Mesosphere inversion layers and stratosphere temperature enhancements

Abstract

[1] It has been known for at least 30 years that vertically narrow thermal layers form within the middle atmosphere. Two types of temperature enhancements, the low-latitude to midlatitude mesosphere inversion layer (MIL) and the highlatitude winter stratosphere temperature enhancement (STE), have both received much attention within the atmospheric science community because of their unexplained formation mechanisms and potential impacts on the middle-atmosphere global circulation. Numerous experimental, numerical, and theoretical studies have attempted to explain certain aspects of these respective thermal layers, but no one theory consistently and satisfactorily describes all the features observed. We present a review of the literature and explicitly propose a classification scheme based on the different formation mechanisms suspected to cause these events. For the MIL we demonstrate that there are two subtypes. The first one is tidally driven and tends to occur above $85 km. This MIL originates from large-amplitude tidal waves propagating into the mesosphere and their subsequent nonlinear interactions with gravity waves, which can often create the appearance of a ''double MIL'' separated by approximately one vertical tidal wavelength ($25 km). The other subtype of MIL is formed by a climatological planetary wave dissipation mechanism that occurs at a zerowind line. The dissipation of the planetary wave tends to generate a mesoscale ($1000 km) inversion layer in the range of 65 -80 km. These two formation mechanisms explain a host of observed characteristics, including the reason behind the downward progression of some MILs and not others, the different climatological nature of the two forms of MIL events, and the relative scarcity of MIL observations at high latitudes. The STE is believed to be generated by an altogether different process, namely, the nonlinear interaction between the polar vortex and planetary waves/Aleutian High. The induced temperatures typically peak around 40 km and often exceed 300 K, generating what appears to be a ''low, hot stratopause.'' When vertical temperature profiles are combined with synoptic analyses, one observes that the STE is the consequence of high-latitude vortex interactions creating a baroclinic atmosphere, i.e., a downward adiabatic compression induced by an ageostropic flow. We summarize the details of the relationship between this feature and sudden stratospheric warmings, as well as the potential for in situ gravity wave generation. We close with a review of currently unexplained MIL/STE features and offer new directions for future middle-atmosphere thermal layer research.

Observations and Modeling of the Upper Mesosphere: Mesopause Characteristics, Inversion Layers, and Bores

“…This behavior appears to be consistent with conclusions drawn by Meriwether and Gerrard [2004]. They distinguish two types of mesospheric TILs: a "higher" TIL, in the altitude range -85-100 km that is tidally driven and amplified by nonlinear gravity-wave interactions; and a "lower" type, below -80 km, that is caused by dissipating planetary waves.…”

“…It therefore seems possible that our observations can be explained in these terms, even though the lower range in Figure 15 is above 80 km. (According to Meriwether and Gerrard [2004], the complexity of the processes involved in TIL formation sometimes obscures the distinction between the two types. For example, variations in the strength of the tide on a particular day could modify the altitude range in question.)…”

“…That is, we wished to determine, for different seasons of the year, the latitudes, local times, and altitudes where they are seen and, conversely, where they are not seen. The discrimination according to LT is important because of the role of the tides, which is not completely understood [e.g., Meriwether and Gerrard, 2004] but clearly a factor in producing them.…”

“…One particularly interesting feature is the temperature inversion layer (TIL). Mesospheric TILs, which have been the subject of great research interest in recent years [e.g., Leblanc and Hauchecorne, 1997;Gardner, 2000, Meriwether andGerrard, 2004;Sassi et al, 2002, Huang et al, 1998, appear with regularity in the SABER data. Each of the temperature profiles shown in Figure 5 in the preceding section has more than one TIL.…”

Influence of gravity waves and tides on mesospheric temperature inversion layers: simultaneous Rayleigh lidar and MF radar observations

“…They have been studied by several authors in the past few decades and most of the important findings are recently reviewed by Meriwether and Gardner (2000) and Meriwether and Gerrard (2004). They are found to occur more frequently in winter months at mid-latitudes (Leblanc and Hauchecorne, 1997) and in equinox months in low-latitudes (Siva Kumar et al, 2001).…”