Study Of Formation And Development Mechanisms Of Sea Fog: Observational Analysis And Numerical Modeling

In this thesis, the formation and development mechanisms of a land radiation fog oc-curred around Cabauw, Netherland on 3 August 1977, two sea fog cases formed over the Yel-low Sea on 11 April 2004 and the East China Sea on 08 March 2005, respectively, are inves-tigated by using almost all available observational data and Regional Atmospheric Modeling System (RAMS) high-resolution modeling results. The modeling of Cabauw land radiation fog shows that the RAMS model has the considerable ability to document the fog formation and development processes well. After examining two sea fog cases observationally and nu-merically, it is indicated that they are quite different cases. Advection, turbulence, radiation and Sea Surface Temperature (SST) played different roles in their formation and development processes.The Cabauw land fog formed near the land surface mainly due to the long-wave radia-tion cooling at night time, which led the air temperature decrease below to dew-point and the air was condensed into fog. The fog developed gradually because of the turbulence effect. The fog near ground was warmed up gradually by the radiation heating from both fog top and ground surface after the sunrise. Warming effect around fog base destroyed the temperature inversion layer and made the buoyancy favorable for the turbulence development, which makes the Turbulent Kinetic Energy (TKE) grow rapidly and the fog top rise consequently. With the cloud water reduction caused by fog top rising and short wave radiation enhancing, the radiation cooling around fog top dismissed gradually, and the temperature inversion layer near the fog top could not be maintained, which caused the fog to disappear after several hours.An extremely dense fog event over the Yellow Sea on 11 April 2004 is investigated ob-servationally and numerically. Observations showed that the almost all area of the Yellow Sea was occupied by fog on 11 April 2004, and the lower level air temperature was higher than SST prior to the sea fog formation. By examining RAMS modeling results, the fog formation mechanism can be drawn as follows. The turbulence near the sea surface was strong enough to enhance the heat exchange between the warmer moisture air and underlying colder sea sur-face, leading the sea fog formation at lower-level. Due to the long wave radiation cooling ef-fect, the fog enhanced and the air temperature fell down below the SST, which makes the evaporation occurs in the fog area. Long wave radiation cools down the fog layer to make it more stable and restrict the of turbulence development. The maximum cloud water center maintains at the fog bottom. Based on the modeling result, the thermal-dynamical budget analysis indicates that the long-wave radiation cooling plays the most important role in the maintaining temperature inversion layer during the development stage. The buoyancy makes against the development of TKE in the fog layer, and the vertical wind shear produces turbu-lence around the fog skirt area. A series of SST experiments indicated that increasing 1oC of SST may enlarge the fog area, and decreasing 1oC of SST may reduce the fog area, respec-tively. The fog over the southern Yellow Sea disappears when turns off the long wave radia-tion emitted by liquid water. The experiments of model micro-physics indicated that the hori-zontal distribution and the sea fog shape in the modeling did not have significant change with the change of cloud-droplet number in model. The experiments of model horizontal resolution showed that the fog formation and development processes were not sensitive to the minor change of horizontal resolution from 8 km to 4 kmDifferent from the sea fog case on 11 April 2004, another sea fog occurred over the Yel-low and East China Seas on 08 March 2005. The fog formed over the East China Sea initially and moved over the Yellow Sea with the synoptic system movement. Based upon the RAMS modeling results, the detailed formation process of the fog can be described as follows. The sea fog occurred because the colder sea surface may cool the air above with much stronger turbulence. The long–wave radiation cooling made the sea fog more denser, and the cloud water center with high value formed near the fog top. In the fog area, the air temperature is higher than SST, because the maximum cloud water formed near fog top and the radiative cooling is not strong enough to cool the air in the fog bottom. The SST experiments indicated that increasing 1oC of SST may make the fog top higher, and decreasing 1oC of SST may make the fog top lower, respectively. The radiation experiments shows that the long wave ra-diation emitted by liquid water plays an important role in the sea fog enhancing.Turbulence, radiation, advection and SST are important factors for the sea fog formation and development. Different characteristics of these two fog cases are mainly due to the rela-tive contributions of these factors. Comparing the two sea fog events, it is found that in the former sea fog case, the TKE is much weaker, the fog top was lower, the area contains higher cloud water was locates near the sea surface, and the air temperature was lower than SST. In the later sea fog case, the TKE is much stronger, the fog top was much higher, the area of higher cloud water was near the fog top, and the air temperature was higher than SST. It is concluded that the formation and development mechanisms of sea fog are so complicated, and much deeper understanding of more fog cases is necessary.