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https://doi.org/10.5194/egusphere-2025-2352
https://doi.org/10.5194/egusphere-2025-2352
03 Jun 2025
 | 03 Jun 2025
Status: this preprint is open for discussion and under review for Geoscientific Model Development (GMD).

Examining Spin-Up Behaviour within WRF Dynamical Downscaling Applications  

Megan S. Mallard, Tanya Spero, Jared Bowden, Jeff Willison, Christopher G. Nolte, and Anna M. Jalowska

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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Megan S. Mallard, Tanya Spero, Jared Bowden, Jeff Willison, Christopher G. Nolte, and Anna M. Jalowska

Status: open (until 29 Jul 2025)

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Megan S. Mallard, Tanya Spero, Jared Bowden, Jeff Willison, Christopher G. Nolte, and Anna M. Jalowska
Megan S. Mallard, Tanya Spero, Jared Bowden, Jeff Willison, Christopher G. Nolte, and Anna M. Jalowska

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Short summary
“Spin-up” is time needed for a model’s result to become effectively free of influence from initial conditions, and it is usually excluded from analysis. Here, spin-up is examined by comparing one decadal simulation to another initialized 20 years prior, in order to determine when their solutions converge. Differences lessen over the first fall and winter, but re-emerge over the following spring and summer, suggesting that at least 1 annual cycle is needed to spin up regional climate simulations.
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