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Current Issue  
2019 Vol. 33, No. 1
Published: 2019-02-28

Role of Differences in Surface Diurnal–Nocturnal Thermodynamics over Complex Terrain in a Squall Line Process
Wei TAO, Xuexing QIU, Ruijiao WU, Kun ZHOU
2019, 33(1): 1-17 [Abstract]( 262 ) HTML PDF (11706 KB)  ( 246 Supplemental Material
Abstract:Squall lines frequently invade the Yangtze-Huaihe River region (YHR), where the complex terrain of rivers, lakes, and mountains plays an important role in the initiation and maintenance of convection. The surface heat flux not only varies with surface conditions, but also changes between day and night. Coupled with the terrain forcing, such diur-nal-nocturnal thermodynamic differences shift the low-level baroclinity, and thus further complicate the convective activities. To investigate the integrated impact of diurnal-nocturnal thermodynamic differences on the development of squall lines over complex terrain including disasters that might ensue, numerical modeling experiments on a squall line in July 2014 were performed by forcing a squall line to pass the YHR separately at daytime and nighttime. The results show that the low-level instability during the day is much larger than that during the night, and is determined predominantly by the shortwave heating of the surface. Specifically, the solar radiation enhances the temperature gradient between the warmland ahead of the squall line and the convectively generated cold pool in the region around Chaohu Lake and the Yangtze River. Such low-level baroclinity sets preconditions in the environment towards the occurrence of deep convection. The increased precipitation and the evaporation of rain in the daytime also enhance the cold pool and the associated downdraft, which further intensify the squall line. Meanwhile, the valley breeze is intensified during the day. Such scenarios promote convection that extends the squall line and the associated heavy precipitation and wind gusts southward. This research may have significant implications for enhancing the squall line prediction capability in the YHR and improving our understanding of the physical mechanisms of convective activities over complex terrain.
Diurnal Variations of Summer Precipitation over the Qilian Mountains in Northwest China
Liangliang LI, Jian LI, Haoming CHEN, Rucong YU
2019, 33(1): 18-30 [Abstract]( 255 ) HTML PDF (4823 KB)  ( 234 Supplemental Material
Abstract:Based on the high-density hourly rain-gauge data from 265 stations over the Qilian Mountains in Northwest China, climatic mean diurnal variations of summer rainfall over different topographies of this area are investigated. Influences of the gauge elevations on the diurnal variation of rainfall are also revealed. Distinct regional features of diur-nal variations in rainfall are observed over the Qilian Mountains. Rainfall over the Qinghai Lake areas shows a single nocturnal peak. A dominant, late-afternoon peak of rainfall occurs over the mountain tops. Over the northeastern and southeastern slopes, a dominant diurnal peak appears in the late afternoon, and an evident second peak is found in the early morning, respectively. The strengths of the early-morning peaks in the rainfall frequency are closely related to the rainfall events with different durations over the two slopes. The early-morning peak is dominant across plains with low elevations. From the mountain tops to the plains, the diurnal peaks of rainfall gradually vary from the domi-nant late-afternoon peak to the dominant early-morning peak with the enhanced early-morning peak in concurrent with the decreasing gauge elevation over the northeastern and southeastern slopes. Further examination indicates that the rainfall at higher elevations over the northeastern and southeastern slopes occurs more readily in the afternoon, compared to the lower elevations. This phenomenon corresponds to the result that the proportion of the rainfall frequency occurring during the early-morning period decreases with increasing elevations over the two slopes.
Climate Sensitivity and Feedbacks of a New Coupled Model CAMS-CSM to Idealized CO2 Forcing: A Comparison with CMIP5 Models
Xiaolong CHEN, Zhun GUO, Tianjun ZHOU, Jian LI, Xinyao RONG, Yufei XIN, Haoming CHEN, Jingzhi SU
2019, 33(1): 31-45 [Abstract]( 207 ) HTML PDF (8450 KB)  ( 114 Supplemental Material
Abstract:Climate sensitivity and feedbacks are basic and important metrics to a climate system. They determine how large surface air temperature will increase under CO2 forcing ultimately, which is essential for carbon reduction policies to achieve a specific warming target. In this study, these metrics are analyzed in a climate system model newly developed by the Chinese Academy of Meteorological Sciences (CAMS-CSM) and compared with multi-model results from the Coupled Model Comparison Project phase 5 (CMIP5). Based on two idealized CO2 forcing scenarios, i.e., abruptly quadrupled CO2 and CO2 increasing 1% per year, the equilibrium climate sensitivity (ECS) and transient climate response (TCR) in CAMS-CSM are estimated to be about 2.27 and 1.88 K, respectively. The ECS is near the lower bound of CMIP5 models whereas the TCR is closer to the multi-model ensemble mean (MME) of CMIP5 due to compensation of a relatively low ocean heat uptake (OHU) efficiency. The low ECS is caused by an unusually negative climate feedback in CAMS-CSM, which is attributed to cloud shortwave feedback (λSWCL) over the tropical Indo-Pacific Ocean.The CMIP5 ensemble shows that more negative λSWCL is related to larger increase in low-level (925-700 hPa) cloud over the tropical Indo-Pacific under warming, which can explain about 90% of λSWCL in CAMS-CSM. Static stability of planetary boundary layer in the pre-industrial simulation is a critical factor controlling the low-cloud response and λSWCL across the CMIP5 models and CAMS-CSM. Evidently, weak stability in CAMS-CSM favors low-cloud formation under warming due to increased low-level convergence and relative humidity, with the help of enhanced evaporation from the warming tropical Pacific. Consequently, cloud liquid water increases, amplifying cloud albedo, and eventually contributing to the unusually negative λSWCL and low ECS in CAMS-CSM. Moreover, the OHU may influence climate feedbacks and then the ECS by modulating regional sea surface temperature responses.
ENSO Features, Dynamics, and Teleconnections to East Asian Climate as Simulated in CAMS-CSM
Bo LU, Hong-Li REN
2019, 33(1): 46-65 [Abstract]( 257 ) HTML PDF (3559 KB)  ( 124
Abstract:This study evaluates the performance of CAMS-CSM (the climate system model of the Chinese Academy of Meteorological Sciences) in simulating the features, dynamics, and teleconnections to East Asian climate of the El Niño-Southern Oscillation (ENSO). In general, fundamental features of ENSO, such as its dominant patterns and phase-locking features, are reproduced well. The two types of El Niño are also represented, in terms of their spatial distributions and mutual independency. However, the skewed feature is missed in the model and the simulation of ENSO is extremely strong, which is found-based on Bjerknes index assessment-to be caused by underestimation of the shortwave damping effect. Besides, the modeled ENSO exhibits a regular oscillation with a period shorter than observed. By utilizing the Wyrtki index, it is suggested that this periodicity bias results from an overly quick phase transition induced by feedback from the thermocline and zonal advection. In addition to internal dynamics of ENSO, its external precursors-such as the North Pacific Oscillation with its accompanying seasonal footprinting mechanism, and the Indian Ocean Dipole with its 1-yr lead correlation with ENSO-are reproduced well by the model. Furthermore, with respect to the impacts of ENSO on the East Asian summer monsoon, although the anomalous Philippine anticyclone is reproduced in the post-El Niño summer, it exhibits an eastward shift compared with observation; and as a consequence, the observed flooding of the Yangtze River basin is poorly represented, with unrealistic air-sea interaction over the South China Sea being the likely physical origin of this bias. The response of wintertime lower-tropospheric circulation to ENSO is simulated well, in spite of an underestimation of temperature anomalies in central China. This study highlights the dynamic processes that are key for the simulation of ENSO, which could shed some light on improving this model in the future.
Boreal Summer Intraseasonal Oscillation in the Asian–Pacific Monsoon Region Simulated in CAMS-CSM
Yanjun QI, Renhe ZHANG, Xinyao RONG, Jian LI, Lun LI
2019, 33(1): 66-79 [Abstract]( 166 ) HTML PDF (2859 KB)  ( 155 Supplemental Material
Abstract:The boreal summer intraseasonal oscillation (BSISO) is simulated by the Climate System Model (CSM) developed at the Chinese Academy of Meteorological Sciences (CAMS), China Meteorological Administration. Firstly, the results indicate that this new model is able to reasonably simulate the annual cycle and seasonal mean of the precipitation, as well as the vertical shear of large-scale zonal wind in the tropics. The model also reproduces the eastward and northward propagating oscillation signals similar to those found in observations. The simulation of BSISO is generally in agreement with the observations in terms of variance center, periodicity, and propagation, with the exception that the magnitude of BSISO anomalous convections are underestimated during both its eastward propagation along the equator and its northward propagation over the Asian-Pacific summer monsoon region. Our preliminary evaluation of the simulated BSISO by CAMS-CSM suggests that this new model has the capability, to a certain extent, to capture the BSISO features, including its propagation zonally along the equator and meridionally over the Asian monsoon region.
An Assessment of ENSO Stability in CAMS Climate System Model Simulations
Lijuan HUA, Lin CHEN, Xinyao RONG, Jian LI, Guo ZHANG, Lu WANG
2019, 33(1): 80-88 [Abstract]( 211 ) HTML PDF (3299 KB)  ( 184 Supplemental Material
Abstract:We present an overview of the El Niño–Southern Oscillation (ENSO) stability simulation using the Chinese Academy of Meteorological Sciences climate system model (CAMS-CSM). The ENSO stability was quantified based on the Bjerknes (BJ) stability index. Generally speaking, CAMS-CSM has the capacity of reasonably representing the BJ index and ENSO-related air–sea feedback processes. The major simulation biases exist in the underestimated thermodynamic damping and thermocline feedbacks. Further diagnostic analysis reveals that the underestimated thermodynamic feedback is due to the underestimation of the shortwave radiation feedback, which arises from the cold bias in mean sea surface temperature (SST) over central–eastern equatorial Pacific (CEEP). The underestimated thermocline feedback is attributed to the weakened mean upwelling and weakened wind–SST feedback (μa) in the model simulation compared to observation. We found that the weakened μa is also due to the cold mean SST over the CEEP. The study highlights the essential role of reasonably representing the climatological mean state in ENSO simulations.
Remote Sensing of Tropical Cyclone Thermal Structure from Satellite Microwave Sounding Instruments: Impacts of Background Profiles on Retrievals
Hao HU, Fuzhong WENG, Yang HAN, Yihong DUAN
2019, 33(1): 89-103 [Abstract]( 277 ) HTML PDF (5444 KB)  ( 405 Supplemental Material
Abstract:A variational retrieval system often requires background atmospheric profiles and surface parameters in its minimization process. This study investigates the impacts of specific background profiles on retrievals of tropical cyclone (TC) thermal structure. In our Microwave Retrieval Testbed (MRT), the K-means clustering algorithm is utilized to generate a set of mean temperature and water vapor profiles according to stratiform and convective precipitation in hurricane conditions. The Advanced Technology Microwave Sounder (ATMS) observations are then used to select the profiles according to cloud type. It is shown that the cloud-based background profiles result in better hurricane thermal structures retrieved from ATMS observations. Compared to the Global Positioning System (GPS) dropsonde observations, the temperature and specific humidity errors in the TC inner region are less than 3 K and 2.5 g kg-1, respectively, which are significantly smaller than the retrievals without using the cloud-based profiles. Further experiments show that all the ATMS observations could retrieve well both temperature and humidity structures, especially within the inner core region. Thus, both temperature and humidity profiles derived from microwave sounding instruments in hurricane conditions can be reliably used for evaluation of the storm intensity with a high fidelity.
Detecting Intensity Evolution of the Western North Pacific Super Typhoons in 2016 Using the Deviation Angle Variance Technique with FY Data
2019, 33(1): 104-114 [Abstract]( 220 ) HTML PDF (9670 KB)  ( 104 Supplemental Material
Abstract:This paper analyzes the complete lifecycle of super typhoons in 2016 in the western North Pacific (WNP) using the deviation angle variance technique (DAV-T). Based on the infrared images from Fengyun (FY) satellites, the DAV-T enables quantification of the axisymmetry of tropical cyclones (TCs) by using the DAV values; and thus, it helps improve the capability of TC intensity estimation. Case analyses of Super Typhoons Lionrock and Meranti were performed to explore the distribution characteristics of the DAV values at the various stages of TC evolution. The results show that the minimum DAV values (i.e., map minimum values:MMVs) gradually decreased and their locations constantly approached the circulation center with enhancement of the TC organization; however, when a ring or disk structure was formed around a TC, significant changes in MMV locations were no longer observed. Nonetheless, when large-scale non-closed deep convective cloud clusters appeared at the early stage or the dissipation stage of the typhoon, the axisymmetry of the TC was poor and the MMV locations tended to lie in the most convective region rather than in the TC circulation center. Overall, the MMVs and their locations, respectively, exhibited a strong correlation with the TC intensity and circulation center, and the correlation increased as the TCs became stronger. Combined with the China Meteorological Administration BestTrack dataset (CMA-BestTrack), statistical analysis of all research samples reveals that the correlation coefficient between the MMVs and maximum surface wind speeds (Vmax) was -0.80; the root mean square error (RMSE) of relative distance between the MMV locations and TC centers was 140.3 km; and especially, when the samples below the tropical depression (TD) intensity were removed, the RMSE of the relative distance decreased dramatically to 95.0 km. The value and location of the MMVs could be used as important indicators for estimating TC intensity and center.
Sea-Salt Aerosol Effects on the Simulated Microphysics and Precipitation in a Tropical Cyclone
Baolin JIANG, Wenshi LIN, Fangzhou LI, Junwen CHEN
2019, 33(1): 115-125 [Abstract]( 217 ) HTML PDF (1864 KB)  ( 157 Supplemental Material
Abstract:We investigate the effects of sea-salt aerosol (SSA) activated as cloud condensation nuclei on the microphysical processes, precipitation, and thermodynamics of a tropical cyclone (TC). The Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) was used together with a parameterization of SSA production. Three simulations, with different levels of SSA emission (CTL, LOW, HIGH), were conducted. The simulation results show that SSA contributes to the processes of autoconversion of cloud water and accretion of cloud water by rain, thereby promoting rain formation. The latent heat release increases with SSA emission, slightly increasing horizontal wind speeds of the TC. The presence of SSA also regulates the thermodynamic structure and precipitation of the TC. In the HIGH simulation, higher latent heat release gives rise to stronger updrafts in the TC eyewall area, leading to enhanced precipitation. In the LOW simulation, due to decreased latent heat release, the temperature in the TC eye is lower, enhancing the downdrafts in the region; and because of conservation of mass, updrafts in the eyewall also strengthen slightly; as a result, precipitation in the LOW experiment is a little higher than that in the CTL experiment. Overall, the relationship between the precipitation rate and SSA emission is nonlinear.
Below-Cloud Aerosol Scavenging by Different-Intensity Rains in Beijing City
Tian LUAN, Xueliang GUO, Tianhang ZHANG, Lijun GUO
2019, 33(1): 126-137 [Abstract]( 199 ) HTML PDF (2147 KB)  ( 144 Supplemental Material
Abstract:Below-cloud aerosol scavenging process by precipitation is important for cleaning the polluted aerosols in the atmosphere, and is also a main process for acid rain formation. However, the related physical mechanism has not been well documented and clarified yet. In this paper, we investigated the below-cloud PM2.5 (particulate matter with aerodynamic diameter being 2.5 μm or less) scavenging by different-intensity rains under polluted conditions characterized by high PM2.5 concentrations, based on in-situ measurements from March 2014 to July 2016 in Beijing city. It was found that relatively more intense rainfall events were more efficient in removing the polluted aerosols in the atmosphere. The mean PM2.5 scavenging ratio and its standard deviation (SD) were 5.1% ±25.7%, 38.5% ±29.0%, and 50.6% ±21.2% for light, moderate, and heavy rain events, respectively. We further found that the key impact factors on below-cloud PM2.5 scavenging ratio for light rain events were rain duration and wind speed rather than raindrop size distribution. However, the impacts of rain duration and wind speed on scavenging ratio were not important for moderate and heavy rain events. To our knowledge, this is the first statistical result about the effects of rain intensity, rain duration, and raindrop size distribution on below-cloud scavenging in China.
Projection of Landslides in China during the 21st Century under the RCP8.5 Scenario
Shuangshuang HE, Jun WANG, Huijun WANG
2019, 33(1): 138-148 [Abstract]( 265 ) HTML PDF (10283 KB)  ( 204 Supplemental Material
Abstract:More and more rainstorms and other extreme weather events occur in the context of global warming, which may increase the risks of landslides. In this paper, changes of landslides in the 21st century of China under the high emission scenario RCP8.5 (Representative Concentration Pathway) are projected by using a statistical landslide forecasting model and the regional climate model RegCM4.0. The statistical landslide model is based on an improved landslide susceptibility map of China and a rainfall intensity-duration threshold. First, it is driven by observed rainfall and RegCM4.0 rainfall in 1980-99, and it can reproduce the spatial distribution of landslides in China pretty well. Then, it is used to forecast the landslide changes over China in the future under the RCP8.5 scenario. The results consistently reveal that landslides will increase significantly in most areas of China, especially in the southeastern, northeastern, and western parts of Northwest China. The change pattern at the end of the 21st century is generally consistent with that in the middle of the 21st century, but with larger increment and magnitude. In terms of the probability, the proportion of grid points that are very likely and extremely likely to experience landslides will also increase.
Simulation and Projection of Near-Surface Wind Speeds in China by BCC-CSM Models
Yajun XIONG, Xiaoge XIN, Xingxia KOU
2019, 33(1): 149-158 [Abstract]( 173 ) HTML PDF (2398 KB)  ( 143 Supplemental Material
Abstract:We evaluated the ability of the Beijing Climate Center models on different horizontal resolutions (BCC-CSM1.1 on approximately 280-km resolution and BCC-CSM1.1m on approximately 110-km resolution) in simulating the near-surface wind speeds (NWS) in China during 1961-2005. The spatial distribution of the annual mean NWS over China is better captured by BCC-CSM1.1m than by BCC-CSM1.1 due to the finer resolution. The weakened NWS over China during 1961-2005 cannot be reproduced by BCC-CSM1.1, whereas BCC-CSM1.1m is able to simulate the decreasing trend of the autumn NWS in North China, although the magnitude is about 1/3 of the observed value. This is attributed to the better performance of this finer-resolution model in reproducing the increase in sea level pressure over Mongolia and North China over the past 45 years. The results suggest that increasing the horizontal resolution of the BCC-CSM model has improved its ability in reproducing the spatial distribution and long-term changes of NWS over China. Future projections by BCC-CSM1.1m under different Representative Concentration Pathway (RCP) scenarios demonstrate that the autumn NWS in North China will decrease during the 21st century under both the middle (RCP4.5) and high (RCP8.5) emission scenarios, with a higher decreasing rate under RCP8.5.
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