Climatology of storm reports relative to upper-level jet streaks

Using quasi-geostrophic arguments, previous works have shown that a thermally direct (indirect) vertical circulation is induced in the entrance (exit) region of a linear jet-streak. The presence of this vertical circulation, which consists of upward motion in the right-entrance and left-exit jet quadrants and downward motion in the left-entrance and right exit quadrants, has been verified by observations and numerical model simulations. Because jet-induced upward vertical motion likely contributes to the development of severe weather, the “four-quadrant rule” is regularly applied by operational forecasters. This study builds on that of Rose et al in 2004, which examined tornado report climatology relative to upper-level jet streaks, by developing a procedure to analyze the climatology of hail and wind reports, as well as tornado reports, relative to upper-level jet streaks. In addition, composite fields, such as upper- and low-level divergence, are analyzed for the jet streak regions to examine whether the fields correspond to what is expected from the “four-quadrant model”, and whether the fields help explain the storm report distributions. Storm report distributions and composite fields are also analyzed for different jet streak orientations and curvatures, as well as composite fields in jet streaks that weren't associated with storm reports.

Preliminary results from the composite analyses reveal that tornado and hail reports have a maxima in the jet exit region centered along the major jet axis (i.e., between the left and right exit quadrants) with a secondary maxima in the right entrance quadrant. Wind reports have a maxima in the right entrance quadrant, with a more diffuse, weaker secondary maxima in the exit regions centered along the major jet axis. Upper-level composite divergence fields generally correspond to what would be expected from the “four-quadrant rule” [i.e., divergence (convergence) in the right-entrance and left-exit (left-entrance and right exit) regions]. However, the jet-induced vertical circulation is better defined in the entrance quadrants where the circulation is thermally-direct. The maxima in the storm report distributions do not necessarily correspond to the areas with maximum upper-level divergence, but correspond much better to an area of low-level convergence associated with frontal boundaries that extends through both exit regions and into the right entrance region. Finally, it is found that that jet streaks approaching from the south to south-south-west are associated with the most storm reports relative to those approaching from other directions.