author = "Essien, Patrick and Wrasse, Cristiano Max and Paulino, I. and 
                         Medeiros, A. F. and Buriti, R. A. and Takahashi, Hisao",
          affiliation = "{Instituto Nacional de Pesquisas Espaciais (INPE)} and {Instituto 
                         Nacional de Pesquisas Espaciais (INPE)} and {Universidade Federal 
                         de Campina Grande (UFCG)} and {Universidade Federal de Campina 
                         Grande (UFCG)} and {Universidade Federal de Campina Grande (UFCG)} 
                         and {Instituto Nacional de Pesquisas Espaciais (INPE)}",
                title = "Seasonal characteristics of small- and medium-scale gravity waves 
                         in the mesosphere and lower thermosphere region over brazilian 
                         equatorial region",
              journal = "Annales Geophysicae",
                 year = "2018",
               volume = "36",
               number = "3",
                pages = "899--914",
                month = "jun.",
             keywords = "Atmospheric composition and structure (airglow and aurora)  
                         electromagnetics (wave propagation)  history of geophysics 
                         (atmospheric sciences).",
             abstract = "This work reports the seasonal characteristics of small- and 
                         medium-scale gravity waves in the mesosphere and lower 
                         thermosphere region observed in the OH NIR airglow images over Sao 
                         Joao do Cariri (7.4oS, 36.5oW). Observations were made from 
                         September 2000 to December 2010, which corresponded to a total of 
                         1496 nights. To investigate medium-scale gravity waves, sequences 
                         of images observed in each night was used to create 701 keograms. 
                         Two techniques were used to calculate for gravity waves 
                         parameters: Keo-FFT analysis for medium-scale gravity waves 
                         (MSGWs) and Fourier cross spectrum for small-scale gravity waves 
                         (SSGWs). The two analytical techniques resulted in 537 and 2343 
                         MSGWs and SSGWs events respectively. The horizontal wavelengths of 
                         MSGWs were concentrated between 100 to 150 km, while that of the 
                         SSGWs were between 10 to 15 km. The observed periods for MSGWs 
                         ranged from 20 to 40 minutes, while the SSGWs had a maximum peak 
                         around 5 to 10 min. The observed horizontal phase speed of MSGWs 
                         was distributed between 60 to 80 10 m/s while the SSGWs showed a 
                         peak around 20 to 40 m/s. Except spring, which the wave events 
                         propagated in all directions, in summer, fall and winter were 
                         northeast and southeast. The horizontal propagation direction of 
                         MSGWs and SSGWs show clear seasonal variations based on the 
                         influence of the background atmospheric conditions such as wind. 
                         The anisotropy observed in the total propagation directions of 
                         MSGWs was northeast which can be attributed to either the source 
                         location of the gravity waves or the filtering process due to the 
                         wind system which depends fundamentally on the seasonal 
                         variations. However, there was no clear anisotropy in the total 
                         propagation direction of the SSGWs which could be due to uniformly 
                         distribution of the sources or filtering processes by the wind 
                         system. Critical level theory for gravity wave filtering was 
                         applied to study the effects of middle atmospheric winds on the 
                         propagation path of the wave events while the average of daily 
                         mean Outgoing Long-wave Radiation (OLR) was used to study the 
                         possible seeding mechanism of the gravity waves. The SSGWs were 
                         found to be filtered out by the mean flow due to their least phase 
                         velocities. Deep Convection is the possible seeding mechanism for 
                         both MSGWs and SSGWs in the summer and fall over the Brazilian 
                         equatorial sector. MSGWs were found to be less susceptible to wind 
                         filtering effects due to their high phase velocities.",
                  doi = "10.5194/angeo-36-899-2018",
                  url = "http://dx.doi.org/10.5194/angeo-36-899-2018",
                 issn = "0992-7689",
             language = "en",
           targetfile = "essien_seasonal.pdf",
        urlaccessdate = "24 nov. 2020"