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(T. F. “Storm” Walsh III)
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Since I did not receive any questions over the past week, I thought I would discuss, somewhat simply, Rapid Intensification of Atlantic Hurricanes. We’ve seen a few hurricanes over the past few years go through this process, most recently, Hurricane Michael of the 2018 Atlantic Hurricane Season. Rapid Intensification (or as we in the meteorological realm refer to as R.I.), by current definition from the National Hurricane Center, is as follows (from a Wikipedia article):
A meteorological condition that occurs when a tropical cyclone intensifies dramatically in a short period of time. The United States National Hurricane Center (NHC) defines rapid intensification as an increase in the maximum 1 minute sustained winds of a Tropical Cyclone of at least 30 knots (35 mph) in a 24 hour period.
The United States National Hurricane Center previously defined rapid deepening of a tropical cyclone, when the minimum central pressure decreased by 42 millibars (1.240 inHg) over a 24-hour period.
Now, in most recent years, we’ve heard the Global Warming crowd, and politicians obsessed with it, come out and state that “Global Warming” is the reason we have such intense hurricanes, and the reason we see rapid intensification of hurricanes. In this “lesson”, I am going to talk of 2 of the “main” factors (2 real reasons) involved in the rapid intensification of hurricanes.
First, let’s begin with how the ocean DOES warm. As was taught in my MET 2010 class at the junior college I attended, the ocean is warmed or heated by INSOLATION (Incoming Solar Radiation)…period! Not by clouds reflecting heat from sunlight, or CO2 trapping heat and the heat traveling down to the ocean surface (remember, heat and warm air travel UPWARD, due to the fact WARM AIR IS LIGHTER THAN COOLER AIR, SO IT RISES). Clouds and dust can block the sun from reaching certain parts of the ocean, thus the ocean NEVER heats the same way each year.
SEA SURFACE TEMPERATURES AND OCEAN HEAT CONTENT
One important factor in the R.I. process, is that SST’s should be near or above 30 deg Celsius (based on my analysis of some SST maps for various hurricanes, it appears the criteria may be 28+ deg Celsius). These extremely warm SST’s should extend down fairly deep. The depth of how far this very warm water extends downward from the ocean surface (along with the temperature of the water) is a determining factor in Ocean Heat Content (OHC), also known as Tropical Cyclone Heat Potential (TCHP). OHC is measured in kJ/cm2. For R.I. to occur, a threshold of 50 kJ/cm2 should support it, however 60+ kJ/cm2 appears to be the optimal value. Lower values MAY sustain a MAJOR hurricane (I’m still researching this), but based on my research over the past few years, it appears the mentioned values are required for R.I. The reason the extremely warm water needs to extend downward a great depth, is to prevent upwelling of colder water. As a hurricane passes over these higher OHC values, it keeps feeding off very warm water. Remember, in meteorology heat = energy. The deeper, warmer water and OHC of 60 and above, provide the needed heat flux to maintain a tropical cyclone. The warmer SST’s aid in adding heat at the surface for the hurricane to work with. As the air is warmed and rises, it eventually cools and condenses, releasing latent heat into the storm, which allows clouds to keep growing vertically as the warm air continues to rise and condense, releasing more latent heat, in which the process keeps repeating. The following graphics of OHC are from 2005 when Katrina traversed the GOMEX, and Matthew of 2016 skirting the FL. coast:
Here is a rather lengthy, but excellent article from the AMS Journals online:
One factor I did not mention is, dry air can affect a hurricane regarding R.I. The atmosphere has to be very moist up through the mid levels. Usually by this stage in the game, a hurricane can mix out some amounts of drier air.
A second important factor is, there MUST be a well established upper level anti-cyclone, or some type of well established outflow channel/pattern over the cyclone. The importance of this is, the warm air being taken in at the surface of the cyclone, MUST be “evacuated” at the top of the storm, in the upper layers of the troposphere. Without this warm air being “carried” away from the storm, the storm kind of “chokes’ on itself. Here is an analogy we tend to use to describe the process. Think of a fireplace, IF the flu is closed, the heat and smoke produced by the fire cannot escape up and out of the chimney. When the flu is open, the heat and smoke can rise up and out of the chimney, which establishes an “inflow” from the room in which the fireplace is located, and an “outflow” at the top of the chimney. Yes, it’s that basic. I myself know the process in how this occurs, but to save time, here is a paragraph from Wikipedia article:
The development of anticyclones aloft occurs in warm core cyclones such as tropical cyclones when latent heat caused by the formation of clouds is released aloft increasing the air temperature; the resultant thickness of the atmospheric layer increases high pressure aloft which evacuates their outflow.
Outflow can also be established if the cyclone slides underneath an already established upper level ridge. Outflow can also be aided if the cyclone is situated to the east of an upper level trof, as east of the trof would be a partial upper level ridge. The latter is pretty much what aided Hurricane Michael from the 2018 season, in the strengthening process and R.I. process. The following animation is of the outflow pattern over Hurricane Michael. The wind flags represent the clockwise flow in the upper troposphere. You’ll notice the flow from around the NW side and moving across Florida, and flow around the SE Quad, flowing over Cuba. This setup was caused by Michael being located to the east of the upper level trof dipping down over TX. and LA., and an upper level trof over western Cuba. With the very warm waters over the Gulf at that time, and high OHC, along with the favorable outflow pattern shown, Michael went through a period of R.I. The favorable outflow pattern remained until landfall, and with the outflow “setup”, air was being evacuated around Michael, and was being drawn away from the storm, by the upper level trof over western Cuba. This is the most likley reason Michael kept intensifying up until landfall.
I hope this is informative for all of you.
Have a blessed evening!
T. F. “STORM” WALSH III
GMCS, USCG (ret)
METEOROLOGIST / HURRICANE SPECIALIST /SEVERE WEATHER SPECIALIST
MEMBER WEST CENTRAL FLORIDA AMS