Mesospheric gravity waves observed near equatorial and low middle latitude stations: wave characteristics and reverse ray tracing results

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Ann. Geophys.,, 39 3, uropean Geosciences Union Annales Geophysicae Mesospheric gravity waves observed near equatorial and low middle latitude stations: wave characteristics and
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Ann. Geophys.,, 39 3, uropean Geosciences Union Annales Geophysicae Mesospheric gravity waves observed near equatorial and low middle latitude stations: wave characteristics and reverse ray tracing results C. M. rasse 1, T. akamura, H. Takahashi 1, A. F. Medeiros 3, M. J. Taylor, D. Gobbi 1, C. M. Denardini 1, J. Fechine 1, R. A. Buriti 3, A. alatun 5, uratno 5,. Achmad 5, and A. G. Admiranto 5 1 Instituto acional de Pesquisas spaciais (IP), C.P. 515, 15-97, ão José dos Campos, Brazil Institute for ustainable Humanosphere (RIH), Kyoto University, Uji, Kyoto, 11-11, Japan 3 Universidade Federal de Campina Grande (UFCG), Av. Aprígio Veloso, Bodocongó, , Campina Grande, Brazil pace Dynamics Laboratory and Physics Department, Logan, UT 315, UA 5 pace cience Center, ational Institute of Aeronautics and pace, Jalan Pemuda Persil 1, Rawamangun, 13, Jakarta, Indonesia Received: June Revised: 17 ovember Accepted: 7 ovember Published: 1 December Abstract. Gravity wave signatures were extracted from OH airglow observations using all-sky CCD imagers at four different stations: Cachoeira Paulista (CP) (.7, 5 ) and ão João do Cariri (7., 3.5 ), Brazil; Tanjungsari (TJ) (.9, 17.9 ), Indonesia and higaraki (3.9, 13 ), Japan. The gravity wave parameters are used as an input in a reverse ray tracing model to study the gravity wave vertical propagation trajectory and to estimate the wave source region. Gravity waves observed near the equator showed a shorter period and a larger phase velocity than those waves observed at low-middle latitudes. The waves ray traced down into the troposphere showed the largest horizontal wavelength and phase speed. The ray tracing results also showed that at CP, Cariri and higaraki the majority of the ray paths stopped in the mesosphere due to the condition of m , while at TJ most of the waves are traced back into the troposphere. In summer time, most of the back traced waves have their final position stopped in the mesosphere due to m or critical level interactions ( m ), which suggests the presence of ducting waves and/or waves generated in-situ. In the troposphere, the possible gravity wave sources are related to meteorological front activities and cloud convections at CP, while at Cariri and TJ tropical cloud convections near the equator are the most probable gravity wave sources. The tropospheric jet stream and the orography are thought to be the major responsible sources for the waves observed at higaraki. Keywords. Meteorology and atmospheric dynamics (Middle atmosphere dynamics; aves and tides; Instruments and techniques) Correspondence to: C. M. rasse 1 Introduction Atmospheric gravity waves (AG) are a very important factor for studies in the dynamical regime of the middle and upper atmosphere. As the gravity waves propagate through the upper mesosphere and lower thermosphere (MLT) regions, the perturbations of the local density and temperature affect the photochemical balance of the surrounding resulting wavelike modulations in the airglow emission rates (chubert and aterscheid, 19; Tarasick and Hines, 199; wenson and Gardner, 199). Gravity waves in the MLT region, then, can be observed by using CCD airglow imagers and the wave parameters, such as horizontal wavelength and phase velocity, can be obtained from them (e.g. Hecht et al., 199; Taylor et al., 1995, 1997; akamura et al., 1999; wenson et al., 1999; Medeiros et al., 1). This technique provides a simple and useful method to investigate the horizontal characteristics of the atmospheric gravity waves and their temporal evolution. Recent progress on the theoretical, numerical and observational studies of gravity waves was reviewed by Fritts and Alexander (3). They also pointed out that ray tracing is one of the useful techniques to investigate gravity wave propagation through the atmosphere (e.g. kermann and Marks, 1997; Brown et al., ; Gerrard et al., ). The reverse mode of ray tracing has also been used to locate the source of the gravity wave disturbances (Bertin et al., 197; Hertzog et al., 1; rasse et al., 3, ). This technique is useful to estimate the source region in the lower heights from the gravity wave signatures observed in the upper mesosphere and lower thermosphere region. Observational studies of the gravity wave characteristics and seasonal variations of the propagation direction have Published by Copernicus GmbH on behalf of the uropean Geosciences Union. 33 C. M. rasse et al.: Mesospheric gravity waves Cariri Cachoeira Paulista (CP) Tanjungsari (TJ) higaraki Fig. 1. orld map showing the location of the four observation sites used to investigate the gravity waves observed in the OH airglow emission. Figure 1. orld map showing the location of the four observation sites used to investigate the been performed bygravity many waves researchers observed fromin several the OH airglow locations emission. (Medeiros et al., ). Only the main peak in the spectrum (e.g. alterscheid et al., 1999; akamura et al., 1999, 3; is used. The period and the phase speed of the gravity wave Hecht et al., 1; Medeiros et al., ). However, little has are determined by applying a 1-D FFT in time to the complex been discussed in the gravity wave source locations. In the -D FFT in space. The peak in the 1-D FFT corresponds to present paper, gravity waves observed by OH airglow imaging the wave frequency. Details of the equipment, data acquisitigate at four different ground-based stations are used to investion and methodology to determine the wave parameters have the wave characteristics, propagation direction, and localize been reported by Medeiros et al. (1). the possible wave source region using a reverse ray tracing method. Observations Gravity wave observations using an all-sky airglow OH imager were carried out at four different stations, Cachoeira Paulista (.7, 5 ), hereafter CP, ão João do Cariri (7., 3.5 ), hereafter Cariri, both in Brazil; Tanjungsari (.9, 17.9 ), hereafter TJ, Indonesia, and higaraki (3.9, 13 ), Japan. Figure 1 shows the map locations of the four observatories. The observations at CP were taken using an all-sky CCD imager, with a 1 field of view, to monitor the spatial and temporal variations of the OH airglow layer. The measurements were made from October 199 to eptember 1999, and 13 gravity wave events were observed. Horizontal wavelengths of the gravity waves are determined by applying a standard -D FFT analysis in the observed airglow images. The advantage of this method is that it is possible to process all of the monochromatic waves present in the images individually. e can localize the gravity wave content in any part of the image by isolating the region of interest, taking its -D FFT and identifying peaks in the frequency spectrum Airglow observations were also carried out at Cariri, where another all-sky CCD imager has been operated. The imager has similar characteristics to that operated at CP. The observations were taken from eptember to October 1 and a total of 3 gravity waves were observed. Medeiros et al. () reported the details of the imager system, data analysis, as well as the determination of gravity wave parameters using the same methodology for CP and Cariri. At TJ, a simple wide view CCD imager has also been operated and measuring the OH airglow emissions since. The camera has a field of view of 1 in the diagonal direction, and 9 and in the north-south and east-west directions, respectively. For the present study the image data 19 taken from eptember to eptember 1 were used, giving a total of 3 nights of observations, of which 7 gravity waves were extracted. Image data were first translated into geographical coordinates, and then differential images between two adjacent exposures were calculated in order to enhance the gravity waves seen in the airglow image (wenson and Mende, 199). Dominant gravity waves were extracted and the main wave parameters were determined if the wave was seen on at least two sequential differential images. akamura et al. (3) reported the details of the Ann. Geophys.,, 39 3, C. M. rasse et al.: Mesospheric gravity waves 331 instrument characteristics, data analysis and the determination of the gravity wave parameters for the data taken at TJ. Prior to the TJ image observations, the same wide view CCD imager was operated at higaraki, Japan, between ovember 199 and May 199. A total of 11 nights with observations and 53 gravity waves were detected in the OH images. The horizontal wavelength and the propagation direction of the gravity waves are determined from the images in the geographic coordinate by applying a -D FFT. The horizontal phase speed can be derived from several successive images, and then the observed wave period is calculated. akamura et al. (1999) has reported the data analysis and the determination of the gravity wave parameters, as well as the climatology of the gravity waves observed at higaraki. It should be noted that due to the instrumental limitation it is not possible to observe waves with horizontal wavelengths larger than the field of view, which is about km at the 9 km altitude, for the imager at CP and Cariri, while at TJ and higaraki it is around km. Also difficult to observe are waves with vertical wavelengths shorter than 1 km (thickness of the emission layer). In practice, further observable horizontal wavelengths may be much less than the imager field of view because of image distortion as the zenith angle increases. The shortest period of gravity waves is limited by the Brunt Väisälä frequency, i.e. around 5 min at the altitude of OH airglow layer ( 7 km). 3 Gravity wave ray tracing model A reverse ray tracing model is used to compute the back trajectory of the observed gravity waves in order to estimate its source region. The ray tracing technique has been widely used in the gravity wave study and the basic ray tracing equations have been described by Lighthill (197) and Jones (199). In the present study we employed the ray tracing model used in rasse et al. (), which followed the previous work of Marks and ckermann (1995). e did not consider any damping mechanisms and did not track the wave action density through the wave trajectory, mainly due to the lack of amplitude information in our data set. For each measured wave event the observed horizontal wavelength, propagation direction, phase speed, period and local time are the launch parameters of one ray. In addition, it is also necessary to know the background atmospheric wind and temperature. ince we have no simultaneous observation of the vertical wind profile, in the present study climatological models, CIRA-19 zonal wind and temperature (Fleming et al., 199), and GM tidal wind parameters (Hagan and Forbes, 3), are used to create the background wind and temperature conditions. The GM model varies horizontally and vertically, as well as temporally, while the CIRA- model does not include temporal variation. This information is needed as a function of height within an area of ±15 degrees in latitude and longitude around each observation site. As the CIRA- model does not provide a meridional wind component, the meridional wind consists only of the tidal wind from the GM. To ensure that the KB approximation remains valid for the ray parameters, the numerical integration was stopped under any of the following conditions: 1) m becomes negative, i.e. the wave cannot propagate vertically; ) m becomes larger than 1 1 (cyc /m ), i.e. vertical wavelength becomes smaller than 1 km and it is close to a critical level; 3) the intrinsic frequency ˆω or ˆω , i.e. m which means that the wave is approaching to a critical level and the wave packet is likely to break. The position where the ray tracing is stopped due to one of the previous conditions will be referred hereafter as the final position of the wave. rasse et al. () discussed the limitation of the reverse ray tracing method and estimated the error range, taking into account the effects of the background wind variation caused by the tidal variability. They found that for 75% of the cases, the error range fell into a region with a radius less than km, and in a few exceptional cases, the ray tracing path stopped in the mesosphere, showing the sensitivity with respect to the wind variation. Results.1 Gravity wave characteristics The observed gravity waves parameters, horizontal wavelength, period and horizontal phase speed are presented in Fig. for all four stations. Figure a shows the gravity wave characteristics observed at CP (3 ), where the horizontal wavelength is distributed mainly between 1 and 3 km, the period ranging from 5 to 15 min and typical phase speed between 1 and m/s. At Cariri (7 ), in Fig. b, the horizontal wavelength ranges mainly between 5 and 5 km, and the observed period varies principally between 5 and 1 min. The horizontal phase speed varies from 1 to 5 m/s. At TJ (7 ), Fig. c, the horizontal wavelength is mainly distributed between 5 and 3 km, the typical observed period is ranging from 5 to 1 min and the phase speed varies between 5 to 75 m/s. At higaraki (35 ), Fig. d, the observed gravity waves showed a horizontal wavelength ranging mainly between 5 and km. The observed period has a maximum occurrence between 5 and 15 min, whereas the phase speed ranges from 1 to 7 m/s. Two airglow imagers used at the Brazilian sites are similar, which makes the comparison of the gravity wave parameters between CP and Cariri easier and reasonable. The same comparison can be done for the image data at TJ and higaraki with the same reason. The horizontal wavelengths observed at CP and Cariri are similar, showing a maximum occurrence between 5 and 5 km; however, the observed periods at CP are longer than at Cariri. It should be noted that at Cariri the phase velocity is faster than at CP. Gravity waves observed at Ann. Geophys.,, 39 3, 33 C. M. rasse et al.: Mesospheric gravity waves umber of vents (A) CACHOIRA PAULITA (3 o ) 7 Horizontal avelength km (B) CARIRI (7 o ) Horizontal avelength km (C) TAJUGARI (7 o ) Horizontal avelength km (D) HIGARAKI (35 o ) 1 1 Horizontal avelengh km umber of vents Observed Period min Observed Period min Observed Period min 3 5 Observed Period min umber of vents Horizontal Phase peed m/s Horizontal Phase peed m/s Horizontal Phase peed m/s Horizontal Phase peed m/s Fig.. Histogram plots showing the distribution of gravity waves parameters at (A) Cachoeira Paulista, (B) Cariri, (C) Tanjungsari and (D) higaraki. The panels show, from top to bottom, the horizontal wavelength, observed period and horizontal phase speed. higaraki and at TJ showed horizontal wavelength ranging higaraki than at TJ. On the other hand, the phase speed at TJ showed a maximum occurrence between 5 and 75 m/s, while at higaraki the phase speeds are distributed mostly between 1 and 7 m/s. It should be noted that the gravity Cachoeira Paulista, (B) Cariri, (C) Tanjungsari and (D) higaraki. The panels show, from top to waves bottom, observed the near horizontal the equator wavelength, have a shorter period observed and period and horizontal phase speed. larger phase velocity compared to those waves observed at low-middle latitudes. The difference between the gravity wave parameters observed at CP, Cariri, TJ and higaraki could not be attributed to the difference between the equipment. The errors of the observed wave parameters are much smaller than the instrumental limitations of the imagers. It is most probable that the observed differences are due to the difference in geographic latitude between the four sites, where the background wind velocities are different. Figure 3 presents the histogram of the seasonal variation of the gravity wave propagation direction. The observation is divided into four periods corresponding to spring (eptember October), summer (ovember February), autumn (March April) and winter (May August) for the outhern Hemisphere. For the orthern Hemisphere, namely higaraki, the period is divided in spring (March April), Figure mostly between. Histogram 5 and 3 km. The plots observed showing period is larger the at distribution summer (May August), of gravity autumn waves (eptember October) parameters at win- (A) ter (ovember February). Figure 3a shows plots of the propagation direction for the gravity waves observed at CP. The wave propagation directions during the summer (ovember February) and winter (May August) are mainly eastward and westward, respectively. In the spring (eptember October) most of the waves propagate in the northeastward direction. During the autumn (March April) the wave propagation is divided into two preferential directions: northwestward and northeastward. Figure 3b presents the propagation direction of the gravity waves observed at Cariri. The propagation is typically eastward during the spring (eptember October). In the summer (ovember February) the preferential propagation is distributed in the northeastward and southeastward directions. During the autumn (March April), the waves propagate preferentially northward and southward. In winter (May August) the waves showed a northward propagation direction. It is noteworthy that the eastward propagation is preferable for all the seasons at Cariri. Figure 3c shows the gravity wave propagation directions at TJ. During the outhern Hemisphere s spring Ann. Geophys.,, 39 3, C. M. rasse et al.: Mesospheric gravity waves 333 r. of vents (A) CACHOIRA PAULITA (3 o ) PRIG (P.-OCT) 1 (I) (B) CARIRI (7 o ) PRIG (P.-OCT) (I) (C) TAJUGARI (7 o ) PRIG (P.-OCT) (I) (D) HIGARAKI (35 o ) PRIG (MAR.-APR.) (I) UMMR (OV.-FB.) (II) UMMR (OV.-FB.) (II) UMMR (OV.-FB.) (II) UMMR (MAY-AUG.) (II) AUTUM (MAR.-APR.) (III) AUTUM (MAR.-APR.) (III) AUTUM (MAR.-APR.) (III) AUTUM (P.-OCT.) (III) ITR (MAY-AUG.) (IV) ITR (MAY-AUG.) (IV) ITR (MAY-AUG.) (IV) ITR (OV.-FB.) (IV) Fig. 3. Azimuthal plots showing the histogram of the seasonal variation for gravity wave propagation directions observed at (A) Cachoeira Paulista, (B) Cariri, (C) Tanjungsari and (D) higaraki. The rows show, from top to bottom, data for spring, summer, autumn and winter in outhern and orthern hemispheres. In each season the corresponding months are indicated in parenthesis. Figure 3. Azimuthal plots showing the histogram of the seasonal variation for gravity wave (eptember October), summer (ovember February) and winter (May August), the waves propagate preferentially to the south. In the autumn (March April), however, no preferential propagation direction is observed. propagation directions observed at (A) Cachoeira Paulista, (B) Cariri, (C) Tanjungsari and (D) higaraki. The rows show from top to bottom data for spring, summer, autumn and winter in southern and northern hemispheres. In each season the corresponding months are indicated in parenthesis. The seasonal variation of the gravity wave propagation direction at higaraki is shown in Fig. 3d. During the orthern Hemisphere s spring (March April) and summer (May August), the gravity waves preferentially propagated eastward and northeastward, respectively. During the autumn (eptember October), the propagation direction is much more evenly distributed, while in the winter (ovember February) the waves propagate mainly to the west, between the southwest and northwest. ot many waves are observed during the autumn, which is also noticed from the other observation sites. Ann. Geophys.,, 39 3, 33 C. M. rasse et al.: Mesospheric gravity waves Table 1. Final position of the back traced gravity waves. Final position Troposphere Mesosphere m Cachoeira Paulista CP (3 ) Cariri (7 ) Tanjungsari TJ (7 ) higaraki (35 ) Mesosphere m Out of range Total.5% 5.%.3%.% 1.% % 7.%.% 1.% 7 3.%.% 1.% % 51.% 5.% 1.% Anisotropy in the wave propagation direction is observed at CP, showing a southeastward direction in summer and a westward direction in winter. On the other side at higaraki the propagation is northeastward and southwestward/northwestward in summer and winter, respectively. o, i
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