9A Ordinary Convection Intro General Thunderstorm Structure

What is a Thunderstorm?

a local storm, produced by a cumulonimbus cloud, that

always is accompanied by lightning and thunder.

What defines a severe thunderstorm?

1. ≥ 1” Hail

(quarter-size)

2. Wind gusts ≥ 50 knots (58 mph).

3. and/or … a tornado!

• *Heavy rain and lightning are NOT a criteria to make a storm “severe”

4 Ingredients for a T-Storm

1. A source of moisture

• Gulf of Mexico, Atlantic Ocean

2. A conditionally unstable atmosphere

• Need lifted parcels to become buoyant (i.e. warmer than the environment as they rise). (Requires cold air aloft)

3. A lifting mechanism

• ‘Push’ to initiate ascent

• Front/dryline, solar heating, convergence,

mountain

4. Optional ingredient:

• Vertical wind shear

• *The storm structure is dependent upon the amount and type of shear (speed vs. direction), but you don’t NEED shear to form a thunderstorm.

Why do thunderstorms form?

– As water condenses it releases latent heat energy.

– Strong updrafts lift a LOT of air. Rapid condensation occurs over a large depth

of the atmosphere, releasing a lot of latent energy.

– This energy helps to drive the storm.

– The stronger (and faster) the updraft, the more quickly condensation can

occur and the more energy is released

Thunderstorm Structure: Updraft

• In order to have convection, there must

be an updraft present (i.e. a thunderstorm ‘tower’).

• An updraft is a vertical channel of rising air, formed by warm, buoyant, unstable air ascending rapidly.

• Updrafts are the first part of a thunderstorm to develop, but not all updrafts become thunderstorms.

• Upward motion can range from about 5 to > 70 ms-1 depending on the amount of atmospheric instability.

Thunderstorm Structure: Inflow

• Inflow is the term given to the environmental air that is being ingested into a thunderstorm.

• This is the surface air that gets sucked into the storm and ‘feeds’ the updraft.

– Often, this air comes from the east or south or southeast of the storm

• For a storm to maintain its vigour, inflow should be warm and moist (buoyant).

• With very strong updrafts, inflow can be

30-40 mph.

• Sometimes ‘inflow bands’ (long trailing bands of clouds at low-levels) form where the inflow meets cooler air.

Characteristics of Inflow Bands

• Orientation: Typically oriented parallel or at a shallow angle to the storm's inflow direction.

• Appearance: Long, curved bands of cumulus or stratocumulus clouds.

• Location: Found on the inflow side (usually the southern or southeastern flank) of the

storm.

• Movement: They move toward the main updraft region of the thunderstorm, feeding it

warm, moist air.

• Duration: Can persist for hours in long-lived supercells or bow echoes

why do inflow bands matter?

• Fueling the Storm: Inflow bands are a visible sign of a thunderstorm being fed by warm,

moist air—essential for the storm’s development and sustainability.

• Storm Strength Indicator: Strong, persistent inflow bands often indicate a powerful and

sustained updraft, which can lead to severe weather—like large hail, strong winds, or

tornadoes.

• Chasing and Forecasting: Storm chasers and forecasters often watch for inflow bands as part

of diagnosing storm structure and severity

Thunderstorm Structure: Downdraft

• A downdraft is a column of cool, dense air that sinks rapidly from a thunderstorm

cloud toward the ground. While updrafts feed the storm with warm, moist air,

downdrafts are the product of cooling processes inside the storm that ultimately

lead to rain, wind, and sometimes even destructive gusts.

• The downdraft is a region of sinking air caused by any or a combination of

the following:

1. Evaporation (causes cooling, cooled air is dense and sinks)

2. The motion of falling rain

3. Melting of ice

4. Pressure effects generated by the storm itself (we will discuss this in

our lecture on supercells!)

how do downdrafts form?

1. Evaporative Cooling:

As rain falls through dry air or into layers of air with low relative humidity, some raindrops evaporate, cooling the surrounding air. Cooler air is denser and begins to sink.

2. Drag Force:

Falling raindrops and hail physically drag air downward with them, enhancing the downdraft.

3. Melting Ice:

Melting hail or graupel absorbs latent heat, further cooling the air and increasing its density

3 types of downdrafts

1. Ordinary Downdrafts:

• Found in single-cell or multicell storms.

• Lead to gusty winds and precipitation.

• Generally not long-lasting or particularly severe.

  2. Downbursts:

• Powerful downdrafts that hit the ground and spread out rapidly.

• Two types:

  • Microbursts: Small-scale (< 4 km), extremely intense.

  • Macrobursts: Larger-scale (> 4 km), still intense.

• Can produce damaging winds over 100 km/h (60+ mph).

• Often mistaken for tornado damage due to similar tree-fall patterns.

3. Rear-Flank Downdraft (RFD):

• Found in supercell thunderstorms.

• Descends behind the main updraft.

• Plays a crucial role in tornado formation, as it helps tighten rotation at the surface.

What Happens at the Surface during the downdraft?

• When the downdraft reaches the surface, it spreads out as a cool, gusty outflow.

• The leading edge of this outflow is called a gust front or outflow boundary.

• This boundary can lift warm air ahead of it, triggering new thunderstorms.

• Downdrafts often leave behind cold pools, areas of cooled air that modify the local environment

what is the Downdraft-Updraft Interaction

• In ordinary storms, the downdraft may choke off the updraft, causing the storm to collapse.

• In organized storms, like supercells or squall lines, updraft and downdraft are separated— allowing the storm to persist and intensify

4 Visual Indicators of Downdrafts:

• Virga: Rain that evaporates before reaching the ground—sign of evaporative cooling aloft.

• Precipitation shafts: Visible columns of heavy rain or hail descending.

• Gust fronts: Rolling clouds or dust fronts at the leading edge of the downdraft’s surface

spread.

• Shelf clouds: Often seen along gust fronts—wedge-shaped and ominous