Atmospheric electrical field measurements near a fresh water reservoir and the formation of the lake breeze

摘要

In order to access the effect of the lakes in the atmospheric electrical field, measurements have been carriedout near a large man-made lake in southern Portugal, the Alqueva reservoir, during the ALqueva hydrometeorologicalEXperiment 2014. The purpose of these conjoint experiments was to study the impact of theAlqueva reservoir on the atmosphere, in particular on the local atmospheric electric environment by comparingmeasurements taken in the proximity of the lake. Two stations 10km apart were used, as they were located upanddown-wind of the lake (Amieira and Parque Solar, respectively), in reference to the dominant northwesternwind direction. The up-wind station shows lower atmospheric electric potential gradient (PG) values than theones observed in the down-wind station between 12 and 20 UTC. The difference in the atmospheric electric PGbetween the up-wind and the down-wind station is 30 V/m during the day. This differential occurs mainlyduring the development of a lake breeze, between 10 and 18 UTC, as a consequence of the surface temperaturegradient between the surrounding land and the lake water. In the analysis presented, a correlation is foundbetween the atmospheric electric PG differences and both wind speed and temperature gradients over the lake,thus supporting the influence of the lake breeze over the observed PG variation in the two stations. Twohypotheses are provided to explain this observation: (1) The air that flows from the lake into the land station islikely to increase the local electric conductivity through the removal of ground dust and the transport of cleanerair from higher altitudes with significant light ion concentrations. With such an increase in conductivity, it isexpected to see a reduction of the atmospheric electric PG; (2) the resulting air flow over the land station carriesnegative ions formed by wave splashing in the lake’s water surface, as a result of the so-called balloelectric effect.These negative ions will form a space-charge density (SCD) that can reduce the atmospheric electric PG.A formulation is derived here in order to estimate the local SCD.