Joint Centre for Mesoscale Meteorology (JCMM)
JCMM INTERNAL REPORT NO.57
by T D Hewson
This paper addresses the long-standing problem of plotting atmospheric fronts objectively. A thermodynamic definition of a front is proposed. Time-independent diagnostic quantities are then devised according to this definition. These can be computed from any gridded dataset. Fronts are then plotted, without human intervention, by utilising standard graphics package facilities, such as contouring, to represent the diagnostics. The methodology includes applying threshold criteria to automatically erase fronts which are thermally weak. By tuning these criteria a good match with subjective fronts is obtained. In addition, the objective surface fronts very often coincide with troughs in surface pressure, implying that a thermodynamic definition is sufficient. The objective front-plotting is further developed to recognise split fronts, and ana- and data-front characteristics. Many examples are presented; these illustrate how objective fronts are potentially a very powerful tool for both forecasting and research. Here at JCMM plots and animations showing objective fronts have been produced on a daily basis for several months now.