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On the origin of atmospheric frontal lines off the east coast of Taiwan observed on space-borne synthetic aperture radar images
Werner Alpers(1), Jen-Ping Chen(2), Chia-Jung Pi(2) and I-I Lin(2) (1) University of Hamburg, Bundesstrasse 53, 20146, Germany
(2) National Taiwan University, No. 1, Roosevelt Road, Section 4, 10617, Other
Abstract
Frontal lines having offshore distances typically between 40 and 80 km are often visible on synthetic aperture radar (SAR) images acquired over the east coast of Taiwan by the European Remote Sensing satellites ERS-1, ERS-2, and Envisat. In a previous paper (Alpers et al. 2007) we have shown that they are of atmospheric and not of oceanic origin. However, in that paper we did not give a definite answer to the question which physical mechanism causes them. In this paper we present simulations carried out with the mesoscale meteorological model MM5 which shows that the frontal lines are associated with a quasi-stationary low-level convergence zone generated by the dynamic interaction of onshore airflow of the synoptic-scale wind with the coastal mountain range of the island of Taiwan. Reversed air flow collides with the onshore flowing air leading to an uplift of air, which often is accompanied by the formation of bands of increased cloud density and of rain bands. The physical mechanism causing the generation of the frontal lines is similar to the one responsible for the formation of cloud bands off the Island of Hawaii as described by Smolarkiewicz et al. (1988). We present four SAR images, one acquired by ERS-2 and three by Envisat, showing frontal lines at the east coast of Taiwan caused by this generation mechanism. For these events the re-circulation pattern, as well as the frontal (or convective) lines that observed, were reproduced quite well with the meteorological model. Thus it is argued that the observed frontal lines are not seaward boundaries of (classical) barrier jets or of katabatic wind fields, which have characteristic quite different from the flow patterns around the east coast of Taiwan as indicated by the SAR images.
Workshop presentation
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