Impact of a simple parameterization of convective gravity-wave drag in a stratosphere-troposphere general circulation model and its sensitivity to vertical resolution

Abstract

Systematic westerly biases in the southern hemisphere wintertime ¯ow and easterly equatorial biases are experienced in the Me te o-France climate model. These biases are found to be much reduced when a simple parameterization is introduced to take into account the vertical momentum transfer through the gravity waves excited by deep convection. These waves are quasi-stationary in the frame of reference moving with convection and they propagate vertically to higher levels in the atmosphere, where they may exert a signi®cant deceleration of the mean ¯ow at levels where dissipation occurs. Sixty-day experiments have been performed from a multiyear simulation with the standard 31 levels for a summer and a winter month, and with a T42 horizontal resolution. The impact of this parameterization on the integration of the model is found to be generally positive, with a signi®cant deceleration in the westerly stratospheric jet and with a reduction of the easterly equatorial bias. The sensitivity of the Me te o-France climate model to vertical resolution is also investigated by increasing the number of vertical levels, without moving the top of the model. The vertical resolution is increased up to 41 levels, using two kinds of level distribution. For the ®rst, the increase in vertical resolution concerns especially the troposphere (with 22 levels in the troposphere), and the second treats the whole atmosphere in a homogeneous way (with 15 levels in the troposphere); the standard version of 31 levels has 10 levels in the troposphere. A comparison is made between the dynamical aspects of the simulations. The zonal wind and precipitation are presented and compared for each resolution. A positive impact is found with the ®ner tropospheric resolution on the precipitation in the mid-latitudes and on the westerly stratospheric jet, but the general impact on the model climate is weak, the physical parameterizations used appear to be mostly independent to the vertical resolution .

The coupled seasonal hindcasts of the South American monsoon

“…The change in the vertical resolution of the AGCM in the COLA-coupled model did not have as comparable an impact as the change in the horizontal resolution which the AGCM had. This result is consistent with an earlier study of Bossuet et al (1998) who also found insignificant impact on the mean climate of their model from changing the vertical resolution. The improvement in reducing the bias from changing resolutions in the COLA model is disappointingly much smaller than the model bias.…”

Review of the formulation of present‐generation stratospheric chemistry‐climate models and associated external forcings

“…Dynamical coupling between dynamics and chemistry is performed via a 6-hourly exchange of fields [Déqué, 2007;Teyssèdre et al, 2007]. GWD in the CNRM-ACM includes a component dependent on tropospheric convection [Bossuet et al, 1998].…”

Bergen Earth system model (BCM-C): model description and regional climate-carbon cycle feedbacks assessment

“…The physical parameterization is divided into several explicit schemes, which each calculate the flux of mass, energy and/or momentum due to a specific physical process. Different from the model description in Déqué et al (1994), the version used in BCM contains a convective gravity drag parameterization (Bossuet et al, 1998), a new snow scheme (Douville et al, 1995), a refined orographic gravity wave drag scheme (Lott and Miller, 1997;Lott, 1999;Catry et al, 2008) and modifications in deep convection and soil vegetation schemes.…”