Temperature gradients and inversions in a forested Cascade Range basin: synoptic-to local-scale controls

Cold-air pooling and associated air temperature inversions are important features of mountain landscapes, but incomplete understanding of their controlling factors hinders prediction of how they may mediate potential future climate changes at local scales. We evaluated how topographic and forest canopy effects on insolation and local winds altered the expression of synoptic-scale meteorological forcing on near-surface air temperature inversions and how these effects varied by time of day, season, and spatial scale. Using ~13 years of hourly temperature measurements in forest canopy openings and under the forest canopy at the H.J. Andrews Experimental Forest in the western Cascade Range of Oregon (USA), we calculated air temperature gradients at the basin scale (high vs. low elevation) and at the cross-valley scale for two transects that differed in topography and forest canopy cover. ERA5 and NCEP NCAR R1 reanalysis data were used to evaluate regional-scale conditions. Basin and cross-valley temperature inversions were frequent, particularly in winter and often persisted for several days. Nighttime inversions were more frequent at the cross-valley scale but displayed the same intra-annual pattern at the basin and regional scales, becoming most frequent in summer. Nighttime temperature gradients at basin and cross-valley scales responded similarly to regional-scale controls, particularly free-air temperature gradients, despite differences in topography and forest cover. In contrast, the intra-annual pattern of daytime inversions differed between the basin and cross-valley scales and between the two cross-valley transects, implying that topographic and canopy effects on insolation and local winds were key controls at these scales.