In this study, structural features of a subtropical squall line that occurred on 17 May 1987 over the Taiwan Straits, were investigated using dual-Doppler data collected during the Taiwan Area Mesoscale Experiment (TAMEX). Fields of the storm-relative wind and reflectivity were derived in a horizontal domain of 45 km × 25 km using an objective analysis scheme with 1 km grid spacing in all three directions. There were ten analysis levels in the vertical ranging from 0.3 to 8.8 km. Vertical velocities were computed from the anelastic continuity equation by integrating downward with variational adjustment.
Results show that many structural features of a subtropical squall line are similar to those for a fast-moving tropical squall line. A low-level jet (LLJ) associated with the frontal system provides the necessary strong shear at lower levels. On the front side of the squall line front-to-rear flow prevails at all levels and is accompanied by shallow rear-to-front flow on the back of the line. There are many individual cells embedded within the squall line. Relatively weak convective downdrafts occur between the cells and behind the main cells. Convective downdrafts on the rear of the main convective updrafts are essential to transport cooler midtropospheric air into the lower layer. Part of the negatively buoyant air from the rear continues to move forward colliding with the advancing high θe air in the boundary layer. As a result, new convective cells form ahead of the old cells, thereby prolonging the life time of the squall line. In the convective region the low-level front-to-rear inflow is lifted at the leading edge to form the main updrafts. The lifted air continues to flow west in the middle and upper levels heading toward the trailing stratiform region. The interaction between the convective updraft and downdraft plays an important role in maintaining the three-dimensional circulation within the squall line.