Abstract
A bow echo is a bow-shaped radar reflectivity pattern that is often accompanied by downbursts at the apex of the bulge. It appears that there are two types of bow echoes documented in the literature, the squall-line type (SLBE) and the single-cell type (CBE). It is not clear that these two types of bow echoes are dynamically similar. This study presents the first complete case study on the CBE, which occurred on 14 July 1982 during the Joint Airport Weather Studies (JAWS) Project. The stormwide kinematic and thermodynamic structure of this storm was documented in Part I of this paper. This paper examines the initiation and evolution of a vorticity couplet using a vorticity-budget analysis to study the bow-echo structure and the bow-echo-microburst relationship.
The elongated-shaped echo assumed a bowed shape below cloud base after the downdraft developed. The bow echo is associated with a cyclonic-anticyclonic vorticity couplet with maximum relative vorticity intensifies of 5×10−3 and −4×10−3s−1 respectively, at the 2.4-km height. This couplet does not exist prior to the initiation of the downdraft. Examination of the vorticity budget shows that the vertical vorticity couplet is generated primarily through tilting of ambient horizontal vorticity by the microburst downdraft. The positive vorticity is enhanced by both the stretching effect and the downward advection of positive vorticity from above that is produced by the updraft through the same mechanism. A particle-trajectory analysis shows that the elongated echo is distorted into a bow shape by the shear vorticity, which exhibits a velocity differential between the center and edges of the echo.
A conceptual evolution model of the CBE is constructed based on the vorticity analysis in which an elongated echo may deform into a bow shape under the following meteorological condition: 1) a nearly unidirectional vertical shear, and 2) downdraft development. The product of the vertical shear and the horizontal gradient of the downdraft determine the strength of the vorticity couplet and hence the extent of the echo deformation. Since the environmental shear is often weak with an airmass thunderstorm, a strong downdraft is required to form a strong vorticity couplet. This may be why a CBE is often associated with strong wind events.
