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Seasonal characteristics of small- and medium-scale gravity waves in the mesosphere and lower thermosphere over the Brazilian equatorial region
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Seasonal characteristics of small- and medium-scale gravity waves in the mesosphere and lower thermosphere over the Brazilian equatorial region
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Journal Article
Seasonal characteristics of small- and medium-scale gravity waves in the mesosphere and lower thermosphere over the Brazilian equatorial region
2018
Overview
The present work reports seasonal characteristics of small- and medium-scale
gravity waves in the mesosphere and lower thermosphere (MLT) region. All-sky
images of the hydroxyl (NIR-OH) airglow emission layer over São João do
Cariri (7.4∘ S, 36.5∘ W; hereafter Cariri) were obtained
from September 2000 to December 2010, during a total of 1496 nights. For
investigation of the characteristics of small-scale gravity waves (SSGWs) and
medium-scale gravity waves (MSGWs), we employed the Fourier two-dimensional
(2-D) spectrum and keogram fast Fourier transform (FFT) techniques,
respectively. From the 11 years of data, we could observe 2343 SSGW and 537
MSGW events. The horizontal wavelengths of the SSGWs were concentrated
between 10 and 35 km, while those of the MSGWs ranged from 50 to 200 km. The
observed periods for SSGWs were concentrated around 5 to 20 min, whereas the
MSGWs ranged from 20 to 60 min. The observed horizontal phase speeds of SSGWs
were distributed around 10 to 60 m s−1, and the corresponding
MSGWs were around 20 to 120 m s−1. In summer, autumn, and winter
both SSGWs and MSGWs propagated preferentially northeastward and
southeastward, while in spring the waves propagated in all directions. The
critical level theory of atmospheric gravity waves (AGWs) was applied to
study the effects of wind filtering on SSGW and MSGW propagation
directions. The SSGWs were more susceptible to wind filtering effects than
MSGWs. The average of daily mean outgoing longwave radiation (OLR) was also
used to investigate the possible wave source region in the troposphere. The
results showed that in summer and autumn, deep convective regions were the
possible source mechanism of the AGWs. However, in spring and winter the deep
convective regions did not play an important role in the waves observed at
Cariri, because they were too far away from the observatory. Therefore, we
concluded that the horizontal propagation directions of SSGWs and MSGWs show
clear seasonal variations based on the influence of the wind filtering
process and wave source location. Keywords. Atmospheric composition and structure (airglow and aurora) – electromagnetics (wave propagation) – history of geophysics (atmospheric sciences)
