around buildings in a fashion similar to a CFD (computational fluid dynamics) code does.

The modified model is tentatively called HOTCFD (Higher Order Turbulence Closure for Fluid Dynamics). The model physics, including second-moment turbulence-closure equations, are almost identical to its original model HOTMAC. HOTCFD was applied to simulate airflows around a cubic, T-shaped, L-shaped, and multiple buildings. The computational domain was 200 m x 200 m x 500 m (vertical). The horizontal grid spacing was 4 m and the vertical grid spacing was 2 m from for the first 20 m above the ground and increased gradually to reach 43 m at the top of computational domain. A total of 51 x 51 x 31 grid points were used.

The previous applications were limited to over a flat terrain. We now consider terrain effects where a terrain following coordinates system was adopted. The way in solving pressure distributions has been modified to accommodate the changes associated with the coordinate transformation. As a first test, a cone shaped, small hill was placed in a computational domain. The elevation distribution was specified by a Gaussian function, where a peak height was 50 m and standard deviation was 25 m.

A three-dimensional Lagrangian puff model RAPTAD was used to visualize the modeled airflows and turbulent motion. For example, puffs released at 25 m above the ground upstream the hill were transported with little dispersion prior to impinging to the hill. Then, some puffs went over and others diverted around the hill. Turbulence was significantly larger downstream of the hill compared with those in the approaching flow. A wake was formed behind the hill in a fashion similar to those formed behind buildings.

As a second test, a short building (8 m high) was placed over a flat area downstream of a hill. Puffs were released in a wake formed in the area between the hill and the building. Some puffs moved upstream to climb up the hill and others traveled downstream over the building. The modeled airflows visualized by puffs showed complex movement due to large turbulence generated by a hill and a building.

The modified mesoscale model HOTCFD successfully simulated wind and turbulence distributions behind obstacles, which showed many characteristics similar to those observed in wind tunnel experiments. In the future, additional tests will be conducted, where a steady state boundary condition will be replaced by a time dependent boundary condition resulted from the diurnal heating and cooling of the ground by solar heating and radiation cooling.