Rain cells in the Gulf of Thailand

The footprints of a line of thunderstorms extend from the inlet of the Mae Nam Chao Praya River into the Gulf of Thailand. ERS-1 acquired the image on 5 June 1992 at 15:50 GMT (22:50 local time). A NOAA/AVHRR image taken on the same day at 07:56 GMT shows a well organised line of cumulonimbus and easterly upperwinds. It can be assumed that this is the line of atmospheric instability that causes the feature on the sea surface. There seems to be a wind downdraft movement toward the south with a continuity of the development of new vertical clouds. The wind-fronts of each cell (probably 5) are clearly visible ( blue arrows ) in the image . The dark areas ( yellow arrows ) inside the footprints are splash down zones of heavy rain, smoothening the wind-roughened sea-surface. But there are also very bright innerward features inside the large footprints, and also further ones to the east, eg. close to the northernmost of the small islands (Island of Sichang). No explanation has been found for this phenomena.

Many ships (bright oblong points) with their wakes, heading south, are visible. Near the Island of Sichang a large number of ships lie at anchor. In this area oil pollution, irregular long black features, is frequent. Just east of the thunderstorm footprints a 30 km long oil discharge is also present. It was most certainly caused by a ship cleaning its tanks.

The coast is marked by dark but clearly separated fish ponds. The inlet of the Menam River also shows rough water, as it is hit by gusts from the thunderstorm. Up-stream, however, the water is calmer and ships are again evident.
The wind regime is very variable in the scene, apart from the wind-downdraft causing the large rounded shapes, the wind direction is likely to be southerly in the right lower part of the image but westerly along the coast west of the Menam. The texture within the wind-streaks suggests also an unstable air-layer, typical of a tropical rainy season.

From the ERS-1 SAR data absolute backscatter (sigma zero) values can be derived of the different zones: brighter areas in the footprints: -5.2 dB (equivalent to wind of 5-7 m/sec), dark areas in the footprints - 14.8 dB (wave damping due to rain), very bright inner features -3.4 dB (no explanation), dark areas outside the footprint -16.7 dB (wind speed less than 3 m/sec), in oil-slicks -20.2 dB (wave damping due to oil film).

ERS Data Utilization Section, ESA/ESRIN

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry