Observations of polar mesospheric summer echoes resembling kilometer-scale varicose-mode flows

Abstract

The mesosphere and lower thermosphere (MLT) region represents a captivating yet challenging field of research. Remote sensing techniques, such as radar, have proven invaluable for investigating this domain. The Middle Atmosphere Alomar Radar System (MAARSY), located in northern Norway (69 N, 16 E), uses polar mesospheric summer echoes (PMSEs) as tracers to study MLT dynamics across multiple scales. recently discovered a spatiotemporally highly localized event showing a varicose mode (simultaneous upward and downward movements), which is characterized by extreme vertical velocities (|w|≥3σ) of up to 60ms-1 in the vertical drafts. Motivated by this finding, our objective is to identify and quantify similar extreme events or comparable varicose structures, i.e., defined by quasi-simultaneous updrafts and downdrafts, that may have been previously overlooked or filtered out. To achieve this, we conducted a thorough manual search through a MAARSY dataset, considering the PMSE months (i.e., May, June, July, August) spanning from 2015 to 2021. This search has revealed that these structures do indeed occur relatively frequently with an occurrence rate of up to 2.5% per month. Over the 7-year period, we observed and recorded more than 700 varicose-mode events with a total duration of about 265h and documented their vertical extent, vertical velocity characteristics, duration, and their occurrence behavior. Remarkably, these events manifest throughout the entire PMSE season with pronounced occurrence rates in June and July, while the probability of their occurrence decreases towards the beginning and end of the PMSE seasons. Furthermore, their diurnal variability aligns with that of PMSEs. On average, the observed events persisted for 20min, while the varicose mode caused an average expansion of the PMSE layer by a factor of 1.5, with a maximum vertical expansion averaging around 8km. Notably, a careful examination of the vertical velocities associated with these events confirmed that approximately 17% surpassed the 3σ threshold, highlighting their non-Gaussian velocity distribution and extreme nature.