Polar mesospheric horizontal divergence and relative vorticity measurements using multiple specular meteor radars

We present the first horizontal divergence and relative vorticity measurements at polar mesospheric altitudes measured from the ground. Our technique relies on combining information from two specular meteor radars (SMRs) separated 130 km at polar latitudes, specifically, the Andenes and Troms⊘ radars in northern Norway. The resulting values are obtained over a region that spans an approximate area of 400 km diameter at mesospheric altitudes. The temporal and vertical resolution are 1 h and 2 km in altitude. The technique not only allows to obtain the gradient terms of the horizontal wind, that in turn are used to derive the horizontal divergence and relative vorticity, but also improves the horizontal sampling compared to single SMRs. Synthetic data are used to qualitatively test the technique and identify potential sources of biases on the resulting measurements. For example, we have found that an apparent large mean vertical velocity is obtained, after averaging many days, if there is a persistent divergent field. We present a climatology of the resulting wind field parameters from 12 years of continuous observations and focus on the summer results. We found a persistent altitudinal pattern in both the horizontal divergence and relative vorticity fields during all northern hemispheric summers. The horizontal divergence is mainly positive decreasing in magnitude below ∼86 km, and the relative vorticity is negative/positive below/above ∼88 km over northern Norway.