Effect of Coriolis Force on Hurricane Tracks

Necessary Ingredients for a Tropical Cyclone

This morning I visited the National Hurricane Center website to see the latest tropical developments. The Houston Area has nothing to worry about at the moment. But while I was on the NHC website, I stumbled on an incredible resource. It discussed – among other things – the necessary ingredients for a tropical cyclone. It’s called the Mariner’s Tropical Cyclone Guide, updated by Dylan Flynn in May 2023.

A Treasure Trove of Information about Tropical Systems

The title sells this electronic booklet short. It’s true that the last quarter of this 86 page booklet discusses how to navigate ships and boats near tropical cyclones. But the rest is a primer on tropical systems themselves. The booklet has four chapters:

  • Tropical Cyclone Basics
  • Climatology
  • Monitoring Tropical Cyclones
  • Tropical Cyclone Evasion

Although the book is targeted at Navy and Merchant Marine personnel, the general public will find the first three parts both informative and educational. The writing is clear, crisp, and concise. And the illustrations are illuminating. Overall, a quick read.

One of the most fascinating discussions started on page 12.

Necessary Ingredients for Development and Intensification

Flynn lists seven environmental conditions necessary for tropical cyclones to form and thrive. Eliminate one, says Flynn, and the whole system starts to break down.

  1. A pre-existing surface disturbance with thunderstorms: Tropical cyclones rely on a build-up of heat energy above them to grow and develop. A thunderstorm complex acts as a vertical transport mechanism for heat, moisture, and the cyclonic turning of winds into the upper levels of the atmosphere. This vertical transport helps tropical cyclones develop.
  2. Warm ocean: Tropical cyclones draw on the heat energy stored in the ocean. Sea surface temperatures of at least 80ºF are needed to support development and intensification. Evaporation of this warm water begins the process of energizing the atmospheric column. The warm seas should extend at least 60 m deep, as the strong winds of a tropical cyclone cause a turbulent sea that mixes the warm surface water with cooler, deeper water.
  3. Low vertical wind shear: Tropical cyclones rely on a vertically stacked structure to grow or maintain intensity. The ideal tropical cyclone has its upper-level circulation directly above the low-level circulation. Changes in environmental wind speed or direction with height will tilt the vertical structure. This tilting inhibits growth and may cause the system to decay.
  4. Unstable atmosphere: Rising air is needed to warm the tropical cyclone core, and an unstable atmosphere is necessary to support rising air.
  5. High atmospheric moisture content: Cloud formation is limited if the atmospheric column is too dry. Rising air will cool but struggle to reach the low dew point. The environment is often stable for dry parcels of air but unstable for moist air.
  6. Upper-level outflow: An exhaust mechanism is needed above a system to perpetuate the strong upward motion. This upper-level mass removal causes the pressure at the surface to drop. As a system develops, low-level cyclonic flow pulls mass toward the center. The flow then turns upward as intense vertical motion associated with thunderstorms. This process is known as “the in-up-and-out” circulation. Without a method to dispose of the mass above a tropical cyclone, low-level converging flow toward the center will halt as the system suffocates.
  7. Adequate Coriolis force: Due to the earth’s rotation, the Coriolis force causes tropical cyclones to spin counter-clockwise in the northern hemisphere and clockwise in the southern hemisphere. This spin is a critical component for development and intensification. Tropical cyclones rarely develop within 5 degrees of the equator, where this force is weakest. See gap in illustration below.

Other Fascinating Discussions

This is just one of dozens of fascinating topics in Flynn’s booklet. Among other things, Flynn discusses:

  • The exact meaning of terms used by NOAA and the National Hurricane Center. For instance, do you know the difference between a potential tropical cyclone, tropical cyclone and tropical depression? See pages 7-8.
  • How tropical cyclones dissipate and transition into extratropical storms. Page 14.
  • The size of the impact area for tropical cyclones. Page 15.
Largest and smallest tropical cyclones on record. Source: NOAA’s Mariner’s Tropical Cyclone Guide by Dylan Flynn. NM stands for nautical miles (about 6,076 ft).
  • What 50 foot waves look like to a sail drone from inside the eye of hurricane. Page 22.
  • How dangers differ in different areas around cyclones.
  • Seasonal variations and the influence of El Niño.
  • Environmental steering currents for cyclones.
  • How to interpret NOAA’s technical charts and tables, such as wave heights/intervals, wind forecasts, etc.

The general public and weather enthusiasts will find a boatload of useful information in this booklet, not just the necessary ingredients for a tropical cyclone.

Posted by Bob Rehak on 7/22/2023

2153 Days since Hurricane Harvey