Analysis Of The Characteristics Of Energy And Water Vapor During The Evolution Of A Plateau Shear Line

Using NCEP global 1°×1° final-analysis data,a plateau shear line was selected in this article,which first occurs in the Southeast of Qinghai province and then move Eastward to the central of Sichuan province,causing a weather influenced process.Using the common dynamic diagnosis quantity and applying the concept of kinetic energy gradient,the structural characteristics of the plateau shear line are simply analyzed at different stages of development.Then the characteristics of total wet energy,disturbance kinetic energy and water vapor at different development stages of the plateau shear line are analyzed.Finally,the decomposition method of kinetic energy based on spatial scale is applied to diagnose the characteristics of the plateau shear line process,and the interaction mechanism between the background field and the disturbance field is discussed based on the transition term of the disturbance kinetic energy and the average kinetic energy.The followings are the main conclusions that were obtained through above chapters:(1)The eastward shift of the mid-latitude trough combined with the westward Western Pacific Subtropical High extension is the common circulation conditions conducive to the formation and development of the plateau shear line.The plateau shear line is in a positive vorticity band and a divergent convergence zone,and is a shallower system.The positive vorticity zone and the convergence zone on the shear line are basically below 400 h Pa.From the point of view of energy,when the plateau shear line occurs and develops,the location of it corresponds with strong geostrophic deviation and great kinetic energy gradient,the horizontal and vertical distribution of the kinetic energy gradient are consistent with the corresponding distribution of divergence,and it can reflect the basic structure of shear line.(2)The plateau shear line is basically located in the warm and humid area.The temperature and humidity on the north and south sides of the shear line are obviously different.Affected by the northerly wind,the north side is realized as a relatively dry and cold zone,while the south side of the shear line,which affected by the humid southwesterly flow,is presented as a warm and humid area.During the formation and mature period of the shear line,there are convective unstable energy on the shear line,especially at the mature period of the shear line,the lower level is unstable and the upper level is stable,near-surface layer is the high energy and high humidity area,shear line along with strong ascending movement.(3)After the plateau shear line is generated,the amount of change in sensible heat energy and the amount of change in latent heat energy are comparable,both of which contribute to the change in total specific energy.When the shear line develops to maturity,the change in latent heat energy dominates.The region where the shear line is located corresponds to a large value area of latent heat energy,indicating that the relevant area is prone to condensation,and the shear line is basically located between the positive and negative advection of sensible heat energy and the positive and negative advection of latent heat energy.(4)The area of great eddy kinetic energy is consistent with the distribution of the shear line,during the early stage of development,the eddy kinetic energy increase obviously.The magnitudes of the advection of the eddy kinetic energy and the barotropic conversion is too small to explain the transformation of the eddy kinetic energy during the evolution of the shear line.The baroclinic conversion and the eddy advection of geopotential dominate the budget of eddy kinetic energy.During the mature period of the shear line,the baroclinic conversion is the biggest and is an important way of energy conversion in the development of plateau shear line,which can contribute to the intensification of the ascending motion on the shear line.Eddy kinetic energy trend item can better predict the development trend of the shear line.Eddy ageostrophic geopotential flux and its divergence have good implications of the formation,disappearing and movement of the plateau shear line.(5)The moisture convergence band is basically located on the shear line at different stages,which provides sufficient moisture conditions for the strong precipitation near the shear line.The area of moisture convergence near the shear line is located in the lower and middle layers of the troposphere(below 400 h Pa).At the mature stage of the shear line,the moisture convergence has the largest thickness and the highest extension height.The convergence zone of the low-level moisture flux divergence can roughly reflect the distribution of heavy precipitation in the next 6 hours.The air block that contributed the most to this precipitation process came from the local area,with a contribution rate of 48%,followed by the Bay of Bengal,which was 26%.(6)The enlargement of the turbulent kinetic energy in the lower troposphere was closely associated with the occurrence and evolution of the plateau shear line, from the formation stage to maturation stage,the increase of turbulent kinetic energy provided energy guarantee for the formation and development of the shear line.The kinetic energy had opposite trends in the mean flow and the turbulent flow,on the formation stage and the developing stage,the mean kinetic energy decrease with time in the lower to middle troposphere.The baroclinic energy conversion dominated the trend of kinetic energy among all the budget terms.On the formation stage of the shear line,the conversion from the mean kinetic energy to the turbulent kinetic energy had an obvious influence on the trend of turbulent kinetic energy in the lower troposphere,the increase of the turbulent kinetic energy and decrease of mean kinetic energy was caused by the interaction between the background circulation and eddy field,constituting a downscaled energy cascade of kinetic energy,This energy cascade conversion is beneficial to the formation of the plateau shear line,which is belong to the mesoscale system.