Extratropical Transition of Hurricane Michael: An Aircraft Investigation

Abstract

In order to better understand the behavior and impacts of tropical cyclones undergoing extratropical transition (ET), the Meteorological Service of Canada (MSC) conducted a test flight into Hurricane Michael. Between 16 and 19 October 2000 the transition of Hurricane Michael from a hurricane to an intense extratropical storm was investigated using a Canadian research aircraft instrumented for storm research. This paper presents the various data collected from the flight with a detailed description of the storm structure at the time when Michael was in the midst of ET. Hurricane Michael was moving rapidly to the northeast, approximately 300 km southeast of Nova Scotia, Canada, during the time of the aircraft mission. A period of rapid intensification had also occurred during this time as the system moved north of the warm Gulf Stream waters and merged with a baroclinic low pressure system moving offshore of Nova Scotia. Consequently, the hurricane was sampled near the period of its lowest surface pressure and maximum surface winds. It is estimated that the aircraft passed approximately 10 km south of the estimated 42.7°N, 59.7°W position of the surface low pressure center at about 1645 UTC 19 October. Sixteen dropsondes were deployed in a single traverse from northwest to east of the storm center, and then back westbound south of the center. Winds were found to be highest on the southeast side of the hurricane where the storm movement adds to the hurricane rotational flow. A southwesterly jet with winds exceeding 70 m s?1 was observed between 500 and 2000 m approximately 85 km southeast of the center. This low-level jet was much deeper than the usual low-level maximum winds found in hurricanes. Michael was observed to have an elevated warm core similar to purely tropical systems, but low-altitude humidity appeared to be eroded by entrainment of dry midlatitude air surrounding the storm, which is typically observed during the ET process. A cloud-profiling 35-GHz radar provided data on the distribution of precipitation across the system, and cloud microphysical probes measured cloud water contents, particle phases, and spectra. Although a wide variety of liquid, mixed phase, and deep glaciated clouds were observed, the glaciated cloud encountered on the northwest side of the center, associated with the most significant precipitation area, was relatively stratiform in nature, with a broad area of high ice water content reaching 1.5 g m?3, and very high concentrations of small ice particles.