Elbow-shape echo pattern and the Mei-Yu frontal rainband

Formation of elbow-shape precipitation echo pattern over the northwest ocean of Taiwan while the surface front approaching the coastal area during the Mei-Yu (May and June) season is examined by using a C-band Doppler radar. The results show that the elbow-shape precipitation echo pattern is composed by two major rainbands with different orientations, i.e., one in the NE-SW direction and the other in the NW-SE direction. Precipitation echoes associated with the NE-SW rainband form in the prefrontal region and reach above 10 km height and show deep convective cellular structure. Precipitation echoes associated with the NW-SE rainband also show convective nature but form at and behind the frontal surface and reveal a much shallower vertical feature. The kinematic structures of these two rainbands are also quite different due to different formation mechanisms. These two rainbands usually merge together while reaching the coast and bring heavy rain to the coastal areas. By comparing the observations of cases with and without the elbow-shape echo patterns, it is found the behavior of the prefrontal low-level jet stream and the postfrontal cold air both play important roles in determining the formation of the elbow-shape echo pattern.