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The East Asian jet stream is an important component of mid-high latitude circulation systems, and its intraseasonal oscillation plays a crucial role in the formation and development of persistent climate anomalies and extreme events over East Asia–Pacific region. This study examines the decadal changes in the intraseasonal variability (ISV) of the East Asian jet stream and investigates the possible reasons with reanalysis and observational datasets. It is found that the zonal winds over East Asia show notable ISV with a significant period of 10–25 days. The variance of 10–25-day filtered East Asian zonal winds exhibits a significant decrease around 2000, which is mainly contributed by the weakened ISV of westerlies within the key region of East Asian polar front jet (EAPJ). Further analysis reveals that the EAPJ experienced remarkable ISV changes in both intensity and location. The ISV of EAPJ intensity was weakened around 2000 due to decreased frequencies of both the extremely strong and weak cases, whereas the ISV of EAPJ location was evidently enhanced, with the southward shifts increased more than the northward ones. Along with the ISV increase (decrease) of EAPJ intensity (location), the dominant mode of EAPJ intraseasonal variation has changed from its intensity change to position shift since the year 2000. The decadal ISV changes and mode transition of EAPJ are closely associated with the modulation of lower-level large-scale circulation over Eurasia. The weakened ISV of meridional winds over Ural region after 2000 is favorable for an intraseasonal circulation pattern change from a continental-scale cyclonic anomalous pattern to a meridional dipole structure across the Eurasia in lower troposphere, resulting in regime shifts of East Asian temperature and its meridional gradient through temperature advection. The intraseasonal temperature gradient patterns display ISV changes parallel to those of EAPJ around 2000, indicating that the thermal anomalies are effective in leading to corresponding intensity and location variations of EAPJ via thermal wind relationship. Furthermore, the modulation of lower-level circulation is closely related to strengthened Arctic warming and weakened temperature variability over Barents sector after 2000, which may further be linked to sea ice reduction in the Barents Sea. [ABSTRACT FROM AUTHOR]

We modify the Zhang‐McFarlane (ZM) deep convection scheme in the Community Atmosphere Model version 5 to couple it with a unified parameterization for boundary‐layer turbulence and shallow convection, that is, Cloud Layers Unified by Binormals (CLUBB). By assuming a lognormal distribution of entrainment rate across the entire moist convective regimes, we link mass fluxes between shallow and deep convection, which are partitioned by the entrainment rate of the shallowest deep convective plume. Hence, a new deep convective closure is established which is coupled to the sub‐grid vertical motion variability in CLUBB. The convection feedback (or memory) effects are also considered to decrease the entrainment spectrum width and enhance the vertical velocity variability that further affect deep convection. Results show that the revised scheme improves the precipitation simulations in terms of the mean state and variability at various timescales, such as the alleviated double‐intertropical convergence zone and more realistic simulations of the seasonal variation of monsoon precipitation over East Asia, Madden‐Julian Oscillation, and precipitation diurnal phase propagations downstream of large terrains. The improvements are still seen in many aspects such as the mean‐state precipitation when turning off the convection feedback impacts in the revised scheme, emphasizing the benefits of using the modified mass‐flux closure. However, the convection feedbacks have considerable effects on the precipitation diurnal cycle simulations over regions with late‐afternoon precipitation peaks. Overall, the revised scheme provides a unified treatment for sub‐grid vertical motions across regimes of boundary‐layer turbulence, shallow convection, and deep convection, leading to better‐simulated precipitation at various timescales. Plain Language Summary: The physical consistency between shallow and deep convection is an important but largely misrepresented aspect in climate models. In the atmosphere, convective updraft plumes usually differ in their fractional entrainment rate, with weaker entrainment favoring a more vertical extension of the plume. Here, we modify the ZM convective parameterization in CAM5 to couple it with a unified parameterization for boundary‐layer turbulence and shallow convection, that is, CLUBB. A continuous distribution is used to represent the spectrum of entrainment rates across the entire moist convective regimes and to link mass fluxes between shallow and deep convection. This couples deep convection to CLUBB and provides a unified treatment for sub‐grid vertical motions across regimes of boundary‐layer turbulence, shallow convection, and deep convection. In addition, the convection memory effects are included to affect deep convection via changing the entrainment spectrum width and vertical velocity variability. The revised scheme performs much better than the original one in the simulated precipitation variability at various timescales, such as the alleviated double‐intertropical convergence zone and improved monsoon precipitation, Madden‐Julian Oscillation, and continental precipitation diurnal variation. Both of the modifications related to the mass flux closure and convection feedback impacts are important for the improved model performance. Key Points: We use an entrainment distribution to link mass flux of deep convection to that of shallow convection related to sub‐grid motion in CLUBBThe convection memory effects are included to affect deep convection via changing the entrainment spectrum width and sub‐grid motionThe revised scheme performs much better than the original one in the simulations of precipitation variability at various timescales [ABSTRACT FROM AUTHOR]

It remains a challenge for climate models to correctly capture the relationship between precipitation and ENSO. This study examines the linkage between the simulated precipitation climatology and ENSO-related precipitation anomaly during boreal winter based on the multi-model ensemble from the Atmospheric Model Intercomparison Project Phase 5 (AMIP5) and perturbed parameter ensemble (PPE) with the Beijing Climate Center (BCC) atmospheric model. The AMIP5 models have large biases in simulating the tropical precipitation anomaly during El Niño, such as the shifts of the Inter-Tropical Convergence Zone (ITCZ) and South Pacific Convergence Zone (SPCZ). The inter-model differences show that the precipitation change in response to sea surface temperature (SST) change increases with enhanced precipitation climatology. The ENSO-related precipitation anomaly can also be related to the spatial distribution of the mean-state precipitation. The simulated ITCZ/SPCZ displacements are significantly correlated with the spatial precipitation-SST relationship in the mean state. Models with stronger mean-state precipitation-SST relationship also produce stronger SPCZ/ITCZ displacements. The mean-state tropical precipitation also has strong impacts on the ENSO-related precipitation anomalies over East Asia. In the BCC PPE, the connection between the mean-state precipitation and ENSO-related precipitation anomaly is overall consistent with that in AMIP5. Parameters associated with the low-cloud and deep convection processes are the most influential ones for the precipitation simulations in BCC. Compared with the version in AMIP5, the new BCC model can better simulate the precipitation climatology and the relationship between ENSO and precipitation over southern China. These results have important implications for model development and model-based climate predictions. [ABSTRACT FROM AUTHOR]

The main advancements of the Beijing Climate Center (BCC) climate system model from phase 5 of the Coupled Model Intercomparison Project (CMIP5) to phase 6 (CMIP6) are presented, in terms of physical parameterizations and model performance. BCC-CSM1.1 and BCC-CSM1.1m are the two models involved in CMIP5, whereas BCC-CSM2-MR, BCC-CSM2-HR, and BCC-ESM1.0 are the three models configured for CMIP6. Historical simulations from 1851 to 2014 from BCC-CSM2-MR (CMIP6) and from 1851 to 2005 from BCC-CSM1.1m (CMIP5) are used for models assessment. The evaluation matrices include the following: (a) the energy budget at top-of-atmosphere; (b) surface air temperature, precipitation, and atmospheric circulation for the global and East Asia regions; (c) the sea surface temperature (SST) in the tropical Pacific; (d) sea-ice extent and thickness and Atlantic Meridional Overturning Circulation (AMOC); and (e) climate variations at different timescales, such as the global warming trend in the 20th century, the stratospheric quasi-biennial oscillation (QBO), the Madden–Julian Oscillation (MJO), and the diurnal cycle of precipitation. Compared with BCC-CSM1.1m, BCC-CSM2-MR shows significant improvements in many aspects including the tropospheric air temperature and circulation at global and regional scales in East Asia and climate variability at different timescales, such as the QBO, the MJO, the diurnal cycle of precipitation, interannual variations of SST in the equatorial Pacific, and the long-term trend of surface air temperature. [ABSTRACT FROM AUTHOR]

A parameterization scheme for the thermal effects of subgrid scale orography is incorporated into a regional climate model (developed at Nanjing University) and its impact on modeling of the surface energy budget over East Asia is evaluated. This scheme includes the effect of terrain slope and orientation on the computation of solar and infrared radiation fluxes at the surface, as well as the surface sensible and latent heat fluxes. Calculations show that subgrid terrain parameters alter the diurnal cycle and horizontal distributions of surface energy budget components. This effect becomes more significant with increased terrain slope, especially in winter. Due to the inclusion of the subgrid topography, the surface area of a model grid box changes over complex terrain areas. Numerical experiments, with and without the subgrid scale topography scheme, show that the parameterization scheme of subgrid scale topography modifies the distribution of the surface energy budget and surface temperature around the Tibetan Plateau. Comparisons with observations indicate that the subgrid topography scheme, implemented in the climate model, reproduces the observed detailed spatial temperature structures at the eastern edge of the Tibetan Plateau and reduces the tendency to overestimate precipitation along the southern coastal areas of China in summer. [ABSTRACT FROM AUTHOR]