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

Ri =shear2ωB22⎛ ∂u⎞ ⎛ ∂v⎞⎜ ⎟ + ⎜ ⎟⎝ ∂z⎠ ⎝ ∂z=⎠g ∂θθ ∂z2where ω B (rad s -1 ) is the Brunt-Väisälä frequency, u and v (m s -1 ) are, respectively, the zonaland meridional components of the horizontal wind vector, z (m) is the altitude, g (m s -2 ) is thegravitational acceleration, and θ (K) is the potential temperature. The condition Ri < 0.25must be met in order to generate turbulence, although once turbulence is initiated it is thoughtthat Ri < 1 is sufficient for it to be maintained (Woods, 1969). Since ω B 2 can be retrievedfrom the radar return signal power (Hooper et al., 2003), Ri for tropopause altitudes andabove can be calculated entirely from MST radar data. Although a full ω B 2 calibration has notyet been carried out for the Aberystwyth radar, the condition Ri < 1 is met for the shear layers(but not shown) using approximate ω B 2 calibration values.The shear layers are associated with inertia-gravity wave activity, which is clearly apparent inthe plots of wind speed and normalised projected wind shown in Figure 1. The sharpness ofthe tropopause is therefore eroded by a combination of mountain wave breaking from belowand inertia-gravity wave breaking from above. Assuming that the altitudes of the staticstability and chemical tropopause levels are coincident prior to the onset of turbulence, themixing is expected to give rise to exchange of upper tropospheric and lower stratospheric air.ReferencesBrowning, K. A., and C. D. Watkins, Observations of clear air turbulence by high powerradar, Nature, 227, 260-263, 1970.Hooper, D. A., and J. Arvelius, Monitoring of the Arctic winter tropopause: A comparison ofradiosonde, ozonesonde and MST radar observations, in Proceedings of the NinthInternational Workshop on Technical and Scientific Aspects of MST Radar, 385-388, Sci.Comm. on Sol.-Terr. Phys. Secr., Boulder, Colorado, 2000.Hooper, D. A., and L. Thomas, Complementary criteria for identifying regions of intenseatmospheric turbulence using lower VHF radar, J. Atmos. Sol.-Terr. Phys, 60, 59-61, 1998.Hooper, D. A., J. Arvelius, and K. Stebel, Retrieval of atmospheric static stability from MSTradar return signal power, in Proceedings of the Tenth International Workshop on Technicaland Scientific Aspects of MST Radar, 2003.Ottersten, H., Atmospheric structure and radar backscattering in clear air, Radio Sci., 4, 1179-1193, 1969.Prichard, I. T., L. Thomas, and R. M. Worthington, The characteristics of mountain wavesobserved by radar near the west coast of Wales, Ann. Geophys., 13, 757-767, 1995.Woods, J. D., On Richardson's number as a criterion for laminar-turbulent-laminar transitionin the ocean and atmosphere, Radio Sci., 4, 1289-1298, 1969.Worthington, R. M., and L. Thomas, Radar measurements of critical-layer absorption inmountain waves, Q. J. R. Meteorol. Soc., 122, 1263-1282, 1996.49