Case study analysis of convectively-generated gravity waves

Deep convection is a source of high frequency gravity waves that may affect the ambient flow in a variety of ways. For example, waves may induce turbulence and mixing near cloud top, thereby contributing to the transport of mass across the tropopause. Waves are a significant source of energy to the middle atmosphere and supply a momentum flux convergence to accelerate the mean flow. Additionally, waves may trigger or modulate existing convection in the troposphere. In spite of the importance of these effects, there remains much to be understood about the mechanisms governing wave generation and their interaction with the surrounding environment.

The purpose of this investigation is to examine the basic properties of convectively-generated gravity waves. Particular attention is paid to the evolution of the wave spectrum from initiation to propagation through the tropopause. Several case studies of deep convection over the United Kingdom are analyzed using both observation and high resolution numerical model simulation. Observations of high frequency waves are provided by a Mesosphere-Stratosphere-Troposphere (MST) wind profiling radar located at Aberystwyth in Wales. Numerical simulation is performed with the (UK) Met Office Unified Model (UM) on a limited area grid with horizontal grid spacing as high as 1 km at which resolution the convective parameterization is disengaged and the wave source is primarily explicit.

Using wavelets, the wave spectrum is demonstrated to be comprised of several distinct peaks that appear in both the MST observations and UM simulations. The spectral peaks exist within a range of horizontal wavelengths from approximately 5 km to 50 km with periods less than 30 min. The origin of these multiple peaks is presently under investigation. Possible causes include a resonant triad interaction or a wave-source interaction. The latter mechanism is examined in idealized model experiments in which a wave generation mechanism is externally imposed (e.g., an injection of vertical velocity of finite duration) against a background environment that allows for wave reflection and refraction near the wave source.