Jiandong Li

In 2023, Yunnan in southwestern China experienced the most severe compound drought–heatwave (CDHW) event on record. To effectively address these extremes, it is crucial to understand the atmospheric physical processes that initiate and sustain them. This study provides a comprehensive diagnosis of the formation and amplification mechanisms of this event. The results indicate that the CDHW in MAM 2023 was exceptionally intense. An anomalously westward‐extending subtropical high placed Yunnan under persistent high‐pressure anticyclonic conditions that suppressed cloud development. As a result, there were marked reductions in cloud cover and cloud thickness, which weakened shortwave reflection and enhanced surface solar absorption. This led to positive anomalies in surface shortwave cloud radiative forcing (SWCF; +15.12 W m ⁻² ) and surface net cloud radiative forcing (NCF; +11.86 W m ⁻² ), while surface longwave cloud radiative forcing (LWCF) exhibited a negative anomaly (−3.26 W m ⁻² ). In MAM 2023, the SWCF value (−55.98 W m ⁻² ) accounted for 27% of the net surface shortwave radiation (NSW; 209.08 W m ⁻² ), whereas the LWCF value (25.31 W m ⁻² ) contributed 32% of the net longwave radiation (NLW; −77.80 W m ⁻² ). Meanwhile, strengthened sensible heat flux (SHF) and reduced latent heat flux (LHF) indicate a transition from a “moist evaporative” to a “dry sensible‐heating” surface regime. These processes together established a positive cloud–radiation–land–atmosphere feedback in which reduced cloudiness enhanced radiative warming, thereby suppressing evaporation and strengthening sensible heating, and further favored cloud reduction, thus sustaining and intensifying the CDHW.

The Tibetan Plateau (TP) surface heating profoundly regulates the Asian circulation, but its roles in regional clouds and surface radiation budget remain unclear. Here, we investigate the impacts of the TP surface sensible heating on the surrounding Asian cloud fraction and surface radiation budget using numerical climate experiments that vary sensible heat exchange. The TP surface sensible heating, as an elevated heating source, can induce a low‐level cyclone surrounding the TP and ascent over the central‐eastern TP, pumping air into the middle troposphere. Consequently, spring cloud fractions significantly increase over the east TP and East China (EC) owing to the TP surface heating, enhancing cloud radiative effects and longwave radiation but reducing shortwave and net radiation at the surface, whereas the opposite changes occur over the west TP. The summer TP impacts exhibit more substantial changes in both magnitude and spatial coverage relative to spring states. Due to the coupling between the upper and lower circulations forced by the TP surface heating, increased cloud fractions extend over the whole TP in summer, with enhanced ascent and cloud radiative cooling dominated by the shortwave component. Meanwhile, cloud fractions decrease over Central and North Asia, accompanied by abnormal descent, weakened cloud radiative cooling, and minimal change in net surface radiation due to a weak offset between longwave and shortwave radiations. Our results indicate that the TP surface sensible heating significantly influences the distribution and magnitude of the surrounding Asian cloud fraction and surface radiation budget via circulation responses.

Large bias exists in the shortwave cloud radiative effect (SWCRE) of general circulation models (GCMs), with unclear contributions from individual cloud properties. Here we present a machine‐learning approach, demonstrated using the FGOALS‐f3‐L GCM, to clarify the bias decomposition. A random‐forest model for calculating SWCRE was developed using observation and model data of cloud fraction (CFR), cloud‐solar concurrence ratio (CSC), cloud liquid/ice water paths (LWP/IWP), top of the atmosphere (TOA) upward clear‐sky solar flux (SUC), and solar zenith angle. Then, following the partial radiation perturbation method, we reveal that the global‐mean TOA SWCRE bias (in W m ⁻² ) is mainly contributed by CFR (+5.67), LWP (−5.92), CSC (+3.28), and IWP (−1.72). Regionally, the relative importance varies according to climate regimes. The large CSC contribution highlights the importance of cloud diurnal variation. The sensitivity to the observational data was discussed, emphasizing the necessity of reducing observational uncertainties for implementing effective model‐improving measures.

The shift in the intertropical convergence zone (ITCZ) and the poleward expansion of the Hadley circulation termini have attracted many investigations, since they affect the hydrological cycle and hence the societies and ecosystems in the tropical and subtropical areas. Using the observed precipitation and three atmospheric reanalysis data sets, different methods have been employed to quantify the changes in the ITCZ position, the Hadley circulation width, terminus position, and center intensity in both hemispheres over the global and seven longitudinal sections. It is found that the ITCZ position from the centroid method is closer to the equator over the global and ocean sections than that from the maximum precipitation method and the mass streamfunction, but the variability between different methods and data sets has significant correlations. The large spread of the ITCZ latitude is mainly from the different methods used. The ITCZ position has shifted away from the equator over 1983–2023, which is consistent across data sets, and the multi-method mean trend from five significant trends is 0.22 ± 0.12°/decade over this period. The south HC branch terminus is expanding poleward; this shift, computed using different methods and data sets, is consistent, and five out of seven are significant. The terminus position shift in the north branch is mixed, and most trends are insignificant except that from P-E. The global mean south branch circulation width has a significant increasing trend, contributed mainly by the northward shift in the ITCZ position; meanwhile, the north circulation width is shrinking insignificantly over 1983–2023. The cross-equatorial atmospheric energy transport AHT and the ITCZ position θ_ITCZ from ERA5 are generally anti-correlated, and the correlation coefficients between AHT and θ_ITCZ from different methods are all significant. The multi-method mean northward shift of θ_ITCZ is 3.48 °PW⁻¹.

Asian large-scale orography profoundly influences circulation in the North Hemisphere. Considerable spring top-of-the-atmosphere (TOA) radiative cooling over Southeast China (SEC) is very likely related to upstream orography forcing. Here we investigate the mechanical and thermal forcings of Asian large-scale orography, particularly the Tibetan Plateau (TP), on downstream East Asian cloud amount and atmospheric radiation budget during March-April using the Global Monsoons Model Intercomparison Project simulations. The thermal forcing drives significant surface heating and a low-level cyclone over the TP, pumping low-level air to the middle troposphere. Ascent and water vapor convergence triggered by the thermal forcing favor air condensation, low-middle clouds, and resultant strong spring cloud radiative cooling over SEC. Moreover, the thermal forcing moves the position of cloud radiative cooling westward towards the TP. The TP’s blocking role weakens low-level westerlies over SEC, but its deflecting role increases downstream high-level westerlies, dynamically favoring cloud formation over SEC and the eastward ocean. In addition, the TP can force ascent and increase cloud amounts over the western and central TP. The thermal forcing contributes to 57.1% of total cloud amount and 47.6% of TOA cloud radiative cooling induced by the combined orography forcing over SEC while the mechanical one accounts for 79.4% and 95.8% of the counterparts over the ocean to the east of SEC. Our results indicate that Asian large-scale orography shapes the contemporary geographical distribution of spring East Asian cloud amount and atmospheric radiation budget to a large extent.

Pronounced climatic differences occur over subtropical South China (SC) and tropical South China Sea (SCS) and understanding the key cloud-radiation characteristics is essential to simulating East Asian climate. This study investigated cloud fractions and cloud radiative effects (CREs) over SC and SCS simulated by CMIP6 atmospheric models. Remarkable differences in cloud-radiation characteristics appeared over these two regions. In observations, considerable amounts of low-middle level clouds and cloud radiative cooling effect appeared over SC. In contrast, high clouds prevailed over SCS, where longwave and shortwave CREs offset each other, resulting in a weaker net cloud radiative effect (NCRE). The models underestimated NCRE over SC mainly due to weaker shortwave CRE and less cloud fractions. Conversely, most models overestimated NCRE over SCS because of stronger shortwave CRE and weaker longwave CRE. Regional CREs were closely linked to their dominant cloud fractions. Both observations and simulations showed a negative spatial correlation between total (low) cloud fraction and shortwave CRE over SC, especially in winter, and exhibited a positive correlation between high cloud fraction and longwave CRE over these two regions. Compared with SCS, most models overestimated the spatial correlation between low (high) cloud fraction and SWCRE (LWCRE) over SC, with larger bias ranges among models, indicating the exaggerated cloud radiative cooling (warming) effect caused by low (high) clouds. Moreover, most models struggled to describe regional ascent and its connection with CREs over SC while they can better reproduce these connections over SCS. This study further suggests that reasonable circulation conditions are crucial to simulating well cloud-radiation characteristics over the East Asian regions.

The net surface energy flux (F_s) is critical to the Earth’s energy budget and surface processes, but its simulation remains uncertain in global and regional scales. This study investigates simulated F_s biases and sources globally and in the Asian monsoon region (AMR) using CMIP6 HighResMIP atmospheric models. Globally, the multi-model mean can reproduce the observed global multiannual mean F_s. The majority of models overestimate the annual mean F_s, net surface shortwave radiation (SW_s) and longwave radiation (LW_s) but underestimate the turbulent heat flux (THF). In AMR, the F_s is predominantly upward during winter and downward during summer owing to the seasonal variation in SW_s and THF. 95% of the winter F_s bias over AMR comes from THF primarily due to the latent heat flux bias. SW_s and THF contribute 40%~90% and 70%~90% to summer F_s bias, respectively. The systematic biases of SW_s and LW_s can be attributed to biases in circulation patterns and cloud cover, while biases in THF are primarily influenced by the near-surface processes. The high-resolution models perform well in F_s, THF, and low-level circulation, particularly in DJF. The winter multi-model mean error is reduced by 21.5%~63.6% in F_s and 25.5%~76.7% in THF across three subregions of AMR. Seven out of nine high-resolution models show higher skill scores of winter F_s and THF than their low-resolution counterparts in SA, with corresponding model number being 8 (F_s) and 7 (THF) in both EA and WNP. This study reveals the advantages of increased horizonal resolution in the F_s simulation.