A day in a hurricane specialist's life

Tropical cyclones: The value of Vaisala Dropsondes and lightning data 15:30 EDT Miami, Florida, USA: a hurricane specialist shows up for the evening shift at the National Hurricane Center (NHC). Hurricane Aidan* is located in the Gulf of Mexico with maximum sustained wind speeds of 135 miles per hour (mph) (217 kilometers per hour) and a minimum central pressure of 941 hectopascals.

Aidan has been rapidly intensifying all day. The storm was only a category one hurricane less than 24 hours ago with maximum sustained wind speeds of 90 mph (145 kilometers per hour).

This is a critical shift for the hurricane specialist. Hurricane Aidan is within 48 hours of landfall somewhere in the northwest Gulf of Mexico along the U.S. coast. It is currently an extremely dangerous category 4 storm with maximum sustained wind speeds of 135 mph (217 kilometers per hour) on a scale of one to five on the Saffir-Simpson Scale. A category five hurricane has maximum sustained wind speeds greater than 155 mph (249 kilometers per hour).

Most numerical weather prediction models used by NHC are projecting landfall near Houston, Texas, the fourth largest city in the U.S. However, two numerical weather prediction models are projecting landfall 300 km east of the Houston area. The specialist needs to issue a hurricane watch when hurricane conditions are expected on the coastline within 36 hours. The specialist is also responsible for issuing tropical storm watches and warnings. A tropical storm watch has already been issued for a large portion of the U.S. coastline in the northwest Gulf of Mexico. A tropical storm warning is issued when tropical storm conditions are expected on the coastline within 24 hours. However, there is a fairly strong outer rainband forming to the north of the center of Aidan. The specialist may need to upgrade the tropical storm watch to a warning during this shift.

Decisions of great importance
The hurricane specialist must answer the following questions during his shift. Is Aidan going to make landfall near Houston? When should he issue a hurricane watch and where? When should he issue a tropical storm warning and where? Will Aidan reach category five intensity or will the rapid intensification stop shortly?

Into the eye of the storm
An Air Force WC-130J reconnaissance plane, better known as a hurricane hunter, has just finished flying through the center of the storm, launching three Vaisala-NCAR Dropwindsondes along the way. The first dropwindsonde (or, simply, dropsonde) was launched into the northeast eyewall of Aidan, the second into the eye, and the third into the southwest eyewall. The first dropsonde measured maximum sustained wind speeds near the surface of 145 mph (233 kilometers per hour). The second dropsonde, launched into the eye of the storm, measured a minimum central pressure of 930 hectopascals. The third dropsonde measured maximum sustained wind speeds near the surface of 140 mph (225 kilometers per hour). Aidan is continuing to rapidly intensify. Unfortunately, the next scheduled hurricane hunter will not be in the storm for another 12 hours.

Vaisala-NCAR Dropsondes provide the only direct profile measure of wind speed and direction, pressure, temperature, and humidity within the core of a hurricane. These measurements are critical for measuring its current intensity and understanding the surrounding environment that influences the development and track of the storm.

The strongest winds in a hurricane are located within a ring of intense weather near the center of the storm known as the eyewall. The eye of the storm is located in the center of a hurricane and may contain little cloud cover and relatively calm winds. The minimum central pressure of a hurricane is found in the eye and is another measure of storm intensity.

Strongest winds at 500 meters
As the hurricane hunter flew in and around the center of the storm, it was taking continuous measurements of the wind at flight level, approximately 10,000 feet (3,048 meters). These measurements showed that the eastern eyewall of the storm contained the highest sustained wind speeds.

The highest wind speeds in the eyewall of a hurricane occur above the surface. If a hurricane is going to make landfall near a large city with numerous high rise buildings, people should not seek shelter on the upper floors of those buildings. Hurricanes are actually a category stronger on the Saffir-Simpson scale on the 25th floor of a high rise building than they are on the ground. Most of the damage caused on the upper floors of high rise buildings during hurricane landfalls is due to windows being blown out.

As the hurricane hunter is returning to its base, it flies through a rainband developing on the north side of Hurricane Aidan. As it flies through the rainband, the Stepped Frequency Microwave Radiometer (SFMR) is taking continuous remote measurements of surface wind speed and measures surface wind speeds just under tropical storm strength. However, flightlevel wind speeds measured by the aircraft show sustained winds of moderate tropical storm strength.

The SFMR estimates surface wind speeds based on the state (or amount of foam) of the ocean surface. Surface wind speed estimates from the SFMR need to be calibrated using Vaisala-NCAR Dropsondes. Similar to flight-level wind measurements, SFMR provides continuous estimates while the aircraft is in the storm. Surface wind speeds are critical to forecasters in gaining a better understanding of storm development and current intensity.

At the same time as the Air Force hurricane hunter is flying through Aidan, a NOAA Gulfstream IV aircraft is flying at 40,000 feet (12,192 meters) and launching dropsondes in the environment surrounding the hurricane.

The Gulfstream IV aircraft launches dropsondes at a much higher altitude than the Air Force hurricane hunter and outside of the hurricane in order to provide numerical weather prediction models with the information they need to improve forecasts. Vertical measurements of wind speed and direction, pressure, temperature, and humidity are immediately sent to the National Center for Environmental Prediction (NCEP) and assimilated into NCEP and other numerical weather prediction models. Research has shown that hurricane track forecasts have improved substantially due to Vaisala-NCAR GPS Dropsonde (RD93) measurements in the environment surrounding the hurricane.

Lightning activity reveals hurricane intensity
As the hurricane specialist waits for the next set of numerical weather prediction model data, including the recent dropsonde measurements assimilated from the hurricane environment, he notices some changes in the lightning activity within Aidan. The outer rainband to the north of the storm is now producing very high lightning rates.

Most of the lightning produced by a hurricane occurs in the outer rainbands of the storm however not all outer rainbands produce lightning. High lightning rates within outer rainbands of a hurricane indicate strong convection that will allow strong winds above the ground to reach the surface in downdrafts. Vaisala-NCAR Dropsonde measurements in the outer rainbands of hurricanes have shown that the highest winds are not at the surface, but also near 500 meters (1,640 feet). Therefore, high lightning rates in the outer rainband of a hurricane may indicate that stronger winds aloft may reach the surface.

The hurricane specialist has also been monitoring an abrupt increase in lightning activity in the eyewall of Aidan over the past nine hours. This increase in eyewall lightning activity has decreased considerably over the last three hours.

Research currently being performed at Vaisala and NHC has shown that numerous hurricanes have produced an abrupt increase in eyewall lightning near peak intensity after a rapid intensification period. This abrupt increase will usually cease after 6 – 9 hours. Rapid intensification ends at about the time the lightning rate ceases in the eyewall.

The latest numerical weather prediction model results using the dropsonde data from the environment outside of the hurricane have just come in. All models now have tracks that take the storm 300 km east of Houston during landfall. The numerical weather prediction models also predict landfall in 36 hours due to an unexpected increase in forward motion that was not shown in previous model runs. It is time to issue the 23:00 EDT advisory for Hurricane Aidan.

The hurricane specialist now needs to process all of this information and come up with the appropriate track and intensity forecast.

(* Hurricane Aidan is a fictitious hurricane name to illustrate actual mission critical actions at the National Hurricane Center, utilizing Vaisala products.)

Author: Nicholas W.S. Demetriades and Ronald L. Holle, Vaisala, USA

Further information:
www.vaisala.com/weather/applications/severeweather/hurricanes