Aircraft observations of the creation of lower positive charges in thunderstorms

The existence of lower positive charges in the so-called tripole thundercloud model has been confirmed by in situ measurements from balloons and by remote measurements from ground-based electric field meters. However, the source of these charges is still not well understood. In this paper, we report on in situ measurements of some distinct lower positive charges using a different platform: instrumented aircraft. Results obtained by interpreting airborne electric field data recorded by two aircraft flying in close formation below, but very near, the estimated 3.4 km mean sea level (msl) cloud base in New Mexico show that, at least in some instances, localized lower positive charge may be the result of deposition by lightning. In one case studied, the deduced charge magnitude of about 1.25 C near 4 km msl is comparable to that obtained by balloons and ground-based electric field meters. A simulation involving aircraft-sampled electric fields in the vicinity of an idealized charge dipole with the instantaneous introduction of a lower positive charge shows good agreement with the observations. The data also show that during the mature stage the horizontal charge distribution in the lower portion of the cloud near cloud base may be quite complex, including side-by-side regions of opposite charge.