Comparison of Model-Based Precipitation Maximization Methods: Moisture Optimization Method, Storm Transposition Method, and Their Combination

In recent years, there has been an increase in interest in estimating probable maximum precipitation (PMP) by numerical weather model (NWM)-based precipitation maximization methods, mainly, the moisture optimization method, storm transposition method, and their combination. This study addresses a quantitative comparison of the effectiveness of each precipitation maximization method regarding the increase in precipitation depths and a discussion on their functions for maximizing precipitation depths. To achieve such a comparison, this study conducted a numerical experiment using the moisture optimization method, which proportionally increases relative humidity using the integrated water vapor transport criterion, and the storm transposition method, which geospatially shifts atmospheric boundary conditions in the weather research and forecasting (WRF) model to maximize atmospheric river (AR)-induced precipitation depths over the Columbia River Basin. Our findings suggest that the storm transposition method should also be considered an essential method in the NWM-based precipitation maximization rather than relying solely on optimizing atmospheric moisture. This study also found that even for ARs with historically small amounts of precipitation over a specified basin, it is possible to increase precipitation depths significantly over the basin by using the storm transposition method. This finding implies that, for maximizing precipitation using the storm transposition method, it is important to select storms that historically did not directly hit the basin but were located around the basin as well, unlike the conventional approach of selecting storms solely based on precipitation depth over the basin. This finding also suggests that the storm transposition method is essential, particularly for estimating long-duration PMP that may cover a whole season, given the contribution of precipitation increase in each single storm event to the snowpack accumulation and reservoir storage for a long duration. Lastly, this study has shown that including the transposition method together with the moisture optimization method can increase precipitation amounts and give a higher upper bound for NWM-based precipitation maximization, especially in AR-dominated regions.