The IMPROVE-1 Storm of 1–2 February 2001. Part I: Development of a Forward-Tilted Cold Front and a Warm Occlusion

Abstract Analysis of observations and the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) are used to study the development of a forward-tilted cold front off the coast of Washington State. The vertical velocity associated with the cold front produced a wide cold-frontal rainband. In the early stage of development the midtropospheric baroclinic zone (or upper cold front) moved forward with time over the warm sector to produce a structure similar to a split front. The movement of the upper cold front was due to horizontal transport and frontogenetical propagation. The frontogenetical propagation was produced by a combination of tilting and diabatic frontogenesis, which resulted in a negative/positive couplet of frontogenesis straddling the baroclinic zone. The lower-tropospheric cold front eventually caught up with the warm front to form a classical warm occlusion. In the initial occluding process the converging frontal zones tilted into a warm-type occlusion configuration due to the presence of a background vertical shear of the horizontal wind component perpendicular to the occluded front. Consequently, as the storm moved over the observing network, the occluded front had the structure of a warm occlusion (tilted forward) in the lower levels. Above the occlusion, the cold front was also tilted forward because it retained its split-front-like structure. Thus, the development of the split front and the warm occlusion were separate processes that occurred in sequence. Although the MM5 captured the basic forward tilt with time of the cold front, some key aspects of the midtropospheric frontal structure were not well simulated. Because diabatic heating was an important contributor to the maintenance and movement of the upper cold front, it is hypothesized that discrepancies in diabatic heating associated with deficiencies in the model’s explicit microphysical scheme may be responsible for deficiencies in reproducing the structure of the upper cold front.