Evolution of the Double‐ITCZ Bias Through CESM2 Development

Abstract

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

Paleodust Insights into Dust Impacts on Climate

“…The two meridional troughs in the SST bias form two convergence centers in SC favoring the excessive ITCZ, even though the local SST bias is negative. The role of the meridional pattern of the SST bias in forming the excessive ITCZ revealed here is consistent with the conclusion in a recent study (Woelfle et al 2019) emphasizing the meridional pattern of southeastern Pacific SST bias in the double-ITCZ problem. The latest release of the CMIP6 models offers little improvement with respect to the excessive ITCZ bias and its relationship with the SST pattern bias.…”

Simulated Tropical Precipitation Assessed across Three Major Phases of the Coupled Model Intercomparison Project (CMIP)

“…Only the two best CMIP6 models for this metric, CESM2 and CESM2-WACCM with r . 0.9 over tropical land, come close to the observational range for r, reflecting a regional improvement in the tropical precipitation pattern of this model (Woelfle et al 2019). Also the root-mean-square errors (RMSE) for precipitation compared to TRMM have decreased on average over the CMIP phases, from 1.85 mm day 21 in CMIP3 to 1.80 mm day 21 in CMIP5 and 1.55 mm day 21 in CMIP6, but again these are larger than the observational uncertainty (Table 3).…”

Changes in Annual Extremes of Daily Temperature and Precipitation in CMIP6 Models

“…Nevertheless, a double-ITCZ bias over the equatorial central and eastern Pacific is evident in estimates of 50-yr Rx1day and Rx5day return values. A spurious double-ITCZ can appear as a result of different interacting factors such as biased sea surface temperatures, surface winds, and erroneous cloud microphysics over the equatorial Pacific (e.g., Dai 2006;Zhang et al 2015;Woelfle et al 2019). Overall, the spatial pattern agreement between models and ERA5 looks slightly better for Rx5day than for Rx1day, with pattern correlation being 0.91 for the latter and 0.88 for the former.…”