## 7 Discussion  
In this study, we are making a qualitative assessment of mesospheric anomalies that can be associated with mesospheric inversion layer, large anomalies that persist several hours to few days. While three main dynamic phenomena [6] have been proposed to explain these large temperature deviations, there is a question about the attribution of their respective origin. Previous observations suggest that there is a continuum in the altitude of these dynamical anomalies. The statistical analysis of the large anomalies occurring in the mesosphere suggests that these features can be grouped into three classes, yielding multiple regions of strong positive temperature lapse rate on various spatial and temporal scales. The statistical analysis of the large anomalies occurring in the mesosphere above the Observatory of Haute-Provence, as seen by systematic lidar sounding for four decades,  
Figure 7: Histogram of the MIL frequency (the percentage of times the MILs occurred) for each cluster from 1978 to 2019.  
suggests that these features can be grouped into three classes. The temperature deviations considered have been separated from smaller anomalies associated either to measurement noise or either to gravity waves. Considering only temperature vertical characteristics, the differentiation between various anomalies with significant positive lapse rates can be found in the altitude of occurrence and their amplitude while the value of the derived temperature positive gradient appears not to be a determinant factor between the different MILs. The seasonal climatology of the MIL occurrence is similar to previous studies and is somehow similar for the different MIL classes. This is not a surprising result, while all the dynamical processes that may be involved in the MIL generation all exhibit a strong seasonal cycle. Whatever the initial cause of MIL generation, either tides, gravity waves, or planetary waves, the dissipation of the gravity waves could maintain a quite stable temperature inversion as the breaking of waves occurs preferably inside and above the inversion layer [2]. [PERSON] et al. [7] seems to indicate that planetary waves could induce MIL. This effect can occur preferably in the lower mesosphere, where the largest anomalies associated with stratospheric warming are observed. These MILs could be associated with the class named Lower MIL. The numerical simulations [PERSON] et al. [46] have confirmed the role of planetary waves, with, however, a regional response. Tides are the largest in the upper mesosphere. However, [PERSON] et al. [54] shows that MILs above California associated with tides occur in the bottom altitude range around 60-65 km but with a very large amplitude. This can be probably associated with the Class 3, named Major upper MILs. Gravity wave breaking occurs above 60 km [40], and then the pure gravity-wave-induced MILs may be associated with the second class called Minor Upper MILs. Thanks to the four decades of measurements above the OHP, the time evolution of the MIL occurrence have been estimated. While all MILs do not reveal any trends that can be associated with any climate change, their evolution has been analyzed according to their classification (Figure 5). A similar behavior can be noted for the lower MIL compare to the global ones. The minor upper MILs reveal a positive trend, while their increase is not significant, as it is mainly observed after 2016. The major upper MILs category shows a nearly constant occurrence, showing that lidar detection is not biased by the detection capability of the lidar that has known some changes since the beginning of the series [23; 24].