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A method to determine the refractive index structure constant, Cn2, from high-resolution radiosonde data has been developed. A full validation of this method was not possible to carry out due to the lack of other datasets, e.g. radar measurements. However, the results obtained present the values and behaviour similar and within the range of those observed by other authors. The statistical behaviour of Cn2 also shows the expected log-normality further confirming the general correctness of the approach. The distributions of turbulent layer thickness are as well within the range of those observed for the outer scale turbulence providing further reassurance on the taken approach. Statistical results were obtained for 4 sites at different latitudes as well as an exponential fit to the median for applications where simplified models for Cn2 suffice. These statistical results show the expected physical impact of the boundary layer, orographic features and local climate.
The authors expect that further work will lead to the full validation of the method and that high-resolution radiosonde data may become of widespread use, due to its availability, to determine turbulence and its parameters.
The authors would like to thank the British Atmospheric Data Centre and the UK MetOffice for providing access to its excellent database of high-resolution radiosonde data. They would like to thank Danielle Vanhoenacker as well for providing the raw data as used by Hugues Vasseur in his paper. Special mention has to be made of Pierluigi Silvestrin for his unwaivering support to this activity and of Gottfried Kirchengast and Per Høeg for the constructive criticism in the many discussions with the authors.
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Table 1: Technical specifications of the Vaisala RS80 radiosonde.
Table 2: Radiosonde stations
Figure 1: Block diagram of methodology
Figure 2: Cn2 for a single radiosonde launch (Camborne 1st January 2002 at 0600 UTC). Data points at 10-21 represent stable layers while scattered data points show Cn2 for turbulent layers
Figure 3: Histogram of Cn2 at a height of 6000 meters. Data covers 4 years (1994-1996, 2002) of 4 daily radiosonde launches in Camborne.
Figure 4: Mean of log Cn2 and 10, 50 and 90 percentiles as a function of height derived from the cumulative distribution of Cn2 for Camborne (data comprising 4 years of 4 daily launches).
Figure 5: The probability of turbulence as a function of height for Camborne (data for 4 years of 4 daily launches).
Figure 6: Percentiles derived from the probability distribution of Cn2 conditioned to having turbulence at Camborne (data for 4 years of 4 daily launches).
Figure 7: Percentiles as a function of height for the thickness of the turbulent layer. The percentiles refer only to turbulent samples.
Figure 8: Mean of log Cn2 and 10, 50 and 90 percentiles as a function of height derived from the cumulative distribution of Cn2 for Lerwick.
Figure 9: Mean of log Cn2 and 10, 50 and 90 percentiles as a function of height derived from the cumulative distribution of Cn2 for Gibraltar.
Figure 10: Mean of log Cn2 and 10, 50 and 90 percentiles as a function of height derived from the cumulative distribution of Cn2 for St. Helena.
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