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

Height distributionMSE are more or less regularly observed in the summer mesosphere, but their occurrence ratedepends markedly on altitude. The strongest mesosphere summer echoes during the threeyears exceeded more than 20 dB signal to noise ratio (SNR).In figure 2a the height distribution of four-year-mean MSE occurrence is presented for SNRgreater than 0 dB. It shows that MSE layers normally occur in an altitude range between 80km and 90 km with a maximum incidence near 85 km. The average occurrence falls rapidlyaway the peak. More than 90 % of MSE occur between 82.5 and 87.5 km. The decrease aboveand below the occurrence maximum is nearly the same.Scatter characteristicsReceived signal power depends linearly on the effective antenna area for a perfect scatteringprocess, and quadratically on the effective antenna area for a plain specular reflection process.If mesosphere echoes were observed at the same location simultaneously with differentantenna arrays it can be estimated whether volume scattering or specular partial reflection isresponsible for the echoes (Zecha et al., 2001).The division of the radar array in six subarrays give the feasibility to use the whole antennawith 144 Yagis for transmission and receiption on the one hand, and a subarray of 24 Yagisfor simultaneous receiption on the other hand. Both antenna configurations have differenteffective antenna areas, but the same efficiency otherwise. Thus the ratios of received powerhint to plain scattering and reflection processes by values of 3.9 dB and 7.8 dB, respectively.Normally the values are between these extrema. In figure 2b the solid line connects themedian values for each height channel. It is evident that in general the scattering charactergain more and more with increasing height.Aspect sensitivityRadar backscatter returns are strongest from the vertical direction and show a pronounceddependency on the zenith angle. Mostly a Gaussian function is assumed and the usualparameter to describe this phenomenon is the half-angular width s . This so called aspectsensitivity can be estimated also without tilting the radar beam but using the characteristics ofthe spatial correlation ellipse of the FCA model (Briggs, 1984) and the radar beam width.Similar to PMSE, mid-latitude mesosphere summer echoes are markedly aspect sensitiv. Thesolid line in figure 2c displays the medians for each height channel. It shows that the aspectsensitivity is very strong especially in the lower part of the MSE layers and becomes smallerwith increasing height.Turbulence parameterThe temporal characteristics of the scatters can be specified by a fading time which is closelyrelated to the total spectral width. This measured total spectral width is determined byturbulent and nonturbulent processes. However, the nonturbulent contribution can becalculated from horizontal wind speed and the half-power beam width of the effective radarbeam assuming that beam broadening is in general the dominating nonturbulent process(Hocking, 1983). The real values of the resulting spectral width σ fluct due to turbulentprocesses are shown in figure 2d. The solid line connects the medians for each height channel.In general the turbulent character tends slightly upwards with increasing height.148