The Kinematics and Dynamics of Extreme Warm–Frontal Passages

Warm fronts are the neglected child of the Norwegian cyclone model. Although abundant research has focused on cold fronts and, to a lesser degree, occluded fronts, studies examining the structure and dynamics of warm fronts are rare. The authors have observed several cases of rapid and large-amplitude (10°C hr-1) warm-frontal passages over Oklahoma. In this study, we focus on two aspects of warm fronts. First, whereas most warm fronts portrayed in the literature are weak, we show a case with an associated temperature gradient rivaling that of the strongest cold fronts. Second, we examine the mechanisms by which warm fronts move. Specifically, how does the warm air displace the denser cold air as the warm front advances?

These two aspects of warm fronts are explored with a strong warm front from 12 January 2005 in Oklahoma. Time series from the Oklahoma Mesonet show large variability of temperature increases across the state during the frontal passage. The temperature increased 13°C in 1 h at Stigler, whereas the temperature increased 11°C in 4 h at Norman. The mesonet locations where rapid temperature increases were observed (e.g., Stigler) were associated with quick increases in wind speed and abrupt change from northeasterly to southerly winds. At stations with a gradual temperature increases (e.g., Norman), the wind shifted slowly to southerly throughout the warming period, and the wind speed increased only slightly.

To diagnose what mechanisms may be causing the differences in warming rates, the 12 January 2005 0600 UTC Rapid Update Cycle – 20km (RUC) was used. The RUC produced a forecast maximum temperature increase of 5°C in a 1 h period between the 1 h and 2 h forecast, only 38% of the observed increase. Nevertheless, the increase was substantially larger than the 1–2°C h–1 increases typically observed with most warm fronts. Therefore, the 0600 UTC run of the RUC was used to estimate the terms in the potential-temperature thermodynamic equation; quasihorizontal potential-temperature advection, vertical potential-temperature advection and a residual term. The preliminary results of the thermodynamic equation show that quasihorizontal potential-temperature advection appears to be a large factor in the temperature increase in the model forecasts. However, the results of the residual term indicate that there are other mechanisms that are important in rapid warm-frontal passages. Mechanisms such as the role of friction, horizontal and vertical mixing, etc, have not been fully explored. We hypothesize that the more extreme warming events are a result of vertical mixing of shallow inversions, a process not well-resolved in models.