Polar mesosphere summer echoes

The first observations of polar mesosphere summer echoes (PMSE) above continental Antarctica were recorded using the VHF radar at Davis during the 2003-04 summer. PMSE are strong radar echoes associated with the extremely cold temperatures that occur in this region during the summer. Much of the early work on PMSE was carried out using northern hemisphere radars. However, since 2003-04, observations of the southern hemisphere PMSE have provided a separate "atmospheric laboratory" to learn more about this phenomenon. Long-term PMSE observations are also thought to provide a proxy for monitoring climate change.

PMSE's manifest themselves as strong radar echo returns. The radar echo strength is often measured using signal-to-noise ratio (SNR). The altitude-time plot of the backscattered SNR on 30 November 2003 of PMSE observed at Davis is illustrated below.

Polar mesosphere summer echoes (PMSE) observed on the Davis VHF radar.

Polar mesosphere summer echoes (PMSE) observed on the Davis VHF radar.

The Davis PMSE events occur at an average peak height of 86 km, with a height range from 81 to 92 km. The peak PMSE height is slightly below the summer mesopause temperature minimum at 88 km, and above the noctilucent cloud (NLC) and/or polar mesospheric cloud (PMC) layer at 83-84 km. The PMSE season typically starts around 19 November and ends near 16 February. PMSE daily occurrence also exhibits a primary (0930 UT) and secondary maxima (0300 UT) with a clear minimum (2000 UT).

Background

Interhemispheric differences

PMSE were first observed using a VHF radar at Poker Flat, Alaska during 1979. Although the climatology of PMSE in the southern polar regions is less well known, it has become apparent that the southern hemisphere PMSE differs from its northern counterpart. The first observations of southern-hemisphere PMSE were carried out at Machu Picchu base on King George Island (62°06'S, 58°28'W) during the late summer of 1992-93.

The interhemispheric differences in the PMSE represent a significant opportunity for their study. They also provide insight into differences in the chemistry and dynamics of the two hemispheres. This increases the importance of any climatological studies that are conducted. It has been suggested that interhemispheric differences in temperature, water vapour and wind are behind the relative dearth of echoes, although there are still questions about whether the temperature structure is dissimilar enough to be the cause.

Noctilucent clouds (NLC)

The linkage between PMSE (radar scatter) and NLC (optical scatter) has been investigated at Davis. Both phenomena are reported to be associated with the presence of increased water vapour content and lower temperatures in the mesosphere (i.e. PMSE with large numbers of sub-visual ice particles (< 10 nm) and NLC with less abundant but more massive ice particles (> 20 nm)). The separation of these two layers results from ice particle growth during the downward sedimentation process. Simultaneous common volume observations of NLC and PMSE have been made at Davis using the LIDAR and VHF radar. These measurements will allow investigation of critical threshold temperature dependencies for both NLC and PMSE occurrence.

PMSE theory

The theoretical explanation for PMSE is still being debated and current interpretations need further experimental verification. Whatever the result, PMSE theory must account for the large summer echo power of PMSE observed by radar and their morphology in both hemispheres. The seasonal occurrence of PMSE is dependent to some extent upon a critical mesosphere temperature threshold (< 140 K), which (given sufficient water vapour content) is linked to the creation of sub-visual aerosols. These aerosols scavenge free electrons in the mesosphere, provide a local variation in electron density, and cause the PMSE scatter necessary for radar detection.

Dr Ray Morris and Dr Damian Murphy