Analysis And Case Study Of Gust Front Weather Characteristics In Guanzhong Area, Shaanxi

The cold sinking airflow in the convective storm reaches the low air and spreads outward,and it merges with the warm and humid air in the low-level environment to cause strong wind.The front edge is the gust front.It is a kind of boundary convergence line,and it is the outflow boundary of thunderstorm.The occurrence of the gust front depends on the stable convective thunderstorm,interacts with the mesoscale convective system,and often brings severe winds to the ground.Therefore,the research on the gust front system is of great significance.In the Guanzhong area of Shaanxi Province,there are many gust fronts in summer.The author used the CB-band data of the new generation Doppler radar in Xi’an,Shaanxi Province,the site information of the automatic weather station,the conventional ground and high-altitude observation data of the MICAPS,and the sounding data,and to explore the environment of the gust front.Conditions,triggering mechanisms,radar echo structure,gust front evolution with time,type of rear thunderstorms and gust fronts,and the causes of windy weather.The 14 gust fronts in the Guanzhong area of Shaanxi Province in the summer of 2016 and 2017 were selected.Statistical analysis was first carried out,and then a typical case was selected for more detailed analysis.The following main features were obtained:(1)According to the 500 hPa circulation situation of the convective weather,the author summarizes three types: Cold vortex groove type,High-altitude trough front type,Subtropical high pressure edge type The presence of 850 hPa trunk and mesoscale convergence lines is a favorable trigger mechanism for convection.The cold advection of 500-850 hPa has a good correspondence with the large-scale area of the ground variable pressure wind.The deeper the high-altitude trough and the stronger the cold advection after the trough,the better the strong wind in the strongest area of cold advection.It also summarizes the two types of stratification curves of the gust front and the thunderstorm matrix.The first type of ‘funnel type’ is characterized by low-level and relatively wet,medium-to-high-level dry layer intrusion,forming a dry and warm cover,and the middle and upper-level dry and cold environment is not configured.The establishment of stable stratification and the release of unstable energy provide favorable conditions;the second is inverted "V" type,the main features are low-layer drying,the layer is dry and adiabatic unstable,the middle and upper layers have wet layers.Convective storms have better organization under medium to strong vertical wind shear conditions,which is conducive to multi-monomer storms,squall lines and supercell storms.Under weak vertical wind shear,the convective storm is poorly organized,and the strong storm formed is only one type of pulse storm.It is more common for the wind conditions of the gust front to be in a weaker vertical wind shear.(2)The changes in ground meteorological elements are similar when the gust front passes,but the intensity is different.After the front,the isotherms,isobars,and relative humidity lines are dense,the wind speed increases sharply,and the wind direction changes suddenly.The surface water vapor conditions in the thunderstorm area are sufficient and the temperature is low,and there is a cold center.The greater the temperature difference between the air that reaches the ground near the ground and the warm air in the environment,the greater the intensity of the outflow,and the temperature center difference before and after the gust front is at least 8 °C.As the thunderstorm matures,the ground cold pool develops and merges,and the scope expands.The cold pool is gradually increasing against the strong wind at the back of the wind front,and the strong wind is always around strong positive pressure and strong negative temperature.Positive and negative temperature changes have a good indication of strong winds.(3)When the gust front crosses the boundary,the outflow airflow has a large horizontal momentum in the middle layer,which will cause strong winds at low altitude or ground.Generally,the wind speed is above 5,and the maximum can reach 10 levels.The maximum wind speed on the ground does not necessarily appear completely in the place where the gust front is just passing,but after a period of gust front.And it is found that the gust front can always bring a strong wind,but it does not necessarily bring about precipitation.Subsequent convective thunderstorms are more likely to produce short-term heavy precipitation than gust front under favorable environmental conditions.(4)The gust front is generally observed on the radar reflectance map at 0.5° and 1.5° elevation angles,and the 1.5° elevation angle is more clear.The narrow-band echoes,which are weak,are generally 5-25 dBz;the horizontal scale is 20-100 km;the width is narrower about 2-5 km;and the height is generally between 0.7-2.1 km.In the radial velocity product gust front mainly manifests as narrow-band echo and positive-negative velocity junction zone(convergence line),sometimes there is speed blur,and it is consistent with the narrow-band echo on the reflectivity map.Whether it is a pulse storm,a multi-unit thunderstorm,a squall line,the gust front is often at the forefront of the thunderstorm movement direction,and the direction of the gust front is generally in the same direction as the thunderstorm.In the radial velocity product gust front mainly manifests as narrow-band echo and positive-negative velocity boundary zone(convergence line),sometimes there is speed blur,and it is more consistent with the narrow-band echo on the reflectivity map.On the vertical profile of the reflectivity,it can be found that the faster the center of the strong echo falls,the stronger the outflow to the ground;the vertical section of the radial velocity,the middle layer has obvious radial convergence,the lower layer and the upper layer divergence.(5)There are four cases of thunderstorm maternal sports gusts in 2016 and 2017,mainly in the mesoscale environment with strong vertical wind shear,and the life history is longer,reaching 1-2 hours.The mature thunderstorm and the front gust front move at the same speed,maintaining a distance of 15-18 km.The gust front triggers a new thunderstorm.After the new thunderstorm develops,the cold outflow supplements the cold air source and driving force at the back of the gust front.The two are positive feedback relations.In 2016 and 2017,there were ten cases of thunderstorm maternal static gusts,mainly in the environment with weak vertical wind shear.When the gust front moves rapidly,thunderstorms are seen as static.As the gust front is far away and lacks the supplement of the new airflow,the two gradually disappear and each other has a negative feedback relationship.