Examining Spin-Up Behaviour within WRF Dynamical Downscaling Applications
Abstract. Spin-up is the period after initialization when a model transitions away from its dependence on initial conditions toward a dynamic equilibrium between driving boundary conditions and its own internal dynamics. Regional climate models (RCMs) are often used to simulate conditions over several decades to inform local adaptation and resilience activities. The spin-up period represents added cost to already resource-intensive simulations, and it is often infeasible to use a spin-up period that produces complete model equilibrium. Therefore, a pragmatic compromise is desired to minimize the effects of spin-up. Here, two overlapping dynamically downscaled simulations using the Weather Research and Forecasting model over the contiguous U.S. (31-year and 11-year integrations) are used to explore convergence associated with model spin up. The shorter simulation is initialized 20 years after initialization of the longer (reference) run, and the runs are analysed over the period covered by both simulations, giving the reference simulation a 20-year period to attain spin-up prior to comparison. After initialization, the shorter run features cooler surface and near-surface temperatures and greater soil moisture compared to the reference simulation. Differences between the runs decrease in magnitude over the first 3 months as autumn transitions to winter; however, these differences re-emerge and reach a secondary peak during the proceeding spring and summer. During this warm season, evaporation and accompanying evaporative cooling increase and temperature differences between the simulations re-emerge. These results support using at least one year of spin-up time in RCM applications to account for the seasonality of spin-up behaviour.
Mallard et al. tackle a key practical issue in regional climate modeling: determining how long a model should be run to remove dependence on initial conditions. This is especially relevant for multi-decade simulations. The work uses a uniquely long reference run (20-year lead) to examine spin-up, extending beyond earlier studies that only looked at shorter lead times (weeks to 2 years). The results (e.g., the need for around a year or more for spin-up, particularly for soil moisture) are valuable for guiding modeling protocols and could influence future downscaling experiment design. The manuscript is well-written and logically organized. Overall, it is a very well-executed study, and I have only one minor point that I’d like addressed before publication.
That is, that the abstract and conclusion note that the results support using at least one year of spin-up. This is accurate but overemphasizes the one-year period. Since this was insufficient in some cases, I suggest clearly quantifying it in some way, in the abstract in particular - for example, acknowledging that some regions may require more.
Also, at line 34, “Georgi” should be “Giorgi”, but this barely warrants comment