T. Aso

An examination of high latitude upper mesosphere dynamic stability using the Nippon/Norway Svalbard Meteor Radar

[1] Using wind measurements from the recently installed Nippon/Norway Svalbard Meteor Radar, (NSMR) at 78°N, 16°E, we have derived wind shears, and, combining these with model Brunt-Vaisala frequencies, have determined estimates of the gradient Richardson Number. These Richardson Number estimates parameterise the degree of stability of the upper mesosphere at a height of around 90 km. We find indications of dynamic instability in spring and autumn, with greater stability in summer.

Atmospheric stability at 90 km, 78°N, 16°E

We employ observations obtained from a meteor wind radar to derive ambipolar diffusion coefficients, neutral temperatures, temperature gradients and, subsequently, Brunt-Väisälä frequencies at an altitude of 90 km over Svalbard (78°N, 16°E). The derived values showed a good agreement with independent measurements at each step of the analysis. Current atmospheric models are based on sparse data obtained at such high latitude, so these results represent a viable alternative for incorporating in subsequent studies of atmospheric dynamics, particularly if the derived monthly variabilities are included. The Brunt-Väisälä frequencies are then combined with wind shear measurements to estimate horizontally averaged gradient Richardson Numbers (Ri). We find Ri to be consistently larger in summer than winter due to wind shears being similarly larger in winter and augmented by the inverse seasonal variation in Brunt-Väisälä frequency. These seasonal variations result in Ri indicative of dynamic stability in summer and instability in winter. The variabilities in wind shear and Brunt-Väisälä frequency are then included to—albeit more qualitatively—illustrate the distribution between stability and static and dynamic instabilities as a function of season, using a novel portrayal pioneered by Zink and Vincent (J. Geophys. Res., 109, doi:10.1029/2003JD003992, 2004). The resulting picture is discussed in the framework of current conceptions of distribution of turbulent energy dissipation with height and season and of current opinion of the mesopause structure at 78°N.