A model study of global variability in mesospheric cloudiness

We have performed microphysical calculations of mesospheric cloudiness which are driven by output (vertical wind, water vapor mixing ratio and temperature) from a two-dimensional (2D) global chemical/dynamical model. The variations in the 2D model output drive variations in the simulated clouds which can be compared with cloud observations. The specific cloud observables we model are ice content, altitude, peak backscatter at 532 nm and albedo at 252 nm. We categorize these parameters in terms of their variations with latitude, solar activity and hemisphere (north vs. south). In agreement with observations, we find brighter clouds in the Northern Hemisphere (NH) relative to the south and at solar minimum relative to solar maximum. Also we find that cloud altitudes are higher in the Southern Hemisphere (SH) relative to the NH. Quantitatively, compared with observations, it appears that the model may overstate the magnitude of these variations. Thus, the entire range of observed cloud altitudes, poleward of 65-70°, is about 2 km (83-85 km), whereas the range in the calculated heights range extends up to 5 km (83-88 km). In addition, the calculated solar cycle brightness change of up to an order of magnitude appears larger than the limited available observations. Since the model H2O variation in the 80-90 km region with respect to solar activity is relatively small (10-40%), it is not the cause of our large model cloud variability. Rather, for both hemispheric variation and solar cycle changes, we suggest that the model temperature variability may be too great.