The basic equations - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 82

the vertical shear is strong) take the form of rolls (in nature frequently termed
horizontal convective rolls) oriented perpendicular to the mean vertical shear vector.
When the mean state is very statically unstable (e.g., when very strongly heated
from below), the most unstable modes take the form of rolls oriented along the
mean vertical shear vector and the growth rates are identical to those when there
is no vertical shear. Consider the above equations (2.281)-(2.285) for the case
when x ¼ 0 (i.e., when rolls are oriented along the vertical shear vector). Then

the equations (2.282)-(2.285) do not include u and only equation (2.281) still
contains u. The effects of vertical shear are contained in (2.281) only in the term
Re w, which represents the advection of vertical shear by the perturbation vertical
velocity. The variables v , w, p, and B may be solved by considering (2.282)-(2.285)
only. Thus, once w and p are solved for, u may then be calculated from (2.281);
the mean shear therefore has no effect on v and w, which make up the roll circula-
tions along the vertical shear vector.
Asai in the early 1970s extended his analyses to include basic states in which
the vertical shear vector turns with height. Wan Shu Wu, Doug Lilly, and Bob
Kerr in the early 1990s numerically examined the effects of some idealized basic
states in which the vertical shear vector is unidirectional and in which the vertical
shear vector turns with height. The results of the latter were that the disturbance
was most likely to be helical when the shear vector curved with height. The
student is referred to the original journal articles for the details.
In the real atmosphere, we frequently observe cloud streets over land during
the day ( Figure 2.17 ), which we interpret as visual manifestations of boundary

@=@

Figure 2.17. Cloud streets over land (over southern Illinois), as viewed from an aircraft, during
the daytime hours on July 2, 1986 (photograph by the author).