COAMPS™ fog forcast during 2003 CBLAST-Low

The prediction of fog presents a particular challenge to the mesoscale community since its formation and dissipation are complex processes closely related to the large-scale weather pattern, local scale turbulence mixing, cloud microphysics and the radiation. This work focuses on a fog episode observed during Coupled Boundary Layers /Air-Sea Transfer in Low Winds (CBLAST-Low) field experiment in Summer of 2003, during which the Navy’s COAMPS has been used to provide real-time forecast. Our objective is to evaluate the fog forecast of COAMPS with respect to this particular episode and understand the interaction among the different physical processes as mentioned previously. The fog episode occurred on the 22nd of August, 2003 and lasted for whole day. The observations from Rawinsonde and the Air-Sea Interaction Tower (located near the shore of Martha Vineyard Island) show that the marine boundary layer is stable with relatively strong low-level wind shear. Although COAMPS forecast provided the similar boundary layer feature, it did not predict any formation of fog (or low-level clouds). The model simulated constant southwest flow to provide moist and warm advection to the fog area. Our analysis reveals that this moist and warm air supply is essential to maintain a moist and stable boundary layer. In addition, our simple one-dimensional model study shows that a relatively low sea surface temperature is critical in providing the cooling necessary for the fog formation. However, the sea surface temperature used in COAMPS is higher than those observed at the buoy and ASIT, indicating that the absence of fog in the COAMPS forecast may be partly due to the error in the SST field. One important feature of the surface turbulence observation is the negative sensible and latent heat fluxes, apparently due to the condensation in the stable surface layer. The negative surface latent heat flux implies that the moisture supply has to come from the horizontal advection in order to maintain necessary condition for long lasting fog. Therefore, the turbulence mixing is likely to be balanced by the horizontal advection process.