The 25-27 May 2005 Mount Logan storm. Part I: Observations and synoptic overview

摘要

In late May 2005, three climbers were immobilized at 5400 m on Mount Logan, Canada's highest mountain, by the high-impact weather associated with an extratropical cyclone over the Gulf of Alaska. Rescue operations were hindered by the high winds, cold temperatures, and heavy snowfall associated with the storm. Ultimately, the climbers were rescued after the weather cleared. Just prior to the storm, two automated weather stations had been deployed on the mountain as part of a research program aimed at interpreting the climate signal contained in summit ice cores. These data provide a unique and hitherto unobtainable record of the high-elevation meteorological conditions associated with an intense extratropical cyclone. In this paper, data from these weather stations along with surface and sounding data from the nearby town of Yakutat, Alaska, satellite imagery, and the NCEP reanalysis are used to characterize the synoptic-scale conditions associated with this storm. Particular emphasis is placed on the water vapor transport associated with this storm. The authors show that during this event, subtropical moisture was transported northward toward the Mount Logan region. The magnitude of this transport into the Gulf of Alaska was exceeded only 1% of the time during the months of May and June over the period 1948-2005. As a result, the magnitude of the precipitable water field in the Gulf of Alaska region attained values usually found in the Tropics. An atmospheric moisture budget analysis indicates that most of the moisture advected into the Mount Logan region was preexisting water vapor already in the subtropical atmosphere and was not water vapor evaporated from the surface during the evolution of the storm. Implications of this moisture source for understanding of the water isotopic climate signal in the Mount Logan ice cores will be discussed.