the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
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.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-2352', Anonymous Referee #1, 19 Jul 2025
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RC2: 'Comment on egusphere-2025-2352', Stefan Rahimi-Esfarjani, 27 Oct 2025
RC comment: GMD-2025-2352, Examining Spin-Up Behaviour within WRF Dynamical Downscaling Application
Stefan Rahimi
Synopsis: The authors compare two WRF simulations, one with 20 years and another with 3 months of spin-up to assess the time scales over which two dynamically downscaled ESMs converge in terms of their soil moisture, temperature, 2-m temperature, and sensible and latent heat fluxes. They also looked at this convergence as a function of NCA region, adding another line of evidence that spin-up time is a strong function of region. While I believe that some within the regional downscaling community will not see this finding as surprising or novel, I believe that it is critical to continue revisiting questions of spin-up length, as many in the dynamical downscaling community (including me) continue to attempt to parallelize experiments across regions with smaller and smaller amounts of spin-up time to finish simulations faster. Continuous testing of spin-up is further warranted given that newer land-surface models (LSMs) continue to roll out more sophisticated and complex solvers, which may behave differently from previous generations of LSMs in terms of spin-up, potentially necessitating continuous updating of community spin-up standards. I really like the idea of giving variable-specific spin-up times in the final table of the manuscript, as it is something that folks downscaling on sub-regional scales can directly point to as a starting point in their spin-up times, and it provides a comparative benchmark to those who will contribute future studies on spin-up time length. I recommend publication of this manuscript following revisions below.
- Question: Why was 1994 chosen instead of 1984 or 2004? Was WY 1995 in the ESM/WRF an average precipitation year? Wet? Dry?
- Line 110: What is the intensity of the spectral nudging employed here, and which relaxation and wavenumber values were used?
- 3: Maybe I missed this, but is the warmness in I94 across south-central Canada explained (Figs. 2, 3)? The following summer’s relative coldness in the 2-m temperature and links to evaporative cooling are well-explained, but I couldn’t seem to find this same physical argument for the preceding winter’s warmness? Is this perhaps a snow difference between the two simulations?
- Lines 249-270: It would be helpful in this table to provide each regions annual precipitation amount given the tie ins with the Cosgrove et al. (2003) finding. WRF has been updated a lot over the past several decades….do the Cosgrove et al. findings still hold up ‘e.g., easier to spin up wet vs. dry soils?’
- 2-6: Would be nice to see the same figures but for precipitation
Citation: https://doi.org/10.5194/egusphere-2025-2352-RC2
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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