Force balances and dynamical regimes of numerically simulated cold fronts within the boundary layer

Abstract

The governing dynamics of numerically simulated cold fronts, as they collapse towards a minimum cross-frontal scale, are identified by determining force balances and the Rossby and Ekman numbers in frontal regions. A hierarchy of numerical simulations of cold fronts is performed with the Weather Research and Forecasting model and individual terms in the horizontal momentum equations are calculated from the model output to construct force balances. An inviscid, three-dimensional, idealized front is simulated at three different horizontal grid spacings (100, 20 and 4 km) and then the experiments are repeated with a planetary boundary layer (PBL) parametrization scheme included. Additionally, a fullphysics simulation of the cold front originally analyzed by Sanders is conducted. The leading edge of the idealized, inviscid cold front is characterized by small Rossby numbers and hence balanced dynamics at all three model resolutions. When the PBL scheme is included, large Rossby numbers and unbalanced flow develop at the leading edge of the cold front, but only when the front is simulated at high resolution; at coarse resolution, balanced dynamics remain. The acceleration is in the cross-front direction and arises due to a localized pressure gradient force. Unlike the inviscid experiments or the coarse-resolution PBL experiments, the front in the high-resolution PBL experiment has a large cross-front thermal gradient, strong forced ascent and sharp surface pressure trough, which enables a large cross-front pressure gradient force to develop. The dynamics of the Sanders front agree qualitatively with the idealized PBL simulation, but much larger Rossby numbers occur at the leading edge of the Sanders front. This finding indicates that the dynamics of the Sanders front are more unbalanced than the dynamics of the idealized simulations.

Sensitivity of idealised baroclinic waves to mean atmospheric temperature and meridional temperature gradient changes

“…The same initial conditions were used e.g. by Sinclair and Keyser (2015), and the zonal jet is described in the paper of Waite and Snyder (2009). To enable others to reproduce our results, we have archived the initial condition files of our experiments, the namelist used in the WRF simulations, and a piece of WRF source code which sets up the initial conditions needed to run the model.…”

The characteristics and structure of extra-tropical cyclones in a warmer climate

“…For example, baroclinic wave simulations have been performed to understand the dynamics of extra-tropical cyclones and fronts in the current climate (e.g. Simmons and Hoskins, 1978;Thorncroft et al, 1993;Schemm et al, 2013;Sinclair and Keyser, 2015). More recently baroclinic life cycle experiments have also been used to assess, in a highly controlled simulation environment, how the dynamics and structure of extra-tropical cyclones may respond to climate change.…”

The characteristics and structure of extra-tropical cyclones in a warmer climate

“…For example, baroclinic wave simulations have been performed to understand the dynamics of extra-tropical cyclones and fronts in the current climate (e.g. Simmons and Hoskins, 1978;Thorncroft et al, 1993;Schemm et al, 2013;Sinclair and Keyser, 2015).…”