Two Types of Strong Local Wind Captured by Simultaneous Multiple-Site Radiosonde Soundings across a Mountain Range

Abstract

A radiosonde observation method is presented, consisting of simultaneous radiosonde observations at closely spaced multiple sites using balloons with varied buoyancies. This method was employed during a strong wind event (Suzuka-oroshi) on the lee side of the Suzuka mountain range, Japan, to derive the detailed structure of the wind as it crossed the mountains. Batches of six radiosondes were launched simultaneously from a line of four sites, using balloons with three different degrees of buoyancy. The four sites were 13 km apart along a 35-km-long transect roughly aligned with the prevailing wind. The observations documented two flow regimes: a downslope flow perpendicular to the mountain range, similar to a windstorm, and an unexpectedly strong low-level jet flowing parallel to the mountain range. The method was more successful at delineating the first regime than the second. The first regime was well simulated by a numerical experiment, but the second regime was not. The vertical wind associated with the downslope windstorm was inferred from the changing slopes of potential temperature isentropes. Comparison of the balloon ascent rates with these isentropes meanders and the simulated vertical wind showed that fluctuations in balloon ascent rate provide reliable information on the vertical direction of the wind. An analysis of the second regime using a long-term meteorological dataset shows that the onset of the low-level jet is related to the synoptic-scale shift in vorticity from positive to negative in the observation area. This vorticity shift appears to be a useful indicator for the low-level jet regime.