Circum-Bohai-Sea Heavy Fog Formation's Observation,Simulation And Influence Mechanism Study

Circum-Bohai-Sea?CBS?region plays an important role in China's coastal development strategy,especially after the national policy proclamation of“Beijing-Tianjin-Hebei integration”and“the Silk Road Economic Belt and the21^st-Century Maritime Silk Road”.The CBS region has developed economy and convenient transportation,as well as the largest comprehensive port group in northern China.The land,sea and air transport networks are dense.However,frequent foggy weather often has a serious impact on economic activities in this area.At the same time,due to the influence of the specific geographical environment and the climate,the fog formation mechanism and boundary layer structure characteristics in CBS region are quite different from those in the other coastal regions in China.Therefore,it is necessary to study the formation process and physical mechanism of this region's fog.In this paper,four winter fog cases during 2006-2016,especially 17-19 December,2016,are investigated.On the one hand,the conventional meteorological observation data,buoy meteorological and hydrological observation data,satellite cloud image inversion data,sounding data,microwave radiometer inversion data,wind profiler radar inversion data,four-component radiometer and supersonic anemometer sampling data and other meteorological data like NCEP FNL?Final?data are used to describe and analyze this event.On the other hand,based on the recognizing of the key characteristics of the fog inversion stratification and water vapor transport,the formation mechanism of the fog case is studied by using the high-resolution numerical simulation results of the WRF?the Weather Research and Forecasting?model.The effects of physical factors such as water vapor transport guided by LLJ?low level jet?and the Bohai Sea underlyings are further analyzed numerically.The main results obtained are as follows:?1?Four cases of persistent CBS fog are analyzed.It is found that these fog processes have similar circulation characteristics,that is,the upper troposphere is straight westerly flow,while the lower troposphere is controlled by the back of the high pressure entering the sea.Therefore,in the early period of fog formation,warm and humid air flow conveyed obviously in the south.Taking 17-19 December,2016fog case as an example,the fog appears in the southwest branch airflow transport belt behind the moving high pressure,and ahead of a shallow trough of the westerlies that was also moving eastwards.Since the high pressure entering the sea becomes stable over waters of south Yellow Sea,with continental low pressure moving eastward continually,extrusion between the high and low-pressure systems generated a stronger horizontal baric gradient.The northeast-southwest LLJ of which the wind speed was over 16m/s at 925 hPa developed.The LLJ carrying warm and wet air was moving northward without interruption until it arrived in the northeast China,which is very favourable for the formation of heavy fog.The further analysis shows that LLJ provides a stable inversion layer about 1200 m thick and continuous vapor transport in the fog area.Due to the strong wind speed during 16-17 December,2016,the vapor flux in CBS region was high,and the growth rate of near-surface specific humidity is positively related with the intensity of LLJ.On the night of 17^th,the northwest weak cold air moved eastward and southward into CBS region,and mixed with the warm and humid air mass in the earlier period to form the frontal fog.The heavy fog first generated in Liaodong Bay,where vapor flux converged.In the process of continuous cold air intrusion,it finally formed a northeast-southwest trend fog belt.?2?Before the fog formed,the increase of vapor in CBS boundary layer can lead to the downward shortwave radiation flux decrease gradually,and the downward long wave radiation flux increase gradually.Then during the fog period,the downward long wave radiation flux increased obviously,and the near-surface atmosphere reached the radiation equilibrium.The turbulent motion is restrained by the inversion layer.Before fog formation,the turbulent kinetic energy and friction velocity has undergone a process from strong to weak,and there is a slight increase after fog generation.During the fog maintenance stage,the atmospheric stability is intermittently unstable near the surface and at the top of the high water vapor concentration region.?3?The numerical simulation further proves that,the formation of CBS heavy fog is closed related to the warm wet transport guided by the LLJ before fog generation,local radiation cooling and sea surface condition.On the one hand,humidity advection causes water vapor to be continuously transported to the CBS region.At first,relative humidity increases at the higher layers,then downward propagates to the earth.Meanwhile,the stable inversion stratification is conducive to the accumulation of water vapor in the lower boundary layer.Due to the decrease of near-surface temperature at 17^th night,the near–surface inversion merges with the thick inversion caused by advection,and gradually formed a double inversion boundary layer structure.The intensity of inversion reaches 1.3?/100 m when fog forms.Strong inversion stratification is helpful for maintenance of fog,while unstable condition is dominant under fog top.As turbulence generates,the fog develops vertically.?4?Water vapor is the most basic material condition for fog formation.The long life-cycle LLJ and its guided warm and wet advection transport are very important for fog formation and maintenance in CBS region.This is confirmed by tracing the source of water vapor in the boundary layer.Based on the WRF outputs and the Hybrid Single Particle Lagrangian Integrated Trajectory Model?HYSPLIT?,the water vapor in the lower and middle boundary layer of CBS region can be traced back southward to the LLJ formed region.The simulated water vapor flux agrees with the air mass back tracking results.The high value area of water vapor flux is located on the west coast of Bohai Sea and the area south of the Bohai Sea,so the liquid water content?LWC?in the southern part of the fog?near surface?is higher than that in the northern part.Meanwhile,due to the southerly airflow in the Liaodong Bay and the area north of the Bohai Sea is thicker than that in the southern part,the height of fog in the northern part of the fog belt is higher.?5?Sensitivity tests of changing water vapor content in the south,topography and different initial time confirm that water vapor transport is very important for CBS fog.The sensitivity experiment of removing the water vapor mixing ratio in each layer of the initial filed and lateral boundary condition south of 36°N shows that,removing south vapor can lead to decrease of the fog area in the Yellow Sea and Bohai Sea decreases significantly;The sensitivity experiment of removing Shandong terrains shows that,due to the removal of topographic disturbance,the corresponding changes of water vapor advection eventually lead to fog area increase in Shandong,decrease in the west coast of Bohai Sea and increase in Liaodong Bay and Northeast China.After removing the topography of Changbai Mountain,the location of the LLJ shifts eastward,and the fog belt moves rapidly eastward.In addition,the forcing field at different initial moments can also affect the transport of water vapor from the south to CBS region,resulting in the difference of local vapor accumulation,and then affect the fog formation.The longer the simulated time is from the fog generation time,the more information of LLJ avection,and the closer the simulated fog area is to the satellite observation.?6?Besides advection transfer of water vapor far away,the contribution of the Bohai Sea surface cannot be neglected.A series of sensitivity experiments of adjusting SST show that,reducing SST can lead to a decrease in the near-surface temperature,an increase in inversion intensity,and a more stable atmospheric boundary layer.In this case,the fog area increases so as to the liquid water content.With the increase of SST,although the surface specific humidity increases with the increase of air-sea flux exchange,the temperature also increases,leading to rising of inversion layer and the atmospheric boundary layer tends to be unstable.As a result,the fog area in the Bohai Sea decreases,but the liquid water content along the coast increases.After changing the Bohai Sea surface to urban underlying surface,the temperature and humidity characteristics of the middle and low layers of the atmospheric boundary also changed,which eventually lead to increase of liquied water content in the near layers of the Bohai Sea and its surrounding areas,and the decrease of fog height in the Bohai Sea.Based on these analyses,the accuracy of fog routine meteorological elements simulation has been improved by introducing air-sea coupling model.