Investigating the role of gravity waves in mesosphere and lower-thermosphere (MLT) inversions at low latitudes

Abstract. The mesosphere and lower-thermosphere (MLT) transitional region, encompassing a height range of 60–100 km, is a distinct and highly turbulent zone within Earth's atmosphere. The region is significant owing to dynamics of atmospheric processes like planetary, tidal, and particularly gravity waves, which contribute to the formation of the mesospheric inversion layer (MIL). Investigating these inversion phenomena is crucial for understanding the dynamics of the middle and upper atmosphere, especially regarding stability and energy transfer. These phenomena are associated with energy transfer processes vital for understanding the overall dynamics of the atmosphere. Despite extensive studies on inversions, the formation mechanisms of mesospheric inversions remain poorly understood. Here, upper and lower inversion phenomena and their causative mechanisms are explored. The study utilizes long-term SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) observations during 2005–2020 over the latitudinal, 3–15° N, and longitudinal, 33–48° E, ranges. The results show that the upper inversion occurs more frequently, with a frequency below 40 %, compared to the lower inversion, which occurs below 20 %. The upper inversion occurs within the height range of 78–91 km, with an inversion amplitude of approximately 20–80 K and a thickness of around 3–12 km. In contrast, the lower inversion is confined to the height range of 70–80 km, with an inversion amplitude of about 10–60 K and a thickness of around 4–10 km. Moreover, the gravity wave indicator potential energy shows high energy (below 100 J kg−1) in the upper-MLT region (85–90 km) compared to the lower-MLT region (70–75 km), with less than 50 J kg−1. Considering gravity waves, the Brunt–Väisälä frequency (N2) stability criteria indicate instability in the upper-MLT region, with very low values compared to the lower-MLT region. This suggests that the high amount of gravity wave potential energy is a consequence of the higher instability in the upper inversion compared to the lower inversion.