Modeling of mesospheric ice layers with LIMA-ICE

Scope

Various layers exist in the summer mesosphere at middle and polar latitudes which owe their existence to water ice particles. These layers are called `noctilucent clouds' (NLC), `polar mesosphere clouds' (PMC), and `polar mesosphere summer echoes' (PMSE) which reflects different observation geometry or entirely different physical processes involved. These layers are very sensitive to temperature (and to a lesser extent on H2O) and are therefore best suited to indicate long term and solar cycle changes. Indeed, variations on decadal time scales of different layer parameters (occurrence frequency, layer brightness etc.) have been reported from ground and satellite based observations. However, the physical and photo-chemical processes involved are not understood and some of the observations contradict expectations.

The new LIMA-ICE model is used to study interhemispheric temperature differences at the summer upper mesosphere and their impact on the morphology of ice particle related phenomena such as noctilucent clouds (NLC), polar mesosphere clouds (PMC), and polar mesosphere summer echoes (PMSE). LIMA-ICE nicely reproduces the mean characteristics of observed ice layers, for example their variation with season, altitude, and latitude.

Major results with LIMA-ICE are that the southern hemisphere (SH) is slightly warmer compared to the NH but the difference is less than 3 Kelvin at NLC/PMC/PMSE altitudes and poleward of 70 N/S. The occurrence frequency and brightness of NLC and PMC are larger in the NH but the differences decrease with increasing latitude. The NLC altitude in the SH is slightly higher by 0.6-1 km, whereas the NLC altitudes itself depend on season in both hemispheres. These interhemispheric differences in NLC/PMC/PMSE characteristics deduced from LIMA/ice basically agree with observations from lidar, satellites, and radars.

Modelled occurrence frequency (daily zonal mean) of PMSE as a function of latitude and season in the NH summer of 2001 (upper panel) and in the SH summer of 2004/05 (lower panel).

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