Wind shear decides what kind of storm you get

Two kinds of shearChange of wind with height, measured as the difference between the wind at the top and bottom of a layer. It is not needed to make a thunderstorm and it decides almost everything about what kind you get., two depths

Speed shear

Wind speed changing with height while the direction stays put. Tilts the updraftThe column of rising air inside a storm. Everything a thunderstorm does, from hail to tornadoes, is downstream of how strong and how organised this is. downshear, which is what gets the precipitation to fall out beside the inflow instead of into it.

Directional shear

Wind direction changing with height. This is what generates horizontal vorticitySpin in the air, about any axis. Wind shear fills the atmosphere with spin about a horizontal axis, doing nothing visible until an updraft stands it upright. with a component along the inflow, and it is where supercellA thunderstorm with a single rotating updraft that can persist for hours. The storm type that produces most strong tornadoes, very large hail and the most damaging winds. rotation comes from in lesson 6.

0 to 6 km shear

Deep-layer shear. The parameter that decides whether a storm can organise into a supercell at all.

0 to 1 km shear

Low-level shear. The parameter that has something to say about whether a supercell is tornadic, which deep-layer shear does not.

Bulk shear is a subtraction, not an addition

Bulk shear over a layer is the vector difference between the wind at the top and the wind at the bottom. Not the difference in speeds. This trips people up constantly, and it always trips them in the same direction: they underestimate it when the direction changes and overestimate it when it does not.

Surface   10 kt from 150°      u = -5.0 kt     v = +8.7 kt
6 km AGL  50 kt from 250°      u = +47.0 kt    v = +17.1 kt

Shear vector                  du = +52.0 kt   dv = +8.4 kt
Magnitude                     sqrt(52.0² + 8.4²) = 52.7 kt

52.7 kt of deep-layer shear is comfortably supercell territory. The threshold, from both idealised modelling (Weisman & Klemp 1982) and observations, is roughly 15 to 20 m/s, or 30 to 40 kt, of 0 to 6 km shear as a necessary condition for supercells.

What deep-layer shear does not tell you

Thompson et al. 2003 pulled proximity soundingsA sounding taken close in space and time to a storm, so it describes the air that storm actually formed in. Collections of these are where most published severe-weather thresholds come from. from the Rapid Update Cycle for a large sample of storms and found that 0 to 6 km shear separated supercells from non-supercells cleanly: there was no overlap between the 10th percentile for supercells, around 15 to 18 m/s, and the 90th percentile for non-supercells. As a mode discriminator it is about as good as a single parameter gets.

The same study found that deep-layer shear does not discriminate tornadic supercells from non-tornadic ones. That job belongs to 0 to 1 km shear and to low-level moisture, which is the lesson 1 point about LCLLifted Condensation LevelThe height at which a rising parcel saturates and cloud forms, which is the cloud base you are looking at. Low bases on a humid day are associated with tornado environments. height arriving from the other direction.

Hodographs, at recognition level

A hodographA plot of the wind at each height, joined up, so the shape of the wind profile is visible at a glance. A straight one means storms that split evenly; a clockwise curve favours the right-moving storm. plots the tip of the wind vector at each height and joins them up. It is the wind profile with height taken out, which sounds like a loss until you see what it makes obvious. Two shapes are worth recognising.

• Straight. Speed shear with little turning. A storm that splits produces a left-moverA storm dividing into two, one turning clockwise and moving right of the mean wind, the other turning anticlockwise and moving left. A clockwise-curving wind profile favours the right-mover and starves the left one. and a right-mover of roughly equal strength, and you get mirror-image storms diverging from where the first one was.
• Clockwise-curved, especially in the lowest kilometre or two. The split is no longer symmetric: the right-mover is favoured and the left-mover withers. This is the classic tornadic-supercell profile, and lesson 6 explains why the curvature does that.

A parameter that aged badly

The Bulk Richardson NumberCAPE divided by a measure of the shear, proposed as one number to say whether supercells are favoured. Observed supercells sit well outside the range the original simulations gave, which makes it a good lesson in reading thresholds carefully. is CAPEConvective Available Potential EnergyHow much energy is available to an updraft, in joules per kilogram. Bigger numbers mean a stronger updraft is possible, and a number on its own says nothing about what kind of storm you get. divided by a measure of the shear, and it was proposed as a single number that would say whether an environment favoured supercells. In the Weisman and Klemp simulations, supercells appeared at BRN values of roughly 10 to 40. That range went into textbooks.

Then observed supercells were checked against proximity soundings and averaged BRN 57 to 78 (Edwards and Thompson, 20th Conference on Severe Local Storms). Not a contradiction of the modelling, and not a reason to throw the parameter away. The simulations were run on a small set of idealised profiles, and the real atmosphere produces a wider variety of them.

Which classification are we using

There are three schemes in circulation and they do not nest neatly. The Field Guide teaches five storm types. Doswell frames convection as a dual spectrum: one spectrum of individual cells running from ordinary to supercell, and a second spectrum of mesoscale systems, with no clean correspondence between them. SPCStorm Prediction CenterThe national office in Norman, Oklahoma that issues severe weather outlooks, mesoscale discussions and watches. It does not issue warnings. uses a radar-based convective mode scheme (Smith et al. 2012), which is what the modern climatologies are built on.

• Discrete right-moving supercell
• Cluster of cells containing a right-moving supercell
• QLCSQuasi-Linear Convective SystemA long line of storms rather than a discrete cell, often bowing forward. Its tornadoes tend to spin up fast, stay brief and give very little warning.
• Bow echoA segment of a line of storms that has surged forward into a bow shape on radar. The apex of the bow is where the strongest straight-line winds are.
• Linear or marginal supercell
• Non-supercell discrete cells and everything else

This course teaches the SPC scheme, for one practical reason: it is the vocabulary the statistics are collected in. When lesson 8 says a given share of tornadoes comes from QLCSs, that number exists because somebody classified thousands of radar images using these categories. A private taxonomy cannot be compared with anything.

Shear relative to the boundary

The single most useful mode-forecasting idea in this lesson is not the magnitude of the shear. It is its angle to whatever is doing the lifting.

• Shear roughly parallel to the boundary favours a linear mode. New updrafts form along the boundary and immediately share a cold poolThe dome of rain-cooled air a storm leaves sitting on the ground under and around itself. Denser than its surroundings, so it spreads out, and what it spreads under decides whether the storm lives or dies. with their neighbours, and the whole thing grows upscale into a line.
• Shear more normal to the boundary, that is, crossing it, favours discrete cells. Storms move off the boundary rather than along it, each keeps its own cold pool, and they stay separated long enough to organise individually.

Dial, Racy and Thompson 2010 quantified this: storm mode about three hours after initiation correlated with the component of the deep-layer shear normal to the initiating boundary, together with the boundary-relative mean wind, and the skill was best using the mean wind and shear angle over roughly 2 to 6 km. This is why two days with identical CAPE and identical shear magnitude can give you a photogenic isolated supercell and a 200-mile squall lineA long, narrow line of thunderstorms moving as one system. It uses the cold air it produces as its own source of lift, which is how it keeps going for hours..