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Xiaohui Wang, Tim Li, and Suxiang Yao

Abstract

While enhanced rainbands progressed northward in East Asia from June to August during regular El Niño decaying summer, strengthened rainbands were only observed in the earlier summer and disappeared in August in the super El Niño composite. The cause of the distinctive feature is investigated through a combined observational and modeling study. The relative roles of the mean state and anomalous heating in causing the northward progression in the regular El Niño group is assessed through idealized numerical experiments. The result shows that the monthly evolving mean state is more important, while the anomalous forcing also plays a role. The distinctive rainfall feature in the super El Niño composite was primarily contributed by 1982-83 and 2015-16 events, whereas the rainband evolution in 1998 resembled the regular El Niño composite.

The cause of the different rainfall pattern in August among the super El Niño events is further investigated. A marked difference lied on the tropical sea surface temperature anomaly (SSTA) and associated anomalous precipitation patterns. A low-level cyclonic (anticyclonic) anomaly appeared south of Japan in August 1983 and 2016 (1998), inducing northerly (southerly) anomalies and thus suppressed (enhanced) rainfall in eastern China. Whereas an anomalous anticyclone in the western North Pacific (WNP) is a typical response to an El Niño during its mature and decaying phases, the formation of a cyclonic anomaly in the WNP resulted from anomalous enthalpy advection associated with the eastward retreat of an anomalous anticyclone triggered by a local cold SSTA belt in August 1983 and from a Pacific Meridional Mode-like positive SSTA pattern in August 2016.

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Zhigang Yao, Jun Li, Jinlong Li, and Hong Zhang

Abstract

An accurate land surface emissivity (LSE) is critical for the retrieval of atmospheric temperature and moisture profiles along with land surface temperature from hyperspectral infrared (IR) sounder radiances; it is also critical to assimilating IR radiances in numerical weather prediction models over land. To investigate the impact of different LSE datasets on Atmospheric Infrared Sounder (AIRS) sounding retrievals, experiments are conducted by using a one-dimensional variational (1DVAR) retrieval algorithm. Sounding retrievals using constant LSE, the LSE dataset from the Infrared Atmospheric Sounding Interferometer (IASI), and the baseline fit dataset from the Moderate Resolution Imaging Spectroradiometer (MODIS) are performed. AIRS observations over northern Africa on 1–7 January and 1–7 July 2007 are used in the experiments. From the limited regional comparisons presented here, it is revealed that the LSE from the IASI obtained the best agreement between the retrieval results and the ECMWF reanalysis, whereas the constant LSE gets the worst results when the emissivities are fixed in the retrieval process. The results also confirm that the simultaneous retrieval of atmospheric profile and surface parameters could reduce the dependence of soundings on the LSE choice and finally improve sounding accuracy when the emissivities are adjusted in the iterative retrieval. In addition, emissivity angle dependence is investigated with AIRS radiance measurements. The retrieved emissivity spectra from AIRS over the ocean reveal weak angle dependence, which is consistent with that from an ocean emissivity model. This result demonstrates the reliability of the 1DVAR simultaneous algorithm for emissivity retrieval from hyperspectral IR radiance measurements.

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Ruibo Lei, Zhijun Li, Yanfeng Cheng, Xin Wang, and Yao Chen

Abstract

High-precision ice thickness observations are required to gain a better understanding of ocean–ice–atmosphere interactions and to validate numerical sea ice models. A new apparatus for monitoring sea ice and snow thickness has been developed, based on the magnetostrictive-delay-line (MDL) principle for positioning sensors. This system is suited for monitoring fixed measurement sites on undeformed ice. The apparatus presented herein has been tested on landfast ice near Zhongshan Station, East Antarctica, for about 6 months during the austral autumn and winter of 2006; valid data records from the deployment are available for more than 90% of the deployment’s duration. The apparatus’s precision has been estimated to be ±0.002 m for the deployment. Therefore, it is possible that this apparatus may become a standard for sea ice/snow thickness monitoring.

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Zhanyu Yao, Wanbiao Li, Yuanjing Zhu, Bolin Zhao, and Yong Chen

Abstract

The Tibetan Plateau is a unique location for studying the global climate and China's severe weather. The precipitation on the Tibetan Plateau can be studied conveniently with the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI). It is shown that the TMI brightness temperature at 85 GHz in the vertical polarization (TB85V) is negatively correlated to the surface rain rate, but a very low value of TB85V does not correspond to very intense surface rain rates on the Tibetan Plateau, a result that is different from what is observed in other areas of the world. For surface precipitation retrieval on the Tibetan Plateau from TMI, the effect from snow cover on precipitation retrieval is removed before analysis of precipitation. Using the dynamic cluster K-mean method, five categories of surface types and rain areas are identified on the Tibetan Plateau: dry soil, wet soil, water area, stratiform rain area, and convective rain area. The precipitation areas are screened by classification before the precipitation retrieval. Two datasets of rain-free areas and precipitation areas are formed after surface classification. Based on the dataset of rain-free areas, the value of TB85V can be simulated well by TB10V, TB19V, and TB21V when it is not raining. By means of the dataset of precipitation areas, it is revealed that the scattering index over land (SIL) is positively correlated and the polarization-corrected brightness temperature at 85 GHz (PCT85) is negatively correlated with the surface rain rate. With SIL, PCT85, and their combinations as retrieval algorithms, three precipitation retrieval formulas are proposed in which the SIL algorithm is most suitable for small rain retrieval, the PCT85 algorithm is most suitable for moderate rain retrieval, and the combined SIL and PCT85 algorithm is most suitable for relatively large rain retrieval on the Tibetan Plateau. By means of two thresholds, 265 and 245 K, for TB85V, the combination of the three formulas is applied to precipitation retrieval on the Tibetan Plateau during the Tibetan Plateau Experiment Intensive Observing Period of 1998, resulting in acceptable and encouraging surface rain-rate retrievals. Intercomparison among the TMI algorithms and the 17 Special Sensor Microwave Imager algorithms from the second Precipitation Intercomparison Project demonstrates that the comprehensive application of the TMI algorithms has good precision and error index and is suitable for precipitation retrieval on the Tibetan Plateau.

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Yao Xu, Hailun He, Jinbao Song, Yijun Hou, and Funing Li

Abstract

Buoy-based observations of surface waves during three typhoons in the South China Sea were used to obtain the wave characteristics. With the local wind speeds kept below 35 m s−1, the surface waves over an area with a radius 5 times that of the area in which the maximum sustained wind was found were mainly dominated by wind-wave components, and the wave energy distribution was consistent with fetch-limited waves. Swells dominated the surface waves at the front of and outside the central typhoon region. Next, the dynamics of the typhoon waves were studied numerically using a state-of-the-art third-generation wave model. Wind forcing errors made a negligible contribution to the surface wave results obtained using hindcasting. Near-realistic wind fields were constructed by correcting the idealized wind vortex using in situ observational data. If the different sets of source terms were further considered for the forcing stage of the typhoon, which was defined as the half inertial period before and after the typhoon arrival time, the best model performance had mean relative biases and root-mean-square errors of −0.7% and 0.76 m, respectively, for the significant wave height, and −3.4% and 1.115 s, respectively, for the peak wave period. Different sets of source terms for wind inputs and whitecapping breaking dissipation were also used and the results compared. Finally, twin numerical experiments were performed to investigate the importance of nonlinear wave–wave interactions on the spectrum formed. There was evidence that nonlinear wave–wave interactions efficiently transfer wave energy from high frequencies to low frequencies and prevent double-peak structures occurring in the frequency-based spectrum.

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Gengxin Chen, Weiqing Han, Yuanlong Li, Jinglong Yao, and Dongxiao Wang

Abstract

By analyzing in situ observations and conducting a series of ocean general circulation model experiments, this study investigates the physical processes controlling intraseasonal variability (ISV) of the Equatorial Undercurrent (EUC) of the Indian Ocean. ISV of the EUC leads to time-varying water exchanges between the western and eastern equatorial Indian Ocean. For the 2001–14 period, standard deviations of the EUC transport variability are 1.92 and 1.77 Sv (1 Sv ≡ 106 m3 s−1) in the eastern and western basins, respectively. The ISV of the EUC is predominantly caused by the wind forcing effect of atmospheric intraseasonal oscillations (ISOs) but through dramatically different ocean dynamical processes in the eastern and western basins. The stronger ISV in the eastern basin is dominated by the reflected Rossby waves associated with intraseasonal equatorial zonal wind forcing. It takes 20–30 days to set up an intraseasonal EUC anomaly through the Kelvin and Rossby waves associated with the first and second baroclinic modes. In the western basin, the peak intraseasonal EUC anomaly is generated by the zonal pressure gradient force, which is set up by radiating equatorial Kelvin and Rossby waves induced by the equatorial wind stress. Directly forced and reflected Rossby waves from the eastern basin propagate westward, contributing to intraseasonal zonal current near the surface but having weak impact on the peak ISV of the EUC.

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Moran Zhuang, Anmin Duan, Riyu Lu, Puxi Li, and Jinglong Yao

Abstract

The Indochina Peninsula (ICP) has a critical effect in shaping the Asian summer monsoon (ASM). However, the seasonal responses of the ASM to the ICP are not fully understood. This study employs a 1° atmospheric general circulation model to examine the different contributions of the ICP’s orography and land–sea contrast to the ASM during the early and late summer. Results indicate that the orographic effect increases South Asian rainfall and reduces the rainfall over the South China Sea (SCS) and North China in early summer, but its influence on monsoonal circulation and rainfall is limited to East Asia in late summer. The impact of the ICP’s land–sea contrast is basically opposite in the two summer stages. With the presence of the ICP, SCS rainfall is enhanced but South Asian rainfall is weakened in early summer. In late summer, however, rainfall from the ICP to the northwestern Pacific is strikingly reduced, accompanied by intensified rainfall over South Asia. Relatively, the orographic effect seems to be more important in modulating the South Asian monsoon in early summer, while the land–sea contrast is dominant in strengthening the SCS monsoon and suppressing the northwest Pacific monsoon via the interaction between the induced local circulation and multilevel ASM subsystems. In late summer, the orographic effect on the ASM is much weaker compared to the land–sea contrast, which plays a critical role by shifting the subtropical high southwestward and through the “thermal adaption” feedback mechanism. Therefore, the orographic impact of the ICP on the ASM differs from that of the land–sea contrast in the two summer stages.

Open access
Yingbin He, Dongmei Liu, Yanmin Yao, Qing Huang, Jianping Li, Youqi Chen, Shuqin Shi, Li Wan, Shikai Yu, and Deying Wang

Abstract

In this paper, an integrated indicator-based system is established to map the suitability of spring soybean cultivation in northeast China. The indicator system incorporates both biophysical and socioeconomic factors, including the effects of temperature, precipitation, and sunshine on the individual development stages of the spring soybean life cycle. Spatial estimates of crop suitability derived using this indicator system are also compared with spring soybean planting areas to identify locations where there is scope for structural adjustment in soybean farming. Results of this study indicate that northeast China is moderately suited to spring soybean cultivation. Areas classified as suitable, moderately suitable, and unsuitable for soybean cultivation, respectively, occupy approximately 9.09 × 104, 11.45 × 104, and 7.99 × 104 km2, accounting for 11.5%, 10.11%, and 14.49% of the total area of northeast China. The Songnen and Sanjiang Plains are identified as the most and least suitable places, respectively, for spring soybean growth. A comparative analysis indicates that the suitable, moderately suitable, and unsuitable areas account for 24.78%, 46.30%, and 28.92%, respectively, of the total area presently under soybean cultivation. The analysis suggests that soybean cultivation in Heilongjiang Province is generally unfavorable, with equivalent percentages of 15.39%, 51.70%, and 32.91%. Results suggest that agricultural structural adjustment may be required to encourage farmers to grow spring soybeans. It is anticipated that this study will provide a basis for follow-up studies on crop cultivation suitability.

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Xudong Liang, Yanxin Xie, Jinfang Yin, Yi Luo, Dan Yao, and Feng Li

Abstract

Dealiasing is a common procedure in radar radial velocity quality control. Generally, there are two dealiasing steps: a continuity check and a reference check. In this paper, a modified version that uses azimuthal variance of radial velocity is introduced based on the integrating velocity–azimuth process (IVAP) method, referred to as the V-IVAP method. The new method can retrieve the averaged winds within a local area instead of averaged wind within a full range circle by the velocity–azimuth display (VAD) or the modified VAD method. The V-IVAP method is insensitive to the alias of the velocity, and provides a better way to produce reference velocities for a reference check. Instead of a continuity check, we use the IVAP method for a fine reference check because of its high-frequency filtering function. Then a dealiasing procedure with two steps of reference check is developed. The performance of the automatic dealiasing procedure is demonstrated by retrieving the wind field of a tornado. Using the dealiased radar velocities, the retrieved winds reveal a clear mesoscale vortex. A test based on radar network observations also has shown that the two-step dealiasing procedure based on V-IVAP and IVAP methods is reliable.

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Dong-Peng Guo, Peng Zhao, Ren-Tai Yao, Yun-Peng Li, Ji-Min Hu, and Dan Fan

Abstract

In this paper, the kε renormalization group (RNG) turbulence model is used to simulate the flow and dispersion of pollutants emitted from a source at the top of a cubic building under neutral and stable atmospheric stratifications, the results of which were compared with corresponding wind tunnel experiment results. When atmosphere stratification is stable, the separation zones on the sides and at the top of a building are relatively smaller than those under neutral conditions, and the effect of the building in the horizontal direction is stronger than that in the vertical direction. The variation in turbulent kinetic energy under stable conditions is significantly lower than that under neutral conditions. The effect of atmospheric stratification on the turbulent kinetic energy becomes gradually more prominent with increased distance. When atmosphere conditions are stable, the vertical distribution of the plume is smaller than that of neutral conditions, but the lateral spread and near-ground concentration are slightly larger than those of neutral conditions, mainly because stable atmospheric stratification suppresses the vertical motions of airflow and increases the horizontal spread of the plume.

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