Processes Leading to Double Intertropical Convergence Zone Bias in CESM1/CAM5

Lee, Wei‐Liang; Chen, Yu-Luen; Hsu, Huang‐Hsiung

doi:10.1175/jcli-d-14-00622.1

Cited by 15 publications

(22 citation statements)

References 52 publications

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“…Similarly, Song and Zhang () attribute the initial bias development to excessive downwelling shortwave radiation. Other studies suggest the double‐ITCZ bias arises because of uncertainty in the entrainment rate leading to deep convection that is too insensitive to large scale subsidence (Hirota et al, ), insufficient rain re‐evaporation within the deep convection parameterization leading to overly strong coupling between the boundary layer and deep convection (Bacmeister et al, ), or overly strong shallow convection leading to an overly moist boundary layer (Wang et al, ).…”

Section: Introductionsupporting

confidence: 90%

“…More recent studies have expanded on the role of cloud and convective parameterizations in the development of the double‐ITCZ bias. Many of these studies suggest the double‐ITCZ bias results from deep convection parameterizations being too sensitive to SSTs and too insensitive to inhibitive large‐scale dynamic forcings (Hirota et al, ; Oueslati & Bellon, ; Song & Zhang, ; Wang et al, ). The enhanced sensitivity of convection to SST supports a coupled feedback cycle which can amplify otherwise small biases (Zhang et al, ), leading to the development of a double‐ITCZ within the first year of integration (Liu et al, ).…”

Section: Introductionmentioning

confidence: 96%

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Sensitivity of Coupled Tropical Pacific Model Biases to Convective Parameterization in CESM1

Woelfle

Bretherton

et al. 2018

J Adv Model Earth Syst

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Six month coupled hindcasts show the central equatorial Pacific cold tongue bias development in a GCM to be sensitive to the atmospheric convective parameterization employed. Simulations using the standard configuration of the Community Earth System Model version 1 (CESM1) develop a cold bias in equatorial Pacific sea surface temperatures (SSTs) within the first two months of integration due to anomalous ocean advection driven by overly strong easterly surface wind stress along the equator. Disabling the deep convection parameterization enhances the zonal pressure gradient leading to stronger zonal wind stress and a stronger equatorial SST bias, highlighting the role of pressure gradients in determining the strength of the cold bias. Superparameterized hindcasts show reduced SST bias in the cold tongue region due to a reduction in surface easterlies despite simulating an excessively strong low‐level jet at 1‐1.5 km elevation. This reflects inadequate vertical mixing of zonal momentum from the absence of convective momentum transport in the superparameterized model. Standard CESM1simulations modified to omit shallow convective momentum transport reproduce the superparameterized low‐level wind bias and associated equatorial SST pattern. Further superparameterized simulations using a three‐dimensional cloud resolving model capable of producing realistic momentum transport simulate a cold tongue similar to the default CESM1. These findings imply convective momentum fluxes may be an underappreciated mechanism for controlling the strength of the equatorial cold tongue. Despite the sensitivity of equatorial SST to these changes in convective parameterization, the east Pacific double‐Intertropical Convergence Zone rainfall bias persists in all simulations presented in this study.

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Section: Introductionsupporting

confidence: 90%

Section: Introductionmentioning

confidence: 96%

Sensitivity of Coupled Tropical Pacific Model Biases to Convective Parameterization in CESM1

Woelfle

Bretherton

et al. 2018

J Adv Model Earth Syst

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“…A double-ITCZ bias in CGCMs can emerge from several factors, such as the choice of convective parameterization scheme (Hirota et al, 2011), convective mixing (Möbis & Stevens, 2012;Song & Zhang, 2018), or from atmospheric transient eddies (Xiang et al, 2018). Additionally, a precipitation bias in CGCM simulations can emerge due to biases in tropical SST pattern associated with ocean current in the model itself (Wang et al, 2015). Indeed, Hwang and Frierson (2013) have shown that biases in cross-equatorial energy transport are linked to precipitation biases south of the equator.…”

Section: Introductionmentioning

confidence: 99%

Tropical Pacific SST and ITCZ Biases in Climate Models: Double Trouble for Future Rainfall Projections?

Samanta

Karnauskas

Goodkin

2019

Geophysical Research Letters

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The double Intertropical Convergence Zone bias remains a persistent problem in coupled general circulation model simulations. Due to the strong sea surface temperature (SST)‐convection relationship in the tropics, precipitation biases are sensitive to background SST. Using historical simulations of 24 coupled general circulation models and an atmospheric general circulation model, we show that cold equatorial SST biases at least exacerbate double Intertropical Convergence Zone biases in the Pacific. A linear regression model is used to demonstrate that improved predictability of precipitation trends is possible with such model‐dependent information as mean‐state SST biases accompanying projected SST trends. These results provide a better understanding of the root of the double Intertropical Convergence Zone bias and a possible path to reduced uncertainty in future tropical precipitation trends.

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“…The inverse sensitivities of the double‐ITCZ and cold tongue biases to southeast Pacific low cloud fraction prompted investigation into other local processes, particularly the role of excessive convective activity and its relationship with local SSTs (e.g., Hirota et al, ; Oueslati & Bellon, ; Song & Zhang, ; Wang et al, ). Biases in simulated convective activity can amplify via coupled feedbacks (Zhang et al, ; Liu et al, ).…”

Section: Introductionmentioning

confidence: 99%

“…This result may be driven by local improvement or by improvements in the meridional gradient of SST which has been shown to be important in determining ITCZ structure in idealized experiments (Neale & Hoskins, ). Further sensitivity studies have suggested processes controlling boundary layer humidity are important for the ITCZ as moister boundary layers favor triggering of deep convection (Bacmeister et al, ; de Szoeke et al, ; Wang et al, ).…”

Section: Introductionmentioning

confidence: 99%

Evolution of the Double‐ITCZ Bias Through CESM2 Development

Woelfle

Bretherton

Hannay

et al. 2019

J Adv Model Earth Syst

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The structure of the east Pacific Intertropical Convergence Zone (ITCZ) as simulated in the Community Earth System Model version 2 (CESM2) is greatly improved as compared to its previous version, CESM version 1. Examination of intermediate model versions created as part of the development process for CESM2 shows the improvement in the ITCZ is well correlated with a reduction in the relative warmth of southeast Pacific sea surface temperatures (SSTs) as compared to the broader tropical mean. Cooling SST in this region enhances the zonal SST and surface pressure gradients and reduces the anomalously southward SST gradient present in boreal spring in early version of CESM2. The improvements in southeast Pacific SST are attributed to increases in low cloud cover and the associated shortwave cloud forcing over the southeast. Sensitivity tests using fixed SST simulations demonstrate the increase in cloud cover between two intermediate model versions, 119 and 125, to be driven by removal of the dependence of autoconversion and accretion rates on cloud water variance as well as the removal of a secondary condensation scheme. Both of these changes reduce drizzle rates in warm clouds increasing cloud lifetime and cloud fraction in the stratocumulus to trade cumulus transition region. The improvements in southeast Pacific shortwave cloud forcing and ITCZ climatology persist through subsequent changes to the cloud microphysics parameterizations. Despite improvements in the east Pacific ITCZ, the global mean ITCZ position and Pacific cold tongue bias strength do not exhibit a systematic improvement across the development simulations. Key Points: • Increased southeast Pacific cloud fraction and improved SST gradients alleviate the double-ITCZ bias in CESM2 • Increased cloud fraction is driven by retuning of liquid cloud microphysics not the implementation of CLUBB boundary layer scheme • Despite improvements in east Pacific precipitation, the global mean ITCZ position and cold tongue biases show no consistent change Supporting Information: • Supporting Information S1

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Processes Leading to Double Intertropical Convergence Zone Bias in CESM1/CAM5

Cited by 15 publications

References 52 publications

Sensitivity of Coupled Tropical Pacific Model Biases to Convective Parameterization in CESM1

Sensitivity of Coupled Tropical Pacific Model Biases to Convective Parameterization in CESM1

Tropical Pacific SST and ITCZ Biases in Climate Models: Double Trouble for Future Rainfall Projections?

Evolution of the Double‐ITCZ Bias Through CESM2 Development

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