Observational Analysis Of Sea Fog And Its Boundary Layer Structure On The Coast Of Southern China

Some degree of difference exists in the formation mechanism and the structure of boundary layer of sea fog at different regions, because of the difference of the regional geography and climate. By using the data of the sea fog during winter and spring season in2007-2012obtained at the Science Experiment Base for Marine Meteorology (SEBMM) at Bohe, Maoming, Guangdong Province (the SEBMM is established by the Guangzhou Institute of Tropical and Marine and Meteorology (ITMM) of the China Meteorological Administration) and other meteorological data, This study is focused on two main points:one is the analysis of the boundary layer structure and air-sea interface exchange characteristics of the sea fog on the coast of southern China, the other is using a factor of temperature difference of surface layer to improve the regional prediction of sea fog on Guangdong coastland, after realized the key feature of the inversion and isothermal phenomenon at surface layer in the sea fog.The major contents and conclusions are present as follows:(1) The sea fog on the coast of southern China mainly occurred during January to April, the peak value is in Mar. sea fog usually occurred between two intrusions of cold air. Main relation to weather map types are the transformed cold high moved to the sea, the cold front and the southwest low. The background of the synoptic system of the region of the sea fog formation has some similar feature. When sea fog occurred, marine warm and wet air flow transported by southeast wind is prevailing at the low level on the coast of southern China. On the contrary, when the sea fog dissipated, cold and dry air flow transported by northeast wind is dominating at the low level.(2) According to the stability definition of saturated wet air, two properties of atmospheric stratification, potential stability and potential instability, have been revealed in the saturated wet air of sea fog. Consequently, the gradient of the potential pseudo-equivalent temperature was equal to zero ((?)θse/(?)z=0) and was defined as the thermal turbulence interface (The height of the thermal turbulence interface was observed between180m and380m). Two layers of turbulence were shown to exist in the sea fog. The thermal turbulence produced by the long-wave radiation was prevalent above the thermal turbulence interface, and the mechanical turbulence aroused by the wind shear was predominant below the interface. Formation, development, and dissipation are the three possible phases of the evolution of the boundary layer structure during sea fog events on the coast of southern China. The thermal turbulence interface was shown to play an important role in these phases. The interface is significant during the formation and the development of sea fog. Also, after sea fog has been elevated into the stratus layer, this interface is the main factor that maintains the layer.(3) On the coast of southern China, the emergence of sea fog was always accompanied by a low cloud layer. Furthermore, sometimes the formation of this low cloud layer was a result of the layered division of the sea fog into two parts. In addition, a close relationship exists between the tops of the fog and the clouds and the inversion layer:The inversion layer (or the isothermal layer) usually corresponds to the tops of the fog and the clouds. Three important factors have a close relationship with the development of the sea fog top on the coast of southern China:the horizontal advection of the water vapor, the long-wave radiation of the fog top, and the vertical turbulence mixing. As a result of long-wave radiation and the ample transportation of water vapor, fog developed in the upward direction. However, if the elevation of the fog top increased too quickly or if the transportation of water vapor decreased, the fog layer was divided by the turbulence and entrainment, resulting in a decrease in the height of the fog top.(4) Air-sea interface exchange characteristics have been analyzed. New results have been investigated from the air-sea temperature difference, the sea surface radiation and the turbulent flux. In2011, three of four process of sea fog appear the phenomenon of the surface air temperature (SAT) is lower than the sea surface temperature (SST). When the SAT is higher than the SST, the downward long-wave radiation (DLR) is higher than the upward long-wave radiation (ULR), the air-sea temperature difference is larger and the phenomenon is more manifest. Conversely, while the SAT is lower than the SST, the DLR is lower than the ULR, too. According to the flux data, momentum flux remains a low value in the sea fog. The sensible flux is mainly the negative value within an order of10W·m-2, which represents a heat transport from the atmosphere to the sea and coincided with the SAT higher than the SST. While the SAT is lower than the SST because of the continuously descent of the SAT, Sensible flux turn into the positive value, which indicates a heat transport from the sea to the atmosphere. The latent flux has a negative value in the mist, while no change rule has been found during the sea fog because of the irregular data. (5)The reason of the SAT is lower than the SST has been explained. The main reason is the mixing mechanism at the sea surface atmospheric layer:the mixing between the northeasterly cold-wet air and the warm-wet air come from the upper level transported by the mechanical turbulence. According to the statistic data of the sea fog at the SEBMM, the cases with the SAT higher than the SST is69%, while the possible cases with the SAT lower than the SST is28%.(6) There is an explicit congruent relationship between the factor of temperature difference of surface layer and the occurrence of sea fog:while sea fog exists, the temperature difference of surface layer has always been a high value during the nearby period. The value of the factor acts as a diminished of the negative value or turn to positive value, which means the SAT is slightly less than or higher than the2m-tempreture above sea surface. This indicates the warm-wet air advection is enhanced and suitable for sea fog to develop. On the contrary, the increasing negative value represents the affect of the cold air advection and result in the dissipation of the sea fog. GRAPES model proved to be good at simulate the factor. Consequently, the accuracy, threat score and Heidke skill score of the sea fog regional prediction are all promoted after using it, which prove the factor is effective and important.