Towards a More Earth-like Circulation in a Dry Dynamical Core Model

Dry dynamical core models are useful for the investigation of dynamical phenomena in which physical processes play a secondary role only. The basic idea of a dry model is to isolate the dynamics from the physics by relaxing temperatures towards analytically prescribed equilibrium temperatures. However, dry models tend to develop somewhat unrealistic circulations and are unfit for the investigation of phenomena in which similarity with the real climate system is crucial. Results from previous studies are therefore highly idealized, for example they employ unrealistic topographic forcing and disallow zonal variations in equilibrium temperature or surface drag. Here, we aim to generate a more Earth-like circulation in a dry model while trying to still keep things simple. We accomplish this by introducing into the model (1) realistic topography, (2) zonal asymmetries in equilibrium temperatures, and (3) zonal asymmetries in surface drag. We investigate the role of these three factors individually and in their combination for the coupled stratosphere-troposphere circulation system. As expected, with topographic forcing the model produces more upward propagating waves and an improved stratospheric polar vortex. Similar improvements result from using zonally asymmetric equilibrium temperatures and surface drag. With all three factors together the circulation is not only most realistic but in some instances the combined effect is also stronger than the sum of the three individual ones, indicating some non-linear behavior. Using all three factors and following an iterative method introduced by Chang (2006) we optimize the equilibrium temperatures and achieve an improved, more Earth-like circulation. The improvements concern temperature distribution, stratospheric polar vortex, tropospheric jet in terms of magnitude and position, and frequency of stratospheric sudden warmings. We demonstrate that the improved model can be used to explore the sensitivity of the coupled stratosphere-troposphere system to climate-change-like surface forcings.