NASA sees Hurricane Isaac make double landfall in Louisiana

On Aug. 29, 2012 at 1 p.m. EDT, Isaac was still a hurricane with maximum sustained winds near 75 mph (120 kmh). Isaac was located about 10 miles northwest (15 km) of Houma, Louisiana and moving slowly. It is moving to the northwest near 6 mph (9 km). Isaac continued bringing heavy rainfall to southeastern Louisiana and southern Mississippi. The threat for dangerous coastal storm surge and inland flooding are expected to continue overnight.

The Tropical Rainfall Measuring Mission (TRMM) satellite flew over Hurricane Isaac twice on the night that Isaac made landfall in Louisiana and headed for New Orleans. TRMM is a joint mission managed by both NASA and JAXA, the Japanese Space Agency.

In the first of the two overflights, the TRMM radar saw two hot towers in the eyewall of Hurricane Isaac just hours before landfall. While hot towers were shooting up in the eyewall over the ocean, Isaac's inner rainband was already lashing Louisiana with heavy rain. Hot towers are common in intensifying tropical cyclones are are a sign that energy is being pumped into the hurricane from the ocean's surface. A "hot tower" is a tall cumulonimbus cloud that reaches at least to the top of the troposphere, the lowest layer of the atmosphere. It extends approximately nine miles (14.5 km) high in the tropics.

Two images were created by Owen Kelley, of NASA's Goddard Space Flight Center, Greenbelt, Md. The background of the first image showed TRMM infrared observations that give a sense of the height of the cloud cover the hides the heavy precipitation inside of of the hurricane. The blue-gray 3D volume contains the light precipitation inside the hurricane, using a 20 dBZ radar-reflectivity threshold.

In the image, an insert reveals details at the center of the hurricane. Two hot towers are indicated by the yellow and orange colors. They are locations where strong updrafts are lifting frozen precipitation above a 14.5 km (9.1 mile) threshold. Water that condenses in updrafts will soon freeze if updrafts lift it above the zero-degree isotherm near 5 kilometers (3.1 miles) altitude. The freezing releases another boost of latent heat, the fuel of hurricanes, following the initial release of latent heat when the water vapor condenses into liquid.

The TRMM radar happened to overfly Hurricane Isaac again just five hours later, shortly after the eyewall made landfall. Robbed of its oceanic source of energy, the eyewall hot towers are gone in this later overflight. Instead of reaching 14.5 km (9.1 mile) altitude, the eyewall merely reaches a 10 km (6.2 mile) altitude, which is indicated by the light green shading at the top of the blue-green volume of light precipitation.

Unfortunately for New Orleans and surrounding areas, TRMM sees that Hurricane Isaac's eyewall was remarkably well organized at that time, despite having made landfall. The insert shows a ring of very intense radar echos in red, echos that exceed 40 dBZ radar reflectivity. The northwest quadrant of his ring of heavy precipitation is almost on top of New Orleans at the time of observation.