الفهرس 
Seasonal Variability of Gravity Wave Poten- tial Energy Density at Various Latitudes
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Abstract
In this study, we review the characteristics of the seasonal cycle of
gravity waves between 2012 to 2020 at different latitudes using temperatures measured by lidar and satellite. We study the energy densities of gravity waves in three different latitudes: Andenes in Norway
(69◦ N), K¨uhlungsborn in Germany ( 54◦ N), and Cairo in Egypt (
30◦ N).
For the first two locations, the data measured by both the lidar
and the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) satellite are used in addition to ERA-5 reanalysis
data. To separate the temperature perturbations caused by gravity
waves, a filter on vertical wavelength (λz < 15 km) and another filter
on wave period (τ < 8 h) were applied. from these temperature fluctuations, the Gravity Wave Potential Energy Density (GWPED) is
calculated. The comparison between the lidar and the satellite temperature analysis showed that both regions show an apparent seasonal
variability of GWPED with a maximum in winter and a minimum
in summer. However, the amplitude of winter to summer variability differs from lidar to satellite. GWPED from reanalysis is smaller compared to lidar, and the difference increases with altitude in winter. All measured and reanalysis data confirmed the larger GWPED
of K¨uhlungsborn over Andenes, but the exact reason for this is not
yet evident.
We used the agreement between the lidar and the satellite to
study the gravity waves over Cairo. Since there are no lidar measurements carried out in Cairo, we used SABER data. Different results from K¨uhlungsborn and Andenes are obtained. GWPED over
Cairo analyzed similarly to K¨uhlungsborn and Andenes showed maximum solstice values and minimum equinoxes values. Winter values
over Cairo are close to those over Andenes and, hence, lower than
the corresponding values over K¨uhlungsborn. Also, the amplitude
of variability between seasons over Cairo is in order of 2. The peak
of summer (August and July in particular) is more pronounced and
persistent in the climatology of both quantities than the winter maximum in high altitudes. Scale heights from EpV and EpM showed large
values of the latter than the former. This difference is an indication
of faster growth of amplitude with height than the energy dissipation
rate.
We investigated the possible sources of GWs over Cairo by studying the behavior of GW with some selected weather patterns, namely,
jet streams, convection. In winter, the sources of GWs are possibly
due to the convection from the Mediterranean sea. In summer, we
have convectively generated GWs due to the African monsoons.
To determine the factors affecting gravity waves in both polar
and middle latitudes, we have studied the effect of North Arctic Oscillation (NAO) and Quasi-Biennal Oscillations (QBO) by studying
their correlation with gravity waves potential energy density at Cairo.
NAO showed a good correlation with winter EpM as well as with
winds at 200 hPa, indicating a teleconnection between GWPED and
the NAO. However, QBO showed almost no connection to the GWs
over Cairo.
A comparison between K¨uhlungsborn and Cairo revealed a significantly higher EpV in winter at K¨uhlungsborn relative to Cairo.
The case is the opposite in summer.
The main reason for such difference is the sources of GWs at
each location. At K¨uhlungsborn, it is the polar vortex in winter. In
summer, particularly in August, the GWs in Cairo are convectively
generated due to the African monsoons. The strong westerly winds
allow for their propagation further up. Such conditions are not feasible for the mid-latitudes. The mean zonal wind is the crucial factor
for the propagation of GWs in both locations in summer.