| Abstract: | Based on the data of regional meteorological stations, ECMWF (European Centre for Medium-Range Weather Forecasts) Reanalysis v5 (ERA5), wind profile network products, S-band Doppler weather radar in China New Generation Weather Radar Network (CINRAD/SA), and X-band phased array weather radar (XPAR), three EF0-EF1 non-mesocyclone tornadoes that occurred in the Yellow River Delta on 10 August 2021 are analyzed in detail. The results are as follows. (1) The tornado process occurs under the background of upper-level dry and cold northwest flow, warm ridge in front of low-level transversal trough, and surface inverted trough vortex. Strong convective instability, vertical wind shear in the 0-6.0 km deep layer, low-level large humidity, and a lifting condensation height of nearly 1 000 m are favorable environmental conditions for the formation of the weak non-mesocyclone tornadoes. The unfavorable condition is that the vertical wind shear in the 0-1.0 km is very weak. (2) The sea breeze front and the gust front trigger convections, and the transversal trough splits and moves southward to strengthen the ascending motion. When the tornado storm affects, the observation elements of the nearby meteorological stations show obvious changes, but the divergence of the wind field shows that the high wind is relevant to the downdraft throughout the storm. (3) The parent storm of tornado is the type of multiple-cell merging and backward propagation storm, and the formation of double tornadoes is related to the development of cell merging. The gust front (outflow boundary) formed by the downdraft of the thunderstorm merges with the sea breeze front, which strengthens the cyclonic small-scale vortex. When the small-scale cyclone encounters the ascending motion strengthened by cell merging, the rotational motion is further enhanced, thus triggering the third tornado. (4) Only cyclonic vortices and storm-top divergence are observed by CINRAD/SA; XPAR observes strong shear and tornadic debris signature during the double tornadoes, and the low-value area of correlation coefficient is obvious. |
