Shear And Convective Storms

  • As vertical wind shear increases, the interaction between wind shear and the cold pool can enhance lifting.
  • Vertical wind shear promotes environmental horizontal vorticity.
  • Cold pool vorticity combined with environmental vorticity (along leading edge of cold pool) promotes deeper lifting on the downshear side of the storm.
  • The amount of lifting on the storm's downshear side is optimized when the cold pool's horizontal vorticity matches the environmental vorticity.
  • Vertical wind shear creates updraft tilt. The resulting downshear tilt of the updraft results in an area of high pressure upshear of the updraft and an area of low pressure downshear.
  • Vertical wind shear (more than buoyancy) has the most fundamental control over convective organization.
  • The organization and maintenance of MCSs is strongly related to the relationship between the cold pool and wind shear.
  • For a given amount of CAPE, the strength and longevity of a squall line increases with increasing wind shear.
  • When low-level wind shear is strong, new cell development is favored downshear of the low-level shear vector.
  • When low-level wind shear is weak, the lifting created by the cold pool circulation alone may be insufficient to allow a surface parcel to reach the LFC, unless the LFC is quite low.
  • The deepest lifting occurs when the horizontal vorticity generated along the cold pool's leading edge is nearly equal in magnitude to, and has the opposite sense of rotation as, the horizontal vorticity associated with the low-level vertical wind shear.
  • Surface to 3km shear values of 10-20 m/s is generally sufficient to promote lifting deep enough to favor new cell development along the downshear portion of a gust front.
  • When the vertical wind shear is strong it initially has a detrimental effect on a developing updraft by inhibiting its development.
  • When the updraft column blocks the environmental flow, it creates a dynamic affect of relative high pressure upshear and low pressure downshear of the updraft. This prompts rising air parcels to turn downshear.
  • Buoyancy gradients on the sides of the towering cumulus create horizontal vorticity on both sides of the rising updraft.
    • In minimal wind shear the horizontal vorticity on both sides is in balance and the updraft rises vertically.
    • When the shear is stronger, the updraft will tilt toward the side of the storm that generating the same sign of vorticity as that associated with the environmental wind shear (tilts downshear).
  • An updraft growing in a sheared environment changes low-level horizontal vorticity into vertical vorticity within the storm, creating a mid-level vortex couplet.
  • Storm Splitting Dynamics:
    • As the downdraft develops, the up-tilted vortex line can be tipped downward in the center, creating another vortex couplet with each new rotation center having an opposite sense rotation as the pair created by the initial updraft.
    • This occurs in all storms growing in a vertically sheared environment, creating transient mid-level rotation centers when the wind shear is weak and significant mid-level rotation when the wind shear is strong.
    • The small-scale rotation centers are associated with lower pressure regardless of the direction in which they rotate.
    • When wind shear is predominantly unidirectional (straight hodograph) the pressure perturbation field associated with the initial mid-level vortex couplet contributes equally to the development of cyclonically and anticyclonically rotating updrafts on the right and left flanks of the storm.
    • This process leads to symmetric storm splitting with the new supercells propagating to the left and right of the mean storm vector.
  • Wind Shear and Supercell Propagation
    • In a sheared environment, relative low pressure is created downshear of an updraft.
    • In sufficient directional wind shear, this contribution to the pressure perterbation field may have an additive effect on the pressure pattern created by the mid-level vorticies, favoring one storm flank for new updraft growth.
    • In clockwise shear, these pressure forces enhance upward vertical motion on the right flank of the storm resulting in a right-moving and cyclonically rotating supercell.