On the short-term variability of turbulence and temperature in the winter mesosphere

Abstract

Four mesosphere-lower thermosphere temperature and turbulence profiles were obtained in situ within 1/4 30min and over an area of about 100 by 100km during a sounding rocket experiment conducted on 26 January 2015 at Poker Flat Research Range in Alaska. In this paper we examine the spatial and temporal variability of mesospheric turbulence in relationship to the static stability of the background atmosphere. Using active payload attitude control, neutral density fluctuations, a tracer for turbulence, were observed with very little interference from the payload spin motion, and with high precision ( < 0.01 %) at sub-meter resolution. The large-scale vertical temperature structure was very consistent between the four soundings. The mesosphere was almost isothermal, which means more stratified, between 60 and 80km, and again between 88 and 95km. The stratified regions adjoined quasi-adiabatic regions assumed to be well mixed. Additional evidence of vertical transport and convective activity comes from sodium densities and trimethyl aluminum trail development, respectively, which were both observed simultaneously with the in situ measurements. We found considerable kilometer-scale temperature variability with amplitudes of 20K in the stratified region below 80km. Several thin turbulent layers were embedded in this region, differing in width and altitude for each profile. Energy dissipation rates varied between 0.1 and 10mWk1, which is typical for the winter mesosphere. Very little turbulence was observed above 82km, consistent with very weak small-scale gravity wave activity in the upper mesosphere during the launch night. On the other hand, above the cold and prominent mesopause at 102km, large temperature excursions of +40 to +70K were observed. Simultaneous wind measurements revealed extreme wind shears near 108km, and combined with the observed temperature gradient, isolated regions of unstable Richardson numbers (0 < Ri < 0.25) were detected in the lower thermosphere. The experiment was launched into a bright auroral arc under moderately disturbed conditions ( Kp 1/4 5).