Analysis On Characteristics Of Atmospheric Boundary Layer During Autumn And Winter Fog Process In Tianjin City

The fog,as a kind of severe weather bring great influence on city transportation,human health and economic and social activities.At the same time the change of underlying surface characteristics of the urbanization also affects the appear and disappear of the fog and its evolution.So it has special important research value and practical significance to give early warning forecast of the fog and the city disaster prevention and mitigation,if we can know the appear and disappear of the fog and its evolution comprehensively and systematically.By using the conventional surface observation data of Tianjin climate observation station from 1953 to 2015,surface hourly visibility and the observation data of 255 m meteorological tower 3 layers of ultrasonic observation data from 2010 to 2011 and Beijing sounding data and MICAPS data to make a relationship analysis between fog and temperature,relative humidity,wind.This papermake an analysis and discussion about background of 6 times large scale circulation background and analysis of characteristics of vertical profile from 2010 to 2011.Then we can get a conclusion.Since 1953,Tianjin city fog annual total days presents the downward trend,especially in recent 10 years in Tianjin city fog in total days down significantly.Tianjin city fog mainly appears in autumn and winter,accounted for 77% of all the year round;there was a significant correlation between air temperature,dew point temperature,relative humidity and the visibility of the fog,and the weak positive correlation between the wind and the visibility.When fog occurs in autumn and winter in Tianjin,relative humidity of surface layer may reach at least 80%,the wind speed of surface layer is slow?less than 2m/s?,the dominant wind is from east to southeast,the temperature is between-2⁵?,dew-point deficit is less than 3?.The fog weather situation during autumn and winter in Tianjin from 2010 to 2011 is mainly divided into three kinds: the first type of upper shallow slot and low pressure on the surface,the second type of upper air circulation straight and low pressure on the surface,the last type of upper shallow slot and high pressure on the surface.Generally speaking,before fog generated,the turbulence of surface layer is relatively stronger,the water vapor of surface layer keeps a high level,and the water vapor is accumulating constantly.The turbulence of surface layer is much weaker when fog keeping step,and it has a weak exchange between sensible heat and the latent heat flux,meanwhile the atmosphere will be neutral partial unstable stratification.Fog dispersal is mainly due to the high altitude of strong cold air injection,the momentum has a high to a low pass,surface layer turbulence exchanged stronger.It has a denotative meaning for the fog forecast if we know the change regulation between water vapor density and turbulent kinetic energy.8 to 12 hours before the fog formation,TKE in boundary layer will fall to the bottom from a high level,the drop is more than 0.9m²·s^-2,and the basic humidity should more than 6g/m³.There are two situations about increasing the humidity.A situation is if keeping increase the humidity slowly and it is more than 8g/m³,the fog will occur.At the same time if there is a humidifying rate of more than 2g·m^-3/3h and maximum value more than 12g/m³,the thick fog will occur.Another case is that a humidifying rate greater than 2.5g·m^-3/3h,and the water vapor density of at least 10g/m³,the fog will occur.The fog disappearing time is generally short.If high and low altitude TKE rises at the same time or high altitude TKE rises first and then low altitude TKE rises later,they both betoken the fog will disappear soon.