Preface to special section on Mesospheric Dynamic and Thermodynamic Studies

[1] The equatorial mesosphere is an especially active region dynamically and chemically, owing to the strong dominance by the diurnal tide, as well as by planetary waves and atmospheric gravity waves (AGWs). The very strong tides are now known to set up vertical gradients of horizontal winds that lead to shear instabilities. Winds induced by breaking waves can complement the tidal winds to increase shears and to cause instabilities and overturning. At times, very strong dynamic features, including mesospheric bores, propagate in the region. Wave breaking at low and middle latitudes induces a stress on the mean flow that drives the global residual circulation system, a process requiring further understanding. This circulation interplays the compositions between the lower atmosphere (troposphere and stratosphere) and the mesosphere and lower thermosphere (MALT) through vertical down- and up-wellings at the winter and summer polar regions, respectively. This process, largely driven by the wave-induced stress at low and middle latitudes, is not currently quantified in magnitude, location, or process. [2] The region is too high for in situ balloon soundings and is too low for in situ satellite measurements. Thus investigations of the MALT are usually performed with either in situ sounding rockets or with passive and active remote sensing techniques. Fortunately, the MALT (80–105 km) is rich in remote sensing signatures that include meteor trails, oxygen recombination leading to chemically induced electromagnetic emissions, and metal constituents. These remote sensing signatures have been and are currently exploited through a wide variety of methods which include Na lidar, radar, and airglow imagers. [3] One of the first detailed multi-instrument campaigns to study the equatorial MALT was a joint ground-based and aircraft campaign, the airborne lidar and observations of the Hawaiian airglow (ALOHA). These results were reported on in the ALOHA-90 special section (Geophysical Research Letters, 18(7), 1991). A more extensive campaign, again including airborne lidar and other optical instruments, took place during the fall of 1993 (ALOHA/ANLC-93). A wealth of new information was reported in a series of papers (Geophysical Research Letters, 22(20), 1995; Journal of Geophysical Research, 103(D6), 1998). [4] On the basis of the success of those campaigns, it was decided to establish a more extensive ground-based observing program in Hawaii. The Maui MALT observations of subtropical mesospheric dynamics began in earnest January 2001 from Haleakala, Maui, Hawaii, with the installation of the Na wind/temperature lidar system which operates through the U.S. Air Force (3.7 m diameter) telescope. The power aperture (PA) product of the Na lidar provides unprecedented capabilities to investigate the vertical and temporal structures of the wind and temperature fields of the subtropical mesopause region at high resolution. The Na wind/temperature lidar, meteor radar, a Rayleigh lidar (on Kauai, Hawaii), and a number of optical imagers offer correlative information to investigate the equatorial mesosphere which is dominated by a strong diurnal tide. The Maui MALT data provide detailed (and contrasting) observations to compare to similar investigations made at the midlatitude site (35°) at Albuquerque, New Mexico, using the Starfire Optical Range (SOR) telescope (3.5 m diameter) at Kirtland Air Force Base [e.g., Tang et al., 2005; Liu and Gardner, 2005]. The Maui MALT studies herein report on specific areas (in ten manuscripts) which are (1) correlative measurements of mesospheric winds using Na lidar and meteor radar; (2) vertical thermal structure; (3) theory of mesospheric bores; (4) observation and analysis of dynamic structure including tides, planetary waves, and gravity waves; (5) vertical fluxes of horizontal momentum carried by small-scale waves; (6) instabilities in the mesosphere, dynamic as well as convective; and (7) wave-induced vertical fluxes of heat and constituents. Below are some of the highlights of key results from these studies that are described in detail in the papers following this preface. [5] Remote sensing of winds is a challenge owing to the small-frequency shift imposed in electromagnetic signatures. Wind measured from the Na lidar and meteor radar were compared and are found to be in good agreement. The root mean square (RMS) radar/lidar wind differences observed in the work of Franke et al. [2005] are in the range of 12–17 ms−1 at altitudes below 96 km. The difference is attributed to the sensitivity the lidar has to small-scale waves, which the radar does not resolve. The quality of the meteor radar winds and the long-term measurements (running continuously, day and night) is suitable to study tides and large-scale waves. The radar also provides background wind measurements for the extraction of intrinsic wave parameters from wave signatures as observed by airglow imagers. [6] Observations of the mesopause thermal structure are observed and compared to SOR measurements using Na lidar. The mean nocturnal background of the mesosphere and lower thermosphere (MLT) thermal structure at Maui and SOR are discussed and are compared to the mass spectrometer incoherent scatter (MSIS) model atmosphere [Chu et al., 2005]. These new insights provide a basis for the future study of the thermodynamic process modeling including mechanical cooling by AGWs and heating by radiative processes. [7] Bores or wall waves are discussed theoretically by Seyler [2005]. In the study by Seyler [2005], analysis and numerical simulations presented show that mesospheric bores are consistent with nonlinear internal gravity waves trapped within a thermal inversion layer. [8] Large-scale waves have been analyzed by airglow and meteor radar [Taori et al., 2005] where a terdiurnal wave is observed. The very strong tides impose a vertical wind which produces height changes in the airglow layers. The layer heights of OH and O2 band were studied by correlating layer temperatures to measured Na temperature profiles [Zhao et al., 2005]. Small-scale waves provide a large vertical flux of horizontal momentum. OH airglow images and meteor radar data were combined to deduce intrinsic AGW characteristics, including their vertical flux of horizontal momentum [Tang et al., 2005]. These observations were compared to those measured previously at the midlatitude SOR observations (35°N). Zonal fluxes are strong and westward in winter and are weaker and eastward in summer. An unexpected result is the meridional fluxes which are strongly northward in summer and southward in winter (i.e., seemingly toward the summer pole). The processes leading to this result are not well understood. [9] The mesopause region is observed to have large winds and shears, as well as large vertical temperature gradients resulting in instabilities. In particular, dynamical instabilities resulting from large wind shears should result in the formation of Kelvin-Helmholtz (KH) billows. These billows have been seen before by in situ chemical release rocket experiments. However, as reported by Hecht et al. [2005], the Maui MALT data of both temperature and winds provide a definitive identification of airglow features known as ripples with KH billows. A key result is that the ripple phase fronts are oriented perpendicular to the wind shear and are advected with the local wind. Furthermore, the high temporal resolution of both the airglow and lidar data allowed measurements of the formation and evolution of the KH billows over their whole lifecycle; an investigation not possible with rocket experiments performed to date. A case study of ripples by Li et al. [2005a] associates the ripples with a breaking gravity wave, which in turn accelerates the atmosphere >70 ms−1 within 1.5 km vertical dimension. This large acceleration resulted in a large shear and shear instability, where the resulting overturning was associated with the ripple structure observed in the airglow. The observational data from Maui and SOR were analyzed for instability characteristics, and these statistics are reported by Li et al. [2005b]. The Li et al. [2005a, 2005b] and Hecht et al. [2005] results together show the chaotic nature of the dynamics in the MALT region. [10] Dissipating waves generate heat and constituent fluxes, an analysis which is reported herein and compared with earlier SOR measurements [Liu and Gardner, 2005]. The vertical profiles of heat flux can be linked with the probability of instabilities. While at SOR the heat flux is mainly below 90 km, which can be linked to a convectively unstable region, the Maui heat flux profile has two peaks in the time-averaged altitude profiles which can be linked separately to convective and dynamical instabilities. Note that the cooling rates due to wave dissipation can be >50 K per day in the mesopause region on average. It is significantly higher than earlier studies had modeled and is also larger than the radiative heating rates deduced by satellite measurements. [11] The teams are indebted to the support of the U.S. Air Force, their contractor at the Maui facility, Boeing, and the National Science Foundation Geo/Sciences Division for support of the projects described herein.