Effects of vernal equinox solar eclipse on temperature and wind direction in Switzerland

摘要

The vernal equinox total solar eclipse of 20 March 2015 produced a maximum occultation of 65.8–70.1 % over Switzerland duringthe morning hours (09:22 to 11:48 CET). Skies were generally clear over the Swiss Alps due to a persistent high-pressure bandbetween the UK and Russia associated with a rather weak pressure gradient over the continent. To assess the effects of penumbralshading on near-surface meteorology across Switzerland, air temperature data measured at 10 min intervals at 184 MeteoSwiss weatherstations were used. Wind speed and direction data were available from 165 of thesestations. Additionally, six Swiss FluxNet eddy covariance flux (ECF) sites provided turbulent measurements at 20 Hzresolution.During maximum occultation, the temperature drop was up to 5.8 K ata mountain site where cold air can pool in a topographic depression. Thebootstrapped average of the maximum temperature drops of all 184 MeteoSwisssites during the solar eclipse was 1.51 ± 0.02 K (mean ± SE).A detailed comparison with literature values since 1834 showed a temperaturedecrease of 2.6 ± 1.7 K (average of all reports), with extreme values upto 11 K. On fair weather days under weak larger-scale pressure gradients,local thermo-topographic wind systems develop that are driven by small-scalepressure and temperature gradients. At one ECF site, the penumbral shadingdelayed the morning transition from down-valley to up-valley wind conditions.At another site, it prevented this transition from occurring at all. Datafrom the 165 MeteoSwiss sites measuring wind direction did not showa consistent pattern of wind direction response to the passing of thepenumbral shadow. These results suggest that the local topographic settinghad an important influence on the temperature drop and the wind flow patternsduring the eclipse. A significant cyclonic effect of the passing penumbralshadow was found in the elevation range ≈ 1700–2700 m a. s. l., but not at lower elevations of the SwissPlateau. This contrasts with an earlier theory that the anticyclonic outflowshould reach as far as ≈ 2400 km from the center of theeclipse, which would have included all of Switzerland during the 2015eclipse. Thus, measurable effects of penumbral shading on the local windsystem could be even found at ≈ 2000 km from the path of theeclipse (that is, Switzerland during the 2015 eclipse), and our results tendto lend support to a newer theory that the anticyclonic cold-air outflow fromthe center of the eclipse only extends ≈ 1600 km outwards,with cyclonic flow beyond that distance.