Observational Studies of Low Altitude Wind Shear (LAWS) formed in the Lee of Mountains.

Abstract

The purpose of this study is to further our understanding of LAWS occurring in the lee of mountains through an observational investigation of the formation processes. The location of the study is Hanamaki Airport, situated in a basin in the Tohoku Region, Japan, where there is many reports of LAWS. A statistical analysis, based on LAWS reports submitted by flight pilots, and surface wind data observed at the airport, revealed the importance of changes in wind direction, as well as wind speed when an aircraft encounters LAWS. Thus, the standard deviation of the horizontal wind vector is defined as a WSI (Wind Shear Index) to evaluate the occurrence of severe LAWS. A case study using upper wind data in the boundary layer, recorded by commercial aircraft flying through severe LAWS systems, pointed out the importance of both a sudden change in wind direction and a descending current, especially close to the ground. To clarify the fine structure and formation process of these wind characteristics, a six-month observation of the boundary layer winds, using a Doppler sodar installed at the airport, was conducted from December, 1996. A WSI analysis was applied to Doppler sodar observations and revealed the following three significant factors when a severe LAWS system is foremd: 1) A strong westerly wind in the middle or upper boundary layer, causing a high WSI current, is observed; 2) The process in which a high WSI peak reaches from the boundary layer down to the ground; 3) and, A descending current is present close to the ground after the high WSI peak touches down.