Fire ignitions related to radar reflectivity patterns in Arizona and New Mexico

Abstract

Over 5400 lightning-ignited wildfires were detected on federal land in Arizona and New Mexico from 1996 through 1998 during the fire season of May through September. The non-uniform and sporadic spatial nature of precipitation events in this region makes the use of rain gauge data a limited means of assessing when and where a cloud-to-ground lightning strike might have ignited a wildfire due to dry lightning. By analysing weather radar reflectivity data with lightning and wildfire data, characteristics of radar reflectivity can be used by fire weather forecasters to identify regions of increased ignition potential. Critical ranges of reflectivity, life span of a reflectivity cell, and storm movement are characteristics of radar reflectivity that are examined in this analysis. The results of this type of analysis can help focus attention of wildfire personnel to particular locations where there is known to be cloud-to-ground lightning in conjunction with radar reflectivity patterns that have been historically associated with wildfire ignition. Results from the analysis show that wildfire ignitions typically occur near the perimeter of a radar echo. The reflectivity values at the ignition location are less than the highest reflectivity located within the echo, and often magnitudes are sufficiently low to suggest that the precipitation is not reaching the ground in this dry region with high cloud bases. Interpretation of the duration, size and level of lightning activity of the radar echo associated with the ignition indicate that ignitions tend to occur in the early stages of a radar echo. However, there are often multiple storm cells having isolated areas of higher reflectivity within a radar echo at the time of ignition. Nearly two-thirds of radar echoes associated with wildfire ignitions moved more than 50 km throughout the echo’s lifetime. These moving storm systems often propagated in a northerly or easterly direction, and ignitions occurred on the leading edge of the storm in over half of the cases that propagated in the same direction. Forecasters can use results from this study to determine where there is an increased potential of wildfire ignitions when similar radar patterns appear in conjunction with lightning activity in the future.