Cooking up a Storm

               With all this activity in the tropics, I guess it would be a good idea to go over what is needed to make, or maintain, a tropical cyclone. This list is by no means perfect, some storms exist without having all the requirements, and sometimes all the environment can be perfect, yet nothing forms. In general though, this list serves as a simple rule of thumb.

1.      WARM WATER. Tropical cyclones are basically giant heat engines, so they need an abundant heat source. This heat comes in the form of the warm water beneath the storm. As waves increase, more surface area develops at the ocean’s surface which will allow more evaporation to occur. This evaporated water vapor will be drawn in towards the center of the storm and rise in the powerful updrafts associated with the storms near the core. As the air rises, it will reach a point where it begins to condense into water droplets (or ice crystals at very high altitudes); this releases a lot of heat, which causes to core to warm. A warmer core means a lower surface pressure, which means a more intense storm that will produce more waves and pull more vapor in. This cycle is how tropical cyclones are able to become incredibly strong and have such low pressure at their centers.

Idealized sketch of movement of heat, water vapor, and air near the core of a tropical cyclone

2.      TEMPERATURE PROFILE. In order for the convection that defines a cyclone to develop, the rate that the environmental air cools with height (called the lapse rate) must be great enough that air from near the surface will remain warmer than the air around it as it rises. As long as it is warmer than the surrounding air, it will be less dense and thus more buoyant than the surrounding air and will rise through it, just like the blobs of goo rising in a lava lamp. This rising air creates the powerful updrafts that form the core of the towering convective clouds in the heart of a tropical cyclone.

temperature profile conducive for tall updrafts, forming tall convective clouds

3.      LATITUDE. As the name implies, tropical cyclones typically form in the tropics. But, they cannot form right on the equator. The reason for this is that there is no source of rotation on the equator, so a developing storm has no way to begin spinning. The force that imparts the rotation is called the Coriolis force, and while the exact definition is a bit complex, the key is that it increases with increasing latitude. In general, storms are unlikely to form within about eight degrees from the equator.

4.      TRIGGER. To get the ball rolling, there has to be some kind of initial disturbance to initiate the storm. This often takes the form of a tropical pressure wave. Perhaps the most famous of these are African Easterly waves which are behind most Atlantic storms and even some East Pacific storms.

5.      SHEAR. Shear, the difference in wind direction/speed between two layers of the atmosphere, is deadly to a tropical cyclone. If shear is too great, this will cause the lower part of the storm’s circulation to detach from its upper circulation (sometimes I refer to this as decapitation). In most cases, shear should be less than about twenty knots in order for a storm to develop.

 

Left: cross-section of storm, with shear increasing down the column.
Right: sketch of heavily sheared storm, dark lines are low clouds, light lines are high clouds.