Proceedings with Extended Abstracts (single PDF file) - Radio ...

winter and a minimum in summer near the tropopause. In the mesosphere, the seasonal cyclechanges having the maxima in winter and summer, also minima in equinoxes incorrespondence with observations [see Gavrilov and Fukao, 1999].In this study, we calculated vertical profiles of the turbulent diffusion coefficientproduced by a spectrum of breaking IGWs. Specifically, we studied the influence of sharpchanging of vertical temperature gradient on the changes of the conditions of IGWpropagation, increasing their destruction and forming a maximum of the turbulent diffusivitynear the tropopause. The mean profiles of wind and temperature are specified using thestandard atmospheric models MSIS-90 and HWM-93 for the geographic coordinates ofobservation site Shigaraki (Japan), from which we have the MU radar data. Below altitude 30km, we improved the vertical profiles of background wind and temperature from the data ofthe reanalysis of radiosonde measurements at the National Center for Atmospheric Researchof Boulder, USA. Fig. 1 shows the vertical profiles of background temperature and wind forJanuary and July at Shigaraki (Japan).Fig. 1. Vertical profiles of background temperature (left) and zonal(middle) and meridional (right) components of wind velocity atShigaraki for January (solid lines) and July (dashes).Fig. 2 represents the corresponding calculated standard deviations of wind velocityproduced by a spectrum of IGWs in the ranges of periods 0.3 – 6 hours and horizontal phasespeeds 3 – 60 m/s and of the coefficient of turbulent diffusion produced by breaking IGWs.One can see that the IGW intensity and the turbulent diffusivity have maxima at altitudes 15 –20 km, where the tropopause is located at different seasons. The measurements of turbulentdiffusivity with the MU radar at Shigaraki show its maxima near the tropopause [Fukao et al.,1994] and our calculated values well enough correspond to the measured ones.Fig. 2. Calculated standard deviation of horizontal wind velocity(left), turbulent diffusivity (middle) and vertical flux of wave energy(right) produced the IGW spectrum for background fields from Fig. 1for Shigaraki in January (solid lines) and July (dashes).In Fig.2, the maxima of IGW amplitudes and turbulence diffusivity in the tropopauseregion are stronger in winter than in summer.235