What Factors Lead To The Development Of Tropical Storms & Hurricanes?
I thought I would give everyone some background on what we look for in terms of environmental conditions and other factors that can lead to the development of tropical storms and hurricanes.
There are generally six items (and sometimes more) that we look for in the tropical Atlantic, Caribbean and Gulf of Mexico that are necessary for tropical cyclone development (called tropical cyclogenesis). They are:
1. Ocean water temperatures that are at least 26 degrees Celsius or 79 degrees Fahrenheit to a depth of at least 50 meters or 150 feet.
2. Moist air at about 10,000 feet.
3. A generally unstable atmosphere.
4. An area of enhanced vorticity (spin) which can be caused by low pressure developing or from a tropical wave or tropical disturbance.
5. Weak vertical wind shear.
6. The area of interest has to be at least 5 degrees of latitude away from the equator.
So, let’s look at each item a little more in depth:
1. Ocean Water Temperatures: Normally, ocean water temperatures of at least 26 degrees Celsius or 79 degrees Fahrenheit going down to a depth of at least 50 meters or 150 feet is considered the minimum to form and maintain a tropical cyclone. These warm waters are needed to maintain the warm core that fuels these tropical systems.
2. Moist Air At About 10,000 Feet: The atmosphere at about 10,000 feet above ground level must contain enough moisture to sustain thunderstorms. Dry air in this level of the atmosphere is not favorable to the continuing development of widespread thunderstorm activity. The more humid the air in the atmosphere, the less the disturbance will have to work to moisten the air in order for tropical cyclone genesis to occur.
3. A Generally Unstable Atmosphere: The atmosphere must cool fast enough with height so that the entire atmosphere is unstable and can support thunderstorm activity. It is this thunderstorm activity which allows the heat stored in the ocean waters to be lifted into the atmosphere and used for tropical cyclone development.
Often times during the hurricane season, you hear me talk about hot and dry Saharan air putting a stop to any development chances. The reason why tropical development is unlikely when the Saharan Air Layer (SAL) is an issue is because the hot, dry air puts a lid on any thunderstorm growth. When the mid-levels of the atmosphere (around 10,000 feet) is warmer than the ground, it stops air parcels from rising into the upper atmosphere. Those parcels of air reach a certain point, hit that lid of warmer air and thus stop rising. We call this a stable atmosphere and it is something that tropical cyclones do not like.
Now, the Saharan Air Layer not only puts a stop to any organized thunderstorm growth which could go on to become a tropical cyclone, but it also leads to the atmosphere to be drier. As I mentioned in point number 3, the more humid air, the more likely tropical cyclone genesis may occur; so, if the atmosphere is very dry, like we would see in a SAL outbreak, then the disturbance has to work very hard to moisten up that atmosphere, reducing it’s energy reserve and reducing the likelihood it may become a tropical cyclone.
4. An Area Of Enhanced Vorticity (Spin): Whether it be an area of low pressure embedded within the Intertropical Convergence Zone (ITCZ), a tropical wave, a broad surface front, or an outflow boundary, a low level feature with sufficient vorticity and convergence is required to begin tropical cyclogenesis. Even with perfect upper level conditions and the required atmospheric moisture and instability, the lack of a surface focus will prevent the development of organized convection and a surface low. Bottom line is that tropical cyclones cannot be generated spontaneously. They require at least a weakly organized system that begins to spin and has low level inflow of moist air.
5. Weak Vertical Wind Shear: There must be low values of vertical wind shear between about 5,000 feet (850 millibars) and 35,000 feet (250 millibars) above the ground. I like to look for 20 mph or less of vertical wind shear between those two heights. Vertical wind shear is the rate of change of wind velocity with altitude. Large values of vertical wind shear disrupt the developing tropical cyclone by removing the rising moist air too quickly, preventing the development of the tropical cyclone. Or, if a tropical cyclone has already formed, large vertical shear can weaken or destroy it by interfering with the organization around the cyclone center.
6. Coriolis Force: A minimum distance of 500 kilometers or 310 miles or 5 degrees of latitude away from the equator is normally needed for tropical cyclogenesis. The Coriolis force imparts rotation on the flow and arises as winds begin to flow in toward the lower pressure created by the tropical disturbance. The existence of Coriolis force allows the developing cyclone to achieve gradient wind balance. This is a balance condition found in hurricanes that allows heat and moisture to concentrate near the storm core; this results in the maintenance or intensification of the cyclone if other development factors are neutral.
Even when all of the aforementioned environmental conditions are favorable, a tropical depression still may not form. For this reason, forecasting the genesis of a tropical depression is a very difficult challenge.
Tropical depression formation does begin with a pre-existing disturbance. This disturbance can come from:
Easterly Tropical Waves: These are inverted troughs of low pressure that track westward across the tropical Atlantic. A trough is defined as a region of relatively low pressure. The majority of tropical cyclones form from easterly tropical waves.
West African Disturbance Line (WADL): This is a line of convection, very similar to a squall line, which forms over western Africa and moves into the far eastern Atlantic Ocean. West African Disturbance Lines normally move faster than tropical waves.
TUTT: A TUTT or Tropical Upper Tropospheric Trough is a trough, or cold core low in the upper levels of the atmosphere, which produces convection. On occasion, one of these develops into a warm-core tropical cyclone; however, given that they are cold cored in nature, they can normally take many days to develop into a tropical cyclone as they have to warm up the atmosphere around them.
Old Frontal Boundary: Remnants of an old frontal boundary can sometimes spawn a tropical cyclone on the tail end (southern or southwestern most side) of the front. In general, the most likely time frame for this to happen is during the early part of the hurricane season or the very end of the hurricane season. The most likely area for tropical cyclones to form off of an old front is in the Gulf of Mexico, the western and southwestern Caribbean or immediately offshore of the US Southeast coast.
Mesoscale Convective System: A mesoscale convective complex or MCS is a large complex of thunderstorms that can grow up to 300 miles in diameter and can last for 6 hours or more. Often times, we see a MCS during the spring and summer track across the Great Plains into the central and southeastern United States. Sometimes within a MCS, a mesoscale convective vortex (MCV) can form.
A MCV is a mid-level low pressure system within an MCS that pulls winds into a vortex. Once the main MCS dies, this vortex can persist and can sometimes take on a life of its own, persisting for up to several days after the main MCS has dissipated. There are times that a MCV can move into the Gulf of Mexico or off of the US Southeast coast and become the seedling that causes the development of a tropical storm or hurricane.
In closing, I hope this insight and background into what factors are needed for tropical cyclone formation is helpful. Often times there are more questions than answers when I try to determine whether a tropical disturbance may or may not become a tropical depression.
Rob Lightbown of Crown Weather