Numerical simulation of fog using non-hydrostatic NWP model with third-order turbulence closure

Numerical simulation of a fog episode is conducted using the Hong Kong Observatory’s Non-Hydrostatic Model (NHM) to study various physical processes in the formation of a shallow saturated layer. It is found that a third-order turbulence closure model and partial condensation scheme implemented in NHM can improve the parametrization of turbulent mixing and condensation processes. Physically, the formation of fog or low-level clouds over the coastal waters is primarily due to advection of warm and humid maritime air mass over a cooler sea surface. The numerical experiments show that the radiative cooling above the low-level cloud top further enhances the mixing process during nighttime and results in condensation of cloud liquid water near the surface. Differences in the forecast low-level clouds, wind and moisture fields within the boundary layer using the original first-order turbulence closure scheme will also be illustrated.