Observing mesoscale dynamics with multistatic specular meteor radars: first climatology of momentum flux, horizontal divergence and relative vorticity over central Europe

Abstract. Continuous and reliable measurements of the mesosphere and lower thermosphere (MLT) are key to further the understanding of global atmospheric dynamics. Observations at horizontal scales of a few hundred kilometers (i.e., mesoscales) are particularly important since gravity waves have been recognized as the main drivers of various global phenomena, e.g., the pole-to-pole residual meridional circulation. Multistatic specular meteor radars are well suited to routinely probe the MLT at these scales. One way to accomplish this, is by investigating the momentum flux, horizontal divergence ∇_H·u and relative vorticity (∇×u)_Z estimated from the Doppler shifts measured by a radar network. Furthermore, the comparison between the horizontal divergence and the relative vorticity can be used to determine the relative importance of gravity waves (i.e., divergent motions) and strongly stratified turbulence (i.e., vortical motions). This work presents the first climatology of all these estimates together, as well as results on the probability distribution of the total momentum flux (TMF), and the comparison between ∇_H·u and (∇×u)_Z , obtained from almost 10 years of continuous measurements provided by two multistatic specular meteor radar networks: MMARIA/SIMONe Germany, covering an area of more than 200 km radius around (53°N, 11°E), and MMARIA/SIMONe Norway, which covers an area of similar size, but around (69°N, 16°E). Among others, our results indicate that at middle latitudes the horizontal divergence and the relative vorticity are balanced around summer mesopause altitudes, while the former dominates over the latter above ~90 km of altitude during parts of the fall transition. At high latitudes, the vortical motions dominate during late spring and early summer. Besides, the strongest 5 % of GWs contribute much more over northern Germany than over northern Norway, where the larger values of the excess-kurtosis indicate that the contribution from the small-amplitude GWs is also more significant at middle latitudes, especially during the summer. In other words, the TMF in the mesosphere and lower thermosphere over central Europe is considerably more intermittent at middle latitudes than at high latitudes.