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Andaman Sea Andaman Sea

Latitude: 8° 48' N - Longitude: 94° 56' E

ERS -1 SAR image acquired over the Andaman Sea west of the Nicobar islands during calm wind conditions. The circular bright patterns with a dark hole in the centre are sea surface manifestations of tropical rain cells. The two arrows inserted into the figure point (1) to the gust front of the rain cell and (2) to the area where the Bragg waves are strongly damped by the turbulence generated by rain drops impinging onto the sea surface. Also visible are sea surface manifestations of packets of internal solitary waves.
Gulf of Thailand Gulf of Thailand

Latitude: 9° 46' N - Longitude: 100° 38' E

Cluster of rain cells over the Gulf of Thailand. The inserted arrows point to the gust fronts of the rain cells and to the areas where the Bragg waves are strongly damped by the turbulence generated by heavy rainfall, respectively.
Gulf of Thailand Gulf of Thailand

Latitude: 10° 39' N - Longitude: 100° 49' E

A string of rain cells with decreasing diameters (marked "1" to "6"). The 6 rain cells are very likely at different stages of their life cycles. The more time has elapsed since the downdraft has first reached the sea surface, the larger is the diameter of the gust pattern at the sea surface. Thus the rain cell 1 should be the oldest, and the rain cell 6 the youngest cell. (See figure below).
Gulf of Thailand - Plan view of surface weather Gulf of Thailand - Plan view of surface weather

Plan view of surface weather, showing locations of the forward-flank and rear-flank downdrafts. The solid line outlines the rainy area usually mapped by radar. Note the hooklike feature. Downdraft and updraft regions are delineated by coarse and fine stippling, respectively. T designates the most likely location where tornados form. [Adapted from Lemom and Doswell (1979), courtesy of R.Davies-Jones.]
South China Sea South China Sea

Latitude: 21° 19' N - Longitude: 116° 21' E

Comparison of an ERS-2 SAR image acquired over the South China Sea showing radar signatures of rain cells with two weather radar images acquired 35 minutes before and 12 minutes after the ERS-2 SAR data acquisition. The weather radar is a C-band radar and is located at Dongsha Island (20° 42'N, 116° 44'E), approximately 100 km away from the rain cells. The radar reflectivity measured by the weather radar is converted into rain rate and plotted in the two figures on the right. Comparison of the ERS-2 SAR and the weather radar images clearly show that the circular patterns on the SAR image are caused by rain cells. From the displacement of the radar signatures in the two weather radar images we conclude that the rain cells moved eastward with a speed of about 5 km/h (reproduced from Melsheimer et al., 2000).
Sulu_Sea Sulu_Sea

Latitude: 10° 08' N - Longitude: 119° 10' E

ERS-2 SAR image acquired over the Sulu Sea near Palawan Island (Philippines). Several small-scale rain cells located at or near an atmospheric front are visible.
Sumatra Sumatra

Latitude: 6° 23' S - Longitude: 104° 30' E

ERS-1 SAR image acquired over the Indian Ocean near the passage separating the Indonesian islands Sumatra and Java (Selat Sunda). A cluster of rain cells forming a common gust front is visible.
South China Sea South China Sea

Latitude: 20° 26' N - Longitude: 117° 35' E

Radar signatures of rain cells of different sizes are visible on this ERS-1 SAR image of the South China Sea. The long line extending almost across the whole image from top to bottom is the radar signature of an internal solitary wave propagating eastward.
# Orbit Frame(s) Satellite Date Time Location
1 24684 3429 ERS-1 03-Apr-1996 12:34
2 14408 3411 ERS-1 18-Apr-1994 03:42
3 14408 3393 ERS-1 18-Apr-1994 03:42
4 15975 3159-3177-3195 ERS-2 11-May-1998 02:46
5 24320 3393-3411 ERS-2 15-Dec-1999 02:26
6 9424 3735 ERS-1 5-May-1993 03:17
7 21076 3177-3195-3213 ERS-1 27-Jul-1995 02:40

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Radar signatures of rain in SAR images of the sea surface result from several different physical processes. The most important processes contributing to these radar signatures are: (1) the backscattering of the microwaves at the short surface waves ("Bragg waves") whose amplitudes are modified by rain drops impinging on the sea surface, (2) the roughening of the sea surface by the wind gusts associated with rain cells, and (3) attenuation and scattering of the microwaves by the rain drops in the atmosphere.

Rain Drops impinging on the sea surface generate ring waves which enhance the sea surface roughness [Moore et al., 1979; Bliven et al., 1997; Craeye et al.1997], but they also generate turbulence in the upper water layer which attenuates the short surface waves [Nystuen, 1990; Tsimplis, 1992]. Analyses of multifrequency SIR-C/X-SAR data acquired over tropical and subtropical ocean areas [Melsheimer et al., 1998a] and laboratory measurements at a wind wave tank have shown that the modification of the sea surface roughness by impinging rain drops depends strongly on the wavelength of the water waves: The net effect of the impinging rain drops on the sea surface is a decrease of the amplitude of those water waves which have wavelengths above 10 cm and a decrease of the amplitude of those water waves which have wavelengths below 5 cm. Unfortunately, the critical wavelength at which the increase of the wave amplitude turns into a decrease is not well defined. It depends on the rain rate, the drop size distribution, the wind speed, and the temporal evolution of the rain event. At the initial stage of the rain event, the turbulence in the upper water layer is not fully developed and thus its damping effect on the water waves is small. On the other hand, after it has stopped raining, the turbulence is not decaying immediately (the life time is of the order of a minute) and it keeps damping the waves even after the rain event has ended. The Bragg wavelength over the ERS SAR lies between 8.2 cm and 6.5 cm. Unfortunately, these wavelengths lie in the transition region where, depending on the rain rate, the drop size distribution, the wind speed, and the time history of the rain event, the impinging rain drops can give rise to either an increase or a decrease of the amplitude of the Bragg waves and thus to an increase or a decrease of the NRCS.

In addition to the modification of the sea surface roughness by the impact of rain drops, the sea surface roughness is also affected by the airflow associated with the rain event. Precipitation from a rain cell usually produces a downward airflow (downdraft) by entrainment and by evaporative cooling under the cloud (see, e.g., Cotton and Anthes, 1989). When the downdraft reaches the sea surface, it spreads radially outward as a strong local surface wind which increases the sea surface roughness. This is shown schematically in the figure below. The outer edge of this airflow is called a gust front. If the ambient wind field is weak and does not disturb this airflow pattern, the radially spreading downdraft is visible on SAR images of the sea surface as a nearly circular bright pattern with a sharp edge [Atlas, 1994a; b]. Such a pattern is usually less bright in the center, where the downdraft reaches the ground and horizontal wind speeds are lowest. The lower the ambient wind speed, the higher is the contrast between such a bright pattern and the surroundings. Therefore, radar signatures of rain cells are often more pronounced over tropical oceans where low wind speeds prevail. When a strong ambient wind field is present, then the radially symmetric airflow pattern is distorted and the resulting radar signature is likewise distorted and shows bright as well as dark areas.

Fig.: Schematic sketch of the downdraft of a rain cell, spreading over the sea surface and causing roughening of the sea surface; (adapted from Atlas, 1994b).


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