Evolution characteristics of the macro-/micro- structure and the boundary layer during a spring heavy sea fog episode in Donghai Island in Zhanjiang

A comprehensive sea fog experiment was carried out in Donghai Island in Zhanjiang from February to March of 2011, with observational instruments including fog droplet spectrometers, visibility meters, wind profilers equipped with RASS, and the 100 m height observational tower. A 15 h heavy sea fog case during 23—24 February 2011 was chosen to understand the evolution of meteorological elements including temperature, humidity and wind fields, as well as dynamic and thermodynamic structures of the boundary layer. Furthermore, the characteristics of microphysical processes in relation to burst reinforcement features and turbulent fluxes exchange were analyzed. The main results are as follow: The observed sea fog occurred when the warm and moist southeast airflows from the South China Sea met the cold water zones near the coast of Guangdong Province. The southeast airflows supplied abundant water vapor and built stable temperature inversion layers for preparation, formation and mature of the heavy fog, with a close relationship between temperature inversion intensity and southeast airflows strength. The fog mostly occurred below 270 m. The approximate isothermal and weak temperature inversion layers existed in the heights from 150 m to 390 m due to an obvious downward airflows from 630 m to 870 m height layers, which prevented the lower layer (below 270 m height) water vapor from mixing with the upper layer (above 390 m height) dry and cool air masses, resulting in a stable and high humidity situation for the fog layer. The average values of fog droplets number concentration (N), liquid water content (W), average diameter (Dave), maximum diameter (Dmax) and effective radius (Reff) were 248 cm-3, 0.102 g/cm3, 5.2 μm, 36.0 μm and 7.0 μm, respectively. The fog droplets number concentration (N) was positively correlated with average diameter (Dave) during the fog initial (i.e. the formation and development phases) and ending (i.e. the dissipation phase) stages, and, besides, a negative correlation mostly occurred in the mature stage between them. The period of 4 h before the heavy fog emerged, when the stable atmospheric structure occurred and sufficient water vapor supplied from the southeast airflows, was regarded as the preparation period of fog burst reinforcement. The fog droplet spectrum boarding was undergone in three stages: active process, stable process and intensive process, with turbulent intensity having little influence on fog burst reinforcement. The rapid dissipation of fog was likely the consequences of joint contributions of the three factors including fog droplet evaporation, gravitational and turbulent collision-coalescence processes, with the distinct decrease in quantities of fog droplets more than 21 μm acted as the important cause of rapid dissipation. The turbulent intensity decreased gradually prior to the fog events, and the turbulent exchange remained weak after the fog formation. The turbulent intensity significantly enhanced when the southeast jets induced wind shear within the near surface layer, resulting in a strong upward sensible heat flux, which was similar to heat exchanges of the upper layers during the maintenance stage of the cold sea fog. The turbulent intensity turned stronger while the fog fell into a decline stage. The markedly rising mean kinetic energy for twice before and during the fog episode respectively were connected with the southeast airflows enhancement. In the maturity phase, fog-topped radiation cooling produces thermal turbulence and wind shear-produced mechanical turbulence were thought to be the possible causes for the enhancement of turbulent kinetic energy.