Frontal activity

Early studies of frontal activity in the Arctic, such as those of Reed and Kunkel (1960) and Barry (1967), were based on manual analysis. While extremely time consuming, manually depicted fronts always contain an element of subjectivity. With the advent of fast computers, thinking has turned to the application of automated methods. Hewson (1998) provides a comprehensive review. Of the various methods that can be found in the literature, one that seems to work fairly well is a thermal front parameter, or TFP. The TFP is defined as:

where t is a thermodynamic variable. The TFP magnitude will be largest where there is a rapid change in the thermal gradient (the first term on the right) with a large component parallel to the direction of the (unitized) thermal gradient (the second term on the right). The minus sign places the frontal boundary on the warm-air side of the concentrated baroclinic zone (corresponding to a ridge line in the field of TFP). The simplest application is to define fronts based on ridge lines of TFP, with the requirement that the TFP exceed a selected threshold value. Further details and application of more robust approaches are described by Hewson (1998).

Serreze et al. (2001) used the TFP approach to assess frontal frequencies over a 20-year period (1979-98) for the region north of 30° N. The intent was to re-examine the concept introduced by Dzerdzeevskii (1945) and Reed and Kunkel (1960) of a "separate" summer Arctic frontal zone (Section 4.1). The study used six-hourly 850-hPa temperature fields from the NCEP/NCAR reanalysis.

Figure 4.12 shows derived frontal frequencies for winter and summer, expressed as the mean number of fronts per day (a frequency of 0.10 means a front was present on 10% of days). Also shown are fields of the mean 850 hPa temperature gradient in K (100 km)-1 based on the 20-year period. The frontal analysis routine is inherently sensitive to strong temperature gradients in areas of extreme topography, the boundaries of high plateaus, and where there are strong land-ocean contrasts. While it turns out that topography and land-ocean contrasts are important for understanding high-latitude frontal activity, this sensitivity can also cause problems. This issue was addressed simply by masking out known problem areas.

The winter frontal frequency field is rather noisy. High frequencies are found over most of North America except northeastern Canada, where the mean temperature gradient is weak. Note the band of high frequencies over the Atlantic Basin, associated with the eastern North American jet and its attendant storm track. There is an analogous feature (extending beyond the latitude bounds of the figure) over the Pacific Basin,

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