Research On Tropopause Vertical Structure And Turbulence Characteristics Over Tibetan Plateau

Atmospheric turbulent motion is a very important physical process in the Tibetan Plateau(TP)Upper Troposphere-Lower Stratosphere(UTLS)in the earth-atmosphere system,impacting significantly on the formation of the East Asian monsoon circulations and the global atmospheric circulations changes.In this paper,the TP tropopause structure,the atmospheric turbulent vertical structure in UTLS and the correlation between both subjects are analyzed using high-resolution radiosondes from 2012 to 2016.The main results obtained are as follows:(1)A novel method for detecting tropopause structure is proposed by fitting the temperature profile with a bi-Gaussian function,improving the definition of cold point tropopause(CPT).Compared with CPT height and potential temperature lapse rate minimum tropopause(LRM)height,the enormous advantages of new method are that both the tropopause height(TH)and the information of transition layer of doubletropopause(DT)structure are able to obtain together.Compared with the temperature lapse rate tropopause(LRT),it is not limited owing to the threshold,meanwhile,improving misdetection rate,so that the DT structure can be detected more accurately and fastly.(2)Based on the new bi-Gaussian method,historical high-resulution radiosondes showed that,there was an obvious high-value TH1 area with low-value thickness in the TP central area(higher about 1 km)compared with the plain area at the same latitude,as well as a significant seasonal variations,namely TH is "high in summer and low in winter",the occurrence frequency and thickness of DT are "low in summer and high in winter"performing.The TH(occurrence frequency and thickness of the DT structure)of the TP in summer is higher(lower)than that in winter.Therefore,it can be inferred that the turbulent characteristics over the TP in summer(winter)play a negative(positive)effect in the formation of the DT structure,and TP’s thermal lifting effect on TH in summer.(3)Spatiotemporal distribution characteristics of Cn2 were estimated in TP’s UTLS in summer by the Thorpe analysis method,showing a "strong in the south and weak in the north" feature as a whole,related closely to the heat source.In particular,Cn2 in the southern TP in 17～20 km above sea level is about an order of magnitude higher than the northern TP on average.Central TP with strong turbulence corresponds to the areas,where possess high-value TH1 and low-value thickness,because the heat source towering into the middle and upper troposphere and enhances turbulent mixing,thereby elevating TH.(4)According to the high vertical resolution thermal radiosondes launched near the central(Lhasa)and marginal(Da Qaidam)areas of the TP during the summers of 2018 and 2020,and the characteristics of the atmospheric turbulent vertical structure were compared and analyzed,confirming the TP’s thermal forcing on turbulence intensity.Affected by TP thermal forcing,the atmospheric refractive index structure constant Cn2 tended to increase and then decrease with increasing height,reaching a maximum at the tropopause at the Lhasa site.Although Cn2 at the Da Qaidam site also tended to increase at the tropopause,the position of the strong turbulent band(STB)was below the tropopause height corresponding to the potential temperature lapse rate minimum.The vertical distribution of Cn2 at the two sites,particularly regarding the position of the STB,was highly correlated with the atmospheric stability Ri)and the thermal mixing scale(LT).The significant correlations among the three parameters(STB,Ri,and LT)indicated that the strong fluctuations in temperature caused by thermal mixing were the dominant factor causing the Ri to be less than its critical value of 0.25.Moreover,the suppression strength involving the upward transport of the heat sources was the main reason for the different turbulent vertical structures and STB positions at the two sites.In the STB of the Lhasa site,the turbulent dissipation rate and eddy diffusion coefficient increased sharply,indicating that the turbulent atmosphere in this central site was highly diffused,and the small-scale turbulence transported the material and energy upward.(5)In addition to the heating effect,the atmospheric turbulent motion over the TP in summer is also affected by South Asian High(SAH)developing frequently on the upper-level.By analysis thermal radiosondes launched at Lhasa site in August 2018,the influence of the high-pressure activities in the lower-(500 hPa)and upper-pressure layer(100 hPa)over the TP on the temperature structure constant CT2 in UTLS is preliminarily discussed.CT2 in the upper troposphere fluctuated by about one order of magnitude within a few days of anticyclone activity.The increase rate of CT2 in the upper troposphere depends on whether a significant "upper level and lower level" is formed;and high-pressure activity near the tropopause accelerates the decline rate of CT2 in the lower stratosphere.