Warm-season diurnal circulations and heat extremes over the northwest U.S.

Abstract

Summer synoptic circulations over the northwest U.S., and their interactions with regional terrain, land/water contrasts, and surface heating, give rise to a variety of fascinating meteorological phenomena, many of which have yet to be explored. Furthermore, it is largely unknown how projected future warming associated with increased greenhouse gases will modify these important features. The work herein seeks to ameliorate this with a comprehensive examination of two important aspects of northwest U.S. summer weather and climate: diurnal circulations and changes to the conditions associated with extreme temperatures under anthropogenic global warming. To simulate regional diurnal circulations, GFS model output was obtained for July and August 2009-2011. These data were categorized into hour of the day, composited, and the resulting files were used to initialize and provide boundary conditions to a WRF (version 3.5) model run. It was shown that, when compared to observations, this WRF run sufficiently simulates average diurnal variability. Using this simulation, the diurnal circulations of the region were described, including several important wind features within the Strait of Juan de Fuca, the Snoqualmie Pass, and the Columbia River Gorge. Also, regional nocturnal low-level wind maxima are described, including one over the northern Willamette valley and another over the high plateau of eastern Oregon. Recent work by the authors has elucidated the physical mechanisms that drive heat extremes over the northwest U.S., including the necessity of a ridge aloft, with associated subsidence and advection warming. Also, easterly flow is crucial for keeping the marine air at bay, and producing downslope flow and adiabatic warming on the western slopes of regional north-south terrain barriers. Given the rising temperatures projected under anthropogenic global warming, how are these conditions, and associated low-level temperature distributions, projected to change? As a first step in addressing this question, changes to the mean and variability of large-scale summer synoptic disturbances and circulations were explored using CMIP5 global climate models. Weaker meridional gradients in 500-hPa geopotential height were found, as well as weaker zonal gradients within troughs and ridges. The variability of high-pass filtered 500-hPa geopotential height, relative vorticity, and omega were all found to be weaker over the mid-latitudes, with close to unanimous model agreement. Offshore/onshore flow was also found to have weaker variability. It was found that regional heat extremes generally warm as fast as the mean, despite weaker variability in large-scale conditions. Ensemble-mean projections suggest that heat extremes for most other areas over North America warm faster than the mean, though model agreement is weak. High-pass filtered 2-m temperature is projected to weaken over the northwest U.S. This is likely a result of weaker synoptic variability and associated offshore/onshore flow. Finally, it was explored how regional temperature distributions affect future changes in threshold exceedances, using dynamically downscaled WRF model output. It was shown that the wide variety of temperature distribution shapes over the region give rise to substantially different values of future threshold exceedances under anthropogenic global warming. Because of positively skewed temperature distributions, western Washington and Oregon experience far fewer exceedances of days above the 97th percentile, when compared to surrounding regions.