Towards transient subgrid-scale gravity wave representation in atmospheric models. Part II: Wave intermittency simulated with convective sources

Abstract

In a companion paper, the Multi-Scale Gravity-Wave Model (MS-GWaM) has been introduced and its application to a global model as a transient subgrid-scale parameterization has been described. This paper focuses on the examination of intermittency of gravity waves (GWs) modeled by MS-GWaM. To introduce the variability and intermittency in wave sources, convective GW sources are formulated, using diabatic heating diagnosed by the convection parameterization, and they are coupled to MS-GWaM in addition to a flow-independent source in the extratropics accounting for GWs due neither to convection nor to orography. The probability density function (PDF) and Gini index for GWpseudomomentum fluxes are assessed to investigate the intermittency. Both are similar to those from observations in the lower stratosphere. The intermittency of GWs over tropical convection is quite high and found not to change much in the vertical. In the extratropics, where non-convective GWs dominate, the intermittency is lower than (comparable to) that in the tropics in the stratosphere (mesosphere), exhibiting a gradual increase with altitude. The PDFs in these latitudes seem to be close to the log-normal distributions. Effects of transient GW-mean-flow interactions on the simulated GWintermittency are assessed by performing additional simulations using the steady-state assumption in the GW parameterization. The intermittency of parameterized GWs over tropical convection is found to be overestimated by the assumption, whereas in the extratropics it is largely underrepresented. Explanation and discussion of these effects are included.