Gravity Wave Sources and Parameterization

Northwest Research Associates, Incorporated, Seattle WA

Investigators

Abstract

This three-year project will improve the parameterization of subgrid-scale gravity wave effects in global models. Gravity wave forcing of the global-scale mean flow is used to correct common deficiencies in modeled stratospheric circulations: (1) A cold-pole problem in the winter stratosphere that has links to errors in temperature-sensitive ozone chemistry in chemistry-climate models and links to errors in planetary wave propagation and reflection, (2) A delayed onset of easterly winds in the springtime that also affects the propagation of planetary waves and the occurrence of stratospheric warmings in the early spring season, and (3) the lack of a quasibiennial oscillation in the tropical lower stratosphere that also affects planetary wave propagation, stratospheric ozone, and propagation of equatorial waves. In addition to the mean-flow forcing effects of gravity waves on the global scale, other effects are emerging as important that are not yet parameterized in global models. In cirrus clouds, for instance, wave vertical motions control crystal sizes, number densities, precipitation rates, and cloud lifetimes, which in turn can have global-scale radiative and ozone chemistry effects. In the troposphere, upward-propagating gravity waves generated by mature convective storms may reflect from higher altitudes back towards the boundary layer and influence further convective initiation there. There are two essential components to gravity wave forcing parameterizations: (1) the specification of the wave source, and (2) the estimation of the wave dissipation as a function of height. There are several methods for parameterization of the wave dissipation with height in use in global models currently, but no clear way to distinguish between them. The approach in this project focuses on the wave sources, and on constraining those rigorously with observations. This will not only make the parameterizations more realistic, but it may also eventually distinguish between the different dissipation methods. Mountain wave sources have been specifically parameterized in global forecasting and climate models for two decades. Because mountain waves are stationary, their dissipation can only drag the mean flow toward zero wind speed. Mountain wave sources are also limited geographically. Waves from other sources are nonstationary so their dissipation can cause either deceleration or acceleration of the mean flow. The focus in this project is on wave generation by convection. Convection is known to generate waves with a broad range of nonstationary phase speeds and is likely the dominant gravity wave forcing mechanism throughout the tropics and in summer midlatitudes. The intellectual merit of the work crosses traditional boundaries in the atmospheric sciences, using tools of cloud-modeling and precipitation radar observations to understand the origin and nature of small-scale waves generated by convection. Through parallel linear model studies and comparison to existing parameterization methods there will be increased understanding of the essential physics of gravity wave generation. Finally, through collaboration with global modeling groups, parameterization improvements to the subgrid-scale gravity wave forcing and feedbacks in global models will be evaluated. The broader impacts of this work will be advances in understanding of gravity wave generation and gravity wave effects across altitude regions ranging from the top of the boundary layer into the upper atmosphere. In addition to the main goal of improving parameterizations of gravity wave mean-flow forcing in global models, results will also be valuable in cirrus cloud studies, and in studies of convection initiation in the troposphere. The project involves training of graduate student and postdoctoral researchers. Initially, the project will be conducted solely by female researchers, a rare occurrence because females are still under-represented in the atmospheric sciences. The project also begins partnerships with two global climate modeling groups, one in the US and and the other in Europe. While subgrid-scale gravity wave effects are not the primary obstacle to improving global weather forecasting and climate models today, many global modeling groups recognize the importance of raising their model upper boundaries to mesospheric altitudes, and this intensifies the need for improved parameterizations of gravity wave effects.