<p>Constraints to do realistic modeling of the electric field at the tip of a lightning leader

In several studies Terrestrial gamma-ray flashes (TGF) have been associated with radio spherics generated by lightning discharges. In particular, a close relationship between TGFs and positive Intra Cloud lightning, which develops in a stepped manner transporting electrons from the main negative toward the upper positive charge layer, have been inferred. In light of these observations the production of TGFs has been proposed to occur during the step in the electric field ahead of the leader tip. Several computer models have been developed to model electron acceleration and multiplication in the strong electric field ahead of a lightning leader. These models assume a strong inhomogeneous electric field in air at sea-level density and pressure. We present new constraints on the vertical length of the leader and the electric field strength that must be considered to do realistic modeling of the leader electric field. Realistic initial conditions are chosen based on in-situ measurements of electric field, vertical extention and the corresponding potential differences in thunderclouds. The observed maximum electric field strength of 50 kV/cm at sea-level, ahead of laboratory leaders, is introduced as an upper constraint on the leader electric field. Scaling the results from sea-level to in-cloud altitudes, constrains the maximum length of the leader channel, sea-level equivalent, to approximately 3 km. We then introduce these constraints to estimates of the leader electric field to determine the region ahead of the leader that has more realistic conditions than others.