Trends in mesospheric ice layers in the Northern Hemisphere during 1961–2013

Abstract

AbstractThe MIMAS ice model is used to study the trend behavior of polar mesospheric clouds (PMCs) responding to long‐term changes in mesospheric temperatures at northern high latitudes for the summer period 1961–2013. This paper presents long‐term PMC variations in terms of ice water content (IWC) at northern high latitudes for monthly July averages that are compared to solar backscatter ultraviolet (SBUV) data available in the period 1979–2013. In general, good agreement is found between long‐term PMC variations from the model and from satellite observations. For example, both model and SBUV data show an increase in ice water content (IWC > 40 g km−2) in the latitude band 74°–82°N with significant IWC trends of 2.8 ± 2.1 g km−2/dec and 2.4 ± 2.0 g km−2/dec, respectively. Modeled trends in IWC are latitudinally dependent. For the entire period 1961–2013 no trend exists at midlatitudes (50°–60°N) but increases poleward to highly significant values of 4.4 ± 0.9 g km−2/dec in the latitude band 74°–82°N. The analysis of trends in extreme PMC events (IWC > 300 g km−2) shows a 23%/dec increase in occurrence frequency at 74°–82°N which is by far the largest trend of all PMC parameters. We discuss the statistical properties of PMC following an exponential distribution. We derive an analytic solution of ice growth from microphysics that allows calculation of ice particle radius as a function of background conditions. As a result, the thermal conditions near 83 km height, with a cooling of −0.58 ± 0.32 K/dec, mainly determine trends in IWC whereas cooling at lower heights, induced by stratospheric ozone, controls to a large extent the long‐term behavior of PMC altitudes.