Moist effects on orographically forced stationary waves

Introduction

Main conclusions •  Orographically forced stationary waves show a clear modification when interactive diabatic heating is included in the model. The modification consists mainly in greater amplitudes and a slight eastward phase shift.

•  A linear stationary wave model could be used to reproduce the non-linear results to a high extent. Both orography, diabatic heating and transient eddy forcing has to be included.

•  The diabatic heating, which is created as a response to the orographically forced stationary waves has strongest contributions by the latent heat release, while the water vapour’s longwave radiation effect only plays a minor role.

•  Latent heat release yields a greater response close to the orography and in the tropical region. These two regions are therefore not changed in the experiments to preserve the energy balance in the model.

Experiments Stationary waves are forced by orography and zonal asymmetries in the diabatic heat fields. A number of experiments were conducted with the Planet Simulator, an EMIC developed at the University of Hamburg, as an attempt to uncouple the dry dynamics from moist diabatic effects. More specifically we try to uncouple the dynamics from water vapour’s effect on long-wave radiation and latent heat release, referred to as rad and lh. The model set-up is an aqua-planet with a single 4000 meter high gaussian shaped topography centered at 45° N, shown as a gray shaded region in the figures.

The following list shows how the diabatic effects are calculated in each experiment. Square brackets mean zonally symmetric contribution to the temperature tendency, i.e. uncoupled from the dynamics.

Difference fields for non-linear and linear simulations at σ = 0.823

There is a great similarity between the difference wave fields in the non-linear and linear simulations. The general wave pattern is reproduced in the linear model but the phases are not exactly the same. The linear amplitudes are in general higher.

The upper (lower) panels show the non-linear (linear) difference fields between experiment (a) and the other experiments. The models are forced by orography (shaded area), diabatic heating and transient eddies. Furthermore, left figures show the resulting wave field for (a)-(b), middle figures (a)-(c) and right figures (a)-(d).

Left figures; the upper (lower) panel shows the non-linear (linear) full wave responses to orography (shaded region), diabatic heating and transient eddies for experiment (a).

Right figure shows a section of the the diabatic heat field for experiment (a) in black and experiment (d) in red.

Full non-linear and linear fields at σ = 0.823 plus diabatic heat fields

The present study examines orographically forced stationary waves in a moist atmosphere on an aqua-planet. Stationary wave theory applies a linear separation of forcing terms into orographic, diabatic and transient-eddy forcing. However, diabatic and transient-eddy forcing cannot be determined from boundary conditions, but result themselves from the stationary waves in the non-linear atmosphere.

In this study we will determine how the diabatic stationary wave forcings adjust as a consequence of orographically forced stationary waves. To this aim, non-linear model experiments are conducted in which the stationary waves were forced either solely by orography or by orography in combination with different diabatic terms. The stationary wave responses are interpreted with a linear model.

*Corresponding author email: marcus@misu.su.se