Infra-red Radiative Cooling/Heating of the Mesosphere and Lower Thermosphere Due to the Small-Scale Temperature Fluctuations Associated with Gravity Waves
We address the effect of an additional infrared radiative cooling/heating of the mesosphere and lower thermosphere (MLT) in the infrared bands of CO2, O3 and H2O due to small-scale irregular temperature fluctuations associated with gravity waves (GWs). These disturbances are not well resolved by present general circulation models (GCMs), but they alter the radiative transfer and cooling rates significantly. A statistical model of gravity wave-induced temperature variations was applied to large-scale temperature profiles, and the corresponding direct radiative calculations were performed with accounting for the breakdown of the local thermodynamic equilibrium (non-LTE). We show that temperature fluctuations can cause an additional cooling of up to 4 K day−1 near the mesopause. The effect is produced mainly by the fundamental 15 μm band of the main CO2 isotope 12C16O2 (626). A simple parametrization has been derived that computes corrections depending on the temperature fluctuations variance, which need to be added in radiative calculations to the mean temperature and the volume mixing ratios (VMRs) of CO2 and O(3P) to account for additional cooling/heating caused by the unresolved disturbances. Implementation of this scheme into the LIMA model resulted in a colder and broader simulated summer mesopause in agreement with recent lidar measurements at Spitsbergen.
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