Representation of the 12–14 March 1993 "Storm of the Century" in a high resolution and dynamically adjusted version of the ECHAM4 General Circulation Model

Between the 12th and 14th of March 1993 on of the most intense extratropical cyclones of recent decades penetrated the East Coast of the U.S.A. It was associated with a severe squall-line over Florida and large amounts of snow in wide regions of the eastern U.S.A. Furthermore it lead to the most widespread disruption of air travel in the history of aviation.

In this study the development of this system in a high resolution general circulation model (with ~100x100 km horizontal resolution (T106)) is investigated. Of particular interest was the ability of the model to reproduce smaller-scale features, such as the structure of the squall-line, the representation of the frontal boundaries in general, local circulations leading for example to a "cold air damming" along the eastern range of the Appalachian mountains or the structure of the convective activity in the region behind the cold front off the East Coast of the U.S.A..

To force the model to the observed state a dynamical adjustment technique based on the "Newtonian Relaxation technique" (nudging) is used. It adds a non-physical relaxation term to the model equations at each time step for the prognostic variables vorticity, divergence, temperature and the logarithm of surface pressure. As "observations" fields from the ECMWF reanalysis are used.

In general the development of the cyclone is well represented in the model simulation. This is true even for quantities which are not forced to the observed state.

The convergence region of the cold front, the development of the fronts in general and the squall line in particular, are realistically represented in the model simulation. The same is true for the qualitative distribution of the different precipitation fields with the exception of the region of convective precipitation occurring behind the cold front. This convection is not represented by the model. Possible reasons are the too coarse horizontal resolution or a too strong subsidence in the "dry slot" behind the cold front.

The "cold air damming" along the eastern range of the Appalachian mountains is not represented by the model. This might be caused by the too coarse horizontal and vertical resolution used for the simulation and the smoothed orography used at T106 resolution.