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Low-latitude mesospheric vertical winds observed using VHF radar

2011; American Geophysical Union; Volume: 116; Issue: D22 Linguagem: Inglês

10.1029/2011jd016385

2156-2202

S. Eswaraiah, M. Venkat Ratnam, B. V. Krishna Murthy, S. Vijaya Bhaskara Rao,

Atmospheric Ozone and Climate

Journal of Geophysical Research: AtmospheresVolume 116, Issue D22 Climate and DynamicsFree Access Low-latitude mesospheric vertical winds observed using VHF radar S. Eswaraiah, S. Eswaraiah Department of Physics, Sri Venkateswara University, Tirupati, IndiaSearch for more papers by this authorM. Venkat Ratnam, M. Venkat Ratnam [email protected] National Atmospheric Research Laboratory, Gadanki, IndiaSearch for more papers by this authorB. V. Krishna Murthy, B. V. Krishna Murthy B1, CEEBROS, Chennai, IndiaSearch for more papers by this authorS. Vijaya Bhaskara Rao, S. Vijaya Bhaskara Rao Department of Physics, Sri Venkateswara University, Tirupati, IndiaSearch for more papers by this author S. Eswaraiah, S. Eswaraiah Department of Physics, Sri Venkateswara University, Tirupati, IndiaSearch for more papers by this authorM. Venkat Ratnam, M. Venkat Ratnam [email protected] National Atmospheric Research Laboratory, Gadanki, IndiaSearch for more papers by this authorB. V. Krishna Murthy, B. V. Krishna Murthy B1, CEEBROS, Chennai, IndiaSearch for more papers by this authorS. Vijaya Bhaskara Rao, S. Vijaya Bhaskara Rao Department of Physics, Sri Venkateswara University, Tirupati, IndiaSearch for more papers by this author First published: 29 November 2011 https://doi.org/10.1029/2011JD016385Citations: 10AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Abstract [1] Using long-term data (January 1998 to December 2009) collected from Mesosphere-Stratosphere-Troposphere (MST) radar located at a tropical station, Gadanki (13.5°N, 79.2°E), India, variability of low-latitude mesospheric vertical wind is investigated for the first time. The emphasis is on sub-daily, monthly, seasonal and annual variabilities in vertical wind in the altitude region 65–85 km. Possible sources of errors in the vertical wind measurement at mesospheric altitudes from the MST radar are discussed. The observed mesospheric vertical wind is generally upward in all the seasons. During winter and summer, significant sub-daily variations are noticed followed by spring and fall equinoxes. The vertical wind can reach occasionally values as high as 5 ms−1 but most of the time (95%) it is, in general, less than ∼2.63 ms−1. The present observations are consistent with the general circulation features in recent models for low latitude locations with northward and upward wind prevailing throughout the year representing part of meridional circulation. Key Points Variability of low-latitude mesospheric vertical wind is investigated The observed mesospheric vertical wind is generally upward in all the seasons Possible sources of errors are discussed 1. Introduction [2] Vertical atmospheric motions play a vital role in different aspects like heat transport, momentum flux and chemical mixing between different layers of the atmosphere. The long-term mean value of the vertical wind velocity is very useful for studying the large-scale circulation pattern [Fritts, 1984] and wave transport phenomenon [Fritts, 1989]. Recently a number of techniques and instruments capable of measuring vertical wind have come into vogue; those include VHF and MF radars, Lidar and FPI (Fabry-Perot Interferometer). Since the pioneering work by Woodman and Guillen [1974], VHF radar has emerged as a promising means for the study of the structure and dynamics of the atmosphere from troposphere to the mesosphere, with a gap in the stratosphere. This technique enables determination of horizontal and vertical winds in the lower atmosphere in the altitude region 2–22 km and in the mesosphere in the altitude region ∼60–85 km, with high temporal and altitude resolutions. [3] The first measurements of mesospheric vertical winds (MVW) at a tropical location were reported using the powerful VHF radar at Jicamarca (11.95°S, 76.87°W) [Woodman and Guillen, 1974; Rastogi and Bowhill, 1976]. They reported that the vertical wind measurement accuracies are of the order of 20 cms−1 and 2 ms−1 in the stratosphere (10–35 km) and in the mesosphere (55–85 km), respectively. Miller et al. [1978] reported the first midlatitude measurements of MVW using the Urbana VHF radar facility located at Urbana (40.12°N, 88.2°W) and they concluded that vertical velocities exhibit presence of gravity waves with dominant periods ∼10 min and amplitudes ranging from less than 1 ms−1 to approximately 5 ms−1. High latitude MVW measurements at Poker Flat, Alaska (65°N, 147°W) were reported by Balsley and Riddle [1984] using MST radar and noticed that the monthly mean vertical wind values varied by ∼0.5 ms−1 with a quasi-sinusoidal annual variation and a maximum upward flow near the winter solstice and maximum downward flow near the summer solstice. More recent studies involving MVW have been on gravity wave momentum flux [Fritts et al., 1992; Hitchman et al., 1992]. Zhou [2000] used Incoherent scatter (IS) radar to study mesospheric vertical winds at Arecibo observatory, Puerto Rico (18.47°N, 66.7°W). He reported that the most dominant turbulent layer in the mesosphere was located at 80 km with a thickness of about a couple of kilometers and that the vertical wind varies between ±0.5 ms−1 in the altitude range 68–88 km. Using EISCAT UHF radar at Ramfjordmoen near Tromso (69.59°N, 19.23°E), Hansen and Hoppe [1996] estimated the vertical winds in the MLT (mesosphere and lower thermosphere) region and concluded that between 70 and 80 km the magnitude of the wind varies between ±5 ms−1 and between 83 and 89 km the wind field is more irregular. They also noticed strong day-to-day variations in the wind field. [4] Meek and Manson [1989] measured the vertical air motions in the upper middle atmosphere (60–110 km) using the MF radar at Saskatoon (52°N, 107°W) and reported large diurnal variations in vertical winds of the order of ∼ 0.2 ms−1 at ∼75 km and ∼110 km and daily mean values varying between 0.2 and 0.5 ms−1. Using Na lidar, Tao and Gardner [1995] and Gardner and Yang [1998] obtained vertical and horizontal winds and temperature simultaneously in the meteor zone (85–110 km) at the Starfire Optical Range (35°N, 107°W) near Albuquerque, New Mexico and studied the heat flux and cooling rate associated with dissipating gravity waves. Höffner and Lautenbach [2009], using mobile scanning iron lidar carried out daytime measurements of vertical winds in the mesopause region (78–95 km) at Kuhlungsborn (54°N, 12°E) and reported variations of the order of ±4 ms−1. [5] Apart from radars and lidars, passive optical instruments have also been used to estimate vertical winds in lower thermosphere region. Recently Kurihara et al. [2009] used Fabry-Perot Interferometer (FPI) at Skibotn (69.3°N, 20.4°E) in Norway and at the KEOPS (67.8°N, 21.0°E) in Sweden and reported large diurnal variations of vertical winds of the order of ±30 ms−1 at mesopause level. Bhattacharya and Gerrard [2010] measured vertical winds in the upper mesosphere and lower thermosphere using a wide–angle Michelson Interferometer (MI) at Resolute Bay (74.5°N, 94.8°W) and reported that the daily mean vertical winds range from 0 ms−1 up to ∼±10 ms−1 at 84 km. Recently Portnyagin et al. [2010] developed an empirical model to calculate the monthly mean prevailing vertical wind for the MLT region (70–110 km). This model estimates the globally distributed mean prevailing vertical wind. Portnyagin et al. [2010] showed that in the mesosphere below (above) 80 km at low-latitudes the direction of wind is generally upward (downward). [6] From the above, it can be seen that there is a relative paucity of mesospheric vertical wind information over tropics. For the first time we have carried out a comprehensive study on mesospheric vertical wind using MST radar at a tropical station Gadanki using long-term data set (January 1998 to December 2009). The main aim of this paper is to study low-latitude mesospheric mean vertical winds using MST Radar. 2. Database [7] The MST Radar [Rao et al., 1995] located at Gadanki is a high power coherent pulsed Doppler radar operating at 53 MHz with a peak transmitter power of 2.5 MW. The radar operates in the Doppler Beam Swinging (DBS) mode for the measurement of winds in the troposphere, lower stratosphere, and mesosphere (∼65–85 km). The main experimental specifications used for the observations related to the current study are given in Table 1. Though observations are available for all the five radar antenna beams (East, West, Zenith-Y, North, and South), we have used only Zenith-Y observations as we are mainly dealing with vertical winds in the current study. VHF radar echoes from the mesosphere result from refractive index irregularities due to electron density fluctuations having scale size of half the radar wavelength (∼3 m) (i.e., through Bragg scattering) and/or from electron density gradients (i.e., through Fresnel reflection/scattering). Atmospheric scatterers are advected with the background air motions and the three dimensional velocity vector can be directly deduced from the Doppler shifts of the radar echoes received in the three independent beam directions of radar antenna. Table 1. Radar Experimental Specifications Used for the Present Study Parameter Specification Operating frequency 53 MHz Power Aperture Product (peak) 3 × 1010 Wm2 Beam width 3° Pulse width 8/16 μs (uncoded) Inter pulse period 1000 μs No. of FFT points 128 No. of Coherent integrations 64 No. of Incoherent integrations 2/4 Range resolution 1.2 km/2.4 km No. of beams One (Zenith Y) [8] A detailed description of the data and signal detectability has been given by Kumar et al. [2007]. The MST radar wind data in the altitude region 65 to 85 km during 1998–2009 comprise of observations on 648 days. The entire data set used in the present study is a combination of two modes of observations namely, Mode I (continuous observations from 1000 to 1600 IST (Indian Standard Time)) and Mode II (between 1200 IST and 1400 IST for about one hour of Common Mode operation (CMO)). A histogram representing the days of data collection spread over the 12 year period (1998–2009) is shown in Figure 1. Out of the 648 day mean profiles 574 are one hour mean profiles and 74 are six hour mean profiles. The total number of hours of data used for the present study is 1018. Figure 1Open in figure viewerPowerPoint Histogram showing the number of days of month wise data availability including both mode one hour and six hour days data and the total number of days of data used over the 12 year (1998–2009) period. 3. MST Radar Data Analysis Procedure [9] As mesospheric echoes from the MST radar are mainly due to fluctuations in electron density and/or electron density gradients, they are confined to daytime. The availability of data is restricted to timings from 1000 to 1600 IST and also to the altitude range of 65–85 km. Mode I data have been used to study the sub-daily variations and both Mode I and Mode II data have been used to study monthly and annual mean variations of vertical winds. [10] Vertical wind velocity profiles of individual days are carefully examined for interference, if any, and the same is removed by a specially made algorithm. After removing the interference and also outliers in the velocity data, percentage of occurrence (PO) of echoes in each range bin is estimated using the relation where SNR is signal-to-noise ratio. [11] After examining several cases it is found that in some cases echoes are highly sporadic and it is difficult to obtain reliable information on vertical wind (See Kumar et al. [2007] for details on how a SNR criterion of −12 dB is arrived at.) Further, a minimum PO threshold criterion has been applied to the data. After trying out several threshold criteria to the data, it is found that reliable estimates of vertical wind can be obtained using a threshold of 20% for PO. Thus, the echoes in range bins with PO less than 20% are omitted. After applying this algorithm, the mean vertical wind for each day of observation is estimated. Although data used for this study have been collected with range resolutions of 1.2 and 2.4 km, the data have been suitably averaged to obtain a uniform range resolution of 2.4 km. [12] Figure 2 shows a typical example of the data on 10 November 1994 with interference and outliers removed. Figure 2a shows the altitude-time plot of SNR after applying 20% PO criteria and after removing the outliers. Note that SNR greater than −12 dB only is plotted in this figure. The echo strength is maximum between 68.4 and 75.6 km and so the vertical wind data extracted in this altitude region can be taken to be reliable. The Doppler width of the zenith beam is shown in Figure 2b. Large Doppler widths can be noticed at times when the SNR is high revealing presence of strong turbulence particularly during 1130–1330 IST at 72 km and 73.2 km. Figures 2c and 2d show the altitude-time variation of vertical wind and daily mean vertical wind, respectively, after applying the threshold criteria. Large upward (positive velocities) and downward (negative velocities) motions reaching values up to 3 ms−1 can be noticed. However, the mean velocities are ∼ 0.5 ms−1 (Figure 2d) with standard deviation of 1.5 ms−1. Note that vertical velocity is high by an order of magnitude at mesospheric altitudes when compared to the troposphere [Jagannadha Rao et al., 2003; Roja Raman et al., 2008]. Figure 2Open in figure viewerPowerPoint Typical example on 10 November 1994 showing the time-altitude plot of (a) Signal-to-Noise Ratio (SNR) after applying minimum (20%) threshold criteria and removing outliers, (b) Doppler width, (c) vertical wind, and (d) profile showing mean vertical winds on the same date along with standard deviations (horizontal bars). [13] In order to study the variability of mesospheric vertical winds, the radar data are separated in to four seasons, namely summer (May– August); fall equinox (September and October); winter (November–February) and spring equinox (March and April) and is shown in the form of histograms in Figure 3 for 73.2 km altitude region from the data collected during the period of 1998 to 2009. The distribution follows nearly Gaussian shape with maximum number of observations within ±1 ms−1. The width of the distribution is broader in summer compared to the other seasons indicating higher variability in summer compared to the other seasons. The mean, standard deviation, median, 25 percentile, 75 percentile, 95 percentile and the number of points used at 73.2 km observed in different seasons are shown in Table 2. Only for 5% of the time the values exceed ∼2.63 ms−1 with slight seasonal dependence and for about 75% of the time, values are more than ∼0.64 ms−1. The high values (>2 ms−1) may be due to manifestation of gravity waves. Figure 3Open in figure viewerPowerPoint Histograms representing the variability of vertical velocity at 73.2 km in (a) winter, (b) spring, (c) summer, and (d) fall equinox seasons observed during 1998–2009. Table 2. Statistics Showing the Mesospheric Vertical Velocity Variability Observed During Different Seasons During 1998 to 2009 by Indian MST Radar Season Mean STD Median P25 P75 P95 N Winter 0.009 1.33 0.027 −0.65 0.73 2.15 4643 Spring 0.040 1.17 0.015 −0.67 0.70 1.88 6448 Summer −0.053 1.57 −0.041 −1.01 0.83 2.63 7440 Fall 0.040 1.14 0.015 −0.58 0.64 1.90 10061 [14] For the configuration of the MST Radar experiment used in the present study with number of FFT points as 128, inter pulse period as 1000 μs and number of coherent integrations as 64, the vertical velocity resolution comes out as 0.35 ms−1. Taking this into account, we considered vertical velocities greater than 0.35 ms−1 in the present study. Since the vertical winds are relatively much smaller than the prevailing horizontal winds, a number of possible observational errors due to factors like beam pointing accuracy, tilted atmospheric structures and rate of change of electron density [Balsley and Riddle, 1984] could affect the measurements. The beam pointing accuracy of the MST radar at Gadanki is 0.1° [Narayana Rao et al., 2008] and the mean horizontal wind between 65 and 85 km over this location is approximately 40 ms−1 [Venkat Ratnam et al., 2001; Kishore Kumar et al., 2008]. These result in an error of 0.07 ms−1 in the vertical wind which is very small compared to the observed vertical winds. Balsley and Riddle [1984] argued that, when the tropospheric mean vertical winds are not biased even though horizontal wind magnitude is comparable to the mesospheric horizontal wind similar to our case. So we can also conclude that ‘significant errors in mesospheric vertical wind do not arise from beam pointing errors (∼0.1°) of the vertical antenna beam’. Regular periodic calibration checks have been carried out on the MST radar antenna system at Gadanki and no significant deviation from the quoted beam pointing accuracy has been noticed during the present study period. [15] If the echoes are from tilted structures within the radar illuminated volume then the contamination from horizontal winds could affect the vertical winds. Similar argument may be applicable to the propagating gravity waves with tilted phase fronts. However, the anisotropy at mesospheric altitudes at this location is reported to be very small [Kumar et al., 2007]. Individual cases with large vertical winds are also checked and found that these high magnitudes of vertical winds are not the manifestation of horizontal winds. This contention is further bolstered by the low and insignificant correlation between horizontal winds and vertical wind (Figure not shown). Since most of the observations considered, correspond to noon time, rate of change of electron density for a given season would be the same and hence contamination of the data through this process can also be considered as insignificant. Thus, based on the above discussion, it is reasonable to conclude that the vertical velocity data obtained from the radar observations can be considered as reliable. 4. Results and Discussion Sub-daily Variation of Mesospheric Vertical Wind [16] The sub-daily variations (using mode I data) of vertical wind for different seasons averaged over the period 1998–2009 are shown in Figure 4. No regular sub-daily variations could be discerned from the figure. The vertical wind is mainly upwards in all the seasons and the variability appears to be less in the fall equinox compared to the other three seasons. Since significant sub-daily variations in vertical wind are observed, a question now arises whether VHF radar observed daytime vertical wind represents the background wind at mesospheric altitudes. Figure 4Open in figure viewerPowerPoint Sub-daily variation of vertical wind observed during (a) winter, (b) spring equinox, (c) summer and (d) fall equinox seasons averaged during 1998–2009. Mean wind panels to the right of each plot show the mean vertical velocity and the standard deviation (horizontal bar). [17] We have used a simple averaging of daytime wind velocity, which is widely used as the mean wind estimation from the MST radar. However, note that by using simple averaging, the observed wind may be biased by diurnal variations due to tides. Since the mesospheric echoes are greatly influenced by the presence of electron density fluctuations, which are prominent during the daytime, the MST radar observations are confined to daytime only. As such the data do not permit obtaining reliable information on diurnal and semidiurnal components and hence to account for these in estimating the vertical wind. However, it is well known that tidal (diurnal and semidiurnal) amplitudes attain large values in the upper mesosphere. This effect might be smaller at lower altitudes (below 70 km) [Nakamura et al., 1996] as compared to those at higher (90 km) altitudes. Until now considerable work has been carried out on the diurnal and semidiurnal tidal effects on horizontal winds over the low latitude region [Deepa et al., 2008; Iimura et al., 2010], but studies on tidal effects on vertical wind component are not done due to non-availability of continuous observation of vertical wind. From the model shown by Portnyagin et al. [2011], it is evident that except in winter, the tidal amplitudes are very small between 70 and 85 km. Based on this, the bias in the daily mean vertical wind obtained with averaging over only the daytime instead of complete diurnal cycle, can be considered to be small, though cannot be ruled out completely. At any rate, the daily averaged vertical wind obtained in the present study from radar observations could be contaminated by effects of tides, however small they are. Seasonal Variation of Mesospheric Vertical Wind [18] After checking each day individual profiles and applying the minimum PO criteria threshold, all the profiles in each season for all the 12 years are averaged in order to obtain seasonal profiles as shown in Figure 5. Figure 5Open in figure viewerPowerPoint Seasonal mean profiles of mesospheric vertical wind during (a) winter, (b) spring equinox, (c) summer, and (d) fall equinox. Horizontal bars indicate the corresponding standard deviations. [19] Inspection of the seasonally averaged vertical wind velocity shows that the vertical wind is upward with no significant seasonal variation. The standard deviations increase with altitude and are generally small in the fall equinox compared to the other seasons. Increase in the vertical wind with respect to altitudes above 80 km is noticed in all the seasons. This may be partly attributed to not addressing the tidal effects which could become significant above 80 km. In comparison with the empirical model of Portnyagin et al. [2010], the magnitude of the vertical wind obtained from the empirical model is in general, about 10 times lower than the reported values in the present study. However, the empirical model shows, in general, upward velocities as the radar observations reported here. Annual Variations in the Mesospheric Vertical Winds [20] Figure 6 shows the composite monthly mean behavior of mesospheric vertical wind. Close inspection of the figure reveals that the mean vertical wind is low and upward most of the time, with no significant seasonal variation as in Figure 5. However, patches of large upward winds particularly during winter months of November, December and February and summer month of June could be seen in Figure 6. Figure 6Open in figure viewerPowerPoint Composite monthly mean variation of mesospheric vertical wind. [21] It is well known that mesosphere dynamics is greatly influenced by gravity waves which are generated in the lower atmosphere and propagate in to the mesosphere. These can induce seasonal variations in the vertical and horizontal wind field at mesospheric altitudes. Gurubaran et al. [2005] have shown evidence of correlation between deep tropospheric convection and diurnal tide in the MLT region indicating the influence of the lower atmosphere on the upper atmosphere. Thus the observed variations in the vertical wind especially at altitudes >80 km could be at least partly due to wave effects. We have examined solar activity effect on mesospheric vertical winds, if any. We found that there is no significant solar activity effect and inter-annual variation (Figure not shown). 5. Summary and Conclusions [22] In this paper, we have presented the results of an analysis of mesospheric vertical wind with the data obtained from MST radar located at the tropical station, Gadanki for more than one solar cycle period (12 years). The vertical wind prevailing over Gadanki is estimated using suitable algorithm to study the sub-daily, monthly, seasonal, and annual variations. Possible sources of errors in the measured vertical wind at mesospheric altitudes are discussed and found that these do not affect the measured wind in any significant manner. The main results are summarized below: [23] 1. Mesospheric vertical winds over Gadanki show upward motion. [24] 2. Mesospheric mean vertical wind velocities can reach occasionally values as high as 5 ms−1 in magnitude probably due to gravity wave effects. However, for 95% of the time, mean vertical winds are less than ∼2.63 ms−1. [25] 3. There are no regular seasonal or sub-daily variations discernible in the vertical wind. [26] Several investigators have studied the variability of vertical wind in the MLT region from various locations mainly by using radio and optical remote sensing methods. All have reported significant variability in the measured vertical wind on a day-to-day basis and sub-daily basis. The variability of vertical wind in mesosphere region from VHF radars at Jicamarca [Woodman and Guillen, 1974; Rastogi and Bowhill, 1976], Urbana [Miller et al., 1978] and Poker Flat, Alaska [Balsley and Riddle, 1984] and IS radar at Puerto Rico [Zhou, 2000] and MF radar at Saskatoon [Meek and Manson, 1989] bears appreciable resemblance with that of vertical wind at Gadanki shown in the present study. [27] The present observations are consistent with the general circulation features shown in the zonal mean meridional wind and vertical winds by the empirical model of Portnyagin et al. [2010] for low latitude locations with northward [Kishore Kumar et al., 2008] and upward wind (present study) prevailing throughout the year representing a part of meridional circulation. However, whether these are linked with double or multi cell structured circulation cells cannot be ascertained with single station data as in the present case. It may be noted here that Balsley and Riddle [1984] while presenting vertical wind observations at a high latitude station (Poker Flat) from VHF radar suggested the possibility of a multi cell meridional circulation. It would be of great importance to see the implication of the observed low latitude mesospheric upward winds prevailing throughout the year on the low latitude mesopause. In this context it may be noted that Venkat Ratnam et al. [2010] reported low latitude mesopause to be high at ∼98 km throughout the year over tropical latitudes. [28] From the present results, it appears that mesospheric mean vertical winds are an order of magnitude greater than the mean tropospheric vertical winds reported by Jagannadha Rao et al. [2003]. It will be interesting to see the correlation between the vertical winds at mesospheric and tropospheric altitudes to see effect of global warming, if any, as tropospheric vertical winds are expected to reveal global warming effects through increase in convection. Acknowledgments [29] We are grateful to the National Atmospheric Research laboratory (NARL), Gadanki, for providing necessary data for the present study. We deeply appreciate the Advanced Center for Atmospheric Science (ACAS) funded by Department of Space (DOS) under RESPOND to S. V. University, Tirupati and University Grants Commission (UGC) by providing fellowship to one of the authors (S.E.) and other necessary facilities to carry out this work. We also thank A.K. Patra for fruitful discussion related to technical aspects of MST radar. We thank all the three reviewers for helpful comments/suggestions. Supporting Information Filename Description jgrd17457-sup-0001-t01.txtplain text document, 406 B Tab-delimited Table 1. jgrd17457-sup-0002-t02.txtplain text document, 376 B Tab-delimited Table 2. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article. References Balsley, B. B., and A. C. Riddle (1984), Monthly mean values of the mesospheric wind field over Poker Flat, Alaska, J. Atmos. Sci., 41, 2368– 2380, doi:10.1175/15200469(1984)041 2.0.CO;2. Bhattacharya, Y., and A. J. Gerrard (2010), Winter time mesopause region vertical winds from Resolute Bay, J. Geophys. Res., 115, D00N07, doi:10.1029/2010JD014113. Deepa, V., G. Ramkumar, T. M. Antonita, and K. K. Kumar (2008), Meteor wind radar observations of tidal amplitudes over a low-latitude station Trivandrum (8.5°N, 77°E): Interannual variability and the effect of back ground wind on diurnal tidal amplitudes, J. Atmos. Sol. Terr. Phys., 70, 2005– 2013, doi:10.1016/j.jastp.2008.07.017. Fritts, D. C. (1984), Gravity wave saturation in the middle atmosphere: A review of theory and observations, Rev. Geophys., 22(3), 275– 308, doi:10.1029/RG022i003p00275. Fritts, D. C. (1989), A review of gravity wave saturation process, effects, and variability in the middle atmosphere, Pure Appl. Geophys., 130, 343– 371, doi:10.1007/BF00874464. Fritts, D. C., L. Yuan, M. H. Hitchman, L. Coy, E. Kudeki, and R. F. Woodman (1992), Dynamics of the equatorial mesosphere observed using the Jicamarca MST radar during June and August 1987, J. Atmos. Sci., 49, 2353– 2371, doi:10.1175/1520-0469(1992)049 2.0.CO;2. Gardner, C. S., and W. Yang (1998), Measurements of the dynamical cooling rate associated with the vertical transport of heat by dissipating gravity waves in the mesopause region at the Starfire Optical Range, New Mexico, J. Geophys. Res., 103(D14), 16,909– 16,926, doi:10.1029/98JD00683. Gurubaran, S., R. Rajaram, T. Nakamura, and T. Tsuda (2005), Inter-annual variability of diurnal tide in the tropical mesopause region: A signature of the El Niño–Southern Oscillation (ENSO), Geophys. Res. Lett., 32, L13805, doi:10.1029/2005GL022928. Hansen, G., and U.-P. Hoppe (1996), Investigation of the upper mesospheric dynamics under late polar summer conditions by EISCAT and lidar, J. Atmos. Terr. Phys., 58(1–4), 317– 335, doi:10.1016/0021-9169(95)00039-9. Hitchman, M. H., K. W. Bywaters, D. C. Fritts, L. Coy, E. Kudeki, and F. Surucu (1992), Mean winds and momentum fluxes over Jicamarca, Peru, during June and August 1987, J. Atmos. Sci., 49(24), 2372– 2383, doi:10.1175/1520-0469(1992)049 2.0.CO;2. Höffner, J., and J. Lautenbach (2009), Daylight measurements of mesopause temperature and vertical wind with the mobile scanning iron lidar, Opt. Lett., 34(9), 1351– 1353, doi:10.1364/OL.34.001351. Iimura, H., D. C. Fritts, and D. M. Riggin (2010), Long-term oscillations of the wind field in the tropical mesosphere and lower thermosphere from Hawaii MF radar measurements, J. Geophys. Res., 115, D09112, doi:10.1029/2009JD012509. Jagannadha Rao, V. V. M., D. Narayana Rao, M. Venkat Ratnam, K. Mohan, and S. Vijaya Bhaskar Rao (2003), Mean vertical velocities measured by Indian MST radar and comparison with indirectly computed values, J. Appl. Meteorol., 42(4), 541– 552, doi:10.1175/1520-0450(2003)042 2.0.CO;2. Kishore Kumar, G., M. Venkat Ratnam, A. K. Patra, V. V. M. Jagannadha Rao, S. Vijaya Bhaskar Rao, K. Kishore Kumar, S. Gurubaran, G. Ramkumar, and D. Narayana Rao (2008), Low-latitude mesospheric mean winds observed by Gadanki mesosphere-stratosphere-troposphere (MST) radar and comparison with rocket, High Resolution Doppler Imager (HRDI), and MF radar measurements and HWM93, J. Geophys. Res., 113, D19117, doi:10.1029/2008JD009862. Kumar, G. K., M. V. Ratnam, A. K. Patra, V. V. M. J. Rao, S. V. B. Rao, and D. N. Rao (2007), Climatology of low-latitude mesospheric echo characteristics observed by Indian mesosphere, stratosphere, and troposphere radar, J. Geophys. Res., 112, D06109, doi:10.1029/2006JD007609. Kurihara, J., et al. (2009), Temperature enhancements and vertical winds in the lower thermosphere associated with auroral heating during the DELTA campaign, J. Geophys. Res., 114, A12306, doi:10.1029/2009JA014392. Meek, C. E., and A. H. Manson (1989), Vertical motions in the Upper Middle Atmosphere from the Saskatoon (52°N, 107°W) M.F. radar, J. Atmos. Sci., 46(6), 849– 859, doi:10.1175/1520-0469(1989)046 2.0.CO;2. Miller, K. L., S. A. Bowhill, K. P. Gibbs, and I. D. Countryman (1978), First measurements of mesospheric vertical velocities by VHF radar at temperate latitudes, Geophys. Res. Lett., 5(11), 939– 942, doi:10.1029/GL005i011p00939. Nakamura, T., T. Tsuda, and S. Fukao (1996), Mean winds at 60–90 km observed with the MU radar (35°N), J. Atmos. Terr. Phys., 58(6), 655– 660, doi:10.1016/0021-9169(95)00064-X. Narayana Rao, T., K. N. Uma, D. Narayana Rao, and S. Fukao (2008), Understanding the transportation process of tropospheric air entering the stratosphere from direct vertical air motion measurements over Gadanki and Kototabang, Geophys. Res. Lett., 35, L15805, doi:10.1029/2008GL034220. Portnyagin, Y. I., T. V. Solov'eva, E. G. Merzlyakov, A. I. Pogorel'tsev, and E. N. Savenkova (2010), Height-latitude structure of the vertical wind in the upper mesosphere and lower thermosphere (70–110 km), J. Atmos. Oceanic Phys., 46(1), 95– 104, doi:10.1134/S0001433810010123. Portnyagin, Y. I., E. G. Merzlyakov, T. V. Solov'eva, I. Pogorel'tsev, E. V. Suvorova, P. Mukhtarov, and D. Pancheva (2011), Height-latitude structure of the vertical component of the migrating semidiurnal tide in the upper mesosphere and lower thermosphere region (80–100 km), J. Atmos. Oceanic Phys., 47(1), 108– 118, doi:10.1134/S0001433811010117. Rao, P. B., A. R. Jain, P. Kishore, P. Balamuralidhar, S. H. Damle, and G. Viswanathan (1995), Indian MST radar: 1. System description and sample vector wind measurements in ST mode, Radio Sci., 30(4), 1125– 1138, doi:10.1029/95RS00787. Rastogi, P. K., and S. A. Bowhill (1976), Gravity waves in the equatorial mesosphere, J. Atmos. Terr. Phys., 38, 51– 60, doi:10.1016/0021-9169(76)90193-8. Roja Raman, M., V. V. M. Jagannadha Rao, M. Venkat Ratnam, G. Kishore Kumar, A. Narendra Babu, S. Vijaya Bhaskara Rao, N. Prabhakara Rao, and D. Narayana Rao (2008), Atmospheric circulation during active and break phases of Indian summer monsoon: A study using MST radar at Gadanki (13.5°N, 79.2°E), J. Geophys. Res., 113, D20124, doi:10.1029/2008JD010341. Tao, X., and C. S. Gardner (1995), Heat flux observations in the mesopause region above Haleakala, Geophys. Res. Lett., 22(20), 2829– 2832, doi:10.1029/95GL02876. Venkat Ratnam, M., D. Narayana Rao, T. Narayana Rao, S. Thulasiraman, J. B. Nee, S. Gurubaran, and R. Rajaram (2001), Mean winds observed with Indian MST radar over tropical mesosphere and comparison with various techniques, Ann. Geophys., 19(8), 1027– 1038, doi:10.5194/angeo-19-1027-2001. Venkat Ratnam, M., A. K. Patra, and B. V. Krishna Murthy (2010), Tropical mesopause: Is it always close to 100 km? J. Geophys. Res., 115, D06106, doi:10.1029/2009JD012531. Woodman, R. F., and A. Guillen (1974), Radar observations of winds and turbulence in the stratosphere and mesosphere, J. Atmos. Sci., 31, 493– 505, doi:10.1175/1520-0469(1974)031 2.0.CO;2. Zhou, Q. H. (2000), Incoherent scatter radar measurement of vertical winds in the mesosphere, Geophys. Res. Lett., 27(12), 1803– 1806, doi:10.1029/2000GL003747. Citing Literature Volume116, IssueD2227 November 2011 FiguresReferencesRelatedInformation