One NASA research program in hurricane intensification utilizes data from the Tropical Rainfall Measuring Mission (TRMM) satellite, whose on-board radar can see all the way into the hurricane and make images of the precipitation. One TRMM goal is to investigate “hot towers,” high rain clouds that reach up 15 kilometers (nine miles), at least to the top of the troposphere, the atmosphere’s lowest layer. These towers indicate areas of the storm where large amounts of latent heat are released from the condensation of water vapor. A group of these towers forming in a portion of the hurricane’s eyewall are sometimes referred to as a “convective burst,” because of their vivid appearance in satellite data.

Hot towers in Hurricane Bonnie, 1998, as imaged by TRMM. The tallest tower here was 18 km (59,000 feet) high, about twice the cruising altitude of a jetliner (Please note that the relative height of the towers has been exaggerated.). The appearance of hot towers is believed to indicate that the hurricane is about to intensify. (image courtesy of NASA)

Hot towers in Katrina, as imaged by TRMM. (image courtesy of Hal Pierce-SSAI/NASA GSFC)

Hot towers are associated with intensification of hurricanes. The TRMM image shows hot towers in hurricane Bonnie, a Category Three hurricane responsible for about one billion dollars of damage when it struck North Carolina in 1998. An especially tall hot tower appeared in Bonnie just as it grew stronger, presumably because Bonnie’s slow speed at the time enabled it to absorb large quantities of heat from the warm ocean water.

And in Katrina, the TRMM radar observed several hot towers (second image). One of these, near the eye wall, was 16 kilometers (ten miles) high, and shortly after TRMM captured this image, Katrina moved up to become a Category 4 storm.

In a very different approach to investigating hot towers, the Convection and Moisture Experiment (CAMEX), conducted by NASA and NOAA, investigates intensity change by flying aircraft into the hurricane and observing the eyewall. The photos show the two kinds of aircraft that fly CAMEX missions, ER-2, a version of the U-2 spyplane, and the DC-8. These two planes observe hurricanes from much higher altitudes than have any previous aircraft.

In hurricane Bonnie, the ER-2’s radar provided much higher resolution than was possible from satellites. The data showed evidence of hot towers when Bonnie intensified, as did radar images from satellites, and moreover the aircraft-bound radar detected strong sinking air currents in Bonnie’s eye at this time. Sinking air in hurricanes had been observed before, but the ER-2 radar images showed just how dramatic this sinking can be in the neighborhood of an unusually large hot tower. Now meteorologists speculate that the sinking is linked to an increase in the temperature of the hurricane’s core, which can be observed with satellite images.

There are other approaches to studying hurricane intensification. For instance, some oceanographers believe that, beyond the temperature at the sea surface, the vertical thermal structure of the ocean could play a role, since the ocean is considerably churned by the passage of the hurricane. The Gulf Stream, a powerful current that originates in the Gulf of Mexico and flows well out into the Atlantic, can meander in the Gulf and then pinch off to form a ring, with depths of several kilometers and radii of more than one hundred kilometers. These rings could inject large amounts of energy into a developing storm in a very short time.

The DC-8 (top) and the ER-2, the two aircraft flown in the CAMEX project (images courtesy of NASA)