Polar lows and their atmospheric environments.

摘要

Conditions for polar low development are not clearly understood. Polar lows were identified using high and low resolution satellite imagery over the Gulf of Alaska region for an eight year period between 1992 and 2000. In addition, NCEP Reanalysis 6-hourly gridded (2.5° × 2.5°) data were utilized for thermodynamic and kinematic analysis of spiraliform polar low case studies between 1997 and 2000. Compositing was also done to create a physical model of the large scale conditions that are needed for polar low genesis and growth (−24 hr, genesis and +24 hr).; The analysis focuses on low and upper level meteorological parameters like specific humidity differences, equivalent potential temperature, and potential vorticity at the time of spiraliform polar low cyclogenesis. The analysis shows that favorable specific humidity difference fields greater than 2.0 g/kg and equivalent potential temperature ridges at both the 925 and 850 hPa levels are important for cyclogenesis. The 850–500 hPa thickness using the threshold height of 3960 m also proved useful in determining spiraliform polar low development. Polar lows favor development when there is an upper level trough and associated cold air advection over the warmer Pacific Ocean. An upper level ridge over the central Pacific Ocean basin helps to increase the winds advecting cold air over the cyclogenesis area.; Finally, polar low frequency and various indices related to atmospheric and oceanographic changes were investigated through statistical analysis to determine linkages between polar lows and atmospheric and oceanographic conditions. Polar low frequency was strongly linked to the Atmospheric Forcing Index (AFI). The AFI is represented by warm sea surface temperatures, an intense Aleutian Low and strong westerly winds which favor polar low genesis. A link between the Pacific-North American Index (PNAI) and polar low frequency was also found. A positive PNAI represents a favorable long wave trough over the region where polar lows form. There was a negative correlation between polar low frequency and positive Arctic Oscillation Index (AOI) values. This suggests when the AO is in a negative phase, Arctic air is able to move over warmer oceanic waters and encourages polar low genesis because of the release of latent and sensible heat that aids in their development. The weakening of the circumpolar vortex also allows more potential vorticity in the upper levels to promote polar low genesis.; The results of this study enhance our ability to understand the dynamics and development of polar lows. This research will provide an avenue for more research on polar lows and the environment in which they form.