Lidar measurements of noctilucent clouds at Río Grande, Tierra del Fuego, Argentina

Abstract

Noctilucent clouds (NLCs) are sensitive tracers of upper-mesospheric temperature, water vapor, and dynamics and thus open windows to study our atmosphere from very large to very small scales, including topics of climate, circulation, waves, and turbulence. NLCs are weaker in the Southern Hemisphere compared to the Northern Hemisphere where there are numerous observations, but no vertical soundings at Southern Hemisphere mid-latitudes were available until now. We determine the properties of NLCs above a Southern Hemisphere mid-latitude site at 53.8° S in southern Argentina. The Compact Rayleigh Autonomous Lidar has provided high-resolution vertical lidar soundings since 2017. Noctilucent clouds are detected every summer, with the earliest (latest) detection on 29 November (29 January), giving 19 events in total of 33.8 h length at an average height of 83.3 km, a maximum brightness of 24 × 10−10 m-1sr-1, an occurrence rate of 7 %, and a maximum in the morning hours (05:00–07:00 UTC, i.e., 02:00–04:00 LT). The latter coincides with a positive amplitude of the semi-diurnal tide of the meridional wind as measured by the Southern Argentina Agile Meteor Radar. The ambient temperature above the site is on average too high to support local ice formation. We find no correlation with the solar flux; indeed, the latest season of 2023/2024 shows the most NLC detections. This leaves transport from more southerly and colder regions and potentially increasing upper-mesospheric water vapor levels as a result of increasing space traffic as possible explanations for the occurrence and unexpectedly large brightness of NLCs above Río Grande.