We thank Gerard Roe for helpful discussions over the course of this project, Beth Tully for her assistance with Fig. 18, and Justin Minder for a very thorough and insightful review. We also acknowledge high-performance computing support from Yellowstone (ark:/85065/d7wd3xhc) provided by NCAR’s Computational and Information Systems Laboratory, sponsored by the National Science Foundation. This work was supported by National Science Foundation Grants AGS-1138977 and AGS-1545927.
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Note that the rain-shadow index according to which these storms were ranked in the top 10 strongest or weakest rain-shadow events was computed from normalized windward and leeward precipitation differences, not from this simple ratio.
This average includes all grid points within the domain of Fig. 3 where the height of the topography is less than 1 km.
The moisture flux includes water in vapor, liquid, and ice phases. Since only fluxes through the western and southern boundaries are considered, the drying ratio does not account for the flux of moisture into the domain from low-level easterly flow, which occurs during WRS storms (Figs. 3a–c). However, the moisture flux associated with such low-level easterly flow is very modest, amounting to no more than 4% of the flux through the western and southern boundaries in each storm. This is likely because the low-level easterly flow tends to be dry and shallow (see section 4 for further evidence of this). As a result, over the eastern boundary as a whole, the moisture flux is westerly at all times during all storms, explaining our decision to calculate the drying ratio based only on the moisture fluxes at the western and southern boundaries.
The cases without a stagnant layer in the lee also show some evolution in rain-shadow strength, presumably resulting from drift in the upstream conditions. However, the magnitude of the change is modest, decreasing from 2.2 at hour 3 to 1.8 by hour 10—similar to the equilibrium value reached after the stagnant air is eroded in Fig. 11.
This scenario appears to be similar to the reverse-rain-shadow example discussed by Mass et al. (2015, their Fig. 9), which likewise exhibits very light precipitation over the eastern slope (~0.1 inch in a 3-h period). This is consistent with our assessment in section 2 that easterly upslope flow has little direct impact on a storm’s overall precipitation pattern.