Momentum and energy fluxes of monochromatic gravity waves observed by an OH imager at Starfire Optical Range,

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. D6, PAGES , MARCH 27, 1999 Momentum and energy fluxes of monochromatic gravity waves observed by an OH imager at Starfire Optical Range, New Mexico G. R. Swenson, R. Haque, W. Yang, and C. S. Gardner Department of Electrical and Computer Engineering, University of Illinois, Urbana Abstract. The intrinsic characteristics of acoustic gravity waves can be deduced when the wave parameters are measured simultaneously with the winds and temperatures so that Doppler effects can be compensated. During February and April, 1995, five nights of OH imager observations of structure were made at the Starfire Optical Range near Albuquerque, New Mexico, simultaneously with Na wind/temperature lidar. The observed (Co) and intrinsic (Cx) phase speeds were deduced for 161 gravity waves from the movement of horizontal structure observed in the images (taken every 2 min) and lidar horizontal wind profiles. The mean of the horizontal wavelengths and horizontal phase speeds for the five nights are Xh km, Co m s -1, and Cx = 61.4 m s -. The mean vertical wavelength, intrinsic period, and vertical phase speed are X z km, -x 7.7 min, and Cz ms-', respectively. Because of their large vertical phase velocities, these waves can propagate to almost 200-km altitude before they are damped by molecular diffusion. The mean vertical flux of horizontal momentum (Fm) was 21.9 m 2 s -2, and the mean zonal and meridional components were -5.3 and +6.1 m s -2, respectively. Monochromatic waves with -x > 1 hour and X z < 20 km were also characterized using the Na lidar wind and temperature profiles [Yang, 1998]. The waves measured simultaneously by the lidar in the same volume of atmosphere observed by the imager had a mean momentum flux of Fm m 2 s -2. The mean zonal and meridional momentum fluxes of the combine data sets are -3.2 and +3.6 m 2 s -2, respectively. 1. Introduction Because acoustic gravity waves (AGWs) play a dominant role in influencing the global circulation in the 80- to 110-km altitude region [see, for example, Fritts and VanZant, 1993; Hamilton, 1996; McLandress, 1998], there is considerable interest in measuring the momentum and energy fluxes associated with these waves. Current global circulation models (GCMs) do not adequately include AGW forcing. Hamilton [1996] addresses a number of ongoing parameterizing studies which include a range of physical constraints on the spectrum. Two studies which offer contrasting approaches include a saturation model parameterization method by Fritts and Lu [1993] and the Doppler-spread theory parameterization by Hines [1991, 1996]. Gardner [1996] summarized most of the current theories of wave dissipation and described methods for testing which of the physical descriptions actually represent what is going on in the atmosphere. Eckermann [1997] identified a number of "untested" assumptions in Doppler spreading theories. The Doppler spreading theory has now evolved to characterize the general spectra in parameterization of AGWs used in global circulation models. An important consideration for model parameterization is the characterization of the actual spectrum of AGWs in the mesosphere. Radar, airglow imagers, and Na lidars have reported gravity wave observations but from mostly different Copyright 1999 by the American Geophysical Union. Paper number 1998JD /99/1998JD parts of the spectrum [Gardner and Taylor, 1998]. A number of studies and numerical models have been developed describing the response of the OH airglow to the atmospheric perturbations [see, for example, Walterscheid et al., 1987; Hickey, 1988a, b; Hickey et al., 1998; Schubert and Walterscheid, 1988; Tarasick and Hines, 1990; Tarasick and Shepherd, 1992; Maklouf et al., 1995]. Most numerical models only describe the ratio of the wave perturbed intensity (I') to brightness weighted temperature (i.e., r/ - (I'/I)/(T x_i/tox_i)), after Krassovsky [1972]. As a measured parameter, r/is independent of the atmospheric perturbation for linear waves [Swenson and Gardner, 1998]. Swenson and Gardner [1998] have developed an analytic model which specifically relates the response of OH intensity and brightness weighted temperature to the AGW amplitudes. The airglow perturbation amplitudes are very sensitive to vertical wavelength, and there is a marked decrease in the response for vertical wavelengths less than 12 km. This Swenson and Gardner model relates the measured OH intensity perturbations (I'/ and brightness weighted temperature T n/tox_i) to the relative atmospheric density perturbation (p'/b) and consequently to the wave amplitude. The ratio of (I'/])/(p'/b) is called the "cancellation factor" (CF), which, for large vertical wavelengths and undamped waves is actually a gain that approaches 3.5. Because of this gain, OH intensity images are especially sensitive to AGW disturbances. CF depends primarily on vertical wavelength and is relatively insensitive to the atomic oxygen density, at least for the simplified chemical approach described by Swenson and Gardner [1998]. For nogrowth waves with vertical wavelengths (Xz) larger than 6 km, 6067

2 6068 SWENSON ET AL.: MOMENTUM AND ENERGY FLUXES Table 1. Horizontal Wavelength Horizontal Wavelength, km Standard Standard Date Mean Deviation Error Feb. 2, Feb. 3, April 1, April 2, April 4, Overall mean* *Overall mean indicates the statistical average for the 161 waves observed. CF(, z) is well approximated by the analytic expression [Swenson and Liu, 1998] CF= --= 3.5{1 - exp [ (, s(km) - 6)2]} (1) 6 km<, s< oc / z ( 2 2 1/2 / h Ti<< T, (7) where r,, 'B, and h are the inertial, buoyancy, and intrinsic periods, respectively. The intrinsic wave motions are imbedded in the mean atmospheric flows. Knowledge of the wind velocity in the direction that the wave is propagating is required to determine the intrinsic horizontal phase speed. The intrinsic phase speed C was derived from the observed phase speed Co and the measured horizontal wind normal to the phase fronts using the equation C i = Co - U. Knowledge of the temperature profile is required to determine the buoyancy period. For the data reported here, the wind and temperature profiles were measured by the Na lidar at the same time and in the same region observed by the imager. In this study we analyze five nights of observations obtained on February 2 and 3 and April 1, 2, and 4, 1995, at the Starfire Optical Range (latitude 35øN, 106.5øW) near Albuquerque, New Mexico. The statistical characteristics of the wave parameters (and standard deviations) as well as the energy and momentum fluxes are examined. The value of CF for, z < 6 km is negligible. By using (1) and the measured intensity perturbation and vertical wavelength, the relative density amplitude of the wave can be computed. p'/h = (2) The horizontal (u') and vertical wind (w') amplitudes are related to relative density amplitude by the AGW polarization relations w'= u': (p'/h): ( '/i)/cf (3). 2 _ f2 As As S (I'/])/CF (4) where Xh is the horizontal wavelength, 9 is the acceleration due to gravity, N is the buoyancy frequency, f is the inertial frequency, and w is the intrinsic frequency of the wave. The vertical energy (Fu) and momentum (FM) fluxes are easily calculated using (3) and (4). Fa = vs po(u,2) = - 2m( 2 re(m2 _f2) + 9,(9/N)2(I'/i)2/(CF) 2 where m is the vertical wavenumber, m = 2 /A:, and 1 (I:/I,)(9/N):(I'/ ) 2 <w'u'>: - (6) Fn = 7 2 (CF) 2 where vs is the vertical group velocity and p{ is the mass density of the atmosphere. The 1/2 factors arise because the fluxes are averaged over one wave period. Note that m is negative for an upward propagating gravity wave. These equations for the fluxes are similar to those derived by Swenson and Liu [1998]. To calculate the ener and momentum fluxes, the horizontal wavelength and relative intensity amplitude are determined directly from the OH images. The vertical wavelength is determined from the dispersion relation using the measured horizontal wavelength and either the intrinsic period (r = Ah/C 0 or, equivalently, the intrinsic horizontal phase speed (C = 2. Observations of Intrinsic Parameters and Data Analysis The imager used to make the OH measurements was similar to that described by Swenson and Mende [1994], where a broadband OH filter ( nm) was used but incorporated a notch at nm to block the 02 atmospheric band, which is also a component of the mesospheric night glow. Images of OH airglow were recorded every 120 s using a 60-s exposure time. The horizontal wavelengths, observed phase speeds, and the I'/I associated with wave disturbances were derived from the images. Each image covered an area of sky corresponding to a radius of kin. Prior to any analysis, movie sequences of OH images were made for each of the five nights, and the wave motions were observed. Conditions of cloudiness were identified, and those images were excluded from the analysis. During the early evening hours, aircraft contrails would also cause some con- tamination in portions of the all-sky image, and those features were also avoided in the analysis. Images were calibrated for aspect and angular resolution using the known star field. The angularesolution was 0.35 ø pixel-, which corresponds to 0.52 km at OH altitudes in the zenith. Monochromatic waves were a ubiquitous feature of the images. Every image contained at least one monochromatic wave, and the observation of several waves was not unusual for the nights studied. Horizontal wavelengths (,,) for monochromatic features were measured at the zenith point using the raw images. Since the movie sequences indicated nearly constant or slowly changing conditions,, was measured in at least two images at the beginning of each hour, and only the short dominant wavelength visually obvious in the image was characterized. The mean,, for the five nights was 28.8 kin, with a standard deviation of 13 km (Table 1). At the Starfire Optical Range the Na lidar measured the wind profile throughout the altitude range of OH emission [Yang, 1998]. The wind represented of that region from which airglow perturbations occur was the lidar measured average between 84 and 89 km. This is the altitude range determined by Hecht et al. [1993] which best correlates with the Na lidar measure-

3 .. SWENSON ET AL.: MOMENTUM AND ENERGY FLUXES g o 'i' /" N 30 ' 80 P.. // t _ 1'K ', b U '-- T... } 90 7½ // 40 / = 0... '";... '"-[ " " t x / 120 x / " (a) (b) ' ' T... ;. / N, / 11.9,15 40,,/ U o lg 9o UT / o19 / 240 ",\ x -" / // oo.-... T ^;/.. ut, //--... ' '"-... ou _- i //,' {,. x 5"\ i?' II i ß!...UT '. 240 kx "-- ':' '":' // o... '%. b... I t 672 /" :". '\ '... :..':' '" ' 0 ': '\ '" '-..: i "': 'i :U'"T :"' ':..../ (c) (d) (e) Figure 1. (a) Polar plot ofwind vector magnitude on February 2, 1995, measured by the Na wind lidar versus direction for UT, where the beginning and end are labeled. The wind velocities are averaged over an altitude range of km. The angle (degrees) is positive from north through east. (b) Same as Figure la except for February 3, 1995, UT. (c) Same as Figure la except for April 1, 1995, UT. (d) Same as Figure la except for April 2, 1995, UT. (e) Same as Figure la except for April 4, 1995, UT. ments of coincident waves. This altitude region was also identified by Swenson and Gardner [1998] and validated by observations from UARS by Lowe et al. [1996]. Magnitude and time histories of the winds in the OH layer are shown in Figure 1. On February 3 the wind was to the west-northwest during the early evening, then rotated through the north and finally to the northeast. The winds on this night were dominated by a largeamplitude, long-period AGW [see Yang, 1998]. In contrast, the winds on the previous night were tidal dominated, and consequently the mean phase progression for the two nights is quite different (Figures la and lb). The three April nights shown in Figures l c-le describe a similar phase progression and magnitude for all nights, suggesting tidal dominance of the wind field. On these nights the wind is to the south in the early evening with a velocity of m s- and later rotates to the northwest. Observed phase speeds were measured from each image using the method described by R. Haque and G. Swenson (Extraction of motion parameters of the gravity wave structure from all-sky OH image sequences taken during Starfire-95, submitted to Journal of Applied Optics, 1998; hereinafter referred to as submitted manuscript). Two images separated in time (by 2 min) are superimposed, and one image is moved with respect to the other until an optimum correlation of the pixel information is obtained. The images were initially time differenced (TD) [see Swenson and Mende, 1994] to provide contrast in moving structure. The TD images were then examined for spatial movement by taking a +25 ø square (---80 km at OH altitude) section of the time-stepped image and placing it in the center of a larger +_43 ø square ( km at OH altitude). The smaller frame was moved in the zonal and meridional direction until a maximum correlation was achieved. As a check on the method, waves were identified at the beginning of each hour, and movements of those waves in the subsequent images were determined carefully against the pixel reference, and these data were used to manually verify that the correlation method was giving the same result. This convinced us that the star patterns were not introducing a bias in the results. This method, of course, is going to give an effective velocity for the waves present. If a single monochromatic feature was present, then that will provide a unique velocity for that wave. For waves which are equal in magnitude and coming from opposite directions, a zero velocity would result. The dominant waves in this case were single, monochromatic features. Figure 2 illustrates one of the images, with the observed phase speed and observed wind vector indicated. Figure 3 is a polar plot of the observed phase speed for all five nights, with a different symbol used for each night. We note that very few waves were ob-

4 6070 SWENSON ET AL.: MOMENTUM AND ENERGY FLUXES ß Figure 2. A time difference all-sky image of OH airglow waves taken on April 4, 1995, at 0708 UT from Albuquerque, New Mexico. The background wind velocity, observed phase speed, and intrinsic phase speeds are labeled on the image. served propagating toward the SE quadrant. The mean Co for mean flow, and not AGW structure. These points are not all nights of observations was 33.4 m s -, with a standard plotted in Figure 6. The mean C considering all wave data for deviation of _+18 m s -. Figure 4 is the histogram of the all nights is 61.4 _ m s -. The magnitudes of C are horizontal wavelength ( ) and observed period ( 'o) as ob- plotted in a polar coordinate in Figure 7. Typically, the waves tained from the observed phase speed. The time sequence of the vectors for U, Co, and C (C = Co - U) are shown (top to bottom) for each of the nights in Figure 5. In this projection the vector is pointing into the line were propagating against the mean flow. The average head wind was 28 m s-. The dispersion relationship can be used to relate C i to, which, combined with wave amplitude, completes the intrinsic in the center of each panel, and time is the horizontal axis, characterization of waves. We note that the mean value of the where LT is indicated on the bottom and UT is indicated on the top. More than 80% of the measured phase propagation directions were directed into the mean flow. Histograms of the magnitudes of U, Co, C, and 'i are shown in Figures 6a-6d, respectively. The nightly mean of the intrinsic phase speeds for each night is indicated in Table 2. Occasionally (-- 10% of the samples, primarily from April 2), the deduced intrinsic phase speeds were small (<5 m s- ). This was considered to be structure being advected by the intrinsic period for the 161 observed waves is 7.7 min. The buoyancy periods ( - ) were calculated for the region km using nightly mean temperature profiles measured by the lidar. These are tabulated in Table 3. The vertical wavelength is then calculated using the measured h, C, Ti, and - pa- rameters for each of the 161 waves using the dispersion relation given by (7). The mean vertical wavelength for all five nights was km, with a standard deviation of _+9.3 km (Table 4).

5 SWENSON ET AL.: MOMENTUM AND ENERGY FLUXES 6071 o 330 l I 30 the potential to influence the mean flow well into the upper E region of the lower thermosphere Momentum Fluxes Wave momentum (F 4) and energy (FE) fluxes were computed and plotted in Figures 9a and 9b, respectively, using (6) and (5), which are similar to the relationships described by lg 90 Swenson and Liu [1998]. Note thathe Vincent [1984] values are given as a mean magnitude. The mean magnitude of the vertical flux of horizontal momentum (Fm) is 21.9 m 2 s -2, and the standard deviation is 9.2 m 2 s -2. The momentum fluxes were averaged for each night to compute the mean zonal and 120 meridional momentum fluxes according to the following for- mulas: ß C on 04/01/95 o ß C on 04/02/95 o C on 04/04/95 o ' ' :- Z FMi COS 0 i (8) i=1 Figure 3. Observed phase speeds (Co) on the five nights February 2 and 3 and April 1, 2, and 4, 1995, in a polar plot. Because the vertical wavelengths are large and the periods are short for these waves, their vertical phase speeds are large. Figfire 8 is a histogram of the vertical phase speed for the waves measured. The average value is 61.5 _ rn s-. These waves can propagate to high altitudes before they are damped by molecular diffusion. Swenson et al. [1995] described the maximum altitude penetration of AGWs observed during the Airborne Lidar and Observations of Hawaiian Airglow (ALO- HA-93) campaign. They derived an equation for the vertical diffusivity (Dmax) required to quench the wave, in terms of intrinsic wave parameters. For our data listed in Tables 1, 2, and 4, equation (2) of Swenson et al. [1995] predicts Dma x - ( z2)/(2 'Ci h) = 2 x 105 m 2 S -1. This corresponds to the value of the molecular diffusion coefficient at 170- to 180-km altitude. In the absence of other damping mechanisms the waves we have observed here, on average, will be able to propagate to 170- to 180-km altitude before the effects of diffusion dominate wave dissipation. These waves, then, have t! 1 FMi sin Oi (9) i=1 where FMi is the flux associated with the ith wave which is propagating at azimuth Oi and n = 161 is the total number of waves. The errors in the flux estimates associated with geophysical variations and errors in FMi and errors in the azimuth angles are ] n I n A l Jt Jt--- E AFMi cos 0 i -- Z FMi A O i sin 0 i (10) i=1 i=1 I n I n A ' ' = -- E AFMi sin O, + - FM,A0, cos 0, (11) i=1 i=1 where AFMi is the total variation in the measured flux (error plus geophysical variations) and A Oi is the error in the measured propagation azimuth. Since the individual waves are mutually independent, the variances of the zonal and meridi- onal flux estimates are 60 Mean 28.8_+13.0 km [ 60 : Mean 18.8_+1.4 minute : : : :. 30 o 30!ii ' Horizontal Wavelength (km) (a) Observed Period (minutes) (b) 60 Figure 4. (a) Horizontal wavelength of gravity waves in the five nights February 2 and 3 and April 1, 2, and 4, 1995, in a histogram. (b) Histogram of observed period of the gravity waves on those same five nights.

6 6072 SWENSON ET AL.: MOMENTUM AND ENERGY FLUXES U (84-89 km) 02/02/ m/s Go 1 O0 m/s 100 m/s Local Time (Hours) U (84-89 km) 02/03/ m/s 100 m/s I I I I I I I I I I /x 100 m/s Local Time (Hours) Figure 5. (a) (top) Observed wind time history U (same as Figure 1), where the direction of propagation is toward the horizontal axis; (middle) observed phase speed (Co) as deduced from wave structure motion observed in images (see text); and (bottom) intrinsic phase speed (C ), as deduced from C = Co - U on February 2, (b) Same as Figure 5a except for February 3, (c) Same as Figure 5a except for April 1, (d) Same as Figure 5a except on April 2, (e) Same as Figure 5a except on April 4, 1995.

7 ß, SWENSON ET AL.: MOMENTUM AND ENERGY FLUXES 6073 U (84-89 km) 04/01/95 / x 100 m/s Go i i i I i i i i i I /\ 100 m/s 100 m/s Local Time (Hours) U (84-89 km) 04/02/95 1 O0 m/s i i i i i i i i i i /$ 100 m/s 100 m/s Local Time (Hours) Figure 5. (continued)

8 6074 SWENSON ET AL.' MOMENTUM AND ENERGY FLUXES U (84-89 km) 04/04/95 1 O0 m/s Go 1 O0 m/s I I I I I I I I I I /x 1 O0 m/s Local Time (Hours) Figure 5. (continued) i n Var (½'½') = Var (FM,) cos: O, /:l + E(F,) Var (z X O,)sin 2 t=l Var (FM) 1 E COS2 0, l=1 the flux are independent of azimuth. If the azimuth angles are approximately uniformly distributed between 0 and 2,r, then the variances are both approximately equal to [Var(FM) + E2(FM)Var(AO]/2n. For our data the rms azimuth error is less than 5 ø. The mean zonal and meridional fluxes and their uncertain- ties were computed using (8)-(13), and the values are tabulated in Table 5. The mean zonal component is -5.3 _+ 0.5 m 2 s-2, and the mean meridional component is +6.1 _+ 0.5 m 2 s-2. [E:(FM) + Var (FM)] Var n Var (½' ') = Var (FM,) sin: 0, i n t=l (A0) + E(F,) Var (A 0,) cos: 0, Var (FM) n 1 E cos20t It [E2(FM)+Var(FM)] + -- II 1% sin2 0, (12) Var(AO) I cos'- O, (13) In deriving the right-hand sides of (12) and (13) we have assumed that the azimuth errors and the mean and variance of 4. Momentum Flux Comparisons The dual-beam Doppler technique is the standard approach for measuring gravity wave momentum fluxcs [Vincent and Reid, 1983]. This tcchnique was applied to the radial winds measured by the lidar. The lidar data were obtained with a temporal resolution of 2.5 min. The profiles were smoothed vertically with a resolution of 500 m before the momentum fluxes were computed. The zonal and meridional fluxes were averaged over the height range km for the same five nights from which the image data were also obtained. The total averaging period was 56 hours. The mean zonal and meridional fluxes are and m: s-:, respectively. These values include the effects of all waves with periods greater than 5 min and vertical wavelengths greater than 1 km. Yang [1998] also used the Na lidar data to investigate the long-period monochromatic waves (r > 1 hour), for the same time period and region observed by the imager. The Na lidar

9 ß. SWENSON ET AL.' MOMENTUM AND ENERGY FLUXES 6075 Mean m/s J 40, 35...,, ":':'::"i.! i. I.,:.. Mean31 t 30P... m ' -.i: -:. : Background Wind Velocity (meter/sec) (a) Observed Phase Speed (meter/sec) (b) ß, i, i : ß " " ' ß I Meanel'4+17'8m/sl i :::::::::::::::::::::::::::::::: IMean sec ; : ' '....: ', ' ;. : : ' '"""'<'m?... : : 52 : :: :: :: s ;: ': : $ :: :::- :: 4c :*s: *::e < *' '. y::: :::-: $ :.p {.:. ; :... ; : ;?.': - T:g :g : - 4; :: -'-'.':':' ;.: [-;:.'. '>.: :;½ :... ß Intrinsic Phase Speed (meter/sec) (c) ' -'-:'..' '-"- ß & " ::--'!... ". -.' ii!:!... 0 '"' : ' ) : :: a"- :...-/... -a? ji ::,: <.., " ': --, i :½ : ::::::::::::::::::::::... : :,,a:..,:,, ::::., :..: :4 ---x-:<. ( :......>..:: : ' *!:::.:.:gj :::::½½ ; ;½ :::,. :;%?,-,_,... :...:.:.:... :;½T:::: :; ::.:...:.:...: I 10 Intrinsic Period (seconds) (d) Figure 6. Histogram plot representing (a) background wind velocity, (b) observed phase velocity, (c) intrinsic phase speed, and (d) intrinsic periods on February 2 and 3 and April 1, 2, and 4, method is sensitive to investigation of AGWs with I < < 20 km, whereas the imager data are sensitive for 12 < < c km [see Gardner and Taylor, 1998]. Yang restricted his study to long-period waves using temporally smoothe data. He used a method which extracted the hodographs of the gravity wave winds and so was able to determine the intrinsic parameters of the waves. By combining the hodograph data with the temperature profiles, he was able to determine, unambiguously, the propagation direction of each wave. For the 91 waves with periods between 1 and 20 hours and vertical wavelengths between 2 and 20 km, he computed momentum fluxes. The mean Table 2. Intrinsic Phase Speed C Intrinsic Phase Speed, m s - Standard Standard Date Mean Deviation Error Feb. 2, Feb. 3, April 1, April 2, April 4, Overall mean* *Overall mean indicates the statistical average for the 161 waves observed. flux magnitude was 13.3 m 2 S -2, the mean zonal flux was 0.16 _ m 2 s -2, and the mean meridional flux was _ m 2 s -2. Because of the high uncertainty compared with the absolute flux values, we can only conclude that the fluxes associated with these long-period waves are small. Figure 10 is a composite plot of the combined data from the monochromatic waves observed by the imager (5 min < C < 15 min) and those monochromatic features extracted by Yang using the Na lidar (Ci > 1 hour). Both the image data reported here and the Yang [1998] data addressed only the momentum fluxes associated with mono- Chromatic waves. It is also important to consider flux contributed by the quasi-random perturbations. These fluxes can be measured using the dual-doppler technique or by analyzing Table 3. Brunt-Vaisala Date Periods Brunt-Vaisala Period, min Feb. 2, Feb. 3, April 1, April 2, April 4, Nightly mean 5.1

10 6076 SWENSON ET AL.: MOMENTUM AND ENERGY FLUXES ß C. on 02/02/95 (Mean m/s)! zx C i on 02/03/95 (Mean m/s) ß C. on 04/01/95 (Mean m/s)! ' C. on 04/02/95 (Mean 57.4+_17.0 m/s)! C. on 04/04/95 (Mean _18.8 m/s) 0! i Figure 7. Intrinsic phase speed (C ) on all five nights (February 2 and 3 and April 1, 2, and 4, 1995) in a polar plot. The y axis is represented as m/s and indicates the magnitude of C. The north and east directions are shown as 0 ø and 90 ø, respectively. 180 the unambiguous horizontal wavenumber spectra of the image perturbations to determine the distribution of wave variance as a function of propagation azimuth. C. Gardner et al. (Measuring gravity wave fluxes with airglow imagers, submitted to Journal of Geophysical Research, 1998; hereinafter referred to as submitted manuscript) have analyzed the two-dimensional (2-D) horizontal wavenumber spectra of the same OH images used to characterize the monochromatic waves reported here. They used the spectral data to calculate the correlation coefficients between the vertical winds and the zonal and meridi- onal winds. The correlation coefficients were then scaled by the rms vertical and horizontal winds measured by the lidar to determine the absolute values of the momentum fluxes. The mean zonal and meridional momentum fluxes for the five nights studied here are _+ 1.5 and 1.5 _+ 1.5 m 2 s -2, respectively (C. Gardner et al., submitted manuscript, 1998). These values have been scaled so that they representhe fluxes associated with waves with periods greater than 5 min. The fluxes measured using all four methods are summarized in Table 6. The values derived from the dual-beam lidar data and the imager spectra are in very good agreement. These values are most representative of the effects of the complete spectrum of gravity waves since they include the important high-frequency waves with periods ranging from 15 min to more than 10 hours. The values derived from monochromatic 4O 35 Table 4. Vertical Wavelengths Vertical Wavelength, km Standard Standard Date Mean Deviation Error Feb. 2, Feb. 3, April 1, April 2, April 4, Overall mean* *Overall mean indicates the statistical average for the 161 waves observed. This was calculated from the measured intrinsic phase speed and the nightly mean *Bv ' : :.,, 0 t I10 Vertical Phase Speed (meter/see) Figure 8. Histogram plot of vertical phase speeds (m/s) for all the nights. i

11 SWENSON ET AL.' MOMENTUM AND ENERGY FLUXES 6077 (rm/lluasm) xnlzt X ouh o (?/rtu) xnl l wnluotuolal

12 6078 SWENSON ET AL.' MOMENTUM AND ENERGY FLUXES Table 5. Vertical Fluxes of Horizontal Momentum Momentum Flux, m 2 s -2 Zonal Meridional D ate Component Component Feb. 2, Feb. 3, April 1, 1995, April 2, April 4, Mean* Summary The true, intrinsic wave characteristics can be calculated when wave parameters are observed simultaneously with the winds so that the Doppler effects on the observed structure can be compensated for. During February and April, 1995, five nights of nighttime observations of OH structure were made with an imager simultaneously with Na wind/temperature lidar measurements with the 3.5-m Starfire telescope in Albuquerque, New Mexico. Observed (Co) and intrinsic (C 0 phase speeds were deduced from the movement of horizontal struc- *Mean indicates the statistical average for the 161 waves observed. waves using the lidar hodograph data are low because they only include the effects of the long-period waves. The average intrinsic period for these waves is 8.8 hours. These waves, which have small vertical velocities, account for only a small fraction of the momentum flux. The values derived from the mono- chromatic waves measured by the imager include only the effects of waves with periods less than about 30 min. The magnitude of the zonal flux is smaller than the dual-beam value, and the meridional flux is larger. Apparently, the longerperiod monochromatic waves not observed by the imager and the quasi-random perturbations contribute the additional westward and southward fluxes necessary to achieve the values derived from the dual-beam lidar technique. When the fluxes associated with the long-period monochromatic waves measured by the lidar are combined with the short-period monochromatic waves observed by the imager, the mean total zonal and meridional fluxes are -3.2 and 3.6 m 2 s -2. By subtracting these values from the dual-beam fluxes, we estimate that the quasi-random wave perturbations and the short-period, shortvertical-wavelength monochromatic waves not observed by the imager or characterized using the lidar wind hodographs must contribute approximately -7.8 _+ 2.1 m 2 s -2 to the zonal flux and -0.8 _+ 2.1 m 2 s -2 to the meridional flux. ß Intrinsic CCD Imager [] Horizontal Phase Speed From Lidar Data ' / / ' 0 40 [- ø (a) ß Momentum Flux From CCD Imager (Net Flux 21.9 m2/s 2) [] Momentum Flux from Lidar Data (Net Flux 13.3 m2/s 2) The intercomparison of measurements made by passive airglow and lidar methods from the same volume of atmosphere, at the same time, were described above. Radar measurements were not made simultaneously, but there is some value to at least comparing these measurements with long-term averages made by medium frequency (MF) radar, which samples this 'øl ø same altitude region. Fincent [1984] has described a long-term (2-year) measurement from two stations (Adelaide and Townsville) in Australia. He reported integral momentum flux (r = hours) measurements of 13 and 12 mw m - - for Adelaide and Townsville, respectively. He reported that -45% of the momentum flux at Adelaide was in the short-period waves, <30 min, and -64% of the momentum flux at Townsville was found in short-period waves, <36 min. Our studies at Albuquerque suggesthat most of the momentum flux is carried in the very short intrinsic waves (r, < 15 min), a finding similar to 180 (b) that of the l/incent [1984] radar study. The magnitude of the Figure 10. (a) A composite plot of intrinsic phase speed obmomentum fluxes for the brief measurement set at Albuquertained using lidar data and charged-couple device (CCD) imque is larger than the mean of the Australia measurements, but ager from StarFire campaign. (b) A composite plot of momentrue number comparisons can be made with a longer period of tum flux on all the dates of the StarFire campaign obtained measurements and is planned for the future. from lidar data and CCD imager ß ; 120

13 SWENSON ET AL.: MOMENTUM AND ENERGY FLUXES 6079 Table 6. Momentum Fluxes Using Different Processes Fluxes, m 2 S -2 Process Zonal Meridional Comments Lidar monochromatic Imager monochromatic Lidar and imager monochromatic Imager spectroscopy Lidar DB period >1 hour period <15 min period <2 hours, Xz < 150 km period >15 min, Xz < 15 km ture observed on all images (taken every 2 min) and lidarmeasured winds. The observed wave speeds were deduced using a method involving the optimum correlation of spatial offsets in sequentially acquired images. The measured mean and standard deviations of the horizon- tal wave lengths and speeds for the five nights were Xh (+ 13) km, C O = 33.4 (_+17.8) ms-', and C = 61.4 (_+17.8) m s -l, which through the dispersion relationship correspond to a Xz (-+9.3) km. It is noted that the deduced Xz is large ( l.3rb_v Ci), since the mean intrinsic period was 7.8 min, or near the Brunt-Vaisala period. It was determined that these large phase speeds can penetrate to km before diffusion processes destroy the upward propagating waves. Calculation of the mean vertical flux of momentum (Fm) was 21.9 (-+9.2) m 2 s -2, with a mean zonal component of -5.3 m 2 s -2 and a meridional of +6.1 m 2 s -2. Approximately one third of the total momentum flux magnitude was anisotropic and two thirds was isotropic, and as such did not contribute to a net momentum exchange with the mean flow. The means of all parameters for the monochromatic waves observed by the imager are summarized in Table 7. The Na lidar measured a mean F m = 13.3 m 2 s -2 for the same time period, from the same volume of atmosphere, but for waves r I > i hour. The ratio of the magnitude of the momentum flux from the two spectral regions, i.e., that measured from OH (T I < 1 hour) to that by Na lidar (T I > 1 hour), was 1.6. The combined weighted zonal and meridional measurements were -3.2 m 2 s -2 and a meridional of +3.6 m 2 s -2. This analysis represents a limited data set (5 days) but contained quality wind data so that true intrinsic phase speeds were deduced. These measurementsuggest, for the first time, the magnitude of the vertical flux measured for the fast waves which perturb OH airglow, using the airglow. Plans are under way to make a 24-month study of the climatology of intrinsic wave parameters (and wave momentum flux) from short-wave AGWs over Albuquerque, New Mexico, using the Starfire Optical Facility. Table 7. Overall Means of Different Parameters Parameters Overall Mean Horizontal wavelength ( Observed phase speed (Co) Relative perturbed airglow intensity (I'/I) Intrinsic phase speed (Ci) Intrinsic period (r ) Brunt-Vaisala period Vertical wavelength ( Vertical phase speed (Cz) Momentum flux (Fro) Zonal component of Fm Meridional component of F m 28.8 km 33.4 m s- 3.8% 61.4 ms min 5.1 min 26.6 km 61.5 ms m 2 s m s m s - Acknowledgments. The measurements reported here were hosted at the USAF Albuquerque, New Mexico, Starfire Range, directed by Bob Fugate. The measurement support and use of the large telescope facility were the main element which led to the data, making this study possible. We are truly grateful for the unselfish, professional support by the facility staff. This work was supported primarily by the National Science Foundation through the grant ATM through the Atmospheric Science Division, Aeronomy, CEDAR program. References Eckermann, S. 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I., Infrasonic variations of the OH emission in the upper atmosphere, Ann. Geophys., 28, , Lowe, R. P., L. M. LeBlanc, and K. L. Gilbert, WINDII/UARS observation of twilight behavior of hydroxyl airglow at mid-latitude equinox, J. Atmos. Ten'. Phys., 58, , Makhlouf, U. B., R. H. Picard, and J. R. Winick, Photochemical- dynamical modeling of the measured response of airglow to gravity waves, 1, Basic model for OH airglow, J. Geophys. Res., 100, 11,289-11,311, McLandress, C., On the importance of gravity waves in the middle atmosphere and their parameterization in general circulation models, J. Atmos. Sol. Ten'. Phys., 60, , Schubert, G., and R. L. Walterscheid, Wave-driven fluctuations in OH nightglow from an extended source region, J. Geophys. Res., 93, , 1988.

14 6080 SWENSON ET AL.: MOMENTUM AND ENERGY FLUXES Swenson, G. R., and C. S. Gardner, Analytical models for the responses of the mesospheric OH* and Na layers to atmospheric gravity waves, J. Geophys. Res., 103, , Swenson, G. R., and A. Z. Liu, A model for calculating acoustic gravity wave energy and momentum flux in the mesosphere from OH airglow, Geophys. Res. Lett., 25, , Swenson, G. R., and S. B. Mende, OH emission and gravity waves (including a breaking wave) in all-sky imagery from Bear Lake, Utah, Geophys. Res. Lett., 21, , Swenson, G. R., C. S. Gardner, and M. J. Taylor, Maximum altitude penetration of atmospheric gravity waves observed during ALOHA- 93, Geophys. Res. Lett., 22, , Tarasick, D. W., and C. O. Hines, The observable effects of gravity waves in airglow emissions, Planet. Space Sci., 38, , Tarasick, D. W., and G. G. Shepherd, Effects of gravity waves on complex airglow chemistries, 2, OH emission, J. Geophys. Res., 97, , Vincent, R. A., Gravity-wave motions in the mesosphere, J. Atmos. Terr. Phys., 46, , Vincent, R. A., and I. M. Reid, HF Doppler measurements of mesospheric gravity wave momentum fluxes, J. Atmos. Sci., 40, , Walterscheid, R. L., G. Schubert, and J. M. Straus, A dynamicalchemical model of wave-driven fluctuations in the OH nightglow, J. Geophys. Res., 92, , Yang, W., Gravity wave studies in mesopause region by using Na wind/temperature lidar, Ph.D. dissertation, Dep. of Electr. and Comput. Eng., Univ. of I11. at Urbana-Champaign, Feb C. S. Gardner, R. Haque, G. R. Swenson, and W. Yang, Department of Electrical and Computer Engineering, University of Illinois, 1308 West Main Street, Urbana, IL (swensonl@uiuc.edu) (Received June 5, 1998; revised October 22, 1998; accepted October 27, 1998.)