TRMM HEAVY RAIN IN HAWAIIAN ISLANDS

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KAUAI INUNDATED WITH PERSISTENT RAINS Persistent rains over the past several weeks lead to the failure of an earthen dam on the Hawaiian Island of Kauai, resulting in 2 fatalities. Six persons are still listed as missing. The rains are a result of a series of upper-level low pressure centers that formed west of Hawaii. These lows draw abundant moisture up from the deep Tropics where it interacts with the topography of the islands. The National Weather Service in Hawaii reported that several locations on the Island of Kauai had rainfall totals in excess of 30 inches for a 3-week period from late February to mid March. During this period Mount Waialeale, one of the wettest if not the wettest spot on Earth with an average annual rainfall of nearly 40 feet, received just over 106 inches of rain.

The Tropical Rainfall Measuring Mission satellite, better known as TRMM, was launched into service in November of 1997 to measure rainfall over the Tropics. The TRMM Precipitation Radar or PR is part of the suite of instruments onboard TRMM. It is the only space-borne precipitation radar of its kind and provides a unique vantage point by viewing rain systems from space. This image shows a snap shot of one of the rain systems that have effected the Hawaiian Islands of late, including Kauai. The isosurface provides a 3D perspective of the precipitation-sized particles associated with the system. The isosurface represents a radar echo intensity of 15 dBZ, which is equivalent to light precipitation, in order to show the maximum extent of the rain (analogous to the outer layer of an onion). A cut away view through the isosurface shows the rain intensity within the system. Underneath is the surface rain rate and cloud pattern. Rain rates in the outer swath are from the TRMM Microwave Imager (TMI), while those in the center are from the PR. The rain rates are overlaid on infrared (IR) data from the TRMM Visible Infrared Scanner (VIRS). TRMM shows that although the storm echoes are rather shallow, generally less than 8km as shown by the vertical scale, they can still be quite intense with reflectivities of up to 50 dBZ (dark red areas in the cut away view), which is equivalent to heavy rain. It means that the bulk of the rain is due to warm rain processes and not as a result of melting ice. Another key to the flooding is the orientation and movement of the radar echoes. Although this particular rain band is located over the channel between the islands of Kauai on the left and Oahu on the right, it shows that these rain bands can be rather long. As the rain showers are drawn north-eastward (along the axis of the rain band) over a particular location, shower after shower passes over the same spot and can lead to large rainfall totals, an effect known as "training". This effect can be especially pronounced when amplified by the topography of an island.

TRMM is a joint mission between NASA and the Japanese space agency JAXA.

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