On source specifications in a stochastic gravity wave parameterization

While stochastic Gravity Wave (GW) parameterizations are promising for improving longstanding climate predictions, there is no consensus regarding the values of tunable parameters. In this study, the GW field is represented using a multiwave scheme, in which the GWs are related to sources. Each source is characterized by the horizontal wavenumber, which is randomly chosen within a given interval, and the phase velocity, which is assumed to follow a centered Gaussian distribution with fixed standard deviation. A control simulation, using the LMDz global climate model together with the present-day parameterization is compared with two variants, that are based on alternative source specifications. These variants are designed using a smaller standard deviation for the phase velocity, which is shown to be a condition for the small scale content of the resulting GW fields to recover that observed in vertical soundings. From on-line simulations, is it found that a vertical spread of the sources allows to restore the inclination of the stratospheric jet and to compensate the warm bias compared to reanalyses. The off-line results obtained with vertically distributed sources reproduce vertical wavenumber spectral tail slopes which vary near the -3 value reported by observations.