A Case Study of Midlatitude Noctilucent Clouds and Its Relationship to the Secondary-Generation Gravity Waves Over Tropopause Inversion Layer

A sporadic case of noctilucent clouds (NLCs) was observed unexpectedly on the night of the 6-7 July 2020 in Beijing (40 degrees 2 ' N, 115 degrees 30 ' E). The noticeable wavy structures and observed ambient temperature (135.54K at the mesosphere and lower thermosphere [MLT]), both indicated that the increase in temperature oscillations could be the cold phase of gravity waves (GWs). The reasons for NLC formation were analyzed based on the observations and model data sets in our study. The real-time synoptic analysis revealed that there were GWs originally generated by a squall line in the troposphere. Due to the blocked effect of a stable tropopause inversion layer (TIL), the GWs broke, leading to strong energy dissipation near the TIL. The reverse ray tracing analysis between the TIL and NLCs' layer revealed the travel distance (206.88 km) and time (49.91 min) of GWs. These findings show that the turbulence over the TIL (at approximately 14.64 km) excited secondary GWs, which propagated upwards toward the mesosphere and probably interacted with diurnal and semi-diurnal tides. The cold phase of the larger-amplitude waves can provide optimal conditions for NLCs forming. Our study highlights the significance of dynamic coupling mechanisms regarding the effects from troposphere to MLT thermal conditions and offers a case study for the increasing occurrences of NLCs at midlatitudes.

Automated summary

A sporadic case of noctilucent clouds (NLCs) was unexpectedly observed in Beijing on the night of July 6-7, 2020. The presence of noticeable wavy structures and a higher ambient temperature at the mesosphere and lower thermosphere (MLT) suggested that an increase in temperature oscillations may have been caused by the cold phase of gravity waves (GWs). Our study analyzed the reasons for the formation of NLCs based on observations and model data sets. It was found that GWs generated by a squall line in the troposphere broke due to the blocking effect of a stable tropopause inversion layer (TIL), resulting in strong energy dissipation near the TIL. Reverse ray tracing analysis revealed the travel distance (206.88 km) and time (49.91 min) of GWs between the TIL and the NLCs' layer. The study also showed that turbulence over the TIL excited secondary GWs, which propagated upwards towards the mesosphere and potentially interacted with diurnal and semi-diurnal tides. The cold phase of larger-amplitude waves provided optimal conditions for NLC formation. This study emphasizes the importance of dynamic coupling mechanisms between the troposphere and MLT thermal conditions, and provides a case study for the increasing occurrences of NLCs at midlatitudes.