1. Motivation:

Both linear wave theory and high-resolution numerical modeling have led to an increased knowledge about the generation of convective gravity waves in the subtropics and tropics. Two primary factors exist that determine the phase speed spectrum of convectively generated gravity waves having short-period and short horizontal wavelength:

(a) the depth of the latent heating region, and,

(b) the tropospheric wind.

Additionally, the tropical dynamical forcing depends on the vertical structure of the zonal mean wind which in turn is associated with the stratospheric gravity wave activity via the Quasi-Biennial Oscillation (QBO).

Usually, these small-scale waves are parameterized in Global Circulation Models (GCMs) using gravity wave parameterizations that assume a uniform gravity wave distribution, one phase speed spectrum distribution, and constant amplitude for all waves (non-orographic gravity wave parametrization). Recent advances to use realistic spectral distributions as those applied in the Whole Atmosphere Community Climate Model (WACCM) are still matter of debate as their experimental verification is complicated.

It is well known in the scientific community, that a spectrum of waves with a broad range of phase speeds is needed to drive the tropical as well as the stratospheric Semi-Annual Oscillation (SSAO). Furthermore, gravity waves are also believed to be partly responsible for driving the Mesospheric Semi-Annual Oscillation (MSAO). Convectively generated gravity waves that are not filtered out in the lower stratosphere propagate upward in the altitude range of the SSAO. Similarly, some gravity waves propagate through the SSAO region to the mesosphere where they often break owing to unstable amplitudes. Near the solstices (around June and December), gravity waves are the largest forcing term of the MSAO. Moreover, they are out of phase with the gravity wave forcing in the SSAO region (largest around March and September). Gravity waves in the tropics contribute to the QBO by breaking near critical levels in the alternating descending westward and eastward shear zones.

Particularly, the experimental determination of the characteristics of these waves from the source region to the middle atmosphere and the associated instability processes are challenging tasks for the currently available observational techniques. Therefore, we propose to deploy HALO to study convectively-generated gravity waves with airborne instruments covering the troposphere as well as stratospheric and mesospheric altitudes.

2. Goals

We propose to conduct airborne investigation of convectively-generated gravity waves in the tropics for studying:

• their excitation in the vicinity of deep convective systems,
• their contiguous impact on air safety,
• their spatial redistribution into the middle atmosphere,
• their long-range influence on the QBO and MSAO by momentum deposition,
• their saturation, and, last but not least,
• their representation in GCMs.