Abstract

AbstractAnalyzing a long simulation and a set of seasonal reforecasts of the Climate Forecast System version 2 (CFSv2), this study demonstrates a large model cold bias in the deep soil layer (100–200 cm) over most of North America continent during summer due to weaker seasonal change in summer. The summer subsurface temperature (SUBT) cold bias influences the land surface temperature (LST) during the summer and subsequent seasons in different ways over different geographical regions in North America: West of the Rocky Mountains, the SUBT's effect on LST is largely overruled by the stronger upstream marine influence from the Pacific. Over the central Great Plains, however, it is a major cause for severe cold LST bias during summer in the model simulation and reforecasts. As a result, model underestimates sensible heat flux into the atmosphere but overestimates latent heat flux. The latter may contribute to an excessive summer rainfall in the region. Over the northeast region, the SUBT cold bias persists to August and September, which causes an additional surface cooling in the fall and helps to bring LST to the freezing point early. This sets up the stage for a prolonged snow‐albedo feedback. In particular, the model long simulation that passes through previous summer and fall demonstrates longer persistence of snow cover over the northeast region than reforecasts initialized in late winter and spring do. A cold bias of the water temperature in the North Atlantic seems also to play a role to prolong cold bias in the northeast region.

Highlights

  • Despite the significant improvement in representing the physical processes that produce land surface properties in state-of-the-art general circulation models (GCMs), including snow, heat fluxes, land surface temperature (LST), soil moisture and vegetation, a number of systematic biases and uncertainties of these properties persist (Koster et al, 2004, 2011; Kim and Wang, 2007; Seneviratne et al, 2006; 2010; Douville et al, 2010; van den Hurk et al, 2011, 2012; Dutra et al, 2011; Santanello Jr. et al, 2018)

  • Consistent with our results over western Eurasia in Climate Forecast System version 2 (CFSv2) simulation (Shukla et al 2019b), it is found that when the solar radiation reduces quickly in the early fall, the cold subsurface temperature (SUBT) bias at 100-200 cm in the northeast region (NER) during August-September causes additional cooling to the upper soil layer and helps to bring LST to the freezing point early in model simulation, which provides a favorable condition for excessive SCF (Fig. 4a-c) in the subsequent

  • Summary and discussion This study investigates the potential impact of deep soil temperature anomalies on the LST

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Summary

Introduction

Despite the significant improvement in representing the physical processes that produce land surface properties in state-of-the-art general circulation models (GCMs), including snow, heat fluxes, land surface temperature (LST), soil moisture and vegetation, a number of systematic biases and uncertainties of these properties persist (Koster et al, 2004, 2011; Kim and Wang, 2007; Seneviratne et al, 2006; 2010; Douville et al, 2010; van den Hurk et al, 2011, 2012; Dutra et al, 2011; Santanello Jr. et al, 2018).

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