Journal of Atmospheric and Solar-Terrestrial Physics xxx (2017) 1–7 Contents lists available at ScienceDirect Journal of Atmospheric and Solar-Terrestrial Physics journal homepage: www.elsevier.com/locate/jastp A case study of convectively generated gravity waves coupling of the lower atmosphere and mesosphere-lower thermosphere (MLT) over the tropical region: An observational evidence S. Eswaraiah a, *, G. Venkata Chalapathi b, K. Niranjan Kumar c, M. Venkat Ratnam d, Yong Ha Kim a, P. Vishnu Prasanth e, Jaewook Lee a, S.V.B. Rao f a Department of Astronomy and Space Science, Chungnam National University, Daejeon 305-764, South Korea Department of Physics, Govt. Degree College, Anantapur 515001, India c Atmosphere and Ocean Research Institute (AORI), The University of Tokyo, Chiba 277-8568, Japan d National Atmospheric Research Laboratory (NARL), Gadanki, Tirupati 517501, India e Department of Physics, Sree Vidyanikethan Engineering College, Tirupati 517102, India f Department of Physics, Sri Venkateswara University, Tirupati 517501, India b A R T I C L E I N F O A B S T R A C T Keywords: Short-period gravity waves Vertical coupling Convection MST Radar LIDAR We have utilized the Gadanki MST Radar and Rayleigh LIDAR to understand the vertical coupling between the lower atmosphere and mesosphere through the short-period gravity waves (GWs). The short-period GWs (20 min–2 h) are noticed both in the troposphere and in the mesosphere during the deep convection. During the convection, the large vertical velocities (>5 m/s) and significant variations in the momentum flux (~3 m2/s2) are noticed in the troposphere and higher fluxes (~45 m2/s2) are evidenced in the mesosphere. The observations suggest the vertical coupling between the lower and middle atmosphere during convection. 1. Introduction It is well understood that the atmospheric gravity waves (GWs) are playing a vital role in understanding the structure and dynamics of the middle atmosphere (Fritts and Alexander, 2003). GWs are most significant in transporting energy and momentum from the lower to the middle and upper atmosphere, and most of the GW sources are located in the troposphere (Fritts and Nastrom, 1992; Fritts and Alexander, 2003). Though the long-period and short-period GWs exist in the atmosphere, the long-period GWs are influenced by the rotation of the earth and the geostrophic adjustment process can act on the ageostrophic component in synoptic-scale wind systems, but the short-period GWs are believed to be generated locally (Sato, 1993). In fact, several sources are thought to generate the short-period GWs (Fritts and Alexander, 2003), however, deep convection (Dhaka et al., 2002; Kumar, 2006; Venkat Ratnam et al., 2008; Dutta et al., 2009; Uma et al., 2011; Das et al., 2012; Niranjan Kumar and Ramkumar, 2008; Niranjan Kumar et al., 2011a) is one of the main sources for the generation of the short-period GWs. The studies on GW generation and propagation and its characteristics are well established in the lower atmosphere in the tropical region (Dhaka et al., 2002; Nath et al., 2009; Niranjan Kumar et al., 2011a and references therein). Previous studies from the present study region have been shown a strong coupling between the troposphere and mesosphere through the planetary waves and intra-seasonal oscillations (Niranjan Kumar et al., 2011b; Niranjan Kumar et al., 2012). Nevertheless, the coupling process using the simultaneous ground-based observations with the two unique instruments (MST Radar and LIDAR) through the short-period GWs and their effects on the mesosphere and lower thermosphere (MLT) is sparsely studied over this region. Further, the GWs gained its importance in changing the MLT thermal structure (Fritts and Alexander, 2003). A vertically propagating GW begins to break at the level where there is a sudden change in the temperature lapse rate or at the critical level (Meriwether and Gardner, 2000; Meriwether and Gerrard, 2004). At the zones of GWs breaking, a large amount of energy and momentum is deposited by the waves (Walterscheid, 1981; Fritts and Nastrom, 1992; Gardner and Yang, 1998; Ramesh and Sridharan, 2012). Later, Lindzen (1981), theoretically demonstrated the significance of GW breaking in the middle atmosphere. It was understood that GWs are the potential constraints in the changing mesosphere temperature and causes the Mesosphere Temperature Inversion * Corresponding author. E-mail address: eswar.mst@gmail.com (S. Eswaraiah). https://doi.org/10.1016/j.jastp.2018.01.005 Received 30 September 2017; Received in revised form 2 January 2018; Accepted 3 January 2018 Available online xxxx 1364-6826/© 2018 Elsevier Ltd. All rights reserved. Please cite this article in press as: Eswaraiah, S., et al., A case study of convectively generated gravity waves coupling of the lower atmosphere and mesosphere-lower thermosphere (MLT) over the tropical region: An observational evidence, Journal of Atmospheric and Solar-Terrestrial Physics (2017), https://doi.org/10.1016/j.jastp.2018.01.005 S. Eswaraiah et al. Journal of Atmospheric and Solar-Terrestrial Physics xxx (2017) 1–7 mesosphere with in-situ measurements are sparsely studied due to lack of simultaneous observations in both the regions. Alexander and Pfister (1995) have shown that GWs propagate away from the deep convection. The effect of the transfer of momentum from one region of the atmosphere to another can be seen from the changes in the mean flow acceleration (Chang et al., 1997). Prabhakaran Nayar and Sreeletha (2003) showed that the momentum fluxes in the troposphere are larger during convection periods when compared to the quiet days, over a tropical region. However, it is not tested experimentally whether the convectively generated GWs will propagate to the mesosphere and affect the mesosphere dynamics over a tropical region or not. We made an attempt in the present study to see the changes in the mesosphere dynamics during convection period. This end, the aim of the present study is to investigate the vertical coupling process by the short-period GWs using the two active groundbased instruments (MST Radar and LIDAR) and quantifying the amount of GW fluxes transported from the lower atmosphere to the mesosphere, during the deep convection. The details of data used and methodology followed for the current study is briefly given in section 2, and the results and discussions are provided in section 3, comprehensive summary, and key findings are emphasized in section 4. (MTI) and the turbulence below and above the inversion layer (Garcia and Solomon, 1985; Hauchecorne et al., 1987; Venkat Ratnam et al., 2003; Ramesh and Sridharan, 2012). Coming to the evaluation of GW role in altering the energy budget of the middle and upper atmosphere, numerous methods are established to estimate the GW momentum flux (Gage, 1983; Worthington and Thomas, 1996). Vincent and Reid (1983) developed a symmetric beam radar method to measure the horizontal momentum flux using only horizontal winds. Dutta et al. (2005) studied the merits and demerits of Symmetric Beam Method (SBM) of Vincent and Reid (1983) and Hybrid Method (HM) of Worthington and Thomas (1996) and noticed that Symmetric Beam Method (Vincent and Reid, 1983) is virtuous at mesospheric altitudes, since the method does not require vertical winds. The vertical winds in the mesosphere are very weak, so this method is more appropriate at mesosphere altitudes. They also found that at the troposphere altitudes, both the methods are well matching. The Symmetric Beam Method (SBM) is widely used to measure the troposphere and mesospheric momentum flux using MST Radar at Gadanki (Eswaraiah et al., 2013 and references therein). Further, Fritts et al. (2012) reported GW momentum flux using different meteor radars with different meteor count and showed the variation of GW momentum flux with meteor count. By using radiosonde data, Zhang et al. (2012) measured the GW momentum flux in the troposphere and stratosphere. The ground-based observational studies on GW momentum fluxes at mesospheric altitudes are well reported at mid and high latitudes, and such studies are sparse in the low latitude MLT region. Recently, Eswaraiah et al. (2013) studied the variability of GW (of periods ~ 20 min to 2 h.) momentum flux from the surface to mesosphere using combined observations of both MST Radar and Rayleigh LIDAR at a tropical station, Gadanki (13.5 N, 79.2 E), India. In their study for the first time, they reported the variability of GW momentum flux in the troposphere, stratosphere, and mesosphere simultaneously using the different techniques using the long-term database (1998–2008). Most of the theoretical and experimental studies publicized that convectively generated short-period GWs in the tropical region is vital for middle atmosphere global circulation changes (Fritts and Alexander, 2003). Hence the assessment of vertical propagation and momentum flux of GWs in both the troposphere and mesosphere over the tropical region is imperative. Further, the case studies on the troposphere and 2. Data and methodology In the present study, the combined observations of MST Radar and Rayleigh LIDAR located in a tropical station, Gadanki (13.5 N, 79.2 E), India are used. This kind of two unique ground-based instruments are rarely existed at the same location and are highly useful to probe the atmosphere from surface to the mesosphere. Short-period GW momentum fluxes are estimated in both the troposphere and mesosphere using the MST Radar wind field while LIDAR temperature profiles are used to represent the GWs in the middle atmosphere. The Gadanki MST Radar is a high power VHF radar with a peak transmitter power 2.5 MW and operates at 53 MHz. The MST Radar provide winds from 1.5 km to 22 km and 65–85 km. More details of MST Radar can be found in Jain et al. (1994) and Rao et al. (1995), and data processing technique in Anandan et al. (2001). As mesospheric echoes from the MST Radar are mainly due to the fluctuations in electron density gradients and irregularities, they are confined to day-time and hence the Fig. 1. Latitude and longitude distribution of brightness temperature observed at different timings on 25 September 2000. Star mark indicates Gadanki (13.5 N, 79.2 E) location. 2 S. Eswaraiah et al. Journal of Atmospheric and Solar-Terrestrial Physics xxx (2017) 1–7 Fig. 2. (a) Time-Altitude plot of signal-to-noise ratio (SNR) observed in the mesosphere. (b) Same as (a) but in the troposphere. (c) Time-Altitude plot of vertical wind observed in the troposphere on 25 September 2000. Fig. 3. Wavelet analysis of MST radar radial velocities at 78 km on 25 September 2000. The left panel (a,b) shows east, west radial velocities and the right panel (c,d) shows the north, south radial velocities. The black line shows 95% confidence level. 3 S. Eswaraiah et al. Journal of Atmospheric and Solar-Terrestrial Physics xxx (2017) 1–7 the observational location, latitude and longitude distribution of brightness temperature observed at different timings on 25 September 2000 are shown in Fig. 1. The brightness temperature is taken as a proxy for the tropical deep convection. It can be clearly observed that deep convection is formed in and around Gadanki region on 25 September 2000 with the closest distance during evening hours (1530–1630 h IST). Deep convection was persistent throughout the observation and moves westward slowly and during 1330–1630 h IST it is very close to Gadanki. During this period the observational site is situated south-east of the convective plume, indicating that the short period GWs can generate both at the south-west and north direction of the observational site including overhead. The observational location is shown with star mark in Fig. 1. The coupling processes between the lower atmosphere and the mesosphere during the deep convection is investigated by the MST Radar observations of the mesospheric echoes and the tropospheric vertical velocities as shown in Fig. 2. The Fig. 2a and b represent the timealtitude plots of Signal-to-Noise Ratio (SNR) in both the mesosphere and the troposphere, respectively, and Fig. 2c is the vertical velocity in the troposphere. It is seen from the Fig. 2a that the strong mesospheric echoes are noticed at higher altitudes (~78–80.4 km) and strong updrafts and downdrafts with the vertical velocities as high as 5 m/s can be observed between 1430 and 1630 h IST in the troposphere (Fig. 2c) during the convection day. The strong vertical velocities are indicative of deep convection in and around the observational site. Moreover, the strong mesospheric echoes during the convection day could be formed due to availability of data is restricted to ~10:00 to 17:00 h Indian Standard Time (IST) (IST¼UTþ05:30 h). The structure and characteristics of MST Radar mesospheric echoes and mesospheric wind estimation are discussed in Kumar et al. (2007) and Eswaraiah et al. (2011). For the present study, a lucid analysis of radar radial velocity profiles of the convection day has been done using the procedure given in Eswaraiah et al. (2013). The reliable estimates of GW momentum flux can be obtained using a threshold of 20% of percentage occurrence (PO). Thus, the echoes in each range bin with PO less than 20% are omitted. For the flux estimation, the symmetric beam radar method of Vincent and Reid (1983) is utilized and the procedure has been discussed in Eswaraiah et al. (2013). The Gadanki Rayleigh LIDAR is used to observe GWs in the stratosphere and mesosphere. The LIDAR employs the second harmonic of Nd: YAG pulsed laser at 532 nm with a pulse energy of ~550 mJ and Rayleigh receiver is used, which employs a Newtonian telescope. The LIDAR provides photon count profiles with an altitude resolution of 300 m and time resolution of 250s (5000 laser shots were integrated for one profile). More details of this instrument and method of temperature analysis can be found from Bhavani Kumar et al. (2001), Siva Kumar et al. (2001) and Venkat Ratnam et al. (2002). The basic method of deriving the temperature profile from the measured photon count is similar to the procedure given by Hauchecorne and Chanin (1980). For the present study, MST Radar observations on 25 September 2000 during the daytime 1000 h–1700 h (IST) and Rayleigh LIDAR temperatures from 25 to 29 September 2000 have been used. The two unique instruments allowed us to study the vertical wave propagation from the lower atmosphere to mesosphere. For obtaining the information on the convection events in and around the observational site Gadanki, the brightness temperature data is used, which is obtained from Multifunctional Transport Satellite (MITSAT-1R), of JMA (Japanese Meteorological Agency), through Kochi University, Japan (http://weather.is.kochiu.ac.jp/archive-e.html) with grid Resolution of 0.05 . Hong et al. (2012), discussed the relation between the brightness temperature and convection. 3. Results and discussion In order to show the manifestation of the convection in and around Fig. 5. (a) Lomb–Scargle periodogram analysis of tropospheric vertical winds obtained using MST Radar at 10, and 16 km, respectively. (b) Rayleigh LIDAR daily mean temperature profiles shifted by 20 K. Fig. 4. Variation of SNR (a) and Spectral Width (b) at 78 km and 80.4 km, respectively, during the period 1000–1700 IST on 25 Sep. 2000. 4 S. Eswaraiah et al. Journal of Atmospheric and Solar-Terrestrial Physics xxx (2017) 1–7 periodogram analysis in the troposphere shows that the short-period GWs (20min-2h) is evidenced during the convection day and their amplitudes are observed above the confidence level, specifies the waves are generating in the troposphere due to deep convection. The daily mean night-time LIDAR temperature profiles (Fig. 5b) during the convection day and normal days, clearly reveals the signature of GWs in temperature perturbations from 50 to 75 km during the deep convection day. In contrast, such features are not seen on normal days. This aspect further supports the existence of the strong link between the lower atmosphere and mesosphere through the GWs, generated by the convection. Due to clouds and bad weather during the convection day, LIDAR has not been operated continuously and hence we could not get time series data in the gap region (30–60 km) to show the GW features. But, the mean profile clearly reflects the wave features. To determine the GWs breaking and deposition of energy and momentum at the mesosphere altitudes, we have estimated the amount of momentum flux carried by the GWs to the mesosphere and their momentum flux in the troposphere. The time variation of vertical flux of zonal and meridional momentum flux on the convection day is presented in Fig. 6. The fluxes are evaluated using the procedure given in Eswaraiah et al. (2013) and averaged over about 15 min. The errors in the momentum flux estimation by the MST radar have been taken care and have been discussed in Eswaraiah et al. (2013). During the peak convection time (1500 h-1630 h IST), the momentum flux in the tropospheric altitudes is significantly higher with the mean magnitudes of ~3 m2/s2 and even more. Much of the time, the zonal momentum flux is eastward (Fig. 6c) and meridional momentum flux is southward (Fig. 6d) in the troposphere. At the mesospheric altitude (78 km), the zonal momentum flux values are perturbed from mean values even before the convection. This might be associated with the GWs propagating radially from the convection plumes existed adjoining to the observational site. The momentum flux values both in the zonal (Fig. 6a) and in the meridional wind (Fig. 6b) reached up to 45 m2/s2. This higher momentum fluxes further indicate the amplitude of GWs the turbulence generated by short period GWs, propagated from the lower atmosphere. High SNR (~30 dB) is evident in the troposphere during the convection time (Fig. 2b). Further, the existence of shortperiod GWs at the mesosphere altitude (78 km) has been shown through the wavelet spectrum of radial velocities in the mesosphere and is depicted in Fig. 3. Fig. 3a and b shows the existence of short period (20 min to 2 h) GWs in the east, west radial velocities, respectively, while Fig. 3c and d exhibits the spectrum in north, south radial velocities. The magnitude of wavelet power is shown in logarithmic values. From the figure, it is observed that in the east and south radial velocities, the wave is strong between 1200 and 1400 h (IST) and continuously exist from 1100 to 1500 h (IST) in other two radial velocities. Hence from Fig. 3, we can claim the existence of short-period GWs, which might have propagated to mesosphere from the lower atmosphere and causes turbulence by wave breaking at mesosphere. Further, to find the relation between the GWs in the mesosphere and turbulence, the variability of SNR and spectral width at the mesospheric altitudes has been displayed in Fig. 4. From the figure it is clear that the SNR (Fig. 4a) and spectral width (Fig. 4b) becomes strong soon after the appearance of strong short-period GWs between 1300 and 1400 h IST (Fig. 3a and d). This implies that the strong turbulence was created after the propagation of GWs from the lower atmosphere to the mesosphere. Hence the strong turbulence in the mesosphere is evident from 1330 to 1500 h IST (Fig. 4a and b). To understand the wave forcing from the lower atmosphere to the mesosphere, we further looked into the tropospheric vertical velocities observed by MST Radar and Rayleigh LIDAR temperature profiles in the upper stratosphere and lower mesosphere during the deep convection day. Fig. 5a shows the Lomb-Scargle (L-S) periodogram analysis of the tropospheric vertical winds obtained from the MST Radar at 10, and 16 km, respectively. Fig. 5b displays the Rayleigh LIDAR daily mean temperature profiles with each profile shifted by 20 K for clarity. Since the LIDAR was not operated continuously during the convection period due to bad weather, we could not get time series profiles. The L-S Fig. 6. Zonal (left panels) and meridional (right panels) momentum fluxes at different altitudes observed on 25 September 2000 in the troposphere (bottom panels) and the mesosphere at 78 km (top panels). 5 S. Eswaraiah et al. Journal of Atmospheric and Solar-Terrestrial Physics xxx (2017) 1–7 Acknowledgements increases exponentially in vertical as the density decreases. Moreover, during normal days we noticed the momentum flux values are less than 1 m2/s2 in the troposphere and in the mesosphere it can reach as high as ~5 m2/s2 (Eswaraiah et al., 2013). The mesospheric fluxes are in accordance with the existence of short-period GWs as shown in Fig. 3. The estimated fluxes at the troposphere altitudes are consistent with the earlier reports over this region (Prabhakaran Nayar and Sreeletha, 2003). It is obvious from Fig. 6, that both the zonal (Fig. 6a) and meridional fluxes (Fig. 6b) in the mesosphere shows large values even before the convection event over Gadanki region, and also peak convection hours. This is probably due to the waves generated by convective region elsewhere (for example from the region south-west of Gadanki as seen in Fig. 1 and reach the mesospheric altitudes over Gadanki) and also at the Gadanki. This is the unique observational study over the tropical region, India, and shows the signature of vertical coupling of the lower atmosphere and mesosphere through short-period GWs. SE acknowledges for financial support by the Korea Polar Research Institute (PE17020), Korea. We deeply appreciate the National Atmospheric Research Laboratory (NARL), Gadanki, India, for providing the data used in the present study. Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi. org/10.1016/j.jastp.2018.01.005. References Alexander, M.J., Pfister, L., 1995. Gravity wave momentum flux in the lower stratosphere over convection. Geophys. Res. Lett. 22, 2029–2032. https://doi.org/10.1029/ 95GL01984. 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Summary and conclusions By utilizing the simultaneous observations of two ground-based instruments namely MST Radar and Rayleigh LIDAR, at a tropical station, Gadanki (13.5 N, 79.2 E), India, the existence and propagation of the short-period (~of periods 20 min to 2 h) GWs during the deep convection and their momentum fluxes has been estimated. The important findings are summarized below; 1. The deep convection is noticed in and around Gadanki, a tropical region, India using the brightness temperature on 25 September 2000. 2. During the convection day, the MST radar observed mesospheric echoes from 1000 to 1700 h (IST) and the large vertical velocities (>5 m/s) in the troposphere during the peak convection. 3. The wavelet analysis of mesospheric radial velocities reveals the existence of short-period GWs in the mesosphere during the convection day, indicating that the mesospheric echoes might be associated with the turbulence caused by breaking of short-period GWs. 4. Spectral analysis of tropospheric vertical winds shows the generation of short –period GWs at the observational location and the LIDAR temperature profiles shows the presence of GWs in the upper stratosphere and lower mesosphere on the convection day while the wave features are not seen in other normal days. 5. The large variations in momentum flux (3 m2/s2) were noticed in the troposphere during the peak convection. In the mesosphere, sudden enhancement in the flux (~45 m2/s2) are noticed during the convection and even at an earlier time than the overhead convection in the troposphere, indicating that the waves might have originated from convection occurring elsewhere in the far away region (from Fig. 1) and propagated to the mesosphere, in addition to the waves generated at the observational site. Thus, the wavelet analysis in the mesosphere and L-S periodogram analysis of tropospheric vertical winds and the LIDAR temperature profiles coherently proves the presence of short-period GWs constantly from the troposphere to mesosphere indicating the strong vertical coupling during the convection. The turbulence in the mesosphere could be due to the GWs that are transporting energy and momentum from the lower atmosphere to the mesosphere. Further study about convectively generated GWs and their propagation using model simulations and ray tracing methods will be an added advantage for the present study, which will be presented in the upcoming publication. Availability of data and materials MST Radar data and LIDAR data can be provided on request to NARL (M. Venkat Ratnam). 6 S. Eswaraiah et al. Journal of Atmospheric and Solar-Terrestrial Physics xxx (2017) 1–7 Rao, P.B., Jain, A.R., Kishore, P., Balamuralidhar, P., Damle, S.H., Viswanathan, G., 1995. Indian MST radar 1. 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