Strong vertical wind shear is important for the development of severe convective storms including the type of thunderstorm, its orientation, and its life span. Wind shear can influence storms in several ways, including speed shear and directional shear. Speed shear is a change in the wind speed with respect to height, while directional shear is the change of the wind direction with height.
A significant increase of the wind speed with height will tilt a thunderstorm’s updraft. This causes the updraft and downdraft to occur within separate regions of the thunderstorm, and can reduce the flux of water loading in the updraft. Since the updraft and downdraft are in separate regions of the storm, the downdraft will not cut off the air flowing up into the storm and will thus strengthen the updraft (this is seen within supercells; a rotating thunderstorm).
Strong upper-level winds within the troposphere can evacuate mass from the top of the thunderstorm, otherwise known as the equilibrium level. This also reduces the precipitation loading and allows the updraft to maintain its strength.
Directional shear in the lower troposphere helps initiate the development of a supercell. This is one of many components that are crucial to the formation of a mesocyclone, and aids in the process of tornadogenesis. Strong low-level tropospheric winds and directional shear will generate high values of storm-relative helicity. Storm-relative helicity is usually calculated in the lowest 1-3 km of a wind field, and high values of storm-relative helicity in a severe storm environment increase the potential for tornadic supercells.
The wind shear environment is important in determining the type of thunderstorms that may occur on any given day. While instability and moisture also help determine the thunderstorm’s precipitation rate (such as rain rates and hailstone growth), the wind shear determines the storm motion and the life cycle of the thunderstorm itself.
Weak Speed Shear and Weak Directional Shear
A storm in this environment will move slowly and is often short lived. These types of thunderstorms are known as airmass thunderstorms, or pulsating thunderstorms. Since the storm motion is slow, the downdraft will undercut the updraft’s strength. Without a persistent region of strong, warm inflow to maintain the storm, it will weaken shortly after genesis.
If a storm under this shear environment forms within a moisture rich boundary layer, ephemeral periods of heavy rain may occur. However, storms in this environment are not likely to reach severe thunderstorm criteria. Severe thunderstorm criteria is as follows: ≥58mph winds , ≥1.0″ diameter hail, or a tornado (note that lightning is not an indicator of a storm’s severity).
The above image is a pictorial depiction of a hodograph. A hodograph is an important tool that meteorologists use when diagnosing the thunderstorm environment. Hodographs are used to plot the change in magnitude and direction of wind with height, and are plotted using polar coordinates.
In the case above, the orange dots indicate a particular point with respect to height (#1 indicates the lowest point above ground and #5 indicates the highest point above ground). To understand how and what this hodograph is depicting, the graph must be read in polar form. Meaning, the top part of the hodograph is north, the right point is east, the bottom point is south, and the left point is west. A visual representation of the atmospheric wind profile can then be read through the hodograph.
Strong Speed Shear and Weak Directional Shear
This situation is often termed as unidirectional shear. Unidirectional shear is where the majority of the mean wind in a thunderstorm environment is flowing in the same direction. Hence, there is weak directional shear. The strength of the speed shear will allow the storm to move. This movement ensures that the storm will last longer than a typical airmass thunderstorm.
Unidirectional wind shear often produces complexes of thunderstorms that evolve into forward-propagating lines or Mesoscale Convective Systems (MCS). Since the storm moves, its outflow generates lift that allows for the formation of newer convection along its periphery. Over time, the linear to quasi-linear evolution of storms can occur. MCS events and squall lines primarily produce strong to severe winds, heavy rain, and even tornadoes (some of which can be strong).
In the hodograph above, the magnitude of the wind increases drastically with height. In this case, the change in the direction of wind with height is lacking. However, there is significant speed shear in place as the wind increases from 35 knots in the low-levels of the troposphere to near 85 knots in the upper-levels. Straight-line hodographs, such as the example above, are often an indicator of the potential for squall line events.
Weak Speed Shear and Strong Directional Shear
When speed shear is weak, the strength of the directional shear is usually of non-significance. Thunderstorms in this environment usually occur in the sub-tropical and tropical regions. An example of this is the sea breeze convection that normally occurs across central and southern Florida, and other regions of the tropics. The hodographs in this environment may look similar to that of an airmass thunderstorm environment, but the change of direction with height may likely be unorganized.
Strong Speed Shear and Strong Directional Shear
In an environment where there is strong speed shear and directional shear, supercells are likely to form. This is the best situation in the atmosphere for the evolution of rotating updrafts. The strength of the speed shear enables a sufficient storm motion and thus helps keep the updraft and downdraft region separated, while the directional shear can generate rotation within the storm. Supercells can produce very large hail, destructive winds, and strong-to-violent tornadoes.
Notice the sharply curved wind profile in the low-levels of the atmosphere in the hodograph above. In addition to its magnitude, the directional change is significant — a necessary ingredient for supercells and mesocyclogenesis. When wind fields resemble strong speed shear and strong directional shear, severe thunderstorms are likely.
Wind shear has a dramatic impact on a weather forecast when thunderstorms are expected. Stronger wind shear can mean the difference between ordinary storms and severe storms. Thus, meteorologists pay special attention to this critical ingredient of wind shear when performing a weather analysis and forecast.