Katrina (August 27, 2005): Changing Gears

GOES-12 Visible imagery of Katrina

GOES-12 Infrared (IR4) imagery of Katrina

GOES-12 Water Vapor (IR3) imagery of Katrina

The mid- to upper-level ridge that had been forcing Katrina to track west-southwest for the past few days began to shift eastward towards Florida on the 27th, allowing the storm to begin a turn to the right. The upper-level anticyclone directly above Katrina remained in place and continued to provide an environment of low wind shear and efficient upper-level outflow. Despite this, the rapid intensification of the day before ended and Katrina's maximum sustained wind speed (MSW) leveled off at 100 kt for most of the day.

Katrina is captured in this true color image from the MODIS unit on the Terra satellite at 11:20 am CDT (1620 UTC) while in the middle of an eyewall replacement cycle. At each cross, the MSW in knots are on the top and the central pressure in millibars is on the bottom. The thick segment of the track line shows the approximate storm's track for 8/27/05. Notice how the center is dominated by a large eye, filled with clouds that also have an "eye" in them.

The pause in intensification was due to an eyewall replacement cycle (ERC). When this process occurs, a concentric ring of convection forms outside of the eyewall. This new "secondary eyewall" robs the original one of much of its warm, moist inflow. Eventually the old eyewall will dissipate completely and the new one will begin to contract and thereby replace the old eyewall. As a result of the need to conserve angular momentum, when the radius of maximum winds suddenly expands out to the secondary eyewall, the storm's maximum wind speed must decrease.

The center of the image above has been enlarged and adjustments have been made to the lighting in order to show more detail. The outer ring is likely the edge of the secondary eyewall, while the clouds inside of it that surround the central dark spot are likely the weakening original eyewall.

In the case of Katrina, the wind speed drop was offset by the overall intensification trend, thus resulting in a stable wind speed for many hours. However, while its maximum wind speed remained the same, the size of the storm increased dramatically. In the 24 hours beginning very late on the 26th, the storm's average tropical storm force wind radius nearly doubled and the average hurricane force wind radius tripled.

These circles depict the average radius of hurricane force winds in Katrina. The colored circles are from the 27th, and the grey circle, from the last advisory on the 26th, is provided for reference.

In this detailed visible loop, the size of the storm is clearly increasing and the transient eye feature Terra captured is briefly visible.

This IR loop (from 8:15 pm on the 26th to 10:45 pm on the 27th) shows Katrina's dramatic expansion,

This water vapor loop (from 8:15 pm on the 26th to 10:45 pm on the 27th) shows Katrina's dramatic expansion, Notice how a "bubble" of moisture keeps the storm separate from the dry air to its' northwest.