Minimize Land-Sea Breeze - East coast of Taiwan

Back to Land-Sea Breeze


East coast of Taiwan East coast of Taiwan

Latitude: 23° 30' N - Longitude: 121° 48' E

SAR strip of five ERS-1 SAR frames acquired over the coastal waters east of Taiwan on 23 April 1995 at 2:25 UTC, i.e., at 11:25 local time. It shows in the central section adjacent to the coast a low-wind speed area which, to the east, is confined by a frontal line, and it shows in the northern and southern sections features which result from the interaction of the Kuroshio current with shallow underwater topography (in the northern section) and with islands (in the southern section). The low wind speed area (dark area in the image) is an area of descending air and the frontal line is an atmospheric front separating the area of descending air from the ambient wind field. They have their origin in the following meteorological processes: During a cloud-free day the coastal land areas are heated up by solar radiation where the air is lifted up. As a consequence, air has to flow in from the ocean which gives rise to sea breeze and to a atmospheric circulation pattern in which the previously uplifted air over land descends over the ocean. The area of descending air typically extends 20 to 50 km from the coast line. The boundary between the area of descending air and the ambient wind field is a zone of convergence where often clouds (cumulus congestus or cumulus nimbus) and rain cells develop. We attribute the fact that the area of descending air (sea breeze area) appears in some sections of the SAR image dark to low wind speed which is below threshold for ripple wave (Bragg wave) generation (2-3 m/s). According to the weather map the wind was blowing with 3-4 m/s from south.
Taiwan Taiwan

Latitude: 23° 30' N - Longitude: 120° 00' E

NOAA-9 AVHRR infrared image (channel 4) of the waters surrounding Taiwan, which was acquired on 24 April 1994 at 1:13 UTC, i.e., 23 hours and 8 minutes later than the ERS SAR image, but approximately at the same time of the day (1 hour and 12 minutes earlier). The weather conditions on 24 April and 23 April were quite similar. Visible is a cloud band which is located approximately at the same position as the front visible on the ERS-1 SAR image. This is a clear indication that the front visible on the ERS-1 SAR image is of atmospheric origin. Note that the cloud band follows closely the coastline which is a clear indication that it has its origin in an interaction of the atmosphere with the island. On both days the sky was almost cloud-free such that the low-land coastal areas could be heated up by solar radiation. Evidence of this is provided also by this infrared image where dark areas represent warm areas.
East coast of Taiwan East coast of Taiwan

Latitude: 22° 40' N - Longitude: 121° 39' E

SAR strip of five ERS-2 SAR frames acquired over the coastal waters east of Taiwan on 18 January 1999 at 2:26 UTC, i.e., at 11:26 local time. It shows adjacent to the coast a broad band of distinct texture, which is approximately 50 km wide. We interpret this band as the sea area, where the air associated with the sea breeze circulation descends to the sea surface. Since this image was acquired during winter, the sea surface temperature was probably higher than the air temperature which gives rise to an unstable air-sea interface and thus favours the generation of turbulence. The inhomogeneous texture of the band is very likely caused by turbulence. The atmospheric front separating the area of descending air from the ambient wind field appears as a bright band on the SAR image with interdispersed black patches in the northern section. The bright band results from strong convergence in the frontal zone which gives rise to increased sea surface roughness. We interpret the bright band in the northern section with interdispersed black patches as sea surface manifestation of he atmospheric front in which rain cells are embedded. According to the weather map the wind was blowing with 3-5 m/s from SE. This wind direction is consistent with the direction of the wind streaks visible on the ERS-1 SAR image.
# Orbit Frame(s) Satellite Date Time Location
1 19716 3105-3123-3141-3159 ERS-1 23-APR-1995 02:25
2 53413 - NOAA-9 24-Apr-1995 01:33
3 19582 3105-3123-3141-
3159-3177-3195
ERS-2 18-Jan-1999 02:26

If you have any comments on these images please write an e-mail to alpers@ifm.uni-hamburg.de.