During the summer of 1992, an experiment was conducted in southwestern France, close to the Pyr¿n¿es, at the Centre de Recherches Atmosph¿riques (CRA) in order to study the evolution of the electric field measured at several levels below thunderclouds. We used a field mill flush to the ground and four field sensors, suspended from an insulated cable and distributed between 0 and 48 m. These altitude sensors separately measure the ambient electric field and the field created by the sensor itself. The Rabelais millimetric radar provides reflectivities and Doppler velocities of cloud and rain systems. Meteorological data like wind velocity, humidity, temperature, and rainfall rate are recorded at the site. Two storm intervals are studied, one on July 30 and one on August 6. Both examples give an idea on how the electric field signature during the development or advection of a convective cloud can be different at the ground and at altitudes of a few tens of meters.

The maxima in the electrostatic activity are visible only on the altitude electric field evolution. The maximum value of the electric field between lightning flashes, close to 30 kV m-1, is detected at 48 m above ground. A vertical gradient of the electric field is observed in this 48-m-thick layer, especially when the field is high. The upward progressive development of this gradient is interpreted in terms of a local charge generated by corona at the ground and rising by conduction. The average charge density in the whole layer reaches around 5 nC m-3 between lightning flashes when the field keeps large values of the same polarity. This density is weaker when the electric field has taken large values of both polarities. Long periods of different electrical activity are observed during both storms (those with large electrostatic fields without any lightning flash and those with lightning activity). The former are associated with a thick stratiform cloud structure, possibly with strong radar echoes above the melting level but without rain reaching the ground; the latter are associated with rain reaching the ground close to the measurement area. These observations suggest that the water liquid phase could play an important role in lightning initiation. ¿ American Geophysical Union 1995