Nonlinear forcing mechanisms of the migrating terdiurnal solar tide and their impact on the zonal mean circulation

Abstract

Abstract. We investigate the forcing mechanisms of the terdiurnal solar tide in the middle atmosphere using a mechanistic global circulation model. In order to quantify their individual contributions, we perform several model experiments and separate each forcing mechanism by switching off the remaining sources. We find that the primary excitation is owing to the terdiurnal component of solar radiation absorption in the troposphere and stratosphere. Secondary sources are nonlinear tide–tide interactions and gravity wave–tide interactions. Thus, although the solar heating clearly dominates the terdiurnal forcing in our simulations, we find that nonlinear tidal and gravity wave interactions contribute in certain seasons and at certain altitudes. By slightly enhancing the different excitation sources, we test the sensitivity of the background circulation to these changes of the dynamics. As a result, the increase of terdiurnal gravity wave drag can strongly affect the middle and upper atmosphere dynamics, including an irregular change of the terdiurnal amplitude, a weakening of neutral winds in the thermosphere, and a significant temperature change in the thermosphere, depending on the strength of the forcing. On the contrary, the influence of nonlinear tidal interactions on the middle atmosphere background dynamics is rather small.

Interaction of Small-Scale Gravity Waves with the Terdiurnal Solar Tide in the Mesosphere and Lower Thermosphere

“…Compared to earlier MUAM versions that use a highly tuned linear Lindzen-type GW parameterization for the middle atmosphere (e.g. Jacobi et al, 2015;Lilienthal and Jacobi, 2019;Samtleben et al, 2019), the winter mesospheric jet is enhanced in the present simulations. In the lower thermosphere, the main impact of the whole atmosphere GW parameterization is to drive a vertical/meridional circulation, mainly by the GWs with high phase speeds, which are able to propagate through the MLT wind jets.…”

Forcing mechanisms of the migrating quarterdiurnal tide

“…MUAM is a 3-D, primitive equation, mechanistic global circulation model based on the earlier Cologne Model of the Middle Atmosphere-Leipzig Institute for Meteorology (COMMA-LIM), described by Jakobs et al (1986), Fröhlich et al (2003b) and Jacobi et al (2006). Recent versions of the MUAM model are described by Lilienthal et al (2017), Lilienthal et al (2018), Lilienthal and Jacobi (2019), Jacobi et al (2019) and Samtleben et al (2019). The model reaches from the surface at 1000 hPa to a log-pressure height of 160 km, with a constant scale height of H = 7 km and a vertical resolution of 2.842 km.…”

“…Above 30 km, the background winds can develop freely in the model and are only affected by the zonal mean temperature nudging below. In contrast to other model experiments (e.g., Pogoreltsev et al, 2007;Pogoreltsev, 2007;Lilienthal et al, 2017;Samtleben et al, 2019), we do not include planetary wave forcing at the lower boundary to avoid undesired wave coupling with tides. In our experiments, we perform ensemble runs with 11 members using ERA-Interim data from 2000 to 2010 in order to describe interannual variability.…”

“…In contrast to the version by Ermakova et al (2017) and Jacobi et al (2017a), latent heat release is not included here. We used a horizontal resolution of 2.5 • × 5.625 • for the model, which differs from the version of, e.g., Lilienthal et al (2017) and Lilienthal et al (2018), in order to be able to better resolve the meridional structure of the QDT. In an earlier model version with a 5 • meridional resolution, essentially only one maximum was seen in the QDT amplitudes per hemisphere (Jacobi et al, 2019), whereas satellite observations (Azeem et al, 2016;Liu et al, 2015) show a more detailed meridional structure.…”

“…The distribution of tendency terms from both gravity wave routines can be summed up to the total acceleration of the mean flow through gravity waves. More information about the gravity wave parameterization included in MUAM is given in Lilienthal et al (2017) and Lilienthal et al (2018).…”