We compare measurements of refractive index structure constant Cn2 and energy dissipation rate $epsilon$ by VHF radar with in situ observations by high-resolution thermosondes during a campaign near Adelaide, Australia, in August 1998. A total of 17 thermosonde soundings was performed within a distance of 120 km to the radar site. The variance between estimates of $langle$Cn2$rangle$ from backscattered radar power and in situ thermosonde observations is found to be comparable to the observed variance of the radar estimates due to temporal and spatial variations of the turbulent parameters (angle brackets denote average over the radar range resolution of 1 km). The high-resolution thermosonde observations allow us to test the applicability of the turbulent volume fraction model to the estimation of $langle$$epsilon$$rangle$ from $langle$Cn2$rangle$. The data indicate that the turbulent volume fraction should not be modeled as the fraction of unstable layers with gradient Richardson number Ri < 0.25.