mesopause structure from thermosphere, ionosphere

Mesopause structure from Thermosphere, Ionosphere, Mesosphere,

Energetics, and Dynamics (TIMED)/Sounding of the Atmosphere

Using Broadband Emission Radiometry (SABER) observations

Jiyao Xu,1 H.-L. Liu,1,2 W. Yuan,1 A. K. Smith,3 R. G. Roble,2 C. J. Mertens,4

J. M. Russell III,5 and M. G. Mlynczak4

Received 29 June 2006; revised 14 November 2006; accepted 17 December 2006; published 2 May 2007.

[1] Thermosphere, Ionosphere, Mesosphere, Energetics, and Dynamics (TIMED)/Sounding of the Atmosphere Using Broadband Emission Radiometry (SABER)temperature observations are used to study the global structure and variability of themesopause altitude and temperature. There are two distinctly different mesopause altitudelevels: the higher level at 95–100 km and the lower level below�86 km. The mesopause ofthe middle- and high-latitude regions is at the lower altitude in the summer hemispherefor about 120 days around summer solstice and is at the higher altitude during other seasons.At the equator the mesopause is at the higher altitude for all seasons. In addition to theseasonal variation in middle and high latitudes, the mesopause altitude and temperatureundergomodulation by diurnal and semidiurnal tides at all latitudes. Themesopause is about1 km higher at most latitudes and 6–9 K warmer at middle to high latitudes aroundDecember solstice than it is around June solstice. These can also be interpreted ashemispheric asymmetry between mesopause altitude and temperature at solstice. Possiblecauses of the asymmetry as related to solar forcing and gravity wave forcing are discussed.

Citation: Xu, J., H.-L. Liu, W. Yuan, A. K. Smith, R. G. Roble, C. J. Mertens, J. M. Russell III, and M. G. Mlynczak (2007),

Mesopause structure from Thermosphere, Ionosphere, Mesosphere, Energetics, and Dynamics (TIMED)/Sounding of the Atmosphere

Using Broadband Emission Radiometry (SABER) observations, J. Geophys. Res., 112, D09102, doi:10.1029/2006JD007711.

1. Introduction

[2] The mesopause, defined as the temperature minimumthat separates the middle atmosphere from the thermosphere,is the coldest region in the terrestrial atmosphere. The meanmesopause location and temperature are established byradiative and dynamical processes [e.g., Leovy, 1964;Holton,1983] and also display large variability due to gravity waves,planetary waves and tides. Both lower atmospheric andthermospheric processes can contribute to the variability.The temperature profile is the most important parameter forcharacterizing this transition from the mesosphere to thethermosphere; two ground-based methods that are currentlyused for determining it are in situ rocket measurement [e.g.,Lubken and von Zahn, 1991; Fritts et al., 2004; Lubken andMullemann, 2003; Lubken, 1999] and sodium lidar measure-ments. The lidars have very high vertical and temporalresolution in the altitude region of 80–110 km [e.g., von

Zahn et al., 1996; Yu and She, 1995; She et al., 1993] but existat only a few sites.[3] A 10-year climatology from lidar measurements indi-

cates that there are two distinct mesopause altitudes, one atabout 100 km during winter and the other at �86 km duringsummer [von Zahn et al., 1996; She et al., 1993; Yu andShe, 1995; Leblanc et al., 1998; She and von Zahn, 1998].These observations, however, were from separate local sitesbased on a limited number of observation days, and mostwere taken during nighttime.[4] The NASA Thermosphere, Ionosphere, Mesosphere,

Energetics, and Dynamics (TIMED) satellite was launchedin December 2001, carrying four instruments for investi-gating the region between 60 and 180 km. The Sounding ofthe Atmosphere Using Broadband Emission Radiometry(SABER) instrument obtains global profiles of temperatureand other parameters [Russell et al., 1999; Mertens et al.,2001]. In this paper we analyze the global altitudeand temperature of the mesopause and their latitudinal,seasonal and diurnal variations as measured by SABER.Simulations with the thermosphere-ionosphere-mesosphereelectrodynamics–global climate model (TIME-GCM) helpin the interpretation of the results and the implications formesospheric dynamics.

2. Data Processing

[5] The data used in this paper for calculating the globaldistribution of the mesopause altitude are temperature pro-

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112, D09102, doi:10.1029/2006JD007711, 2007

1State Key Laboratory of Space Weather, Chinese Academy ofSciences, Beijing, China.

2High Altitude Observatory, National Center for Atmospheric Research,Boulder, Colorado, USA.

3Atmospheric Chemistry Division, National Center for AtmosphericResearch, Boulder, Colorado, USA.

4NASA Langley Research Center, Hampton, Virginia, USA.5Department of Physics, Hampton University, Hampton, Virginia, USA.

Copyright 2007 by the American Geophysical Union.0148-0227/07/2006JD007711

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files from the SABER instrument onboard the TIMEDsatellite. The height of the TIMED orbit is about 625 km,the inclination is 74.1�, and the period is about 1.6 h. BecauseTIMED is a slowly precessing satellite, it takes about 60 daysto complete a full 24-hour coverage of local time.[6] SABER measures temperature from the lower strato-

sphere to the lower thermosphere by limb observation ofCO2 infrared emission. The inversion takes into accountdepartures from local thermodynamic equilibrium (LTE)and variations in CO2 mixing ratio (measured by SABERduring daytime). Mertens et al. [2004] present comparisonsof preliminary SABER observations to in situ falling spheremeasurements at high latitudes. The comparisons indicatethat the mesopause temperatures are similar between the twobut that the SABER mesopause altitude is lower than thatdetermined by the falling sphere method by about 2–4 km.This difference persists in more recent versions of theSABER retrieval (Version 1.06). The exact cause of thisdiscrepancy is not clear, though recent study suggests thatnon-LTE retrieval with additional CO2 v-v exchange pro-cesses can bring the SABER summer mesopause altitudescloser to ground base observations [Kutepov et al., 2006].The present study uses Version 1.06 Level 2A data. Fouryears of SABER data from February of 2002 to February of2006 are used in this paper.[7] The SABER temperature inversion provides profiles

on pressure levels. The effective field of view of theSABER instrument gives a vertical resolution of about2 km [Mertens et al., 2001]. Altitude information for eachtemperature measurement is a standard data product thatwas obtained by integrating the temperature hydrostaticallyfrom a reference position in the stratosphere, using NCEPanalyses. In our analysis, the mesopause location for eachprofile is identified by determining the minimum tempera-ture. Mesopause temperature and altitude data are binned in10 degrees of latitude (from 90�S–90�N) and 1 hour oflocal time, and the binning is performed every 5 degrees .There are two hours of missing data around noon in almostevery latitude [Zhu et al., 2005]; at high latitudes, the lengthof the missing midday measurements becomes longer: up to3–4 hours varying w ason. The SABER data not

marked as missing were used in this study. In other words,we did no extra screening to remove profiles or individualtemperature measurements that were outliers.[8] In order to analyze the mesopause altitude for the full

24-hours local time, we apply 60-day windows on theSABER data. Averages over shorter periods include onlya subset of local times, which can affect the mean altitudeand temperature of the mesopause. Analysis labeled by aparticular month includes 30 days data in that month plus15 days before and after the month. The data used extendfrom February 2002 through February 2006, with averagingperiods centered on each month.[9] For averaging over all local times, the mesopause

temperatures and altitudes were first sorted into the 24 localtime bins and averaged. Then values for all local time binsthat were not empty were averaged. Note that a few binsaround local noon were never filled. This introduces a biasinto the diurnal means, especially in the tropics where thediurnal temperature perturbations are largest.

3. SABER Mesopause Altitude and Temperature

[10] In this study we are going to focus only on theclimatological mean temperature structure and its variationwith solar local time. We thus average the temperature alongthe latitude circle for each local time hour and each latitudebin over the 60-day period. This averaging will remove orreduce the nonmigrating tidal components, but the migrat-ing components will remain. The confidence level of theaveraged temperature can be estimated. At June solstice atthe mesopause for the local time bin 0–1 hour, for example,the analysis error estimates for a confidence level of 95%are 2.4 K, 2.4 K, and 5.8 K for the latitudes of 0�, 45�N, and75�N, respectively. The mesopause altitude and temperatureare then calculated from these averaged profiles. It shouldbe noted here that, in this analysis, we take the minimumtemperature of the vertical profile as the mesopause. Whenmore than one local minimum is present in a profile, as canhappen in the presence of tides or other waves, the smallestamong these local minima determines the mesopause.Figures 1 and 2 give the results for the March and

Figure 1. The global distribution of mesopause (a) altitude (units are km) and (b) temperature (units are K),for a 60-day period centered around March based on 4-year-averaged Sounding of the Atmosphere UsingBroadband Emission Radiometry (SABER) temperature.

D09102 XU ET AL.: MESOPAUSE STRUCTURE

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September equinoxes for the 4-year-averaged results. Themesopause altitude is between 95 and 100 km aroundequinoxes, but dips below 90 km for a few hours at lowlatitudes during the morning and at middle latitudes duringnight. The range of mesopause temperature is from 160 to185 K; it is cooler in the equatorial region and warmertoward both poles.[11] The temporal and spatial variations of the mesopause

altitude and temperature suggest that these variations areprobably associated with atmospheric tides, particularly thediurnal tide. Both mesopause altitude and temperature havea 24-hour periodicity at low latitudes and midlatitudes. Therapid latitudinal change of the mesopause altitude andtemperature at around 20� is consistent with the phasechanges of the diurnal (1, 1) mode. The strongest modula-tion of both the altitude and temperature occurs at theequator (peak-to-peak 15 km and 30 K, respectively, forMarch) and also at latitudes between 30–40�, again con-sistent with the diurnal (1, 1) mode. The modulation islarger in March than in September, agreeing with thevariations of the diurnal tide amplitude [McLandress,2002]. The semidiurnal signature is also more evident inSeptember (Figure 2) than in March (Figure 1). The tidalmodulation of mesopause height and temperature is alsoconsistent with the model study by Berger and von Zahn[1999]. These features are similar for every year from 2002to 2005.[12] Figure 3 demonstrates the rapid shift of mesopause

altitude due to tidal perturbation. The vertical distancebetween the two low-temperature values (at 30�N in March)is �20 km, similar to the vertical wavelength of theobserved migrating diurnal tide [McLandress et al., 1996].At LT 0.5 hour, the minimum temperature occurs at 84.7 km,that is, at the lower elevation of the two temperatureminima. In the following hour (LT 1.5 hour), the higherelevation minima is now colder, and therefore the meso-pause altitude jumps to 101 km.[13] For the solstice period at June (Figure 4), there are

two distinct ranges of mesopause altitude, with the higherrange between 95–102 km and the lower between 82 and85 km. The boundary between the two altitude ranges islocated between 30–40� ring the day and 25–30�N at

night. The nighttime mesopause positions are in goodqualitative agreement with the bimodal mesopause altitudesderived from nighttime potassium lidar measurements inJune of 1996 [von Zahn et al., 1996], except that thesummer mesopause altitude is lower in SABER than inthe lidar observations by about 2–4 km. This discrepancy issimilar to that between the SABER results and rocketmeasurements as mentioned in section 2. Note from Figure 4that the lowest mesopause altitude (�83 km) is not at thehighest latitude of the observation, but at around 40–50�N,while the lowest temperature is at the highest latitudesampled (124–135 K, depending on local time, at 80�N).

Figure 2. (a, b) Similar to Figure 1 but for September.

Figure 3. The temperature profiles at four local times atthe latitude of 30�N in March of the 4-year average. Notethat LT 23.5, 0.5, 1.5, and 2.5 hours refer to hourly meansbetween LT 23–24, 0–1, 1–2, and 2–3 hours, respectively.


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At middle and low latitudes, both mesopause altitude andtemperature display modulations with a 12-hour period anda clear northward progression with strongest modulationnear 40�S (peak-to-peak 5 km and 25 K, respectively).These characteristics suggest that the modulation is closelytied to the migrating semidiurnal tide, which peaks aroundsolstice.[14] The general features are similar in December

(Figure 5), although the minimum summer mesopause inthe polar region (133–143 K) is warmer than that in June(124–135 K), and the lowest mesopause altitude is about1 km higher. A thorough comparison of winter mesopausealtitude and temperature between December and June is notpossible because there are no observations at most localtimes poleward of 50� in the winter hemisphere. With thedata available, it appears that the winter mesopause tem-perature in December is generally warmer than that in June.[15] Figure 6 shows the latitudinal and annual variation of

mesopause altitude and temperature, where the mesopauseis determined from temperature profiles averaged over the24 hours of local time for each month. The modulation bytides is thus in principle removed although a tide that variessignificantly during the 60-day period can leave a residual

signal. From May to August poleward of 40�N, the diur-nally averaged mesopause altitude is between 83 and84.5 km. The equatorward extension of the region of lowmesopause altitude is especially prominent in May–July (toabout 35�N). In August, the lowest mesopause altitude of82 km is found between 45�N and 55�N. During the borealsummer, the lowest diurnally averaged mesopause temper-ature is 126 K (at 80�N in July). This is in generalagreement with a set of falling sphere temperature profilesmeasurement at Longyearbyen (78�N) by Lubken andMullemann [2003], although the diurnal average mesopausealtitude from SABER is again slightly lower than thosefrom the falling sphere measurements. In the austral winter,the mesopause altitude increases to 98 km and the temper-ature minimum reaches as high as 188 K in the polar region.In November, December, January and February the regionof low mesopause altitude extends poleward from 40�S,with the minimum value of about 82 km at 55�S. The lowestmesopause temperature of about 135.5 K is found at 80�S inJanuary.[16] Hemispheric asymmetry is evident in Figure 6: the

summer mesopause altitude at high latitudes is higher in theSH than in the NH by about 1 km, and the summer

Figure 4. (a, b) Similar to Figure 1 but for June.

Figure 5. (a, b) Similar to Figure 1 but for December.


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mesopause temperature in the polar region is higher in theSH than in the NH by about 5–10 K. To better illustrate thisasymmetry, we overplot the latitudinal distribution of themesopause altitude (Figure 7a) and mesopause temperature(Figure 7b) derived from the 60-day average aroundDecember solstice (at day 350) and June solstice (at day 170).It is clearly seen from Figure 7 that the mesopause altitudeduring austral summer (solid line) is 1 km higher than thatduring the boreal summer (dashed line), and the summermesopause temperature in the polar region is warmer in theSH than in the NH. Figure 8 shows that the austral summeris warmer not only at the mesopause but also at all altitudesbetween 20 and 90 km at the polar region. The hemisphericdifference in summer mesopause temperature agrees withthat found by Huaman and Balsley [1999] from analysis ofHRDI temperature data. Hemispheric differences of about1 km are also found in the polar mesospheric cloud (PMC)altitudes from lidar measurement [Chu et al., 2003, 2006]; thePMC altitudes are strongly affected by temperature but also

are quite sensitive to water vapor, nucleation processes, andvertical wind.[17] It is evident from Figure 7 that around December

solstice in both hemispheres, the mesopause altitude ishigher at most latitudes, and that the mesopause is warmerat middle and high latitudes. The results for high latitudes inthe winter hemisphere should be treated with cautionbecause of very poor local time coverage there (Figures 4and 5). At middle-low latitudes, the mesopause altitude isbetween 96 and 100 km for the whole year, and thedistribution of mesopause temperature is relatively uniform.[18] The transition time between the two mesopause

altitudes is also studied using SABER data. Figure 9 showsthe daily mesopause altitude and temperature at 5 latitudesfor 4 years. From Figure 9a we can see that at the middle-and high-latitude regions there are about 120–130 dayscentered around the summer solstice when the mesopausealtitude is at the lower level, which is around 85 km in bothhemispheres. The summer mesopause altitude in the SH

Figure 6. The diurnally averaged global distribution from the 4-year-averaged results of (a) themesopause altitude and (b) the mesopause temperature. Each monthly data point represents 60 days ofobservation centered on the specific month. Contour intervals are 1 km (Figure 6a) and 5 K (Figure 6b).

Figure 7. Latitudinal distribution of (a) mesopause altitude and (b) mesopause temperature, from thediurnally averaged temperature profiles for December (solid line) and June (dashed line). The June curveshave been flipped so that left to right on the latitude axis indicates south to north for the December curvesand north to s or the June curves.


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shows more variability than in the NH. In other seasons themesopause altitude is at the high level in middle and highlatitudes. At the equator the mesopause altitude is at thehigh level in all seasons. Owing to the missing data at thelatitudes of 80�N and 80�S, we cannot determine the time ofthe transition between the two mesopause levels so we willinvestigate this at 50�N and 50�S. We take the variation ofthe temperature at 50�N in 2005 as an example (Figure 10).Figure 10 shows that from day 125 to day 130, themesopause altitude jumps from the higher mesopause levelto the lower level. During the period from day 250 today 255, the mesopause altitude jumps back from the highlevel to the low level. Therefore the transitions between thetwo levels of mesopause are fast (several days) and theyoccur �40 days after the spring equinox and �15 daysbefore the fall equinox. The temperature transitions, on theother hand, are more gradual (Figure 9b).[19] We recognize that the summer mesopause altitudes

derived from SABER temperature profiles may be system-atically low by 2–4 km, especially in the high latitude, asmentioned earlier. This bias, however, should affect thedetermination of summer mesopause in both hemispheressimilarly.[20] Larger error may occur at higher latitudes (>52�) in

the analysis because the polar regions are only alternativelyobserved half of a year owing to the yawing of the TIMEDsatellite. The yaw cycle is 60 days, and only one polarregion is observed in each yaw cycle.

4. Asymmetry Between the Northern andSouthern Hemispheres

[21] As shown in section 3, the SABER measurementsindicate that the mesopause is warmer around Decembersolstice than June solstice at middle and high latitudes

(Figure 7) at and below the altitude of the summer meso-pause altitude (Figure 8). Possible mechanisms for thishemispheric asymmetry are discussed in this section.[22] The thermal structure of the mesosphere/mesopause

is affected by radiative processes, by heating throughexothermic chemical reactions, and by dynamical processes.Adiabatic warming and cooling due to vertical motion isresponsible for the strong departure from the radiativeequilibrium [e.g., Brasseur and Offermann, 1986; Mlynczakand Solomon, 1993; Berger and von Zahn, 1999; Holton,1983]. Variation in any of the processes could lead to thehemispheric asymmetry of the mesopause temperature andaltitude. Chu et al. [2003] showed that at most altitudes thesummer mesosphere is warmer at the South Pole (SP) thanthat at the North Pole (NP) in simulations made with theTIME-GCM numerical model. It was determined from thesimulations that the difference is mainly caused bythe greater solar flux in January than in July due to theEarth’s orbital eccentricity. Therefore the warmer meso-sphere during austral summer is a consistent feature inmultiple-year TIME-GCM simulations.[23] Figure 11 shows the latitudinal and seasonal varia-

tion of the mesopause altitude and temperature derived frommonthly mean temperature profiles of TIME-GCM simula-tion for the year 2004. The variation is in general agreementwith those of the SABER measurements (Figure 6). Themesopause temperature from the simulation is generallywarmer at most latitudes in December/January than in June/July. Similar to the SABER observations (Figure 8), thetemperature below the mesopause is also higher duringaustral summer in the simulation. Detailed comparisons,however, show disagreement between the observation andsimulation. For example, in the simulation the temperatureis generally lower at low latitudes and the polar mesopauseis colder in summer and warmer in winter than those in the

Figure 8. Profiles of diurnally averaged temperature from SABER at 80� for both hemispheres (solidline for Southern Hemisphere and dashed line for Northern Hemisphere) and two summer months (leftprofile shows December and June; right profile shows January and July).


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Figure 9. Daily (a) mesopause altitude and (b) mesopause temperature, at five latitudes during 2002 toFebruary of 2006.


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observations. The meridional temperature gradients aregenerally larger at high latitudes in the simulation that inthe observations. It is also difficult to resolve the �1–2 kmhemispheric difference of mesopause altitude with certaintyin the model because the vertical resolution of the modelnear the mesopause is about 2 km.[24] An asymmetry in the gravity wave forcing of the

circulation is another possible cause of the hemisphericasymmetry of the mesopause altitude and temperature,especially in the summer hemisphere. As is well establishedin previous studies, the gravity wave breaking in themesosphere drives a circulation from the summer to thewinter hemisphere; this circulation generates adiabatic cool-ing and warming, respectively, in the summer and wintermesosphere [e.g., Andrews et al., 1987]. The winter meso-pause location is not controlled by gravity wave forcing inany significant way, but rather tied to the minimum tem-perature near 100 km owing to weak solar heating. This isbecause (1) the adiabatic warming associated with thegravity wave driven circulation is spread over a deepervertical layer because of the broader spectrum of waves[Garcia and Solomon, 1985] and is not sufficient at any

single level to change the sign of the thermal gradients, and(2) most models indicate the bulk of the gravity wavemomentum driving and associated adiabatic warmingoccurs below 100 km, which will accentuate rather thanalter the temperature minimum above.[25] In the summer hemisphere, however, the mesopause

is determined by the lower temperature from two competingcooling mechanisms: the net radiative balance and adiabaticcooling [Chu et al., 2005]. From both observations andmodel simulations, it is clear that the summer mesopausealtitude at high latitudes is always at lower altitudes (below90 km) and thus that the adiabatic cooling is reliably strong.Model studies [e.g., Garcia and Solomon, 1985] alsoindicate that the summer mesopause altitude coincides withthe strongest upward vertical wind of the residual circula-tion (Figure 1 in that paper) and occurs near the altitude ofthe strongest forcing by gravity wave breaking (Figure 6 inthat paper). Therefore the summer mesopause altitude andtemperature are dependent closely on the strength of thegravity wave sources in the troposphere and on the wavefiltering by the wind below the mesopause. Studies byVincent [1994] and Dowdy et al. [2001] suggested that the

Figure 10. The variation of temperature at 50�N around (left) March equinox and (right) Septemberequinox. The dashed lines show the mesopause altitudes.

Figure 11. (a, b) Similar to Figure 6 but from thermosphere-ionosphere-mesosphere electrodynamics–global climate l (TIME-GCM) simulation of the year 2004.


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gravity wave activity is weaker in the austral summer; theweaker gravity wave driving and warmer temperature in themesosphere are consistent with their measurements of asmaller mean vertical wind.[26] Large uncertainties in gravity wave sources, gravity

wave launch level, the parameterization of gravity waves,and limited vertical resolution make it difficult to accuratelydetermine the summer mesopause and its hemisphericasymmetry in global models. For example, the disagreementbetween the observed summer mesopause structure(Figure 6) and the simulation (Figure 11) may be due tothe uncertainty in the latitudinal distribution of gravitywave. This is because gravity wave drag plays a determin-ing role in the departure of the summer mesopause fromradiative equilibrium; the radiative and chemical processesare of secondary importance there. In contrast, the wintermesopause structure is mainly determined by the balancebetween radiative cooling, chemical heating and diffusivetransport of heat from the thermosphere. The uncertaintymay also explain why the summer mesopause altitudes varyamong a range of global models with different gravity waveparameterization schemes [e.g., Garcia and Solomon, 1985;Berger and von Zahn, 1999; Siskind et al., 2003]. Thewinter mesopause, at the same time, has been consistently at�100 km in all these models.[27] The sensitivity of mesopause temperature to the

gravity wave filtering was demonstrated by Siskind et al.[2003], who showed that the greater eastward wind in theSouthern Hemisphere troposphere during summer leads tostronger breaking of westward gravity wave components inthe troposphere and lower stratosphere and greater filteringof the eastward wave components. As a result of the greaterfiltering, there are fewer eastward wave components thatcan propagate into the mesosphere. The eastward forcing isthus weaker in the summer mesosphere and the summermesopause in the SH is warmer. Because the troposphericwind mainly affects the gravity wave components withrelatively lower phase speed, the hemispheric difference is

more pronounced with a narrower gravity wave phase-speedspectrum.[28] The sensitivity of the mesopause to gravity wave

source strength is tested here through three TIME-GCMsimulations. In all three simulations, the gravity wavespectra of phase speed specified at the lower boundaryof the model (�10 hPa) are anisotropic with the spectral peakat 20 ms�1 (eastward). As discussed by Liu and Roble[2002], the gravity wave spectra was specified to beanisotropic in order that the jet reversal in the summerhemisphere occurred at a lower altitude than that in winter,as observed. Observational support for such an anisotropyin the equatorial region was provided recently by Kovalamet al. [2006]. The magnitudes of the gravity wave momen-tum fluxes are different for the three cases: that in the‘‘weak’’ gravity wave case is half of the ‘‘base’’ case andthat in the ‘‘strong’’ gravity wave case is twice as large. Thesimulations are otherwise identical. It is shown in Figure 12that within this range of gravity wave fluxes, a stronger fluxleads to a lower breaking altitude for the waves while aweaker flux leads to a higher breaking altitude. The positionof the mesopause altitudes will change accordingly: lowerfor stronger gravity wave fluxes. The mesopause altitudes athigh latitudes (poleward of 40�) in the summer hemisphereare highest in the weak gravity wave case (87–90 km) andlowest in the strong gravity wave case (at 80–83 km). Themesopause altitudes are between 81–86 km in the basecase. With the scale height near the summer mesopausearound 4 km, the change of the mesopause altitudes in thesethree numerical experiments suggest that the mesopausealtitude decreases by three quarters to one scale height whenthe momentum flux of the wave source doubles (increasedby 100%).[29] Accompanying the lowering of the summer meso-

pause altitudes, the summer mesopause temperature becomeswarmer. This can be associated with the increasing air densityat lower altitudes. With the doubling of gravity wavemomentum flux, the wave breaking altitude (and the summer

Figure 12. Zonal mean temperature at December solstice from three otherwise identical TIME-GCMsimulations, with the gravity wave momentum fluxes at the lower boundary in the (left) ‘‘weak’’ gravitywave case and the (right) ‘‘strong’’ gravity wave cases 50% and 200% of that in the (center) base case.The dashed lines indicate the mesopause heights at corresponding latitudes. Contour interval is 10 K.


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mesopause altitude) decreases by about 0.75–1 scaleheight from z0 (the base mesopause height) to zd (themesopause height with doubled gravity wave fluxes). At zd,the momentum deposition nearly doubles while, with r(zd) =r(z0) exp((zd � z0)/H), the air density is about 2.11 to2.72 times as large as the air density at z0. Therefore theincrease of the momentum flux deposition is offset by theincrease of the air density at the breaking altitude zd andthe acceleration/deceleration rate at zd is 94%–74% ofthat at z0. The strength of the residual circulation and theadiabatic cooling rate are directly related to the acceleration/deceleration rate and will be smaller at the lower altitude. At70�S, the increase of the mesopause temperature is about5 K for each doubling of the momentum flux.

5. Conclusions

[30] In this study we use TIMED/SABER temperatureobservations to analyze the global distribution of the me-sopause altitude and temperature. The results indicate thatthere are two distinct ranges of mesopause altitude, a higherone between 95 and 100 km and a lower one below�86 km.These results are in general agreement with the mesopausestructure observed by lidar [von Zahn et al., 1996; She et al.,1993] although the summer mesopause altitude is lower inSABER than either lidar or falling sphere data. The meso-pause temperature varies from �126 K in the summer polarregion to �190 K in the winter polar region. Aroundequinox, the diurnally averaged distribution of mesopausealtitudes is quite uniform and mainly in the higher altituderange and the mesopause temperature is between 180–190 K. Around solstice the mesopause position at middleand high latitudes in the summer hemisphere is at the loweraltitude while it is at the higher altitude at other latitudes andin the winter hemisphere. Therefore there is strong seasonalvariation of the mesopause altitude and temperature in themiddle- and high-latitude regions, while at low latitudes themesopause altitude and temperature are quite uniformthroughout the year. In addition to seasonal variation, themesopause altitude and temperature are also strongly mod-ulated by the migrating diurnal tide at equinox and thesemidiurnal tide at solstice.[31] SABER temperature measurements from 2002 to

February of 2006 show that the mesopause altitude is higherat most latitudes by 1–2 km and the mesopause temperaturewarmer by about 7–8 K at middle and high latitudes aroundDecember solstice than June solstice (136.8 K and 129.2 Kat 80�). The temperature is also warmer at most altitudesbetween 20 km and the mesopause during austral summerthan that during boreal summer. This difference is likelycaused at least in part by the greater solar flux in December/January than in June/July due to the Earth’s orbital eccen-tricity, as discussed by Chu et al. [2003]. The mesopausealtitude and temperature, especially during summer, are alsosensitive to troposphere gravity wave sources and gravitywave filtering below the mesopause, and they could con-tribute to the hemispheric asymmetry of the mesopause.There are still large uncertainties in quantifying the hemi-spheric differences of gravity waves from observations andnumerical studies.[32] We identify three sources of error that could affect

the analysis of the S data: errors in the non-LTE

temperature retrieval due to uncertainties in parameters;slow precession in local time of the TIMED satellite; andmissing data, especially at high latitudes due to yawing ofthe TIMED satellite. Further comparisons with other obser-vations are thus desirable to better quantify the mesopausestructure and variability.

[33] Acknowledgments. We would like to thank Xinzhao Chu andQian Wu for helpful discussions. This research was supported by theNational Science Foundation of China (40523006, 40225011, 40336054)and the National Important Basic Research Project (2006CB806306). Thiswork was also supported in part by the International Collaboration ResearchTeam Program of the Chinese Academy of Sciences. H.L.L.’s effort is inpart supported by the NASA Sun Earth Connection Theory Program.Support for A.K.S. and partial support for J.Y.X. to visit NCAR wasprovided by the NASA Office of Space Science. The National Center forAtmospheric Research is operated by the University Corporation forAtmospheric Research under the sponsorship of the National ScienceFoundation.

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��H.-L. Liu and R. G. Roble, High Altitude Observatory, National Center

for Atmospheric Research, P.O. Box 3000, Boulder, CO 80307-3000, USA.([email protected])C. J. Mertens and M. G. Mlynczak, NASA Langley Research Center,

Hampton, VA 23681, USA.J. M. Russell III, Department of Physics, Hampton University, Hampton,

VA 23668, USA.A. K. Smith, Atmospheric Chemistry Division, National Center for

Atmospheric Research, Boulder, CO 80307, USA.J. Xu and W. Yuan, State Key Laboratory of Space Weather, Chinese

Academy of Sciences, P.O. Box 8701, Beijing 100080, China.


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