A review of the mesosphere inversion layer phenomenon

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 105, NO. D10, PAGES 12,405-12,416, MAY 27, 2000 A review of the mesosphere inversion layer phenomenon John W. Meriwether Department of Physics and Astronomy, Clemson University, Clemson, South Carolina Chester S. Gardner Department of Electrical and Computer Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois Abstract. An active topic of current research in aeronomy is the study of the dynamics of the mesosphere and lower thermosphere (MLT) from 60 to 130 km, especially in regard to the influences that govern variability. The physical processes of this region are diverse and complex with strong coupling between the MLT and the adjacent atmospheric regions brought about largely by the propagation and dissipation of atmospheric gravity waves (GWs) from sources above and below. The measurements of MLT winds and temperatures required for such studies represent daunting technical challenges. At low and midlatitudes the mesosphere inversion layer (MIL) phenomenon, a - 10 km wide region of enhanced temperatures (AT K), is observed with great regularity in both the upper mesosphere (60-70 km) and the mesopause ( km). Observations are largely based upon Rayleigh and Na temperature lidar systems but coherent radar observations have shown that the MIL phenomenon is linked to layers of turbulence occurring in both the topside and the bottomside regions. GW activity is believed to play an important role in the development of a linkage between the MIL and the tidal structure through GW coupling that results in an amplification of the tidal thermal structure. This linkage is readily evident for the upper MIL but is seen only occasionally for the lower MIL. Further study of MIL properties should emphasize continual 24 hour temperature observations, especially for the lower MIL region, to confirm the linkage of the development of the MIL to the MLT tidal structure. Introduction An interesting and poorly understood feature of the mesosphere and lower thermosphere (MLT) region is the phenomenon of the mesospheric inversion layers (MILs). This name derives from the positive lapse rate observed for the bottomside portion of the temperature profiles of these layers. Schmidlin [1976] was the first to report the MIL as an oftenobserved integral part of the MLT thermal structure. Further explorations with a variety of remote sensing and in situ techniques, including the Rayleigh lidar [Hauchecorne et al., 1987; Jenkins et al., 1987], Na temperature lidar [She et al., 1990; Bills et al., 1991; She et al., 1993; Bills and Gardner, 1993; States and Gardner, 1998; Chen et al., 2000], satellite photomerry [Clancy et al., 1989, Leblanc and Hauchecorne, 1997], and falling spheres [Schrnidlin, 1976, Labken et al., 1994] have all frequently detected the MIL phenomenon. Careful intercomparison of temperature profiles by these different techniques [Labken et al., 1994] shows excellent consistency among the several sets of measurements. The MIL is clearly a physical phenomenon rather than a measurement artifact. MILs are observed at low to midlatitude regions for two different altitude regions, which at midnight tend to be separated by the equivalent of one diurnal tidal wavelength at - 70 km and Visiting Research Scientist at the MIT Haystack Observatory, Westford, Massachusetts. Copyright 2000 by the American Geophysical Union. Paper number 2000JD /00/2000JD ~ 95 kin. Each layer typically displays a width of-10 kin. The measured amplitudes of the enhancements above the MLT "background" thermal profile are typically K but can be as high as 100 K [Meriwether et al., 1994]. Generally, lidar observations of both the lower and the upper MILs have been constrained to nighttime periods, and the role of tides, which may be significant in contributing to the observed development of either MIL, is difficult to assess without full 24 hour coverage [States and Gardner, 2000a, b]. The lower MIL is a particularly characteristic and persistent feature of the winter MLT thermal structure [Hauchecorne et al., 1987]. Events with amplitudes greater than 10 K are observed more than 80% of the time in December and January. In midsummer the occurrence rate drops to about 30%. Amplitudes greater than 30 K are observed more than 20% of the time in winter but are rare in summer. The mean monthly amplitude is greater than 20 K in winter and between 5 and 10 K in summer [Hauchecorne et al., 1991; Leblanc and Hauchecorne, 1997]. The altitude of the MIL peak temperature also changes seasonally. It is highest in summer (-75 km) and lowest in winter (~65 km) [Hauchecornet al., 1991; Whiteway et al., 1995]. Satellite observations have shown that the phenomenon can extend over large geographical areas (millions of km2). The upper MIL is typically seen with an amplitude ranging from 10 to 35 K and is also a persistent feature of the mesopause region [She and Yu, 1993]. In most cases the observations have been restricted to a narrow range of local times at night, but Na temperature measurements extending over 24 hours have been obtained successfully [States and Gardner, 2000a, b; Chen et ai., 2000]. Inspection of the Na temperature profile for data averaged over 24 hours shows little indication of the MIL feature which, in 12,405

2 12,406 MERIWETHER AND GARDNER: MESOSPHERE INVERSION LAYERS contrast, is readily seen in individual profiles for data integrated [Andrews et al., 1987; Wehrbein and Leovy, 1982; Rodgers et al., over a few minutes [States and Gardner, 2000a]. These results 1992], by turbulent heating caused by breaking gravity waves present convincing evidence that the development of the upper (GWs)( ~1-5 K/d) [Fritts and van Zandt, 1993; Labken et al., MIL is related to the MLT tidal behavior [States and Gardner, 1993], and by dynamical cooling associated with the vertical 2000 a, b]. downward transport of heat by dissipating waves (--30 K/d) Several review papers have been written in recent years [Walterscheid, 1981a; Weinstock, 1983; Gardner and Yang, concerning more general aspects of middle atmosphere dynamics 1998]. Many of these processes produce thermal perturbations [Holton et al., 1995; Hocking, 1996; Hamilton, 1996; most pronounced near 90 km. Thus to summarize, the McLandress, 1998; Hamilton, 1999]. Our review is aimed at combination of these studies over the past decade has produced summarizing the current state of our understanding of the MIL considerable progress in achieving a clear understanding of the regarding climatology and the various mechanisms that have been underlying thermal structure of the MLT region. proposed for the production of this curious phenomenon. Because the MIL phenomenon is found at low and midlatitudes, 3.MIL Vertical Structure and Temporal Behavior this review will not touch upon the interesting but complex thermal behavior of the MLT region for high latitudes [Duck et The papers by Hauchecorne et al. [1991], Leblanc and al., 1998; Gerrard et al., 1998; Whiteway et al., 1997]. The Hauchecorne [1997], Whiteway et al. [ 1995], Yu and She [ 1995], morphology of the high-latitude thermal structure is closely Meriwether et al. [1998], and States and Gardner [2000 a, b] coupled to the stratospheric phenomenon of the polar vortex and provide an excellent overview of the characteristics of the lower the PW activity that underlies the formation of stratospheric warmings. 2. Physical and Chemical Processes Important to the MLT Energy Budget and upper MILs with respec to the vertical thermal structure and temporal behavior. Most literature references to the MIL refer to the lower MIL observed at midnight at altitudes between about 60 and 70 km. This is largely because the general instrumental limitations of Rayleigh lidar systems or the falling sphere technique have prevented the extension of the observational range to altitudes above 80 km. The development of Na temperature lidar systems, particularly during the past decade [She et al., Understanding the production sources and climatology that governs the behavior of the MIL requires the achievement of a 1990; Bills et al., 1991], made possible the extension of deeper understanding of the highly complex MLT region in temperature measurements routinely to 105 km. These regard to the workings of the diverse physical processes. Many dynamical, chemical, and radiative processes play important roles measurements have revealed a second inversion layer near 95 km which is linked to tidal activity and chemical heating [Dao et al., in defining the MLT thermal structure. The mean atmospheric 1995; States and Gardner, 1998, 2000 a, b; Berger and yon Zahn, background temperature generally decreases monotonically from the stratopause near 50 km to the mesopause near 100 km. This negative lapse rate is largely determined by radiative balance, i.e., the balance between absorption and emission of solar radiation [e.g., Gavrilov and Roble, 1994; Roble, 1995]. Below 95 km, mesospheric ozone absorbs UV radiation at wavelengths in the Hartley band between 200 and 300 nm and is the main source of solar heating in this region. Because the ozone mixing ratio 1999]. The question arises as to whether the lower MIL may also be related to the propagation of thermal tides. To illustrate an example of the two MIL structures, Figure 1 shows a series of composite temperature profiles obtained by Dao et al. [1995] for balloon-borne sensor profiles (ground-30 km) combined with Rayleigh (25 to 80 km) and Na temperature (80 to 105 km) lidar measurements obtained in Hawaii during the 1993 Airborne Lidar and Observations of the Hawaiian Airglow decreases above the stratopause (-50 km), the mean temperature (ALOHA) Campaign. Analysis of these low-latitude results decreases monotonically with increasing altitude in the mesosphere. Above 95 km the combined absorption of solar EUV and UV radiation by [O] and [02] from the continuum region below 100 nm and from the spectral regions of the Schumann-Runge bands and continuum ( nm) and the Lyman-cx line (121.5 nm) is the dominant heat source, so in the thermosphere, the temperature again increases monotonically shows that the MILs at 90, 70, and 50 km all appear to be propagating downward with a phase speed representative of the diurnal tide. However, the observed MIL temperature amplitude is considerably larger (~5 times) than that predicted [Hagan, 1996; Hagan et al., 1997] using the global scale wave model (GSWM), which is based upon a tidal model combined with an estimate of the affect of GW interactions with the tidal structure. with increasing altitude. Below 95 km, tropospheric IR It is generally regarded to be representative of most tidal model absorption by water vapor and stratospheric absorption by ozone play a significant role in the mesosphere by forcing tidal activity. Near the mesopause there are several other important radiative, predictions. Examination of the GSWM predictions [Hagan, 1996; Hagan et a l., 1997] revealed that the combination of the diurnal and semidiurnal tidal temperature oscillations will chemical, and dynamical processes that influence the temperature generate a lower MIL, exhibiting a downward phase propagation structure while contributing to the mesospherenergy budget and complicating this overly simplified picture. The energy budget of the mesopause region is dominated by the absorption of solar UV radiation by 02 and 03 (~10 K/d) [Mlynczak and Solomon, 1993], by chemical heating from consistent with that of the diurnal tidal component. The amplitude of the GSWM MIL is ~ 1-2 K, which is 10 to 30 times smaller than the amplitudes detected in most observations of the lower MIL [Meriwether et al., 1998]. Midlatitude Rayleigh lidar measurements have detected exothermic reactions involving odd-oxygen and odd-hydrogen occasionally a downward phase progression of the lower MIL species ( K/d) [Mlynczak and Solomon, 1991; Mlynczak and Solomon, 1993; Reise et al., 1994; Meriwether and Mlynczak, consistent with the phase speed of the diurnal tide [Leblanc and Hauchecorne, 1997; Meriwether et al., 1998]. Generally, 1995] which include quenching of excited photolysis products however, the lower MIL does not exhibit a phase progression. [Mlynczak and Solomon, 1993; Shimazaki, 1985], by radiative cooling associated with infrared emissions of CO2 (- -15 K/d) Thus a link between the lower MIL and the thermal tidal structure within the MLT region is seen on isolated occasions, but this

3 MERIWETHER AND GARDNER: MESOSPHERE INVERSION LAYERS 12,407 loo Temoerature (K)+ Offset( o 4O U T (hr) Figure 1. Composite temperature lidar profiles at 30-min intervals starting from 2030 LT (0830 UT). The pluses correspond to the altitudes at which Rayleigh lidar temperature is set equal to Na W/T. Note the maxima near 90 and 65 km and their downward phase progression [from Dao et al., 1995, Figure 1]. bond is often not detected, perhaps because the strength of the coupling interaction may vary. be regarded as "fossil layers" that arise from the sampling of the MLT thermal structure over a fraction of the diurnal cycle. The duration of time chosen for the averaging of Rayleigh lidar observations of the lower MIL events, generally ~1 hour, 4. Climatology of the MIL and Possible Link will modify the MLT thermal structure removing to some extent to Planetary Wave Activity the high spatial frequency structure. This shortcoming of the measurement technique needs to be taken into account when evaluating the temporal behavior of the lower MIL. The long averaging times are required at the altitudes of the lower MIL because the sensitivity of Rayleigh lidar systems is generally too weak to observe thermal fluctuations of a few degrees magnitude with a temporal resolution less than a few minutes. Moreover, the correct approach to studying temporal variations, especially in regard to MLT thermal tides, would require the determination of The climatology of the lower MIL at midlatitudes has been studied from the ground with Rayleigh lidar systems [Leblanc and Hauchecorne, 1997; Hauchecorne et al., 1991] and from space [Clancy and Rusch, 1989; Leblanc and Hauchecorne, 1997]. Midlatitude climatologies, based upon ground-based lidar studies of the upper MIL have been carried out by Bills and Gardner [1993], Senfi et al. [1994], and Yu and She [1995]. The horizontal spatial scale of the lower MIL is known to span many the mean vertical thermal structure <T, calculated by averaging hundreds of kilometers [Hauchecorne and Maillard, 1990]. The data from 24 hour observations of the MLT structure within the analysis reported by Leblanc and Hauchecorne [1997], based altitude range of 50 to 80 km. This is generally not possible for Rayleigh lidar measurements due to the problems of daytime observations. Deviations of temperature observations relative to <T may thus be contaminated by the residual temporal variations of any large-scale structure (semidiurnal, diurnal, PW, GW ) which remain for observations limited by partial coverage of the diurnal cycle. upon UARS/ISAMS and UARS/HALOE temperature data, shows s{gnificant longitudinal and latitudinal structural variations in the lower MIL amplitude. The latitudinal variations are largely symmetric about the equator which may be symptomatic of a tidal influence. The profiles such as those reported by Meriwether et al. [1998] show a region of cooling relative to the climatological In contrast, the sensitivity of the Na resonance temperature profile (such as that of the upgraded Mass Spectrometer and technique is such that the upper MIL can be observed almost always with excellent temporal resolution. States and Gardner [2000a] found that nighttime temperature profiles almost always exhibit a MIL near 95 km, thereby producing the double mesopause structure reported by She et al. [1993]. When these same observations are averaged with the daytime data to determine the mean temperature profiles, hardly any indication of a MIL can be seen in the averaged temperature profile. Thus Incoherent Scatter (MSIS) model published by Hedin et a1.[1990]) on the bottomside of the stratopause region. The depression ranges from 10 to 20 K. Such depressions relative to the CIRA climatological model (a model similar to that of MSIS) were also noted in the climatology study reported by Leblanc et al. [1998] and in the Utah observations reported by Meriwether et al. [1998]. Leblanc et al. [1998] averaged numerous series of nighttime lidar measurements to examine this further and the upper MILs observed in nighttime Na lidar observations can concluded that the CIRA model is too warm for the winter lower

4 12,408 MERIWETHER AND GARDNER: MESOSPHERE INVERSION LAYERS stratosphere. This may imply a need to account for thermal effects caused by propagating PWs that may often reach the lower stratosphere region during winter. Na temperature data reported by States and Gardner [2000a] show that the midnight upper MIL layer is highest (~ km) during midsummer and lowest during midwinter (~90 km). The largest amplitudes (> 10 K) occur in December and January. This behavior is similar to that observed for the lower MILs. This is latitudes of GW forcing of the zonal wind circulation of the MLT region. Measurements of winds and temperatures from space and ground have been compared with results from improved global circulation models of the MLT, which has been based upon the continued development of the GSWM. This work has shown that for midlatitudes the diurnal tide dominates the MLT region near the vernal equinox and is much weaker near the solstice where the semidiurnal tidal component becomes dominant [Burrage et al., 1996; Shepherd et al., 1998; Williams et al., 1998, Manson et al., 1999]. Manson et al. [1999] suggested that our understanding of the MLT global circulation in terms of the comparison of the GSWM modeling with coherent radar observations of the diurnal tide might be classified as very good to excellent. In contrast, progress in modeling the behavior of the semidiurnal tidal component has been more limited. The thermal variability of the mesosphere at midlatitudes over the time scales of a few weeks is substantial. Both planetary and tidal waves are mixed in with the annual variations of solar This requirement of continual measurements of temperature and winds over 24 hours over the entire MLT region represents an enormous challenge. Only recently has this been achieved for a Na wind/temperature lidar facility [Yu et al., 1997; States and Gardner, 1998; Chen et al., 2000] with observations that apply to the upper portion of the mesosphere. Because of the limitations of the Rayleigh lidar technique and a low power-aperture product, Rayleigh lidar measurements have been constrained to the night evidence that seasonal changes in the phase and amplitude of the and to altitudes below 80 km. Measurements with the Na lidar diurnal tide may be responsible for the seasonal variations in the heights and amplitudes of both lower and upper MILs. A dominant annual cycle for temperature is observed at midlatitudes for altitudes below 65 km: a warm stratopause during the summer and a cold winter stratosphere and lower mesosphere during winter. Between 65 and 98 km the variation of the annual cycle is opposite that of the stratopause: i.e., warm in winter and cold in summer [She et al., 1995]. Stratospheretropospher exchange (see review by Holton et al. [1995]) from the tropics to the polar regions takes place through the Brewer- Dobson meridional circulation cell that features upwelling in the technique are limited by the density distribution of the Na layer to altitudes of about 80 to 105 km. Simultaneous measurements by both techniques have been rare, and the development of a cohesive understanding of the processes involving both MIL regions has been hampered by the lack of such measurements. If there is a tidal linkage involving the lower MIL, an important question is: why is the thermal amplitude of the MIL so much larger than the amplitude predicted by the GSWM? This question is especially compelling since the predicted GSWM winds compare favorably with UARS and ground-based medium frequency (MF) radar measurements of tidal wind amplitudes and tropics and the summer polar region and downwelling at winter phases [Hagan et al., 1997, 1999]. However, the thermal mid-latitudes and the winter polar area [Plumb and Eluszkiewicz, amplitude of the MIL as measured by lidar systems is always 1999]. It is interesting that the latitudinal extent of frequent substantively greater, typically a factor of 5, than what is appearances of MIL events coincides with the region of predicted by tidal models such as the GSWM [Meriwether et al., subsidence at midlatitudes within the "surf zone" that denotes the 1998; States and Gardner, 1998; Gille et al., 1991; Dao et al., 1995]. Part of the answer to this question is that the model predictions apply to the overall global tidal structure while ground-based measurements refer to the temperature structure of a local region. Thus if the MIL is a result of GWs coupling into the thermal tidal structure, then the observed MIL amplitude determined from Rayleigh and Na temperature lidar measurements and therefore representing a spatially localized quantity would be larger than that predicted by GSWM calculations. However, the lower MIL has an amplitude, especially for winter examples, which is always much larger than the GSWM tidal predictions. This suggests that the appearance of the upper MIL feature is the product of a fundamental underlying phenomenon. A question that arises is what is the coherent radar wind observational equivalent of the enhanced MIL temperature amplitude? One answer is that it is not clear just what the wind signature of the MIL phenomenon would be. Would the wind speeds be large in correspondence to the large temperatures? If the tidal linkage is as important as it appears to be in contributing heating and GW forcing to produce a nonuniform behavior of cooling from winter to summer in the upper mesosphere and heating from winter to summer in the stratopause region. There remains much in the details of the stratospheric and MLT global circulation in its many variations to be examined and studied in future investigations. to the MIL evolution, then this expectation should be true. Moreover, it is not clear that good radar measurements can be attained for MIL events. Coherent radar systems do not ordinarily measure winds below km during the night, and so the nighttime radar wind signature of the lower MIL cannot be observed. Coherent radar observations of the upper MIL event represent an altitude range (> 90 km) for which the direction and 5. Production Mechanisms for MILs speed of the MLT winds are changing rapidly, which may represent a problem in interpreting the radar results. The origin of the MIL is not understood in spite of the many years that have passed since the first MIL discovery was reported Furthermore, comparison of groundbased observations with GSWM model predictions inherently implies averaging of the by Schmidlin [1976]. Possible physical mechanisms proposed for ground-based data to remove short-term variations. This the production of the MIL structure are diverse. A confusing factor is that it is not clear that the lower and upper MIL structures are both generated by the same mechanism. Perhaps a averaging process may reduce the observed amplitudes of radar winds. Studies conducted by the coherent radar and meteor radar instruments have tended to emphasize the tidal response key reason for the slow progress in developing an understanding determined upon numerical fits of the radar observations to 12 of this phenomenon is the importance of being able to average and 24 hour harmonics. Future coherent radar work should over the entire diurnal cycle to obtain a MLT thermal profile for which possibl effects of tidal activity have been averaged out. consider examining transient behavior that might be related to the upper MIL phenomenon.

5 ß ß MERIWETHER AND GARDNER: MESOSPHERE INVERSION LAYERS 12,409 1oo go ß!, i ß i, O0, i ß!,!, oo!, )0,!, i i ß i, )o o 7O 80 ' 7{} -,o Diffusion is0 4O I.1', I 3,0 ' Temperature (K) 130 ' ' ' -' '! o 1oo 200 :3o0 (,',,'/,) -15' ' -5' 5,, 15,. 25 Heatinil Rates (K/day) Figure 2. (a) Limits of uncertainty (short dashed ) for the observed temperature profile of November 7/8, 1991, compared with radiative equilibrium (long dashed) and a simulation (solid) using the eddy diffusion coefficient profile of (b). (c) Corresponding heating/cooling rates due to IR radiative transfer, turbulent diffusion, and dissipation [from Whiteway et al., 1995] Breaking Gravity Waves Whiteway et al. [1995] observed numerous lower MILs from At first it seemed that the MIL region of enhanced their Toronto Rayleigh lidar and noted frequent occurrences of temperatures above the thermal background could be attributed to events in which the topside lapse rate approached that of the the breaking of GWs. When such events occur, they are usually adiabatic limit,--10 K/km. Their interpretation of this finding accompanied by regions of zonvective or shear instabilities. was that this represented the signature of a well-mixed turbulent Convective instabilities arise when the gradient of the total layer. They constructed a model to simulate the several heating potential temperature becomes negative while the dynamic or and cooling functions within the topside region of the MIL, which shear instability occurs when the Richardson number, R, = N2/ were caused by the absorption of solaradiation accompanied by (du/dz) 2 (where N is the Brunt-Vaisala frequency, u is the wind the processes of emission and exchange of IR radiation due to speed, z is the height), is between 0 and 1/4. Hauchecorn et al. CO2 cooling, turbulent diffusion, and dissipation. The results of [1987] described a model in which a succession of breaking GWs these calculations are presented in Figure 2, which shows that the would generate the lower MIL through the gradual accumulation vertical distribution of the eddy diffusion coefficient for heat ot' heat. They assumed that a GW becomes unstable at the height transport, a quantity reaching values as high as 300 m 2 s -, a level where the zonal wind begins to decelerate as part of the 3 times higher than the globally averaged eddy diffusion development of a critical layer arising from the matching of the coefficient derived by Strobel et al. [1985]. wave phase speed with the background wind. The decrease in N However, such high values for the eddy diffusion coefficient caused by the higher temperatures in the MIL reduces stability, are not supported by the in situ rocket measurements of density causing the waves to break. The turbulent heating, arising from fluctuations carried out by Liibken et al. [1993] for summer and the breaking waves, provides a feedback mechanism that then maintains the MIL. Such a mechanism in a broad sense looks to be valid, as 11o numerous reports have been published linking the observations of GW activity with the appearance of MIL events. Sica and 'l'l ot 'iey [1996] utilized their high-power-aperture product loo Rayleigh lidar to look for instances of super adiabatic lapse rates and found that the upper mesosphere is highly populated with numerous vertically thin "hot" regions that were determined to be convectively unstable. Thomas et al. [1996] found coherent radar echoes generated by turbulent regions coincided with both the.. 9o 80 topside and the bottomside regions of the lower MIL. They - concluded that convective instability, induced by breaking GWs, generated turbulent regions causing the topside radar echoes observed. A different mechanism, which is suggested to be the 7O shear instability induced by large vertical gradients in the background wind field, causes the observed echoes within the bottomside region of a MIL. Meriwether et al. [1994], using 60- Rayleigh lidar observations conducted at the Wright Patterson Air Force base in Ohio, found that the thermal structure for the lower 10 ' t 10" MIL included long-period wave activity with downward phase K m [m2/s] propagation for winter events. Their supposition for the MIL Fio,,ro Mo n nrnfil e nf th t,, kulen t 4;cc,,o;... cno: _, origin was that the lower MIL anomaly was simply the breaking for summer (dashed line) and winter (solid line). The hatched of a slow-moving long-period inertio-gw similar to those that ea shows the range of K,, calculated for the CIRA model [from have been seen frequently [Tsuda et al., 1990]. L bken, 1997, Figure 11 ].

6 12,410 MERIWETHER AND GARDNER: MESOSPHERE INVERSION LAYERS winter rocket probes launched from And0ya (69øN). These results presented in Figure 3 show that typical values of this parameter are consistent with Strobel et al.'s upper limit of 100 m 2 s '. The typical value of the inferred heating rate due to wave dissipation is of the order of a few degrees per day, considerably weaker than hitherto projected [Hocking, 1996]. Liibken' s [ 1997] most recent work shows that the heating can approach 10 K/d near the summer mesopause at 90 km. Thus the intensity of turbulence, observed locally by rocket sensors, is considerably less than that required to supporthe GW breaking mechanism proposed by Hauchecorn et al. [1987] and Whiteway et al. [1995], suggesting that insufficient thermal energy would be released in such events. A caviat, however, is that the models such as those by Whiteway or Hauchecorne are necessarily of mesoscale dimensions applying to the MIL spatial extent of hundreds of kilometers. The rocket measurements measure a local quantity that is determined by the intensity of the turbulence encountered by the rocket. Thus, the large eddy diffusivity postulated in Whiteway's calculations looks to be unrealistic for mesoscale dimensions, since it exceeds the localized quantity observed by the rocket experiments by at least a factor of 3. The dynamics of dissipating GWs will also transport heat downward. Walterscheid and Schubert [1990] applied _sophisticated numerical modeling techniques to simulate the overturning and breaking of a GW with a horizontal scale of 300 km and found that the thermal cooling effects can be significant, as much as K. Gardner and Yang, [1998] used the large aperture Na W/T lidar at the Starfire Optical Range in New Mexico to determine the divergence of this vertical heat flux. The results showed a substantial dynamical cooling of the region between 90 and 95 km (~-30 K/d) and heating below about 87 km. The transport of heat below 80 km by dissipating GWs may contribute to the formation of MILs near 70 km. Ongoing studies will help to determine the importance of this downward heat flux in contributing to the development of the lower MIL Gravity Wave-Tidal Interactions The work of Sica and Thorsley [1995] and Thomas et al. [1996] indicates that GW activity plays an important role in the development of the lower MIL. Liu and Hagan [1998, 2000] have developed what looks to be a promising amplification mechanism based upon the interactions of GWs with the tidal structure. This work followed up the original pioneering work of Walterscheid [1981b, 1984] who pointed out the importance of GW interactions with the mean flow in possibly producing modification of the mean flow through this coupling. Liu and Hagan considered the linkage of the MIL to the progression of the diurnal tidal mode through the MLT region (observed frequently for the upper MIL and occasionally for the lower MIL) as a significant clue. They used this insight to investigate whether a wave-wave interaction might explain the enhancement of the tidal amplitude observed. Their study was based upon numerical experiments utilizing a two-dimensional nonlinear numerical model of GW propagation to study the interaction of a GW with the GSWM diurnal tidal wind field. The goal was to determine whether the interaction between a GW and the GSWM tidal structure might have an impact upon the stability of the GW and thus in turn the wave-breaking dynamics for the GW. Similar work has just recently been published by Alexander and Dunkerton [1999] who presented a new spectral parameterization designed to improve the characterization of the mean-flow forcing caused by breakir g GWs. Liu and Hagan found that the local dynamical cooling and turbulence heating functions were enhanced through the meanflow modification of the GW stability. The consequent development of heating and cooling structures demonstrated descending phase dependence in accord with the phase descent of the tidal wave. Figure 4 presents the vertical profiles of temperature variations for a gravity wave with a intrinsic phase speed of 33 ms '1 westward. Here it can be seen that both MIL layers are affected by the wave-wave interaction. With just the wave breaking in the absence of the tidal wind field, the net effect is a cooling (as originally predicted by Walterscheid [1981a]) which arises because the dynamical cooling function calculated in the Liu and Hagan's model is less dependent upon the extent of GW saturation than the heating function. The background tidal wind affects the GW heating function giving rise to the transfer of momentum flux. Through a critical layer interaction the GW deposits momentum in the mean flow. Additional GWs that are generated the criticalayer through this interaction propagate to higher altitudes and end up interacting with the higher cycle of the tidal structure to produce amplification of the thermal at plitude of the tidal component in these regions as well. When the GSWM diurnal tidal wind field is included, the results obtained are consistent with the lidar observations of increased temperatures for both the lower and upper MIL regions. The schematic drawn in Figure 5 illustrates the mechanism involving the GW interaction with the tidal wind field. In the region of dynamical cooling the GW becomes unstable and breaks, accelerating the mean flow. This acceleration increases the shear relative to the region below and increases the downward heat flux giving rise to increased heating on the bottomside of the GW. This model predicts convective instability for the region above the maximum tidal wind perturbation and shear instability for the region below, a behavior consistent with the findings of Thomas et al. [1996]. These simulations reveal that the temperature perturbation may be as large as 20 K with the descent dependent upon the phase speed of the tidal wind structure. The region of heating corresponds to the region of mean-flow interaction where high shear develops as a result of the momentum transfer from the GW to the mean flow. Liu and Hagan [1998] and Liuet al. [2000] point out further that the extent of the development of the temperature enhancement at other latitudes may be different as a result of the change in the phase relation between the background tidal wind and the temperature. Recent work by Leblanc et a/.[1999a, b] reported on the comparison of nighttime lidar observations obtained at Table Mountain (34.4 øn) and Mauna Loa (19.5 øn) with daytime results from the Upper Atmosphere Research Satellite measurements of upper mesosphere temperatures by the High- Resolution Doppler Imager (HRDI) interferometer and with GSWM and GSWM 98 model results. They developed a method of "constrained wave adjustment" using an iterative comparison of a constructed temperature profile (derived from a composite combination of 12 hour and 24 hour tidal oscillations ) with the 10 hour nighttime lidar and the daytime HRDI averages to overcome the lack of continual measurements over 24 hours. As the authors point out, this is a delicate calculation to make. The application appears to be successful for the special case of the upper mesosphere given the particular morphology of nighttime tidal behavior in which the diurnal tidal component has its warm phase between 70 to 90 km close to the midnight hours. Comparison of these results for both lidar observatories and the HRDI data sets with the GSWM 98 model found as before

7 MERIWETHER AND GARDNER' MESOSPHERE INVERSION LAYERS 12,411 -loo -50 Temperature o verietio K) 1oo 12o 110 loo 9o 8o Time (see) Figure 4. Vertical profiles of temperature variation at 20 time steps between 6,000 and 30,000 s (interval: 1200 s). Each profile is offset by 10 K. The two thick lines indicate the profiles at 12,000 and 18,000 s [from Liu and Hagan, 1998, Figure 4]. that the amplitudes calculated by the GSWM model are weaker than the amplitudes derived from these observations. Both the HRDI and the lidar observation show the downward propagation of warm temperatures, similar to that presented by Dao et effects of the dissipation generated by the interactions of the GW spectrum with the tidal structure. Using the results from these comparisons, Leblanc et al. [1999 a] argued that the nighttime temporal behavior of the lower MILs can be explained solely on a/,[1995] and Meriwether et a1.[1998]. The weak amplitudes of the combined effects of the semidiurnal and diurnal tidal the GSWM are increased when the more recent GSWM 98 model is applied, but the discrepancy still remainsignificant (factors of 2 to 5) for the 60 to 80 km region. The main difference between components. However, the uncertainty in the extraction of the amplitudes of the tidal components from the nighttime lidar observations left open the question as to whether the differences the two tidal models is that the GSWM 98 model includes an in amplitude between the GSWM 98 tidal predictions and the improve description of the ozone and water vapor composition observations are significant. Thus 24 hour Rayleigh lidar for the troposphere and stratosphere regions. It also includes a modified representation of the background wind profile and the measurements of temperatures for the lower MIL region are crucial to assess the need for the possible inclusion of an Z2 -DS, namical cooling Zl / / Heating Background and tidal wind, % Gravity wave propation Figure 5. Schematic plot of a propagating gravity wave interaction with background and tidal wind. Zo, Z, and Z2 are selected heights with one vertical wavelength of the gravity wave. Thin arrow, gravity wave wind; thick solid arrow, background and tidal wind; thick dash arrow, composite wind [from Liu and Hagan, 1998, Figure 5].

8 12,412 MERIWETHER AND GARDNER: MESOSPHERE INVERSION LAYERS additional mechanism that might explain the observed enhanced values. The wave-tidal coupling mechanism of Liu and Hagan [1998] and Liu et al. [2000] would potentially provide the means for making up the difference. 105 Annual Mean Temperature 5.3. Possible Linkage of Chemical Heating to the Upper MIL Mlynczak and Solomon [1993] developed a global model of the chemical heating in the mesopause region arising from exothermic reactions involving the odd-oxygen and oddhydrogen species created by UV photolysis of molecular oxygen and ozone. They found that the associated net heating rate at midlatitudes peaks near 90 km at values of approximately 10 K/d. Meriwether and Mlynczak [1995] suggested that this chemical heating might be related to the production of MILs. Quenching of the excited atomic and molecular products (such as OH, O, and O2), resulting from the photolysis of ozone and from the reaction of molecular oxygen and ozone and ozone and atomic hydrogen, provides a peak heating rate of-5-10 K/d at 90 km. Because ozone concentrations increase during nighttime hours, chemical heating is approximately 8 K/d larger at night than during daytime. Heating associated with quenching of excited species such as OH is also maximum at night near 90 km [Shimazaki, 1985; Mlynczak and Solomon, 1993]. The chemical heating and quenching components are 180 ø out of phase with the direct solar heating associated with UV absorption by 03 below 90 km and by O2 above 95 km. Consequently, nighttime temperature profiles of this region typically exhibit the upper MIL near km. In this region the phase of the diurnal tide is also near midnight which further enhances the nighttime upper MIL. This is illustrated by the profiles plotted in Figure 6a, which were derived by averaging in local time the data obtained throughout the year at Urbana, Illinois [States and Gardner, 2000b]. Only the midnight profile exhibits the classic MIL structure. Notice that the diurnal mean profile plotted in Figure 6b shows little evidence of the MIL. Figure 6c is the difference between the midnight profile and the diurnal mean. Although these data were obtained by extensive Annual Mean Temperature Structure O I { " ' Mean D!_urnal -_ Temperature (K) Figure 6b. Annual mcan temperature profile at Urbana, Illinois ( 0 N) obtained by averaging the data in local time over the whole year and over 24 hours (diurnal mean) and just over the n{ght (nighttime mean) [from States and Gardner, 2000a]. averaging, the peak-to-peak amplitude of the midnight upper MIL is still almost 12 K. These observations are plotted against height and time of year in Plate 1. These results illustrate that at midnight the upper MIL appear at heights 5 to 10 km higher during the summer. This behavior is similar to the finding reported for the lower MIL of highel' heights during the nightime hours [Hauchecorn et al., and is one more indication that both the lower and upper MILs may be produced by a common mechanism. In summary, while chemical heating plays a significant role in contributing to the thermal structure of the upper MIL at night, it is also clear that this mechanism of chemical heating is of no importance for ''1... _.. ' ' i ' ' ' I ' ' ' I ' ' ' I ' ' '"i i :! iannu l Meah Day] i i i 6 h - Ibiurna Mea O Temperature (K) Figure 6a. Midnight, 0600, 1200, and 1800 temperature profiles observed at Urbana, Illinois (40 øn). These data were obtained by averaging the seasonal data set in local time to produce the annual mean day profiles [from States and Gardner, 2000b] Temperature (K) Figure 6c, Difference between the midnight profile plotted in Figure 6a and the diurnal mean profile.

9 MERIWETHER AND GARDNER: MESOSPHERE INVERSION LAYERS 12, !!00-1! _ 95 lo J F M A M J J A S 0 N D Plate 1. Seasonal distribution of the peak altitude of the midnight MIL in the mesopause region.

10 12,414 MERIWETHER AND GARDNER: MESOSPHERE INVERSION LAYERS the development of the lower MIL. At these lower altitudes there face challenges in three areas. One lies in the direction of further is no significant production of excited species related to odd modeling of wave-tidal coupling that may underlie the MIL hydrogen, and the contribution provided by chemical heating is negligible. formation and development. What role if any does PW activity and mesopause ducting [Walterscheid et al., 1999] play in this regard? Furthermore, a realistic computation of the contribution 6. Conclusions and Future Directions of the possible wave-wave linkage toward MIL formation should take into account the reality that GWs with a variety of periods A major shortcoming of present-day lidar and radar studies of the MLT region is the piecemeal approach by which studies have been conducted. It is clear that observations of the complete and wavelengths suffuse the winter upper mesosphere and lower thermosphere regions in large numbers. Such analysis must include the possible influence represented by the filtering of the MLT temperature and wind profiles are essential for source spectrum by the background vertical distribution of neutral understanding the relationships between the lower and the upper MILs, atmospheric tides, PWs, and GWs. An example of the winds through critical layer interactions [Hickey et al., 1998]. Thus simultaneous measurements of the wind profile in the possible payoffs represented by this approach is the simultaneous vicinity of the lower MIL are desirable to evaluate the possible series of MLT wind and temperature Na lidar measurements role of vertical shears in the formation of the lower MIL events. combined with OH imaging observations conducted during the Since these measurements between 60 and 80 km lie below the ALOHA 93 campaign in Hawai [Qian et al., 1998; Huang et al., 1998; Gardner and Taylor, 1998]. It seems evident now that atmospheric tides play an important altitude range of most MF or meteor radar systems, the best instrument for these wind measurements is the Doppler lidar system, which is well within the current capability of lidar role in the formation of MILs. One interesting specific case technology to develop and operate for routine measurements. example is that analyzed by Huang et al. [1998]. This featured A second challenge lies in exploring further the question of the development of a sudden increase in the MLT temperature how the formation and development of the MIL phenomenon profile that appeared just prior to the onset of what appears to be convective instability. Huang et al. suggested that this onset was triggered by the interaction of the gravity wave with the diurnal tide through a critical layer interaction. While breaking GWs are common in the MLT region, it is unlikely that the thermal effects from the dissipation of these waves are sufficient to explain the simultaneous appearance of contribute to the variability of the MLT winds and temperatures in the region of 100 to 130 km, which during the night is often populated by thin sporadic layers of ionization. Here in this region, the physics are complicated by the need to take into account the coupling between the neutral atmosphere and the ionosphere, and the details of such coupling st 11 remain to be elucidated by instrumental studies [Hocking, 1996]. Chemical both the lower and the upper MILs. Notably, the observed release measurements have demonstrated that the winds in this temperature amplitudes of MILs are much larger than the tidal amplitudes predicted by current tidal models. The recent theoretical works of Liu and Hagan [1998] and Liu et a/.[1999; 2000], regarding the development of a numerical tworegion are often times observed to be larger than predicted by theoretical numerical models such as those based upon global general circulation models [Larsen and Odom, 1997; Larsen et al.,!995]. It may be true that these results are tied in with the dimensional model of GW-tidal coupling, point strongly to the 10roduction and evolution of the MILs. Thus the strong conclusion that breaking GWs may play a significant role by amplifying the temperature amplitude of the tidal structure and thus contributing to the large MIL enhancements observed. Should this mechanism be the major forcing for the large MIL amplitudes observed, much work remains to be done both experimentally and theoretically to understand how this mechanism applies to both MILs. The upper MILs at midlatitudes are a strictly nighttime phenomenon caused by the combined affects of chemical heating and the phase progression of the tidal structure (possibly enhanced by the coupling to GW activity). The upper MILs development of lower and upper MIL events may imply the simultaneous development of enhanced winds and temperatures at the next cycle of the diurnal tidal structure at ~ 130 km near midnight. This enhancement may either be the result of the natural phase progression of the tidal structure through the lower thermosphere or through the mechanism of wave-tidal coupling as described by Liu and Hagan [1998; 2000]. Measuring winds and temperatures in the altitude region of 105 to 130 km by ground-based techniques is a difficult instrumental challenge. It can be presently met only by incoherent scatter radar (ISR) facilities for daytime measurements [Tetenbaum et al., 1990; evident in nighttime thermal profiles are absent when temperature Salah, 1994; and Goncharenko et al., 1998] at midlatitudes. Such profiles are averaged over 24 hours. If the amplitude of the lower M!Ls should still prove to be significant when Rayleigh lidar data are averaged over 24 hours, then this would imply that other mechanisms, such as the thermal effects of GW breaking [Andreassen et al., 1994; Liu et al., 1999; Alexander and Dunkerton, 1999] do participate significantly in the lower MIL production. From the technological standpoint, Rayleigh 24 hour measurements are feasible to carry out. Such measurements are planned for the Millstone Hill lidar facility using a large aperture steerable telescope constructed to operate with a small field of view to reduce the daylight background to the maximum extent possible. Aside from the need to bring together lidar and radar 24 hour measurements can be made by ISRs (Sondrestrom and ElSCAT) located at high latitudes provided that auroral precipitation add sufficient ionization to the E-region to enable nighttime MLT measurements [Johnson, 1990; Brekke, 1994]. Additional observational efforts of the type involving the union of ground-based Rayleigh/Na wind and temperature lidars with ISR facilities similar to those carried out recently at Sondrestrom and Arecibo with Na density lidar are thus desirable [Heinselman et al., 1998; Friedman et al., 2000]. Finally, the remaining challenge lies in recognizing that MLT morphology studies of ground-based lidar or radar wind and temperature measurements as they pertain to MIL events must maintain a global perspective. Allowance must be made for the instruments to make combined wind and temperature possibl existence of large-scale stationary and nonmigrating tidal measurements of the MLT region and especially to check on the phenomena, for the possibility of mesopause ducting that may internal consistency of these measurements, future MIL studies channel wave energy latitudinally, and for the possible influence

11 MERIWETHER AND GARDNER: MESOSPHERE INVERSION LAYERS 12,415 of PWs that may contribute to the variability of the MLT wind and temperature structure, especially during the winter season. Understanding the role played by these large-scale effects in contributing to the development of MIL events will require the combination of global satellite MLT coverage with results from a lidar and radar observatory network measuring MLT winds and temperatures and all-sky mesosphere imaging systems observing the horizontal structure of mesopause GW activity. The launch of the Thermosphere Ionosphere Mesosphere Energetics Dynamics (TIMED) spacecraft scheduled for late 2000 should indeed usher in an era featuring such unified studies of the MLT region. dynamics: Observations at Sondrestrom, Greenland, Geophys. Res. Lett., 25, , Gille, S. T., A. Hauchecorne, and M.-L. Chanin, Semidiurnal and diurnal effects in the middle atmosphere as seen by Rayleigh lidar, J. Geophys. Res., 96, , Goncharenko, L. P., and J. E. Salah, Climatology and variability of the semidiurnal tide in the lower thermosphere over Millstone Hill, J. Geophys. Res., 103, 20,715-20,726, Hagan, M. E., Comparative effects of migrating solar sources on tidal signatures in the middle and upper atmosphere, J. Geophys. Res., 101, 21,213-21,222, Hagan, M. E., J. L. Chang, and S. K. Avery, GSWM estimates of nonmigrating tidal effects, J. Geophys. Res., 102, 16,439, Hagan, M. E., M.D. Burrage, J. M. Forbes, J. Hackney, W. J. Randel, and X. Zhang, GSWM-98: Results for migrating solar tides, J. Geophys. Acknowledgments, This work was supported by NSF grant ATM- Res., 104, , from the Upper Atmosphere Facilities to Clemson University Hamilton, K., Comprehensive meteorological modelling of the middle and by NSF grant ATM from the Aeronomy Program to the atmosphere: A tutorial review, J. Atmos. Sol. Terr. Phys., 58, University of Illinois. 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