Abstract/Summary

Surface atmospheric electricity measurements, particularly of the vertical electric field (or Potential Gradient, PG) began in the 1770s, becoming more widespread in the UK during the nineteenth century. In the twentieth century, PG measurements were systematically obtained by the Met Office, at their observatory sites of Kew, Eskdalemuir and Lerwick. These records’ importance is now increasingly recognized, due to the inherent global atmospheric electric circuit (GEC) information, which is an embedded part of the climate system. Earlier data has the limitation that, until about 1960, preferred daily PG data values were selected using the geomagnetically-informed approach of identifying quiet and disturbed days. It is now known that classification by local weather conditions, identifying the data recorded during “fair weather” (FW) or dry, “no hydrometeors” (NH) circumstances, is superior for obtaining GEC signals. However, the necessary weather information is only available at a subset of PG measurement sites globally. For other sites, meteorological reanalysis – and many such data products are available spanning different times and scales –offers a new approach for retrospective weather-based classification of PG data. This work investigates applying ERA5 reanalysis to selecting PG data obtained during FW and NH conditions by using the Lerwick site as a testbed, comparing with direct weather observations (including wind speed, precipitation, cloud base height, and pressure) which were originally used for PG data selection. ERA5-based PG data selection is shown to yield quality improvements, especially for data downgraded by the geomagnetic approach e.g. our method reclassifies 20% of disturbed weather data as fair weather. This offers a general route to improving long-term atmospheric electricity data obtained at non-meteorological sites globally.