Abstract

Estimates of the vertical mass eddy diffusion coefficient K are necessary to understand the dynamics and photochemistry of a planetary atmosphere. Most previous estimates for the atmosphere of Venus^1–10, pertain to the upper atmosphere. In the lower atmosphere (80 km), information on small-scale (1 km) temperature fluctuations has been provided by radio scintillations observed during radio occultation by the atmosphere^11–18. Here we relate the vertical mass eddy diffusion coefficient to these radio scintillation observations so that reliable estimates can be obtained near the tropopause level of 60 km for the first time. Using the recent Pioneer Venus radio occultation measurements¹⁶, and assuming that the turbulence is generated by shear instability, we find that K = 4 10⁴ cm² s^-1, the energy dissipation rate = 20 cm² s^-1, and the dissipation rate of temperature fluctuations N = 10^-3 K² s^-1. These results indicate that small-scale turbulence probably plays a major part in vertical transport near the tropopause of Venus.