Weather and Climate Dynamics
Weather and Climate Dynamics
Weather and Climate Dynamics
Preprints
https://doi.org/10.5194/wcd-2020-61
https://doi.org/10.5194/wcd-2020-61

  22 Dec 2020

22 Dec 2020

Review status: this preprint is currently under review for the journal WCD.

The signature of the tropospheric gravity wave background in observed mesoscale motion

Claudia Christine Stephan1 and Alexis Mariaccia2,a Claudia Christine Stephan and Alexis Mariaccia
  • 1Max Planck Institute for Meteorology, Hamburg, Germany
  • 2Université Claude Bernard Lyon 1, Villeurbanne, France
  • anow at: Laboratoire Atmosphères, Observations Spatiales, Guyancourt, France

Abstract. How convection couples to mesoscale vertical motion and what determines these motions is poorly understood. This study diagnoses profiles of area-averaged mesoscale divergence from measurements of horizontal winds collected by an extensive upper-air sounding network of a recent campaign over the western tropical North Atlantic, the Elucidating the Role of Clouds-Circulation Coupling in Climate (EUREC4A) campaign. Observed area-averaged divergence amplitudes scale approximately inversely with area equivalent radius. This functional dependence is also confirmed in reanalysis data and a global freely-evolving simulation run at 2.5 km horizontal resolution. Based on the numerical data it is demonstrated that the energy spectra of inertia gravity waves can explain the scaling of divergence amplitudes with area. At individual times, however, few waves can dominate the region. Nearly monochromatic tropospheric waves are diagnosed in the soundings by means of an optimized hodograph analysis. For one day, results suggest that an individual wave directly modulated the satellite-observed cloud pattern. However, because such immediate wave impacts are rare, the systematic modulation of vertical motion due to inertia-gravity waves may be more relevant as a convection-modulating factor. The analytic relationship between energy spectra and divergence amplitudes proposed in this article, if confirmed by future studies, could be used to design better external forcing methods for regional models.

Claudia Christine Stephan and Alexis Mariaccia

 
Status: open (until 10 Feb 2021)
Status: open (until 10 Feb 2021)
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Claudia Christine Stephan and Alexis Mariaccia

Claudia Christine Stephan and Alexis Mariaccia

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Short summary
Vertical motion on horizontal scales of a few hundred kilometers can influence cloud properties. This motion is difficult to measure directly, but can be inferred from the area-averaged mass divergence. The latter can be derived from horizontal wind measurements at the area’s perimeter. This study derives vertical properties of area-averaged divergence from an extensive network of atmospheric soundings and proposes an explanation for the variation of divergence magnitudes with area size.