Abstract
A 24-h numerical simulation is used to study features of the wedge-shaped pressure ridge and the coastal front, which occur in eastern United States during winter and often cause substantial errors in operational models. Specifically, the Penn State/National Center for Atmospheric Research, Mesoscale Model (MM) is used as a diagnostic tool to investigate the mesoscale structure of these phenomena.associated with the Appalachian ice-storm of 13–14 January 1980.
The MM, which used 15 vertical levels (8 below 800 mb), a 50-km, horizontal mesh size, and a multi-level boundary-layer parameterization, produced a better forecast of boundary-layer temperature and sea-level pressure than did the National Meteorological Center's operational model for this case-study period. To the east of the mountains the MM maintained the wedge-ridge pattern and produced the low-level, northerly flow which is characteristic of the damming region. The MM also provided reasonable vertical profiles of temperature and wind, including a low-level, mountain-parallel jet which is commonly observed along mountain ridge during cold-air damming. However, despite the model's fairly sophisticated surface energy equation and moderate spatial resolution, the low-level temperature simulation was still 1-5°C too warm in some areas to the cast of the mountains.
The moderate horizontal and vertical resolution did allow the MM to develop the coastal front which appeared in the Chesapeake Bay-New Jersey area by the end of the 24-h study period. Despite the small-scale nature of a coastal front, synoptic and mesoalpha-scale forcing in the model created a realistic, coastal baroclinic zone. The simulated, low-level temperature gradient and convergence zone compared favorably with surface observations. The simulated coastal front, separating easterly maritime air from the cold northerly flow inland, diminished in strength with height and disappeared by 940 mb. In the damming region, observations suggested and model results showed an “extended coastal front”. a sloping inversion separating the trapped surface-based cold air from the warm onshore flow above. This feature appears to be at least partially responsible for the low-level jet.
Model-generated, kinematic trajectories showed the strongly sheared, three-dimensional character of the flow within the damming region. These model results and observations indicate that a relatively large number of model layers would be required below ∼800 mb in order to adequately model the thermal and wind field structures.
