The relationship between polar mesospheric clouds and their background atmosphere as observed by Odin-SMR and Odin-OSIRIS

摘要

In this study the properties of polar mesospheric clouds (PMCs)and the background atmosphere in which they exist are studied usingmeasurements from two instruments, OSIRIS and SMR, on board the Odinsatellite. The data comes from a set of tomographic measurements conducted bythe satellite during 2010 and 2011. The expected ice mass density and cloudfrequency for conditions of thermodynamic equilibrium, calculated using thetemperature and water vapour as measured by SMR, are compared to the ice massdensity and cloud frequency as measured by OSIRIS. We find that assumingthermodynamic equilibrium reproduces the seasonal, latitudinal and verticalvariations in ice mass density and cloud frequency, but with a high bias of afactor of 2 in ice mass density.To investigate this bias, we use a simple ice particle growth model toestimate the time it would take for the observed clouds to sublimatecompletely and the time it takes for these clouds to reform. We find adifference in the median sublimation time (1.8 h) and the reformation time(3.2 h) at peak cloud altitudes (82–84 km). This difference implies thattemperature variations on these timescales have a tendency to reduce the icecontent of the clouds, possibly explaining the high bias of the equilibriummodel.Finally, we detect and are, for the first time, able to positively identifycloud features with horizontal scales of 100 to 300 km extending far belowthe region of supersaturation (  2 km). Using the growth model, we concludethese features cannot be explained by sedimentation alone and suggest thatthese events may be an indication of strong vertical transport.