Gravity waves generated by tropical convection: generation mechanisms and implications for global circulation models
Convectively generated gravity waves influence the circulation of the middle atmosphere by depositing momentum to the flow at their breaking levels. Global Circulation Models do not resolve convectively generated gravity waves and therefore the gravity wave spectrum generated by convection must be parameterized. In order to properly parameterize such a spectrum the mechanisms of wave generation by convection must be understood.In this study gravity wave generation by tropical convection is explored by studying the response to thermal forcings of varying complexity. Firstly we examine the gravity wave response to a thermal forcing using 2D linear Boussinesq equations. Subsequently we perform model simulations with specified heating of more complex forcings. Lastly we perform squall line simulations to verify the relationships found using the simpler methods.Throughout the study we separately study the response to the oscillatory and the steady-state components of the thermal forcing region and we examine how environmental wind interacts with each of those components. We show that the momentum flux phase speed spectrum of gravity waves generated by the oscillatory component of the forcing in motionless basic state can be determined from the knowledge of the vertical and horizontal scales of convection and its temporal distribution. The gravity wave spectrum is symmetric in the east-west directions in a motionless atmosphere, however this is drastically altered by the presence of mean wind in and above the heating region. We present a method of describing the changes in the momentum flux phase speed spectrum under various environmental shear distributions both in and above the forcing region.The steady-state component of the heating produces upward propagating gravity waves only in the presence of environmental shear. The gravity wave response to such forcing is similar to the response to flow over a mountain, where the vertical wavelength of gravity waves is determined by the environmental buoyancy frequency and the mean wind.In our study we show that all the features observed in the spectrum of gravity waves from simulated squall lines can be explained in terms of wave generation by the steady and oscillatory components of the forcing, however at this point it can not be concluded that gravity waves in all convection are solely generated by the oscillatory and steady-state components of the heating region.
- Atmospheric sciences