LAUSR

Abstract. In 2003, a decreasing trend was reported in the long-term (1962–2001) fair weather atmospheric electric potential gradient (PG) measured in the Széchenyi István Geophysical Observatory (NCK; 47∘38′ N, 16∘43′ E), Hungary, Central Europe. The origin of this reduction has been the subject of a long-standing debate, due to a group of trees near the measurement site which reached significant height since the measurements have started. Those trees have contributed to the lowering of the ambient vertical electric field due to their electrostatic shielding effect. In the present study, we attempt to reconstruct the true long-term variation of the vertical atmospheric electric field at NCK. The time-dependent shielding effect of trees at the measurement site was calculated to remove the corresponding bias from the recorded time series. A numerical model based on electrostatic theory was set up to take into account the electrostatic shielding of the local environment. The validity of the model was verified by on-site measurement campaigns. The changing height of the trees between 1962 and 2017 was derived from national-average age–height diagrams for each year. Modelling the time-dependent electrical shielding effect of the trees at NCK revealed that local effects played a pivotal role in the long-term decrease. The results suggest that earlier attempts could not quantify the shielding effect of the trees at NCK accurately. In this work it is found that the reconstructed PG time series at NCK exhibits an increase between 1962 and 1997 followed by a decaying trend since 1997. It is pointed out that long-term variation in summertime and wintertime PG averages should be analysed separately as these may contribute to trends in the annual mean values rather differently.