Spin-up time and internal variability analysis for overlapping time slices in a regional climate model

Abstract

Long-term regional climate simulations are computationally very 8 costly. One way to improve their computational efficiency is to split them into 9 overlapping time slices, which can then be run in parallel. Although this proce10 dure reduces the cost, sufficient spin-up must be left at the start of each slice. 11 In any case, discontinuities will occur due to internal variability where two dif12 ferent slices join. In this study, we explore the relative role of spin-up time and 13 internal variability in the discontinuities of overlapping time slice simulations 14 and their effect on the simulated climate. This analysis has implications also 15 for non-overlapping time slices, commonly used in very high resolution climate 16 modelling, where long transient simulations cannot be afforded. We show that 17 discontinuities are negligible for surface and upper-air variables, but they are 18 noticeable in variables with long response times, such as soil moisture or snow 19 depth. For these variables, differences between the slices are mainly attributed 20 to internal variability, but also to insufficient spin-up time, depending on the 21 region. In general, the results show that the overlapping time slice approach 22 is valid to accomplish long term regional climate simulations.