Changes in measured lightning flash count and return stroke peak current after the 1994 U.S. National Lightning Detection Network upgrade - 2. Theory

A model of return stroke detection by the U.S. National Lightning Detection Network (NLDN) magnetic direction finder (MDF) sensors is used to approxim ate the pulse width criterion modification made to the sensors during the 1 994 upgrade. Decreasing the pulse width detection criterion used by the MDF sensors increases their effective detection range, which increases their s ensitivity to weak flashes (because of NLDN network geometry, increasing se nsitivity has little effect on detection of strong flashes). Consequently, we observe an increase in the weak flash counts. The increased detection of weak flashes accounts, in part, for the decrease in mean peak currents obs erved in subsequent years to 1994. In addition to decreasing the mean peak current of detected positive and negative flashes, the NLDN upgrade has app arently had the unwanted effect of increasing the contamination of the posi tive CG flash data with cloud flashes.