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(T. F. “Storm” Walsh)
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Greetings to everyone!
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This post is the fourth in the series of Hurricane Tutorials. Today’s tutorial may be a bit lengthy, but will touch more in depth on Tropical Cyclone formation, including how Tropical Waves form, the criteria needed for Tropical Cyclone development, and Rapid Intensification of hurricanes.
By now, we’re all familiar with the African Easterly Wave, or Tropical Wave. In simplistic terms, a tropical wave is basically an inverted trough, in which winds are converging east of the wave axis, and diverging west of the wave axis. The more organized waves, should they maintain for 24 hours, can be classified as a Tropical Disturbance (Tropical Disturbance: A discrete system of apparently organized convection originating in the tropics or subtropics, having a non-frontal migratory character and maintaining its identity for 24 hours or more).
TROPICAL WAVE DIAGRAM
These waves develop in part, due to the complex nature of the African Easterly Jet (AEJ) during June through Oct. If you click the following graphic, it will take you to a brief explanation of the AEJ:
AFRICAN EASTERLY JET
The next graphic is linked, and if you click on it, will give a fairly decent explanation on the development of Tropical Waves. The article is from the Trinidad and Tobago Weather Center:
TROPICAL WAVE FORMATION:
As these waves move generally westward, they can potentially develop into a Tropical Depression, Tropical Storm, and Hurricane (also known by the term Tropical Cyclone).
For a Tropical Cyclone to to develop, as a general rule, there are SIX main criteria which need to be met. I just discussed one of the six, regarding a “disturbance” or existing low pressure that can develop. An existing disturbance DOES have to be present. The following is pretty much a complete list of “disturbances” that can lead to cyclogenesis, which has been nicely explained by one of my colleagues, Rob Lightbown of Crown Weather Services:
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.
The following graphic is linked, and will take you to the six main criteria needed for cyclogenesis (Tropical Storm Formation):
RAPID INTENSIFICATION OF HURRICANES:
As we have seen in some past hurricane seasons, especially during active, above average seasons, rapid intensification (R.I.) of hurricanes. We know in order for a hurricane to be able to sustain, upper level winds need to be not too strong (low wind shear, or lack thereof). This is one of the key factors for a storm to survive. Another key factor is the temperature of the seas surface (or in this case, below the sea surface). In general, for tropical cyclone formation, the SST’s (Sea Surface Temperatures) in general, need to be 26.5C 0r 80F. I’m sure those of you who follow my site have at one time or another heard me speak of “Tropical Cyclone Heat Potential (TCHP) or Ocean Heat Content (OHC). This refers to how far below the ocean surface the 26.5 or warmer isotherm extends. TCHP is measured in kilojoules per square centimeter (kJ / cm2). As a hurricane moves into an area of higher TCHP, and as long as all other atmospheric conditions are favorable (i.e. lack of wind shear, upper level anticyclone overhead, abundant moisture from the surface to the mid level of the atmosphere, etc.), rapid intensification can occur. Upper level outflow, or development of an upper level anticyclone over the storm is paramount in aiding in rapid intensification and steady intensification up to landfall. The following animated GIF shows why Hurricane Michael was able to go through R.I., and keep intensifying up to landfall:
HURRICANE MICHAEL UPPER LEVEL OUTFLOW
Rapid intensification, as defined, is a meteorological situation where a tropical cyclone intensifies dramatically in a short period of time. The United States National Hurricane Center defines rapid intensification as an increase in the maximum sustained winds of a tropical cyclone of at least 30 knots (35 mph; 55 km/h) in a 24-hour period (from Wikipedia). In order for this to occur, a hurricane, based on my research, needs to traverse an area of TCHP with a value of 50+ kJ / cm2. The higher the value, the further down the 26.5 or greater isotherm extends. This acts as high octane fuel for a hurricane, in that more heat energy, and moisture enter the storm. Also, in a higher TCHP environment, upwelling of colder SST’s is usually not a factor, or is slower to occur.
The following graphic is from the NOAA Physical Oceanographic Division and displays the TCHP for Feb. 14, 2020. The outlines with the greenish color fill indicate where the 50 kJ / cm2 values begin. This graphic I have linked to an article by Caitlyn Kennedy from NOAA:
TROPICAL CYCLONE HEAT POTENTIAL
Have a blessed evening!
T. F. “STORM” WALSH III
GMCS, USCG (ret)
METEOROLOGIST / HURRICANE SPECIALIST /SEVERE WEATHER SPECIALIST
MEMBER WEST CENTRAL FLORIDA AMS