Principles of Convection

BACKGROUND When vertical shear is weak, the main influence on convective updrafts & downdrafts is bouyancy. As the vertical wind shear increases the environmental winds are the main influence on convective storm organization and duration. This module shows the effects of vertical wind shear on convective storms. Using hodographs we can assess vertical wind shear.

COOL POOL INTERACTIONS A cool pool is the thunderstorm outflow, which can trigger new cells if upward motion is present to lift warm air to the LFC (level of free convection). A strong vertical wind shear must be present for this to occur. The enhanced lifting that results is described by horizontal vorticity, which is similar to a paddle wheel being pushed by vertical wind. Horizontal buoyancy gradients generate horizontal vorticity. (Using the shear as your finger curl & the thumb as the vorticity vector, we can use the right hand rule to find positive vorticity into the page or negative out of the page).

COOL POOL INTERACTS W VORTICITY This would result in a strong jet of vertical motion in between the regions. This would drag all the air to the stronger vortex. As shown, the right side is favored for deep lifting. On the left, because the environmental vorticity has the same sign as the vorticity created by the cold pool on its upshear side dragging air over the pool w minimal lift. However, on the downshear side, to the right, the environmental vorticity and the cold-pool induced vorticity have opposite signs. This produces a more vertical jet of air, resulting in deeper lifting. The optimal state for deep lifting is when the cool pool & wind shear are balanced, producing the greatest vertical jet. When cool pool > wind shear, the air will be lifted to the height of the cold pool. When wind shear > cool pool, the air ahead of the cold pool will be dragged up and then back downshear.

UPDRAFT INTERACTIONS Updraft tilt can be caused by an updraft with weak vertical momentum which is easily tilted by vertical wind shear. When the vertical wind shear is deep, the horizontal vorticity associated with the deep shear layer adds to the horizontal vorticity associated with the buoyancy gradient in the updraft. This causes the storm to tilt to the side with the same sign of horizontal vorticity as the environment This updraft column blocks environmental flow creating High pressure upshear & low pressure downshear. This gradient also makes rising parcels lean downshear (storm tilt).

ISOLATED STORMS & SHEAR In ordinary cells, the lift produced by the gust front in weak shear conditions is insufficient to consistently generate new cells on its own so the cells move with the environmental wind. In multi-cell systems, the gust front still spreads outward, but bc of high vertical wind shear new cell growth is favored on the downshear side of the cold pool where the lift is the greatest. Strong, low-level, vertical wind shear produces the strongest & longest living multi-cell systems. Optimal condition for the generation of new convective cells is when the cool-pool vorticity is equal to the vertical wind shear vorticity. When strong shear is curved, mid-level rotation occurs and the resulting pressure forcing can create new updrafts on the lateral end of the storm. A sustained, rotating updraft is a primary characteristic of supercell storms, which are often associated with severe weather. Clockwise curvature=right moving storms. Counter- clockwise curvature = left moving storms.

STORM SYSTEMS & SHEAR Vertical wind shear also influences evolution of large storm systems as it interacts with the cold pool to create the most organized mesoscale convective systems. A squall line oriented perpendicular to the low-level shear will be the strongest & longest lived. Severe bow echoes (a small segment of a strong isolated storm) are most often observed in environments with moderate-to- strong low-level shear and very high CAPE. Bow echo and supercell environments have much overlap, bow echos form when there is high vertical wind shear at low levels, supercells at high vertical wind shear at deep levels.