Abstract
Abstract. According to current understanding, adiabatic cooling and heating induced by the meridional circulation driven by gravity waves is the major process for the cold summer and warm winter polar upper mesosphere. However, our calculations show that the upward/downward motion needed for adiabatic cooling/heating of the summer/winter polar mesopause simultaneously induces a seasonal variation in both the O maximum density and the altitude of the [O] peak that is opposite to the observed variables generalized by the MSISE-90 model. It is usually accepted that eddy turbulence can produce the [O] seasonal variations. Using this approach, we can infer the eddy diffusion coefficient for the different seasons. Taking these results and experimental data on the eddy diffusion coefficient, we consider in detail and estimate the heating and cooling caused by eddy turbulence in the summer and winter polar upper mesosphere. The seasonal variations of these processes are similar to the seasonal variations of the temperature and mesopause. These results lead to the conclusion that heating/cooling by eddy turbulence is an important component in the energy budget and that adiabatic cooling/heating induced by upward/downward motion cannot dominate in the mesopause region. Our study shows that the impact of the dynamic process, induced by gravity waves, on [O] distributions must be included in models of thermal balance in the upper mesosphere and lower thermosphere (MLT) for a consistent description because (a) the [O] distribution is very sensitive to dynamic processes, and (b) atomic oxygen plays a very important role in chemical heating and infrared cooling in the MLT. To our knowledge, this is the first attempt to consider this aspect of the problem.
Highlights
The thermal balance of the mesosphere and lower thermosphere (MLT) is controlled by radiative heating due to absorption of solar UV radiation by O2 and O3, by chemical heating from exothermic reactions, by radiative cooling associated with infrared emission of CO2, and heating and cooling induced by dynamic processes
According to the SOCRATES model used by Smith, the maximum adiabatic heating is found to be 16 K/day in the winter polar mesosphere and the maximum adiabatic cooling rate is found to be −14 K/day during the summer polar mesopause
The goal of this paper is to consider the contradiction between the observed seasonal variations of atomic oxygen density distributions in the upper mesosphere and lower thermosphere (MLT) and the impact of upward/downward motion, responsible for the cold summer and warm winter mesopause, on the [O] distribution
Summary
The thermal balance of the mesosphere and lower thermosphere (MLT) is controlled by radiative heating due to absorption of solar UV radiation by O2 and O3, by chemical heating from exothermic reactions, by radiative cooling associated with infrared emission of CO2, and heating and cooling induced by dynamic processes The latter includes compression/expansion caused by downward/upward motion associated with the gravity wave-driven meridional circulation, as well as direct heating due to the gravity wave dissipation and turbulent diffusion from breaking gravity waves and/or the Kelvin–Helmholtz instability (KHI), caused by sheared flow. According to the SOCRATES model used by Smith, the maximum adiabatic heating is found to be 16 K/day in the winter polar mesosphere and the maximum adiabatic cooling rate is found to be −14 K/day during the summer polar mesopause This cooling/heating is caused by mean vertical motions with a magnitude of ±2 cm/s.
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