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North Atlantic surface analysis, 18 UTC 10 Sep 2020

the "ingredients" that support tropical cyclone formation


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The National Hurricane Center (NHC) defines a tropical cyclone as "a warm-core non-frontal synoptic-scale cyclone, originating over tropical or subtropical waters, with organized deep convection and a closed surface wind circulation about a well-defined center. Once formed, a tropical cyclone is maintained by the extraction of heat energy from the ocean at high temperature and heat export at the low temperatures of the upper troposphere."

Thus a tropical cyclone (TC) does not have fronts, it forms over ocean water at lower (tropical & subtropical) latitudes, its center is warmer than the air outside of the storm at the same altitude, it's made up of strong thunderstorms with cold cloud tops (deep convection), and it produces surface winds that clearly spin around a specific location. But how does such a storm form in the first place? Which factors contribute to "the ability to support deep convection in the presence of a low-level absolute vorticity maximum"? (Where low-level absolute vorticity maximum is a fancy way to refer to a pre-existing, or "seed," disturbance.)

[The above link directs you to a section in the MetEd textbook on tropical meteorology. Other links on this page also take you to MetEd, which is a completely free resource but a free account is required to access its content.]

Example: Infrared satellite imagery showing Hurricane Otis (2023) as a tropical disturbance (left) and then as a tropical depression (right):

Ingredient #1: warm ocean water

To power the strong thunderstorms that make up a TC, warm ocean water is needed. How warm? The typical threshold is 26°C (78.8°F) because water that is at least this warm provides latent heat that can be released in thunderstorms. The below map shows August-September SSTs averaged during 1991-2020 from NOAA's OISST dataset. The black line represents where the temperature is, on average, 26°C. Click the image for a larger version.

When air with less than 100% relative humidity is above a body of water, evaporation will occur. Evaporation increases the relative humidity of that air. The more moist the air becomes, the lighter it gets at the same temperature, and being lighter helps it rise. When that air rises, it cools because atmospheric pressure decreases with height, and eventually the rising air becomes saturated (100% relative humidity). Condensation then occurs, which releases energy, and that energy is in the form of latent heat. The now-warmer air continues rising, which supports thunderstorms.

(Why is more moist air lighter than more dry air? Our atmosphere is mainly made up of nitrogen (N2) and oxygen (O2) molecules. The molecular weight of N2 is 28 and O2 is 32. On the other hand, water vapor (H2O) has a weight of 18! Since air has a limit to the number of molecules that can be present in a given volume at a particular temperature and pressure, the addition of H2O molecules displaces the other (heavier) molecules, reducing mass and thus making the air parcel lighter. More details can be found here!)

Ingredient #2: atmospheric moisture

If near-surface tropical air gets moisture from the warm ocean below, why do other parts of the troposphere also need to be moist? Well, if a cumulonimbus cloud develops and the air around it is dry, that dry air will mix into the cloud (a process called entrainment). When dry air mixes into a cloud, the water droplets making up the cloud evaporate. Evaporation takes energy to happen, in this case from the air where the evaporation occurs, which will lower that air's temperature. Air that's now cooler than its surroundings will sink, and sinking air is bad for a TC. This is why moist air near the surface and up into the mid-levels of the troposphere tends to help a TC.

The above map shows ERA5 relative humidity averaged over the 850-500-hPa layer for August-September 1991-2020. Click the image for a larger version.

Ingredient #3: low vertical wind shear

Warm ocean water and high humidity aren't enough for a TC to form. Low vertical wind shear is also needed because strong shear will disrupt thunderstorms in the disturbance that could become a TC. Vertical wind shear is a change in wind speed and/or direction with height:

adapted from Stull's Practical Meteorology, Fig 14.49

The left column depicts directional shear because the wind direction (red arrow) points different directions at different altitudes. We usually estimate shear between pressure levels, such as 200 and 850 hPa. The right column depicts speed shear because the wind direction doesn't change but the length of the arrow (wind speed) does change.

The above map shows ERA5 vertical wind shear magnitude computed between 200 and 850 hPa and averaged for August-September 1991-2020. Click the image for a larger version. The black line represents where the wind shear is, on average, 20 kt. Generally, shear above 20 kt will weaken TCs that have already formed or delay, perhaps even prevent, a disturbance from becoming a TC... but not always.

Ingredient #4: atmospheric instability

"Static stability" refers to the ability of air parcels to continue rising if pushed upward (or continue sinking if pushed downward). Since atmospheric pressure decreases with altitude, a rising blob of air will expand due to that decrease. This expansion results in the air parcel cooling at what's called the dry adiabatic lapse rate. But if the air parcel reaches 100% relative humidity (in other words, becomes saturated), condensation will occur as it keeps rising and cooling. That condensation releases latent heat! So the drop in temperature from rising will be offset by the added energy by condensation.

If the air's drop in temperature is slower than the surrounding air's drop in temperature, it'll end up warmer as it rises and thus keep rising! But a stable atmosphere will resist this outcome. Why? Because when conditions are stable, a rising air parcel does not end up warmer than its surroundings, and thus it can't continue going upward.

Read more about the need for instability, moisture, and warm water here (NWS JetStream).

Did you know? We can capture the impacts of SST, atmospheric instability, and some aspects of moisture through one parameter called "potential intensity." The higher the potential intensity, the more conducive the environment is to a strong hurricane. It can be referred to as "the maximum speed limit" for a TC. Click here to learn more about potential intensity and tcpyPI!

Ingredient #5: distance from the equator

We live on a rotating planet, and this rotation induces something called the Coriolis effect. Air gets deflected from its initial trajectory based on its latitude, and the higher the latitude, the larger the deflection. As a result, the Coriolis effect is weakest near the equator. The background "spin" provided by the Coriolis effect helps tropical disturbances develop the closed wind circulation necessary to become a TC. In the below map, the darker the green shading, the more Coriolis acceleration occurs at that latitude.

(Click here for a version of this plot with units and a colorbar.)

Ingredient #6: a "seed" disturbance

I keep mentioning these seed disturbances. That's because TCs don't form spontaneously. Some type of thunderstorm-powered weather feature needs to exist first, like an African easterly wave or a mesoscale convective system. The existence of such a feature at low latitudes does not guarantee a TC will develop, but warning centers such as the NHC will monitor disturbances that have the potential to undergo tropical cyclogenesis, the term used for the process of a disturbance becoming a TC.

Example: Infrared satellite image from the NHC's 2-day tropical weather outlook (TWO) showing the disturbance ("1", 70% chance) that would later develop into Hurricane Otis (2023):

(page last modified May 2024)