Abstract
AbstractPrevious studies showed that high-resolution GCMs overestimate land precipitation when compared against observation-based data. Particularly, high-resolution HadGEM3-GC3.1 shows a significant precipitation increase in mountainous regions, where the scarcity of gauge stations increases the uncertainty of gridded observations and reanalyses. This work evaluates such precipitation uncertainties indirectly through the assessment of river discharge, considering that an increase of ~10% in land precipitation produces ~28% more runoff when the resolution is enhanced from 1° to 0.25°, and ~50% of the global runoff is produced in 27% of global land dominated by mountains. We diagnosed the river flow by routing the runoff generated by HadGEM3-GC3.1 low- and high-resolution simulations. The river flow is evaluated using a set of 344 monitored catchments distributed around the world. We also infer the global discharge by constraining the simulations with observations following a novel approach that implies bias correction in monitored rivers with two methods, and extension of the correction to the river mouth, and along the coast. Our global discharge estimate is 47.4±1.6×103km3yr−1, which is closer to the original high-resolution estimate (50.5 × 103km3yr−1) than to the low-resolution (39.6 × 103km3yr−1). The assessment suggests that high-resolution simulations performbetter in mountainous regions, either because the better-defined orography favours the placement of precipitation in the correct catchment, leading to a more accurate distribution of runoff, or the orographic precipitation increases, reducing the dry runoff bias of coarse resolution simulations. However, high-resolution slightly increases wet biases in catchments dominated by flat terrain. The improvement of model parameterizations and tuning may reduce the remaining errors in high-resolution simulations.
Highlights
The hydrologic cycle is a closed system that describes the circulation of water between ocean, atmosphere, and land
Given that our focus is on the evaluation of precipitation sensitivity to model resolution, we proposed an indirect method of evaluation, through the assessment of river flow, a natural integrator of the water balance at the catchment scale, whose observations are useful to understand the water budget in catchments with low density of in situ measurements of precipitation and/or evapotranspiration
With the final goal of advancing the understanding of precipitation biases in low and high-resolution HadGEM3-GC3.1 simulations submitted to CMIP6-HighResMIP, we have 1) assessed the river discharge in monitored rivers, and 2) extended the knowledge acquired in step 1 to ungauged rivers, to produce a global discharge estimate
Summary
The hydrologic cycle is a closed system that describes the circulation of water between ocean, atmosphere, and land. It is known that AMIP and COUPLED present different atmospheric dynamics, due to their differences in sea surface temperatures (observed versus simulated), that may alter the water budget components For this reason, the methodology followed in this paper (analysis of water budget, river routing, bias correction, etc.) is done for each individual simulation; some results are presented as ensemble mean, given that the GCM is more sensitive to resolution than to the use of coupling with the ocean. Both AMIP and COUPLED present a clear increase of precipitation and runoff at HR, AMIP shows larger differences. On the basis of these results, and given the uncertainty of observational products discussed in section 2, we assess the simulated river discharge at the catchment scale, using it as a proxy to understand precipitation biases
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