Generation Mechanisms of Convectively Forced Internal Gravity Waves and Their Propagation to the Stratosphere

Abstract

Characteristics of gravity waves induced by mesoscale convective storms and the gravity wave sources are investigated using a two-dimensional cloud-resolving numerical model. In a nonlinear moist (control) simulation, the convective system reaches a quasi-steady state after 4 h in which convective cells are periodically regenerated from a gust front updraft. In the convective storms, there are two types of wave forcing: nonlinear forcing in the form of the divergences of momentum and heat flux, and diabatic forcing. The magnitude of the nonlinear source is 2 to 3 times larger than the diabatic source, especially in the upper troposphere. Three quasi-linear dry simulations forced by the wave sources obtained from the control (CTL) simulation are performed to investigate characteristics of gravity waves induced by the various wave source mechanisms. In the three dry simulations, the magnitudes of the perturbations produced in the stratosphere are comparable, yet much larger than those in the CTL simulation. However, the sum of the quasi-linear perturbations generated by the nonlinear and diabatic sources compare well with the mesoscale circulations and gravity waves in the CTL simulation. Through the spectral analysis, it is found that the stratospheric gravity waves in all simulations have similar vertical wavelengths (6.6?9.9 km), horizontal wavelengths (10?100 km), and periods (8?80 min). Also, the magnitudes of gravity waves in the quasi-linear dry simulations are comparable with each other in spite of the differences in the magnitude of the nonlinear and diabatic sources. This is because the vertical propagation condition of linear internal gravity waves, in both the troposphere and stratosphere, restricts wave sources in the horizontal wavenumber (k) and frequency (?) domain, and therefore all of the forcing cannot generate gravity waves that can propagate up to the stratosphere. Compared with the diabatic sources, the nonlinear sources are inefficient in generating linear gravity waves that can propagate vertically into the stratosphere. These results suggest that wave generation mechanisms cannot be accurately understood without examining the vertical propagation condition of the gravity waves. Also, the ?effective? wave sources are of comparable magnitude, yet mostly out of phase. Therefore, although the wave amplitudes produced by simulations with nonlinear forcing and diabatic forcing are about 2 to 3 times too large, their sum compares well to the control simulation.