Structure of the Downdraft under the Melting Layer of a Front Observed on 11 May 1994

The structure of a downdraft under a melting layer of a front was analyzed. On 11-12 May 1994, during the TAPS period, a frontal system accompanied by a weak cyclone was observed at Tsukuba as it moved along the southern coastal line of the Kanto district. The upper front was observed as a stable layer at Tsukuba. The front was observed at an altitude close to a 0 °C layer. This situation lasted for about 12 hours. It is suggesting that cooling due to melting of ice particles maintained and intensified the stable layer. This resulted to fix the front to the altitude close to the 0 °C layer. In the stratiform cloud area, a precipitation band appeared at around 20:45UTC and reflectivity of the melting layer increased to 45 dBZ. This precipitation band was narrow with a width of about 20 km in the direction perpendicular to the band. Downdraft was produced under the layer of intense reflectivity of the precipitation band. This downdraft was generated mainly as a result of cooling due to melting of falling ice particles. The maximum speed of the downdraft was estimated to be 0.2 ms-1. This was the same order of magnitude as the simulated value in the stratiform cloud band behind a squall line (Biggerstaff and Houze, Jr. 1991, 1993, Caniaux et al. 1994). The downdraft induced convergence around the height of the melting layer and divergence at lower layers. With this convergence and divergence across the precipitation band, wind fields under the melting layer were deformed. Due to this deformation, wind speed before and after passage of the precipitation band was not uniform over the Tsukuba point. At 3.5 km altitude, a SW wind intensified from 12-13 ms-1 to 19-21 ms-1 due to the convergence. At altitude 0.5 km, an ENE-NE wind intensified from 2-3 ms-1 to 7-8 ms-1, which corresponded to the divergence. Horizontal wind fields calculated from Doppler velocity also shows discontinuity of wind across the precipitation band, which extended from SSE to NNW. At 0.5 km altitude, an ENE wind of about 10 ms-1 was detected on the western side of the band and a weak easterly on the eastern side. This change corresponded to wind divergence. At 3.5 km altitude, a SW wind of about 15-30 ms-1 occurred on the western side and one of 5-10 ms-1 on the eastern side of the band. This corresponded to the convergence.