Cold Fronts with and without Prefrontal Wind Shifts in the Central United States

摘要

Time series of cold fronts from stations in the central United States possess incredible variety. For example, time series of some cold fronts exhibit a sharp temperature decrease coincident with a pressure trough and a distinct wind shift. Other time series exhibit a prefrontal trough and wind shift that precedes the temperature decrease associated with the front by several hours. In early March 2003, two cold fronts passed through Oklahoma City, Oklahoma (OKC), representing each of the above scenarios. The cold front on 4 March was characterized by a coincident sharp wind shift, pressure trough, and a strong temperature decrease of 10℃ in 2 min. On the other hand, the cold-frontal passage on 8 March was characterized by a prefrontal wind shift occurring over a 7-h period before the temperature decrease of 10℃ in 2 h. Twelve hours before frontal passage at OKC, both fronts had the same magnitude of the horizontal potential temperature gradient and Petterssen frontogenesis. By the time of frontal passage at OKC, the magnitude of the horizontal potential temperature gradient for the 4 March front was double that of the 8 March front, and the frontogenesis was nearly four times as great. The simultaneity of the surface horizontal potential temperature gradient and deformation and convergence maxima (coincident with the wind shift) was primarily responsible for the greater strength of the cold front in OKC on 4 March compared to that on 8 March. Whether a prefrontal wind shift occurred was determined by the timing and location of cyclogenesis in the central United States. On 4 March, a cyclone was adjacent to the slope of the Rocky Mountains and developed on the cold front as it moved through Oklahoma, permitting greater frontogenesis and resulting in a cold-frontal passage at OKC with a simultaneous temperature decrease and wind shift. On 8 March, the cyclone moved eastward through Oklahoma before the arrival of the cold front, resulting in a prefrontal wind shift associated with the northerlies behind the cyclone, followed by the frontal passage. A 2-yr climatology of cold-frontal passages at OKC supports the results from the two cases above, indicating that the timing and location of cyclogenesis was responsible for these two different cold-frontal structures. These results imply that, for situations resembling those of this study, the prefrontal trough is not directly associated with the cold front, but is caused by external processes related to the lee troughing.