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2023 Vol. 40, No. 10

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Original Paper
Formation of Low-Level Jets over Southern China in the Mei-yu Season
Xuanyu LIU, Guixing CHEN, Sijia ZHANG, Yu DU
2023, 40(10): 1731-1748. doi: 10.1007/s00376-023-2358-5
This study examines the diversity of low-level jet (LLJ) formation and related physical processes over southern China. A total of 171 LLJ formation events with enhanced daily southwesterlies and early-morning maximum wind speeds were observed during the mei-yu seasons of 1989–2018. The LLJs can be further categorized into four types based on the increases in the daily mean and diurnal amplitude of the low-level winds. Analysis of the synoptic-scale disturbances shows that the two types of LLJ formation (Q1 and Q4), which feature large increases of daily southerly wind components, are mainly induced by west-east dipole patterns of pressure change, in association with the enhanced southwest vortex and/or the western Pacific subtropical high (WPSH). In contrast, the other two types (Q2 and Q3), which feature relatively large increases in their daily westerly components, are related to a northwest-southeast dipole pattern of pressure change due to the mid-latitude trough and the WPSH. We further analyze the considerable variations in the diurnal thermal forcing among the LLJ formation events. The strong (weak) daytime heating of solar radiation leads to relatively large (small) increases in the diurnal amplitude of low-level winds in Q1 and Q2 (Q3 and Q4) types. Therefore, different combinations of synoptic-scale disturbances and diurnal thermal forcings are found to account for the diversity in LLJ formation and associated differences in downstream rainfall patterns. These results help to improve our understanding and prediction of the formation of LLJs.
Monsoon Break over the South China Sea during Summer: Statistical Features and Associated Atmospheric Anomalies
Minghao BI, Ke XU, Riyu LU
2023, 40(10): 1749-1765. doi: 10.1007/s00376-023-2377-2
This study identifies break events of the South China Sea (SCS) summer monsoon (SCSSM) based on 42 years of data from 1979 to 2020, and investigates their statistical characteristics and associated atmospheric anomalies. A total of 214 break events are identified by examining the convection evolution during each monsoon season. It is found that most events occur between June and September and show a roughly even distribution. Short-lived events (3–7 days) are more frequent, accounting for about two thirds of total events, with the residual one third for long-lived events (8–24 days). The SCSSM break is featured by drastic variations in various atmospheric variables. Particularly, the convection and precipitation change from anomalous enhancement in adjoining periods to a substantial suppression during the break, with the differences being more than 60 W m−2 for outgoing longwave radiation (OLR) and 10 mm d−1 for precipitation. This convection/precipitation suppression is accompanied by an anomalous anticyclone in the lower troposphere, corresponding to a remarkable westward retreat of the monsoon trough from the Philippine Sea to the Indochina Peninsula, which reduces the transportation of water vapor into the SCS. Besides, the pseudo-equivalent potential temperature (\begin{document}$ {\theta }_{\mathrm{s}\mathrm{e}} $\end{document}) declines sharply, mainly attributable to the local specific humidity reduction caused by downward dry advection. Furthermore, it is found that the suppressed convection and anomalous anticyclone responsible for the monsoon break form near the equatorial western Pacific and then propagate northwestward to the SCS.
Interdecadal Enhancement in the Relationship between the Western North Pacific Summer Monsoon and Sea Surface Temperature in the Tropical Central-Western Pacific after the Early 1990s
Kui LIU, Lian-Tong ZHOU, Zhibiao WANG, Yong LIU
2023, 40(10): 1766-1782. doi: 10.1007/s00376-023-2200-0
This study reveals the strengthened interdecadal relationship between the western North Pacific summer monsoon (WNPSM) and tropical central-western Pacific sea surface temperature anomaly (SSTA) in summer after the early 1990s. In the first period (1979–91, P1), the WNPSM-related precipitation anomaly and horizontal wind anomaly present themselves as an analogous Pacific-Japan (PJ)-like pattern, generally considered to be related to the Niño-3 index in the preceding winter. During the subsequent period (1994–2019, P2), the WNPSM-related precipitation anomaly presents a zonal dipole pattern, correlated significantly with the concurrent SSTA in the Niño-4 and tropical western Pacific regions. The negative (positive) SSTA in the tropical western Pacific and positive (negative) SSTA in the Niño-4 region, could work together to influence the WNPSM, noting that the two types of anomalous SSTA configurations enhance (weaken) the WNPSM by the positive (negative) phase PJ-like wave and Gill response, respectively, with an anomalous cyclone (anticyclone) located in the WNPSM, which shows obvious symmetry about the anomalous circulation. Specifically, the SSTA in Niño-4 impacts the WNPSM by an atmospheric Gill response, with a stronger (weaker) WNPSM along with a positive (negative) SSTA in the Niño-4 region. Furthermore, the SSTA in the tropical western Pacific exerts an influence on the WNPSM by a PJ-like wave, with a stronger (weaker) WNPSM along with a negative (positive) SSTA in the tropical western Pacific. In general, SSTAs in the tropical western Pacific and Niño-4 areas could work together to exert influence on the WNPSM, with the effect most likely to occur in the El Niño (La Niña) developing year in P2. However, the SSTAs in the tropical western Pacific worked alone to exert an influence on the WNPSM mainly in 2013, 2014, 2016, and 2017, and the SSTAs in the Niño-4 region worked alone to exert an influence on the WNPSM mainly in Central Pacific (CP) La Niña developing years. The sensitivity experiments also can reproduce the PJ-like wave/Gill response associated with SSTA in the tropical western Pacific/Niño-4 regions. Therefore, the respective and synergistic impacts from the Niño-4 region and the tropical western Pacific on the WNPSM have been revealed, which helps us to acquire a better understanding of the interdecadal variations of the WNPSM and its associated climate influences.
Near Homogeneous Microphysics of the Record-Breaking 2020 Summer Monsoon Rainfall during the Northward Migration over East China
Long WEN, Wei ZHANG, Cha YANG, Gang CHEN, Yajun HU, Hao ZHANG
2023, 40(10): 1783-1798. doi: 10.1007/s00376-023-2242-3
Knowledge of the raindrop size distribution (DSD) is crucial for disaster prevention and mitigation. The record-breaking rainfall in the summer of 2020 caused some of the worst flooding ever experienced in China. This study uses 96 Parsivel disdrometers and eight-year Global Precipitation Measurement (GPM) satellite observations to reveal the microphysical aspects of the disastrous rainfall during its northward migration over East China. The results show that the nearly twice as heavy rainfall in Jiangsu Province compared to Fujian Province can be attributed to the earlier-than-average northward jump of the summer monsoon rainband to the Yangtze-Huaihe River valley. The persistent heavy monsoon rainfall showed similar near-maritime DSD characteristics, with a higher concentration of small raindrops than the surrounding climatic regimes. During the northward movement of the rainband, the DSD variables and composite spectra between the pre-summer rainfall in Fujian and mei-yu rainfall in Jiangsu exhibited inherent similarities with slight regional variations. These are associated with similar statistical vertical precipitation structures for both convective and stratiform rain in these regions/periods. The vertical profiles of radar reflectivity and DSD parameters are typical of monsoonal rainfall features, implying the competition between coalescence, breakup, and accretion of vital warm rain processes. This study attributes the anomalously long duration of the mei-yu season for the record-breaking rainfall and reveals inherent homogeneous rainfall microphysics during the northward movement of the summer monsoon rainband. The conclusion is statistically robust and would be helpful for accurate precipitation estimation and model parameterization of summer monsoon rainfall over East China.
Progress of MJO Prediction at CMA from Phase I to Phase II of the Sub-Seasonal to Seasonal Prediction Project
Junchen YAO, Xiangwen LIU, Tongwen WU, Jinghui YAN, Qiaoping LI, Weihua JIE
2023, 40(10): 1799-1815. doi: 10.1007/s00376-023-2351-z
As one of the participants in the Subseasonal to Seasonal (S2S) Prediction Project, the China Meteorological Administration (CMA) has adopted several model versions to participate in the S2S Project. This study evaluates the models’ capability to simulate and predict the Madden-Julian Oscillation (MJO). Three versions of the Beijing Climate Center Climate System Model (BCC-CSM) are used to conduct historical simulations and re-forecast experiments (referred to as EXP1, EXP1-M, and EXP2, respectively). In simulating MJO characteristics, the newly-developed high-resolution BCC-CSM outperforms its predecessors. In terms of MJO prediction, the useful prediction skill of the MJO index is enhanced from 15 days in EXP1 to 22 days in EXP1-M, and further to 24 days in EXP2. Within the first forecast week, the better initial condition in EXP2 largely contributes to the enhancement of MJO prediction skill. However, during forecast weeks 2–3, EXP2 shows little advantage compared with EXP1-M because the increased skill at MJO initial phases 6–7 is largely offset by the degraded skill at MJO initial phases 2–3. Particularly at initial phases 2–3, EXP1-M skillfully captures the wind field and Kelvin-wave response to MJO convection, leading to the highest prediction skill of the MJO. Our results reveal that, during the participation of the CMA models in the S2S Project, both the improved model initialization and updated model physics played positive roles in improving MJO prediction. Future efforts should focus on improving the model physics to better simulate MJO convection over the Maritime Continent and further improve MJO prediction at long lead times.
Evaluating Parameterizations for Turbulent Fluxes over the Landfast Sea-Ice Surface in Prydz Bay, Antarctica
Fangyuan CHENG, Qinghua YANG, Changwei LIU, Bo HAN, Shijie PENG, Guanghua HAO
2023, 40(10): 1816-1832. doi: 10.1007/s00376-023-2299-z
It is crucial to appropriately determine turbulent fluxes in numerical models. Using data collected in East Antarctica from 8 April to 26 November 2016, this study evaluates parameterization schemes for turbulent fluxes over the landfast sea-ice surface in five numerical models. The Community Noah Land Surface Model with Multi-Parameterizations Options (Noah_mp) best replicates the turbulent momentum flux, while the Beijing Climate System Model (BCC_CSM) produces the optimum sensible and latent heat fluxes. In particular, two critical issues of parameterization schemes, stability functions and roughness lengths, are investigated. Sensitivity tests indicate that roughness lengths play a decisive role in model performance. Based on the observed turbulent fluxes, roughness lengths over the landfast sea-ice surface are calculated. The results, which can provide a basis for setting up model parameters, reveal that the dynamic roughness length (z0m) increases with the increase of frictional velocity (u*) when u* ≤ 0.4 m s−1 and fluctuates around 10−3 m when u* > 0.4 m s−1; thermal roughness length (z0t) is linearly related to the temperature gradient between air and sea-ice surface (ΔT) with a relation of lg(z0t) = −0.29ΔT−3.86; and the mean water vapor roughness length (z0q) in the specific humidity gradient (Δq) range of Δq ≤ −0.6 g kg−1 is 10−6 m, 3.5 times smaller than that in the range of Δq ˃ −0.6 g kg−1.
Increasing Surface UV Radiation in the Tropics and Northern Mid-Latitudes due to Ozone Depletion after 2010
Fei XIE, Yan XIA, Wuhu FENG, Yingli NIU
2023, 40(10): 1833-1843. doi: 10.1007/s00376-023-2354-9
Excessive exposure to ultraviolet (UV) radiation harms humans and ecosystems. The level of surface UV radiation had increased due to declines in stratospheric ozone in the late 1970s in response to emissions of chlorofluorocarbons. Following the implementation of the Montreal Protocol, the stratospheric loading of chlorine/bromine peaked in the late 1990s and then decreased; subsequently, stratospheric ozone and surface UV radiation would be expected to recover and decrease, respectively. Here, we show, based on multiple data sources, that the May–September surface UV radiation in the tropics and Northern Hemisphere mid-latitudes has undergone a statistically significant increasing trend [about 60.0 J m–2 (10 yr)–1] at the 2σ level for the period 2010–20, due to the onset of total column ozone (TCO) depletion [about −3.5 DU (10 yr)–1]. Further analysis shows that the declines in stratospheric ozone after 2010 could be related to an increase in stratospheric nitrogen oxides due to increasing emissions of the source gas nitrous oxide (N2O).
A Multi-Domain Compression Radiative Transfer Model for the Fengyun-4 Geosynchronous Interferometric Infrared Sounder (GIIRS)
Mingyue SU, Chao LIU, Di DI, Tianhao LE, Yujia SUN, Jun LI, Feng LU, Peng ZHANG, Byung-Ju SOHN
2023, 40(10): 1844-1858. doi: 10.1007/s00376-023-2293-5
Forward radiative transfer (RT) models are essential for atmospheric applications such as remote sensing and weather and climate models, where computational efficiency becomes equally as important as accuracy for high-resolution hyperspectral measurements that need rigorous RT simulations for thousands of channels. This study introduces a fast and accurate RT model for the hyperspectral infrared (HIR) sounder based on principal component analysis (PCA) or machine learning (i.e., neural network, NN). The Geosynchronous Interferometric Infrared Sounder (GIIRS), the first HIR sounder onboard the geostationary Fengyun-4 satellites, is considered to be a candidate example for model development and validation. Our method uses either PCA or NN (PCA/NN) twice for the atmospheric transmittance and radiance, respectively, to reduce the number of independent but similar simulations to accelerate RT simulations; thereby, it is referred to as a multi-domain compression model. The first PCA/NN gives monochromatic gas transmittance in both spectral and atmospheric pressure domains for each gas independently. The second PCA/NN is performed in the traditional spectral radiance domain. Meanwhile, a new method is introduced to choose representative variables for the PCA/NN scheme developments. The model is three orders of magnitude faster than the standard line-by-line-based simulations with averaged brightness temperature difference (BTD) less than 0.1 K, and the compressions based on PCA or NN methods result in comparable efficiency and accuracy. Our fast model not only avoids an excessively complicated transmittance scheme by using PCA/NN but is also highly flexible for hyperspectral instruments with similar spectral ranges simply by updating the corresponding spectral response functions.
Evaluation of the Predicted Particle Properties (P3) Microphysics Scheme in Simulations of Stratiform Clouds with Embedded Convection
Tuanjie HOU, Baojun CHEN, Hengchi LEI, Lei WEI, Youjiang HE, Qiujuan FENG
2023, 40(10): 1859-1876. doi: 10.1007/s00376-023-2178-7
To evaluate the ability of the Predicted Particle Properties (P3) scheme in the Weather Research and Forecasting (WRF) model, we simulated a stratiform rainfall event over northern China on 22 May 2017. WRF simulations with two P3 versions, P3-nc and P3-2ice, were evaluated against rain gauge, radar, and aircraft observations. A series of sensitivity experiments were conducted with different collection efficiencies between ice and cloud droplets. The comparison of the precipitation evolution between P3-nc and P3-2ice suggested that both P3 versions overpredicted surface precipitation along the Taihang Mountains but underpredicted precipitation in the localized region on the leeward side. P3-2ice had slightly lower peak precipitation rates and smaller total precipitation amounts than P3-nc, which were closer to the observations. P3-2ice also more realistically reproduced the overall reflectivity structures than P3-nc. A comparison of ice concentrations with observations indicated that P3-nc underestimated aggregation, whereas P3-2ice produced more active aggregation from the self-collection of ice and ice-ice collisions between categories. Efficient aggregation in P3-2ice resulted in lower ice concentrations at heights between 4 and 6 km, which was closer to the observations. In this case, the total precipitation and precipitation pattern were not sensitive to riming. Riming was important in reproducing the location and strength of the embedded convective region through its impact on ice mass flux above the melting level.
Classification of Hailstone Trajectories in a Hail Cloud over a Semi-Arid Region in China
Xiaoli LIU, Kerui MIN, Jianren SANG, Simin MA
2023, 40(10): 1877-1894. doi: 10.1007/s00376-023-2156-0
The growth trajectory of hailstones in clouds determines the ground intensity and spatial distribution of hailfall. A systematic study of hail trajectories can help improve the current scientific understanding of the mechanisms by which hail forms in semi-arid regions of China and, in doing so, improve the quality of hail forecasts and warnings and help to prevent and mitigate disasters. In this study, the WRFv3.7.1 model was employed to provide the background field to drive the hailstone trajectory model. Cluster analysis was then used to classify hail trajectories to investigate the characteristics of different types of hail trajectories and the microphysical characteristics of hail formation. The differences in hail trajectories might be mainly due to differences in the background flow fields and microphysical fields of hail clouds in different regions. Comparative analysis revealed that as the maximum particle size of ground hailfall increased, the maximum supercooled cloud water content and the maximum updraft velocity for the formation and growth of hailstone increased. The larger the size when the hailstone reaches its maximum height, the larger the ground hailstone formed. Overall, the formation and growth of hailstone are caused by the joint action of the dynamical flow field and cloud microphysical processes. The physical processes of hailstone growth and main growth regions differ for different types of hail trajectories. Therefore, different catalytic schemes should be adopted in artificial hail prevention operations for different hail clouds and trajectories due to differences in hail formation processes and ground hailfall characteristics.
Seasonal Forecasts of Precipitation during the First Rainy Season in South China Based on NUIST-CFS1.0
Sinong LI, Huiping YAN, Jing-Jia LUO
2023, 40(10): 1895-1910. doi: 10.1007/s00376-023-2318-0
Current dynamical models experience great difficulties providing reliable seasonal forecasts of regional/local rainfall in South China. This study evaluates seasonal forecast skill for precipitation in the first rainy season (FRS, i.e., April–June) over South China from 1982 to 2020 based on the global real-time Climate Forecast System of Nanjing University of Information Science and Technology (NUIST-CFS1.0, previously known as SINTEX-F). The potential predictability and the practical forecast skill of NUIST-CFS1.0 for FRS precipitation remain low in general. But NUIST-CFS1.0 still performs better than the average of nine international models in terms of correlation coefficient skill in predicting the interannual precipitation anomaly and its related circulation index. NUIST-CFS1.0 captures the anomalous Philippines anticyclone, which transports moisture and heat northward to South China, favoring more precipitation in South China during the FRS. By examining the correlations between sea surface temperature (SST) and FRS precipitation and the Philippines anticyclone, we find that the model reasonably captures SST-associated precipitation and circulation anomalies, which partly explains the predictability of FRS precipitation. A dynamical downscaling model with 30-km resolution forced by the large-scale circulations of the NUIST-CFS1.0 predictions could improve forecasts of the climatological states and extreme precipitation events. Our results also reveal interesting interdecadal changes in the predictive skill for FRS precipitation in South China based on the NUIST-CFS1.0 hindcasts. These results help improve the understanding and forecasts for FRS precipitation in South China.
Improved Microwave Ocean Emissivity and Reflectivity Models Derived from Two-Scale Roughness Theory
Lingli HE, Fuzhong WENG
2023, 40(10): 1923-1938. doi: 10.1007/s00376-023-2247-y
The Geometrical Optics (GO) approach and the FAST Emissivity Model (FASTEM) are widely used to estimate the surface radiative components in atmospheric radiative transfer simulations, but their applications are limited in specific conditions. In this study, a two-scale reflectivity model (TSRM) and a two-scale emissivity model (TSEM) are developed from the two-scale roughness theory. Unlike GO which only computes six non-zero elements in the reflectivity matrix, The TSRM includes 16 elements of Stokes reflectivity matrix which are important for improving radiative transfer simulation accuracy in a scattering atmosphere. It covers the frequency range from L- to W-bands. The dependences of all TSRM elements on zenith angle, wind speed, and frequency are derived and analyzed in details. For a set of downwelling radiances in microwave frequencies, the reflected upwelling brightness temperature (BTs) are calculated from both TSRM and GO and compared for analyzing their discrepancies. The TSRM not only includes the effects of GO but also accounts for the small-scale Bragg scattering effect in an order of several degrees in Kelvins in brightness temperature. Also, the third and fourth components of the Stokes vector can only be produced from the TSRM. For the emitted radiation, BT differences in vertical polarization between a TSEM and FASTEM are generally less than 5 K when the satellite zenith angle is less than 40°, whereas those for the horizontal component can be quite significant, greater than 20 K.
Data Description Article
CAS FGOALS-f3-L Model Datasets for CMIP6 DCPP Experiment
Shuai HU, Bo WU, Yiming WANG, Tianjun ZHOU, Yongqiang YU, Bian HE, Pengfei LIN, Qing BAO, Hailong LIU, Kangjun CHEN, Shuwen ZHAO
2023, 40(10): 1911-1922. doi: 10.1007/s00376-023-2122-x
The outputs of the Chinese Academy of Sciences (CAS) Flexible Global Ocean-Atmosphere-Land System (FGOALS-f3-L) model for the decadal climate prediction project (DCPP) of the Coupled Model Intercomparison Project Phase 6 (CMIP6) are described in this paper. The FGOALS-f3-L was initialized through the upgraded, weakly coupled data assimilation scheme, referred to as EnOI-IAU, which assimilates observational anomalies of sea surface temperature (SST) and upper-level (0–1000-m) ocean temperature and salinity profiles into the coupled model. Then, nine ensemble members of 10-year hindcast/forecast experiments were conducted for each initial year over the period of 1960–2021, based on initial conditions produced by three initialization experiments. The hindcast and forecast experiments follow the experiment designs of the Component-A and Component-B of the DCPP, respectively. The decadal prediction output datasets contain a total of 44 monthly mean atmospheric and oceanic variables. The preliminary evaluation indicates that the hindcast experiments show significant predictive skill for the interannual variations of SST in the north Pacific and multi-year variations of SST in the subtropical Pacific and the southern Indian Ocean.