Research On The Influence Of Turbulent Parameter Uncertainty On Typical Marine Atmospheric Duct Numerical Simulation

Atmospheric duct is a kind of abnormal refraction phenomenon in the air,which exerts great influence on the propagation of electromagnetic wave.Hence it has high value and huge potential in military applications.Atmospheric duct forms under different weather conditions and weather situations.The causes analysis,development mechanism and numerical prediction of atmospheric duct have long been the difficulty and hotspot among the research in military marine meteorology.This paper selects strong ducts induced by super typhoon“Lupit”(0920)and subtropical high pressure circulation background in the open sea of northwest Pacific in 2016 for further research,with the main contents including duct cause analysis,turbulent parameter uncertainty analysis in planetary boundary layer(PBL)parameterization schemes and its influence on atmospheric duct simulation.Based on the high-resolution global positioning system(GPS)data and European Centre for Medium-Range Weather Forecasts(ECMWF)reanalysis data,the cause of strong ducts induced by super typhoon“Lupit”(0920)are analyzed.The results show that the ducts were mainly formed in the weak subsidence area on the northwest side of typhoon.There are several prerequisites for their formation.First of all,high pressure belt on the middle level of 850hPa fractured.Southward airflow which was induced by and located after a though,on the east of the continental high and on the west of typhoon continuously conveyed the dry air from northern continent to the west of typhoon,contributing the formation of noteworthy dry tongue.Secondly,high humidity of lower air over the ocean was maintained by the water vapor transported from the ocean through turbulence flow,cooperated with the convergence suction effect caused by the typhoon.Thirdly,the high pressure belt on the lower level of1000hPa prevented the continental dry air from moving southward,which helped forming sharp decrease of moisture with altitude and ducts.Also,inversion and sharp decrease of moisture in the entrainment layer at the top of convective PBL promoted the formation of duct.On the basis of the WRF model(version3.8),further numerical simulation research of strong ducts induced by super typhoon“Lupit”(0920)is carried out.Then the simulation results are compared with GPS dropwindsonde observations.The results show that the WRF model is capable of simulating tropical cyclone(TC)ducts.These ducts are mainly composed of elevated ducts and distributed over the sea.Compared with TC intensity and eye wall structures,TC ducts simulation effects are more closely related to the simulation of TC inner-core size and spiral rainband.PBL physical processes exert great influence on the development of atmospheric duct.Improving and optimizing PBL parameterization schemes is an important way to improve the numerical simulation accuracy of atmospheric duct.Generally speaking,among all the PBL parameterization schemes provided by WRF model(version3.8),YSU scheme and ACM2 scheme(both of them belong to nonlocal K approach scheme)show better simulation results of two kinds of duct(TC duct and anticyclone duct in the open sea).But the simulation of duct characteristic parameters(especially strength and height)is still not quite satisfactory.The research shows that the profile shape exponent pfac is the most important parameter in regard to duct simulation(its default value is 2.0)in these two schemes.Small value of pfac results in larger turbulent heat and water vapor exchange coefficient(K_h)in PBL and higher altitude where K_h maximum value exists.More importantly,the absolute value of K_h vertical gradient becomes distinctly larger when simulating with a smaller pfac,which obviously means thinner entrainment layer and higher PBL top height.As a result,the simulated duct height is elevated and the strength is maintained or even strengthened.This study suggests that,for TC ducts,it is more appropriate to set pfac as 1.0 and 1.5respectively in the YSU scheme and ACM2 scheme because TC ducts are influenced by not only the thermal turbulent mixing induced by unstable stratification(convective boundary layer),but also the mechanical turbulent mixing induced by strong wind shear(condition of strong wind).In other words,TC ducts are influenced by stronger turbulent mixing than normal conditions.Based on the high-resolution global positioning system(GPS)data and European Centre for Medium-Range Weather Forecasts(ECMWF)reanalysis data,the cause of strong ducts induced by strong anticyclone in the open sea of northwest Pacific in2016 are analyzed.The results show that the anticyclone downdraft transformed dry air from high altitude to low altitude,promoting the formation of sharp decrease of moisture with altitude and ducts.Also,anticyclone downdraft strengthened the capping inversion at the top of convective PBL,promoting the formation of ducts.These anticyclone ducts can be simulated well by YSU scheme and ACM2 scheme,with the profile shape exponent pfac being the most important parameter in numerical simulation,which is the same as the TC ducts.Although the mechanical turbulent mixing in open sea anticyclone duct case is weaker than TC duct case,interestingly,it is also appropriate to set the profile shape exponent pfac as 1.0 and 1.5 respectively in these two schemes,which may be attributed to the strong capping inversion,caused by anticyclone downdraft.