Sequence of surface meteorological variables with the passage of winter cold fronts in the United States east of the Rocky Mountains

Abstract

According to the 80-year-old Norwegian cyclone model, fronts are pressure troughs, extensions of a mid-latitude cyclone. Thus, a cold frontal passage is evidenced at the surface by a minimum in air pressure and a corresponding wind shift. In addition, decreases in both temperature and humidity also occur. This conceptual model assumes that these changes are nearly coincident in time; however, this is usually not the case. Strict adherence to the model is the basis for the highly subjective single line drawn on a synoptic chart to indicate a front. To evaluate the behavior of meteorological variables with cold frontal passage, data was collected for seven American cities east of the Rocky Mountains, including NMC/NCEP 3-hourly surface analyses and NCDC archived hourly surface observations during three consecutive winters (October 1-March 31). Surface analyses were used to estimate the time of frontal passage. Using these estimates as a guide, the hourly observations were employed to determine the specific hours at which each meteorological variable demonstrated a change that could be conclusively attributed to the frontal passage. The four meteorological variables scrutinized include temperature, moisture (dew point), wind (direction), and pressure. Significance of means tests were applied to assess the statistical significance of the differences between the timing of each pair of variables (temperature vs. wind, dew point vs. pressure, etc.). Statistically significant differences mathematically validate the sequence of the means and suggest the likely repeatability of the sequence. At all locations and for all subsets of fronts, the pressure rise and the wind shift consistently precede the decreases in temperature and dew point. The timing of the pressure rise, however, is nearly indistinguishable from the wind shift, and likewise, there is rarely any significant difference between the decrease in temperature and the decrease in dew point. While all the variables are essentially coincident in time for some fronts, there is a frequent disparity between the pressure rise/wind shift and the temperature/dew point decrease. The data thus suggest a double-boundary frontal zone, a surface pressure trough/wind shift line followed by a temperature/moisture boundary.