It is perhaps no surprise that the 2005 hurricane season, which shattered so many records, concluded the year with an active storm in the Atlantic-- Tropical Storm Zeta. On the 30th of December, 2005, Tropical Storm Zeta, the record 27th named storm of the season, was born in the central Atlantic from a remnant area of low pressure previously associated with an old frontal boundary. These boundaries can induce convection, which in turn can transform an area of associated low pressure from one with midlatitude characteristics into one with tropical charactersitics. This type of tranformation in not uncommon in the later stages of the hurricane season.

Over the next several days, Zeta moved in a general westward direction, initially northwestward then southwestward, across the central Atlantic well away from any land areas. Zeta has remained a tropical storm with only slight fluctuations in intensity.

For the past 8 years, the Tropical Rainfall Measuring Mission satellite (known as TRMM) has used its array of passive and active sensors to estimate rainfall over the global Tropics from space. TRMM has also proven to be a valuable platform for monitoring and studying tropical cyclones (e.g., tropical storms and hurricanes), especially over remote parts of the open ocean such as in the case with Zeta in the central Atlantic.

The first image was taken by TRMM at 8:01 UTC (3:01 am EST) on 3 January 2006 as it flew over Zeta in the central Atlantic. It shows a top-down view of the horizontal rainfall intensity pattern associated with Zeta. Rain rates in the central part of the swath come from the TRMM Precipitation Radar (PR), the only radar that can measure precipitation from space. Rain rates in the outer swath are from the TRMM Microwave Imager (TMI). The rain rates are overlaid on infrared (IR) data from the TRMM Visible Infrared Scanner (VIRS). This image from TRMM shows that Zeta is a small storm with a fairly well-developed circulation as evidenced by the curvature in the rain features (blue and green arcs). However, the storm is very asymmetrical with most of the moderate intensity rain (green areas) northeast of the center. The actual center of circulation is located on the southwest corner of the cloud mass (lower left portion of the white area) and appears to be surrounded by a partial eye (small, tightly curved green arc). At the time of this image, Zeta was a tropical storm with sustained winds estimated at 50 knots (58 mph) by the National Hurricane Center (NHC).

The final set of images were taken at 00:34 UTC on January 4th (7:34 pm EST January 3rd). The top-down view shows a similar overall pattern with the bulk of the rain northeast of the center (large continuous green and blue area) as a large rainband [longer green arc with areas of embedded heavy rain (red areas)] feeds in from the eastern side of the storm. The center itself is still on the southwest corner of the cloud mass (white area) and is now surrounded by a ragged ring of light (blue) to moderate (green) rain. The cloud pattern (white area) shows that the center of the storm is nearly separated from the rainband by an intrusion of cloud free air.

The final image shows a 3D perspective of Zeta looking west constructed from TRMM PR data taken at the same time. The PR can look inside the storm to shows details of the storm's structure. The isosurface corresponds to small precipitation-sized particles. The 3D perspective shows how high up into the atmosphere the particles are. The red tops show areas of deeper convection that are associated with the areas of heavier rain (red areas in the previous image). The center is nearly surrounded by a ring of moderately deep convection (green annulus). A gap separates the center convection from the higher tops associated with the rainband (green ridge with red tops). At the time of these last images, Zeta was still a tropical storm with sustained winds estimated at 55 knots (63 mph) by NHC. The system was moving to the southwest. Zeta is expected to turn northwestward and weaken.

TRMM is a joint mission between NASA and the Japan Aerospace Exploration Agency (JAXA).

Images produced by Hal Pierce (SSAI/NASA GSFC) and caption by Steve Lang (SSAI/NASA GSFC).