Application of an interactive parameterization for polar mesospheric clouds and comparison with observations.

Physics

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0305 Aerosols And Particles (0345, 4801, 4906), 0340 Middle Atmosphere: Composition And Chemistry, 0342 Middle Atmosphere: Energy Deposition (3334)

Scientific paper

A new model of Polar Mesospheric Clouds (PMCs) and the global atmosphere is used to study the interaction of PMCs with the climate of the summer mesopause. The model combines a two dimensional chemical/dynamics model with a parameterization for PMCs that assumes a single particle size. We investigate the sensitivity of the model to the assumed particle size and ice aerosol heating. Ice aerosol absorption of terrestrial and solar IR radiation is shown to lead to atmospheric heating rates which can exceed 10K/day. This heat is dissipated partially by increased upwelling above the cloud layer and also by a small (2- 5K) temperature increase. Inclusion of the temperature increase in the model acts to reduce the calculated ice mass by approximately 30%. In addition, the model ice mass is seen to vary with solar activity and assumed particle size. We also compared our model predictions with ozone data from HALOE. The HALOE data show a clear perturbation in the regions where PMCs occur. Above the cloud layer, ozone increases by 25-65% while just below, there is a suggestion of a small ozone decrease. This is presumably due to the dehydration and resultant decrease in HOx radicals above the cloud layer. Below the cloud layer, we expected H2O and HOx to be enhanced which should lead to lower ozone. Paradoxically, however, when ice aerosol heating is included in the model, the resultant temperature increase acts to reduce the ozone response. This reflects the well known anticorrelation between ozone and temperature. Thus the model is in best agreement for the case with no IR aerosol heating, which is inconsistent with expectations. Our calculations point to the need for high precision temperature measurements in the presence of PMCs, which will be provided by SOFIE on AIM.

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