Distinguished Professor University of California, Davis UC Davis
This study aims to better understand the forcing mechanisms of warm season precipitation in the US Northern Rockies, which was recently reported to be a strong driver of wildfire area burned over the region. The logistic regression analysis showed that the occurrence of the wetting rain over the US Northern Rockies can be well explained using the 500hPa-geopotential height (Z500) anomaly over the selected western North American (WNA) region and integrated water vapor transport (IVT) anomaly over the US Northern Rockies (ROC AUC=0.83). Composite analysis of Z500 and IVT fields showed that the average Z500 was found to be 83.8 m higher on non-wetting rainy days than the average Z500 on wetting rainy days over the WNA-region (p < 0.001); the average IVT over the US Northern Rockies was found to be 53 kg m-1 s-1 lower on non-wetting rainy days than the IVT on wetting rainy days (p < 0.001). The event-based analysis using the Weather Research and Forecasting (WRF) model with the latitudinal Atmospheric Boundary Condition Shifting method quantified precipitation responses to increasing Z500 with associated IVT changes over the region. As an average of the three target events, the event cumulative US Northern Rockies-average precipitation decreased by 33.6 mm (SD 2.7 mm) with a Z500 increase of 100 m over the WNA-region. Furthermore, using the decision tree-based machine learning algorithms, Z500 interannual variability was positively correlated, especially with the central and western sectors of the Northern Pacific SST patterns.
