The storm that will not look severe, has no rotation to report, lasts half an hour, and puts a tree through somebody's roof. Most of our summer severe weather comes from this.

In this lesson you will

  • Describe the pulse-storm environment and its hazards.
  • Define downburst, microburst and macroburst, and distinguish wet from dry microbursts.
  • Identify the Southeast wet-microburst setup: moist low levels under a dry mid-level layer.
  • Recognize visual precursors and link them to the safety instruction in module 8.
  • Explain why a downburst's damage pattern differs from a tornado's.

The pulse storm

Take lesson 1's ingredients at their humid-summer settings: rich moisture, large CAPE, sea-breeze lift. Then take lesson 3's fourth ingredient away. With almost no shear the updraft and downdraft sit on top of each other and the cell has a short, violent, self-terminating life. That is a pulse storm, and it is what most June through September convection along the Gulf Coast and through the Southeast is.

The Field Guide's figures for it, used verbatim: a 30 to 45 minute life; brief small to moderate hail; downburst winds usually under 70 mph; a possible weak tornado; isolated damage.

"Usually under 70 mph" and "isolated" are why these storms are underrated. The damage is scattered rather than swathed, so it does not read as an event, and each individual cell is only dangerous for a few minutes. But those minutes land on somebody, and there are a hundred of these storms a summer.

Downburst, microburst, macroburst

downburst
A strong downdraft that reaches the surface and produces damaging outward winds. The general term; the two below are size classes of it.
microburst
A downburst less than 4 km across. Typically lasts 5 to 10 minutes, and winds can exceed 100 mph in the strongest.
macroburst
A downburst larger than 4 km across. Longer-lived, generally less extreme at its peak, and covers far more ground.

Fujita coined all three terms. The size cut at 4 km is his, and it is arbitrary in the way useful definitions usually are: it marks roughly where a downburst stops being small enough to slip between observing stations and start being resolvable.

The Southeast setup, in one number

Dry microbursts are a High Plains phenomenon: a deep, dry sub-cloud layer, virga rather than rain, and a dust foot when it lands. East of the Rockies, in a humid boundary layer, the same process produces wet microbursts instead, and the environment that produces those was measured directly in northern Alabama during the MIST field program in 1986.

The discriminator Atkins and Wakimoto found was the drop in equivalent potential temperature, theta-e, between the surface and the mid-level minimum. Theta-e bundles temperature and moisture into one quantity, so a large drop with height means a moist boundary layer sitting under notably drier mid-levels, which is exactly the profile that lets falling rain evaporate hard.

Afternoon surface to mid-level theta-e dropWhat the MIST sample showed
Greater than 20 KMicroburst days
Between 13 and 20 KNeither group. The ambiguous middle.
Less than 13 KThunderstorm days that produced no microbursts

Atkins & Wakimoto 1991, from the 1986 MIST program in northern Alabama. The middle band is genuinely undetermined rather than marginal: the study separated two populations and did not fill the gap between them.

The mechanism is lesson 2's evaporative cooling with the volume turned up. Rain falls out of the storm into a dry layer, evaporates into it, chills that air below the temperature of its surroundings, and the resulting dense parcel accelerates all the way to the ground and splashes outward.

What it looks like before it hits

  • A rain foot. The bottom of the rain shaft splays outward near the ground instead of falling vertically. That splay is outflow already spreading, and it is the clearest single visual cue.
  • A ring of dust, spray or debris expanding outward from a point under the storm. On water it is unmistakable and it is the reason mariners get almost no warning.
  • A rapidly descending precipitation core. A shaft that visibly drops rather than drifts. What you are watching is the load letting go.

What to do about any of that is module 8, which covers positioning and safety and is the module that will keep you alive. The relevant preview: a wet microburst gives you a very short amount of notice, and the safe distance is larger than the photograph makes it look.

Divergent, not convergent

A downburst and a tornado leave different signatures on the ground, and the difference is the single most useful thing a spotter can report about damage. A downburst pushes air outward from a point, so trees fall in a diverging, fanned or starburst pattern all pointing away from where the core landed. A tornado is a convergent, rotating flow, so debris and tree-fall vectors converge toward and cross the track, often lying in opposite directions on either side of it.

Damage survey technique proper is module 5. What belongs here is why the two patterns differ, which is that they are pictures of two different flows.

Where downburst science came from

Aircraft accidents. Fujita and Byers analyzed the 1975 crash of an airliner on approach to JFK and identified the spearhead echo and the downburst behind it. The concept was resisted for years and then vindicated by dedicated field programs, and the result is the on-board and terminal wind shear detection that every commercial flight now depends on. It is one of the clearest cases in meteorology of an observation nobody wanted to believe preventing a recurring disaster.

On the central Gulf Coast

Sea-breeze-initiated pulse convection is the June to September pattern along the central Gulf Coast, and the reports it generates are almost all wind and almost all brief. Two cases from that stretch are worth knowing, both with measured gusts, which is what makes them citable:

  • 2 August 2012, Pensacola. An early-afternoon storm produced a wet microburst around 12:30 to 12:45 pm, with a measured gust of 49 mph just west of Pensacola Regional Airport, downed power poles and 2 to 3 inches of rain.
  • 27 March 2009, Baldwin County. A University of South Alabama surface station measured 86 mph near Robertsdale in a spring bow echo and downburst event. Same physics, different season, and a reminder that the 70 mph figure above is a typical value rather than a ceiling.

Check yourself

A summer sounding shows a moist boundary layer, a notably dry layer in the mid-levels, and a surface to mid-level theta-e drop of 24 K. What does that favour?

  1. Wet microbursts, because falling rain has a deep dry layer to evaporate into
  2. Supercells, because a dry mid-level layer increases deep-layer shear
  3. Nothing in particular, because theta-e says nothing about downdraft strength
  4. Dry microbursts with virga, because the mid-levels are dry
Hint

Compare the number against the MIST bands, then ask what a dry mid-level layer does to falling rain.

Answer

Wet microbursts, because falling rain has a deep dry layer to evaporate into

24 K is above the 20 K threshold that characterised microburst days in the MIST sample. The mechanism is evaporative cooling: rain from a moist boundary layer falls into much drier air aloft, evaporates, chills that air, and the dense parcel accelerates down. It is wet rather than dry because the boundary layer here is moist, so the rain reaches the ground rather than evaporating away as virga.

Sources

  1. NWS Weather Spotter's Field Guide (YPA-201154, June 2011), section 2, the pulse storm and downburst wind safety
  2. NSSL Severe Weather 101, damaging winds
  3. Atkins & Wakimoto 1991, Wea. Forecasting 6, 470–482: wet microburst activity over the southeastern United States
  4. Wakimoto 1985, Mon. Wea. Rev. 113, 1131–1143: forecasting dry microburst activity over the High Plains
  5. Fujita & Byers 1977, Mon. Wea. Rev. 105, 129–146: spearhead echo and downburst in the crash of an airliner
  6. Fujita 1985, The Downburst, SMRP Research Paper 210, University of Chicago
  7. Caracena, Holle & Doswell 1989, Microbursts: A Handbook for Visual Identification, NOAA/ERL/NSSL, 35 pp. Public-domain text; several photographs are third-party credited
  8. NWS Mobile event summary, 2 August 2012 Pensacola microburst
  9. NWS Mobile event summary, 27 March 2009 Baldwin County wind event