Foehn-like wind with a traditional foehn effect plus dry diabatic heating from the ground surface contribute to high temperatures at the end of a leeward area

Abstract

oehn wind is an important factor in the occurrence of many extreme high temperature (EHT) events in geographically complex regions. In this study, we verified the hypothesis that a foehn-like wind contributes to high temperatures at the end of the leeward (eastward) area using three difference approaches: field experiments, numerical experiments, and statistical analysis. According to the hypothesis, the foehn-like wind has the features of the sum of a traditional foehn effect with adiabatic heating, plus dry-diabatic heating from the ground surface along the fetch of the wind. Field experiments conducted at seven observational points on Nobi Plain, Japan, where a mesoscale westerly wind blew, revealed that the westerly wind clearly had the features of a traditional foehn effect in the western part of the Nobi Plain. In addition to field experiments, a simplified estimation using a simple mixed-layer model demonstrated that the wind was further heated by dry-diabatic heating (sensible heat supply) from the ground surface along the fetch (especially in urbanized areas in the eastern region of the Nobi Plain) of the wind. This diabatic heating effect along the fetch of the wind on the high temperature at the end of the leeward area was also supported by both additional numerical experiments and a statistical analysis. These results proved that the hypothesis is correct, and indicated that ground conditions and the land use and land cover (LULC) in the windward area were strongly related to air temperature at the end of the leeward area, where an extremely high temperature was observed.