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Keywords:

  • line convection;
  • squall;
  • surface observations;
  • frontal gradients

ABSTRACT

Large gradients in surface parameters are often observed across cold fronts exhibiting line convection. Such gradients may constitute a hazard to aviation, owing to the associated strong horizontal wind shear and abrupt pressure surges. Significant along-front variability in the magnitude of cross-frontal gradients is usually observed, making quantitative forecasting of expected parameter changes and anticipation of future locations of associated hazards challenging. In this paper, operationally available minute resolution data are analysed for three recent cases of strong cold fronts, in order to explore the magnitude and along-frontal distribution of parameter changes. Cross-frontal temperature fall, sea level pressure rise and wind veer were calculated and mapped. In two of the cases, coherent regions of larger parameter changes were resolved, which could be traced for several hours. This raises the possibility of providing more quantitative short term forecasts of expected parameter changes at downstream locations by extrapolation. Swathes of larger parameter changes usually corresponded to tracks of intense line convection segments. However, significant variability in the relationship between radar reflectivity and the magnitude of parameter changes was found, both within individual cases and between cases. In the third case, the typically small scale of line convection segments precluded the resolution of coherent areas of larger parameter changes, though the general areas in which parameter changes were locally large could still be delineated. Generally, cross-frontal temperature falls exceeding 1.25 °C, pressure surges exceeding 0.4 hPa, and wind veers exceeding 30° could be confidently (>90%) identified by the analysis.