Mechanisms of tropical precipitation biases in climate models
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Mechanisms of tropical precipitation biases in climate models Hanjun Kim1 · Sarah M. Kang1 · Ken Takahashi2 · Aaron Donohoe3 · Angeline G. Pendergrass4 Received: 28 January 2020 / Accepted: 2 June 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract We investigate the possible causes for inter-model spread in tropical zonal-mean precipitation pattern, which is divided into hemispherically symmetric and anti-symmetric modes via empirical orthogonal function analysis. The symmetric pattern characterizes the leading mode and is tightly related to the seasonal amplitude of maximum precipitation position. The energetic constraints link the symmetric pattern to the seasonal amplitude in cross-equatorial atmospheric energy transport AET0 and the annual-mean equatorial net energy input NEI0 . Decomposition of AET0 into the energetics variables indicates that the inter-model spread in symmetric precipitation pattern is correlated with the inter-model spread in clear-sky atmospheric shortwave absorption, which most likely arises due to differences in radiative transfer parameterizations rather than water vapor patterns. Among the components that consist NEI0 , the inter-model spread in symmetric precipitation pattern is mostly associated with the inter-model spread in net surface energy flux in the equatorial region, which is modulated by the strength of cooling by equatorial upwelling. Our results provide clues to understand the mechanism of tropical precipitation bias, thereby providing guidance for model improvements. Keywords Tropical precipitation · Model uncertainty · Double ITCZ problem · Energetic constraints · Cold tongue bias · Atmospheric shortwave absorption
1 Introduction Most of the state-of-the-art coupled general circulation models (CGCM) overestimate precipitation south of the equator and underestimate it near the equator: this bias is termed as the double intertropical convergence zone (ITCZ) problem (Mechoso et al. 1995; Lin 2007). Despite steady improvements in resolution and model parameterizations, the double Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00382-020-05325-z) contains supplementary material, which is available to authorized users. * Sarah M. Kang [email protected] 1
School of Urban and Environmental Engineering, Ulsan National Institute of Science and Technology, UNIST‑gil 50, Ulsan 689‑798, Republic of Korea
2
Servicio Nacional de Meteorología e Hidrología del Perú (SENAMHI), Lima, Peru
3
Applied Physics Laboratory, University of Washington, Seattle, WA, USA
4
National Center for Atmospheric Research, Boulder, CO, USA
ITCZ problem has been persistent in CGCMs across generations of climate models (Zhang et al. 2015). This model bias in tropical precipitation has various climatic consequences. For example, Ham and Kug (2014) showed that the climate models with excessive precipitation over the central/eastern Pacific ITCZ tend to simulate a slower phase transition of the El Niño-Southern Oscillat
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