MLT
This is a region where processes balance one another in a manner not found anywhere else in the atmosphere and is frequently considered to be a distinct region. In the past few years, much has been discovered about MLT through further observations and enhanced numerical modeling capabilities. This review will outline the dynamics of the MLT, focusing on recent advancements and current questions.
The ideas will be demonstrated mainly with the outcomes of simulations performed using the Whole Atmosphere Community Climate Model. WACCM is part of the Community Earth System Model, a suite of model components at the National Center for Atmospheric Research. WACCM is a global climate model that covers the Earth’s surface up to the lower thermosphere. It also contains interactive chemistry with realistic surface concentrations and natural and anthropogenic trace gas trends.
External forcing by solar variation and energetic particle fluxes are also included. Model results presented here are based on long integrations with WACCM version 3.5. The version does not contain interactions with the oceans but rather specified sea surface temperatures based on observational records of recent decades. Comprehensive comparisons between WACCM and other models are given by Eyring et al. and WMO. There is also a higher-top version of the model, medical,
urgent care, dermatologist near me, medical billing and coding, american medical association, ems, ultrasound machine WACCM-X, which has an upper boundary at 2.5 × 10−9 hPa.
Measurements and Their Limitations
The MLT dynamical conditions have been remotely measured using ground- and space-based instruments and in situ using rockets. None of the instrumentation can provide a complete picture, but collectively, they present a more balanced picture of the fundamental state and its variations than is available with any one measurement technique. Ground-based and rocket-borne instruments have limited geographic coverage but can offer high vertical resolution. Continuous ground-based observations provide a lot of information about local time variations.
Satellite sensors offer a global or near-global view but with sparse local time sampling, and several also offer sparse spatial resolution. The most important dynamical fields that can be derived from observations are kinetic temperature, neutral density, horizontal wind vectors, and small-scale perturbations to airglow emissions due to waves. Specialized measurements like turbulence have also been made.
Passive ground-based observations exploit the light radiated in the MLT to infer atmospheric properties. The light sources employed include the diverse airglow emissions, which, depending on the chemical processes generating the light, are from different altitudes. OH Meinel emissions, which occur in a layer around 87 km, are employed to infer temperature and aspects of the small-scale wave evolution. Other used emissions include the O2 atmospheric band, the sodium D line, and the oxygen green line. These observations are restricted to nighttime and clear or light-overcast sky conditions. Visible polar mesospheric cloud observations in high-latitude summer offer a short-duration but dramatic way of viewing waves in the MLT.
Climatology
The word climatology is used to describe multiyear means and their seasonal changes. The climatology can be used to forecast overall conditions to be observed at a given location and time of year. An older but widely utilized reference for the climatology in the mesosphere is the MSIS mass spectrometer incoherent scatter empirical model. The latest variant is referred to as NRLMSIS. Since further new satellite observations have been piling up at an accelerating pace since this was published in 2000, the MSIS model does not convey many aspects regarding the MLT dynamics, constitution, and variation.
The NRLMSIS-00 uses the empirical model containing parameters to quantify the seasonally, solar-cycle, and geomagnetic activity-dependent variations. The associated Horizontal Wind Model provides horizontal wind climatological maps. The HWM07 incorporates seasonal and local time changes caused by planetary waves and tides under solar quiet periods. There is also the alternative for the upper thermosphere during solar active periods.
The MLT climatology itself has been developing partly as a result of novel observations but partly because the climate itself is dynamic. The MLT region has year-to-year variability and long-term trends caused by evolving external forces and varying atmospheric composition.
Waves
Generally speaking, a disturbance propagating in space and time is what is referred to by the word wave. The three aspects of waves are key generation, propagation, and dissipation. The generation is accorded lesser discussion in this paper since the majority of wave activity in the MLT starts below, that is, in the troposphere or stratosphere. But secondary waves, which are generated from the interactions between waves, may be produced anywhere within the system, even in the MLT.
Waves will not couple with the background atmosphere except when they are transient or decaying. This theorem of noninteraction, formulated by Andrews and McIntyre, holds for Rossby waves, gravity waves, and tides. For each of these waves, medical center, nephrology, otolaryngology, endocrinology, medical store, emrs, pulmonology the conditions for propagation are a function of wave characteristics as well as of the background in which the wave is propagating. The background zonal wind is especially significant due to its high velocity and large seasonal and year-to-year fluctuations.
Another idea that helps discuss wave behavior is the critical layer, alternatively referred to as the critical level. This is a location in the atmosphere in which the phase speed of a wave is the same as the background wind speed. Waves will not propagate through this area; they will be attenuated or, under some conditions, reflected. The critical layer theory is most effective in anticipating the propagation of gravity waves and planetary waves since they possess phase speeds whose magnitudes fall within middle atmosphere meridional and zonal wind ranges.
Conclusion
Knowledge of the dynamics of the MLT region is continually refined with the establishment of new measurements and analysis methods and numerical models incorporating improved descriptions of the physical processes. This review paper presents an overview of the fundamental state and processes of large-scale dynamics. The review presents some of the advancements over the last decade.
The advent of several global whole atmosphere numerical models permits the study of interactions of the MLT with the troposphere and stratosphere below and the thermosphere above, as well as dynamics-chemistry interactions. Nonmigrating tides are now recognized as being important. The new appreciation follows from new observations that have made it possible to more fully characterize the global structure of these tidal modes.
Proof for linkages in tides and mean temperatures between the winter stratosphere and summer MLT is proof of global coupling. Uncertainties remain, but it seems that the coupling mechanism involves wave propagation, changes in background wind and temperatures, and large-scale circulation. The recent release of new data for the variations of spectrally resolved solar flux with the 11-year solar cycle has brought into question the solar cycle and the atmospheric response. Temperature and trace species perturbation observations during active periods in NH winter have revealed strong transport and mesospheric changes previously unobserved. Characterization of the transport and dynamics during these times has resulted in an improved understanding of the vertical coupling in the polar winter middle atmosphere.
Two long-duration satellites, TIMED and Envisat, have offered global measurements of the middle atmosphere for improved characterization of climatology, interannual variability, and the interactions of chemistry with dynamics. This has been especially useful in the last decade due to highly dynamic and variable processes in the NH winter middle atmosphere.
The network of ground-based measuring systems has also expanded; the synergy between global satellite observations and high-resolution but localized ground-based data has been utilized in certain studies but is poised for greater use to define and interpret coupling across horizontal scales. Future progress in understanding the MLT will be significantly diminished if there is a termination of, or a hiatus in, satellite observations covering the MLT region. This is a significant threat to progress that cannot be replaced by improved ground-based observing systems and cutting-edge numerical models.