Yuan Wang

The cold pole and westerly wind biases associated with an overly strong polar vortex are typical systematic biases in climate models, indicating the insufficient stratospheric wave drag. To investigate the effects of orographic gravity wave drag (OGWD) on the stratospheric atmospheric circulation, two sets of experiments are performed by the middle‐atmosphere version of Beijing Climate Center Atmospheric General Circulation Model, employing different OGWD parameterization schemes with and without the second‐order Wentzel‐Kramers‐Brillouin (WKB) corrections to the surface wave momentum flux (SWMF) caused by wind profile shear and curvature. In the simulation with the WKB‐corrected OGWD scheme, the cold pole bias is reduced up to 2°C, and the associated westerly wind bias diminishes up to 4 m s⁻¹, particularly during the austral winter. Changes in the SWMF can further affect the vertical transport of orographic gravity waves. The enhanced SWMF in the modified scheme transports more wave momentum flux upward. Consequently, more wave momentum flux is transported into the upper stratosphere, enhancing the wave breaking there. The OGWD‐induced meridional circulation is strengthened over Antarctic, contributing to the alleviation of the cold pole and westerly wind biases. In addition, the upward propagation of planetary Rossby waves in the mid‐high latitudes of the Southern Hemisphere is enhanced, which contributes to the reduction of the westerly wind biases of the polar vortex as well. Overall, the WKB‐corrected scheme is effective to alleviating the delayed breakdown of the polar vortex in Antarctica.

An extreme high-temperature event occurred in North China on 22 June 2023, with the maximum temperature reaching 41.8 °C. The high-temperature centers preferentially occurred at the foothills of the Taihang and Yanshan Mountains, indicating an important role of the underlying orography. In the present work, we study the orographic effects of this extreme high-temperature event according to high-resolution numerical simulations using the Weather Research and Forecasting model. The results show that the presence of the mountains in North China contributed notably to the high-temperature event, which can enhance the 2 m air temperature by up to 3 °C. In the daytime, the enhancement of temperature is primarily due to the diabatic heating of sensible heat flux at the terrain surface caused by solar shortwave radiation, whereas the well-known foehn effect has little contribution. Indeed, the dynamically forced downslope flow of foehn is totally suppressed by the upslope flow of the thermally driven mountain-plain circulation. In the nighttime, the sensible heat flux at the terrain surface changes to weakly negative, given the cooling of land surface longwave radiation. As a result, the enhancement of near-surface temperature at the terrain foothill is dominated by the adiabatic warming of downslope flow. Yet, the near-surface temperature far away from the mountain is enhanced by the subsidence warming of a synoptic anomalous anti-cyclone, which is induced by the diabatic heating over the mountains in the daytime. These findings help improve the understanding of the thermal and dynamical effects of orography on the occurrence of high-temperature events.

The northeastern China cold vortex (NCCV) is the main weather system affecting Northeast China. Based on the precipitation products from the dual-frequency precipitation radar (DPR) onboard the Global Precipitation Measurement core observatory (GPM) satellite, the precipitation structures and microphysical properties for different rain types in 6432 NCCVs from 2014 to 2019 were studied using dynamic composite analysis. Our results show that the precipitation in NCCVs is dominated by stratiform precipitation. Regions with high stratiform and convective precipitation frequency have a comma shape. The growth mechanism of precipitation particles changes at ~4 km in altitude, the lower particles grow through collision (more pronounced in convective precipitation), and the upper hydrometeors grow through the Bergeron process. Additionally, the precipitation structures and microphysical properties exhibit great regional variations in NCCVs. The rainfall for all rain types is the strongest in the southeast region within an NCCV, mainly characterized by higher near-surface droplet concentration, while precipitation events occur more frequently in the southeast region for all rain types. There are active rimming growth processes above the melting layer for convective precipitation in the western region of an NCCV. In the southeast region of an NCCV, the collision growth of droplets in both types of precipitation is the most obvious.

Northeast China cold vortex (NECV) is the major influencing weather system in northern China. Yet the impacts of complex terrain on the evolution of NECV remains poorly understood. This work studies the influence of subgrid orographic drag (SOD) on a heavy‐rain‐producing NECV occurred in July 2011 using the Weather Research and Forecasting model. A series of numerical experiments are conducted with different parameterizations of SOD including turbulent orographic form drag (TOFD), flow blocking drag (FBD), and mountain wave drag (MWD). Results show that the NECV intensity is overestimated in the absence of SOD parameterization, accompanied with too‐low geopotential height (GPH) and too‐strong horizontal winds. The parameterization of TOFD can significantly decelerate the 10‐m winds, whereas the FBD and MWD play a minor role. However, the influence of TOFD is overwhelmed by FBD and MWD in the troposphere, especially the latter. This implies that the breaking of mountain waves play a more important role in weakening the NECV than the low‐level flow blocking. The lower‐tropospheric MWD directly weakens the convergence and ascent motion of the NECV, producing an anti‐cyclonic circulation that uplifts the GPH under the constraint of quasi‐geostrophic vertical vorticity. This MWD‐induced circulation indirectly weakens the NECV in the mid‐upper troposphere by producing a warm advection that counters the low‐level cold advection to the southwest of the NECV. The findings provide useful insights into the impacts of complex terrain on the NECV and highlight the importance of topographic drag parameterization in the simulation and short‐range forecast of NECV.

Weak tropical cyclone (TC) center positioning is difficult work in operational forecasting. In the present study, a TC-red-green-blue (TC-RGB) composite was designed by using satellite multichannel observations (reflectance, brightness temperature, and brightness temperature differences). Compared with single channel images, TC-RGB composites can clearly show the exposed low-level circulation (LLC) of weak TCs under large vertical wind shear. Based on the guidelines of TC-RGB composites for TC center positioning, we repositioned 83 western North Pacific (WNP) TC cases during 2017–2019. Then, the comparisons of TC center positions were made between the TC-RGB composite and the Regional Specialized Meteorological Centre-Tokyo (RSMC-Tokyo), the Joint Typhoon Warning Center (JTWC) and the China Meteorological Administration-Shanghai Typhoon Institute (CMA-STI). Via case analysis of TC Kalmaegi (2019), it was found that the best-track data from the RSMC-Tokyo, JTWC and CMA-STI would have over 100 km biases at the early stage of TC life history. Taking all the 83 TC cases into account, the results show that the average center position biases and standard deviations for weak TCs under small vertical wind shear in the daytime are 5 km larger than those under large vertical wind shear at nighttime. When considering the 83 TC cases with clear LLC centers, the difference of these two biases is 10 km. The average biases are mostly above 20 km in the areas south of 18° N and north of 36° N over the WNP. Conversely, in the areas between 18° N and 36° N over the WNP, they are mostly below 20 km.

Embankment is an important water retaining structure and an important part of ensuring flood control safety. The traditional method of embankment safety and stability evaluation usually puts forward the stability early warning value for a single monitoring data, trying to find out the correlation between stability and monitoring data from a statistical point of view, which may get a one-sided evaluation conclusion, and also does not make full use of the existing monitoring and detection data information. Based on Bayesian theory, a monitoring and detection information fusion model integrating structural analysis, data mining, and comprehensive prediction is established. Based on structural analysis and considering the overall stability of the embankment, the forward analysis and inverse analysis fusion platform of detected monitoring are built, and the data mining model is constructed by response surface method, it avoids large-scale and cumbersome structural analysis and calculation. An engineering case is selected to apply the evaluation model. The evaluation model can integrate monitoring and detection information at the same time, predict the probability density distribution of the overall stability of the embankment, capture abnormal data, and evaluate the safety state of the embankment, which provides beneficial support for the management decision of the embankment.

The northeastern China cold vortex (NCCV) has an important influence on the regional rainstroms over East Asia. Using the National Centers for Environmental Prediction Final (FNL) reanalysis dataset and the Global Precipitation Measurement product, an improved objective algorithm for identifying heavy precipitation NCCV (HPCV) events from 2001 to 2019 was designed. The climatological features of 208 HPCV events were investigated. The number of HPCV events showed an upward linear trend, with the highest frequency of occurrence in summer. The most active region of HPCVs was in the Northeast China Plain of 40–55° N. Most HPCV events lasted 2–5 days and had radii ranging from 250 to 1000 km. About 79% of the HPCV events was initiated (dissipated) in the definition region (35–60° N, 115–145° E). The initial position was close to the western boundary of the definition region, and the final position was mainly in the first and the fourth quadrants. The HPCV events that moved into the study region had a longer duration than those initiating in the region. The monthly variation of the strongest precipitation in the HPCV events showed a bimodal pattern, with two peaks in June and October. The location of the strongest precipitation was mostly concentrated within 2000 km southeast of the HPCV systems and it had a strong positive correlation with the center of HPCV events.

In order to investigate the key characteristics of mesoscale convective systems (MCSs) initiated over the Tibetan Plateau (TP) in recent years and the main differences in circulation and environmental factors between different types of MCSs, an automatic MCS identification and tracking method was applied based on the data from China’s Fengyun satellite and precipitation estimates. In total, 8820 MCSs were found to have been initiated over the TP during the summers from 2013 to 2019, and a total of 9.3% of them were able to move eastward out of the TP (EO). The number of MCSs showed a monthly variation, with a maximum in July and a minimum in June, while most EOs occurred in June. Compared with other types of MCSs, EOs usually had a lower cloud-top temperature, a greater rainfall intensity, a longer life duration, more rapid development, larger areas of rainfall and convective clouds, longer tracks and a wider influence range, indicating that EOs are more vigorous than the other types of MCSs. The movement of MCSs is mainly due to the mid- to high-level dynamic conditions, and moisture is an essential factor in their development and maintenance.

The mechanism of convection initiation (CI) occurring in the Southwest Xinjiang, Northwest China is investigated using quantitative budget analysis of vertical momentum for the first time. The Weather Research and Forecasting (WRF) model is used to reproduce and analyze the CI events. The observations showed that many CIs occurred continuously, with an intense mesoscale convective system eventually forming. The overall features of the CIs were well captured by the simulation. Lagrangian vertical momentum budgets, in which the vertical acceleration was decomposed into dynamic and buoyant components, were performed along the backward trajectories of air parcels within two convective cells. The results showed that the buoyant acceleration is the major contributor in both the slow and rapid lifting period of the CI, while the dynamic acceleration also showed a considerably positive effect only during the rapid lifting period. The buoyant acceleration during the slow lifting period was due to the warm advection generated by the radiative heating near the mountainous area on the south side of Tarim Basin in the afternoon. The buoyant acceleration during the rapid lifting period was from the latent heat release within the convective cell. Further decomposition of the dynamic acceleration showed that the vertical twisting related to the vertical shear of horizontal wind almost completely dominated the dynamic acceleration, while the horizontal curvature and extension showed very weak contribution. These findings provide some new insights into the roles of buoyant and dynamic forcing in the mechanism of CI in Southwest Xinjiang.

Cloud optical/microphysical properties (COMPs) with a high spatiotemporal resolution are required for monitoring the evolution of convective clouds, studying aerosol-cloud-precipitation interactions, and evaluating cloud microphysics parameterization schemes in weather and climate models. This study applied the bispectral method, commonly used for COMP retrievals, to Advanced Himawari Imager (AHI)/Himawari-8 observations. The AHI daytime cloud optical thickness (τ) and cloud-top particle effective radius (Re) retrievals were then evaluated with the Collection-6.1 Moderate Resolution Imaging Spectroradiometer cloud product (MOD06) for a three-month period from June 1, 2017, to August 31, 2017. Overall, the AHI-retrieved COMP results are in good agreement with the MOD06 product. The correlation coefficients of τ and Re between the AHI retrievals and MOD06 products are 0.76-0.89 and 0.63-0.87, respectively. Overestimations are mainly observed in the 1.6-μm Re (Re1.6) and ice-phase 3.9-μm Re (Re3.9) retrievals. There are τ dependencies in the retrieval biases of τ and Re1.6, as well as angular dependencies in the retrieval biases of τ and ice-phase Re3.9. Moreover, the monthly mean composites for three COMP retrievals from the MOD06 and AHI algorithms are compared. Significant differences are only found between the two Re1.6 products. This study also finds that AHI-retrieved Re and τ values have a diurnal variation over land but not over oceans.