The “Striated Delta” Cloud System–A Satellite Imagery Precursor to Major Cyclogenesis in the Eastern Australian-Western Tasman Sea Region

Abstract

A distinctive upper-tropospheric cloud pattern, frequently a precursor to major extratropical cyclogenesis within the eastern Australian–western Tasman Sea region, has been identified from an examination of Geostationary Meteorological Satellite imagery. This cloud feature is usually triangular- (or delta-) shaped, approximately symmetrical, composed of cirrus level cloud tops, and associated with transverse banding. Thirty-seven of these cloud systems, referred to as “striated deltas” (SDs), were detected within the study region between 1980 and 1991. Approximately 75% of these were associated with the development of storms of at least gale force intensity within 24 h of formation of the SD. Most of these intense developments were characterized by a 12-h period of explosive deepening. Three types of SD development are observed. The cloud system usually forms as a discrete feature; this process can be either rapid or gradual. The SD can also form as a subcomponent of a major meridional cloud band. Striated deltas associated with intense cyclogenesis are normally generated east of a high-amplitude 1000-500-hPa thickness trough and within diffluent north to northwesterly flow in the exit region of a strong thickness jet streak. The relationship between the SD and the surface pattern differs substantially between oceanic and continental event. Case studies in which major surface low deepening occurred are presented to demonstrate these variations. Events in which intense cyclogenesis does not occur are also examined. The synoptic and geographic environment in which such cloud systems form are found to vary from that observed in intense developments. A qualitative investigation into the source mechanism of the striations suggests that their most probable cause is gravity wave activity due to the geostrophic adjustment process.