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2020 Vol. 37, No. 7

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News & Views
Could the Recent Taal Volcano Eruption Trigger an El Niño and Lead to Eurasian Warming?
Fei LIU, Chen XING, Jinbao LI, Bin WANG, Jing CHAI, Chaochao GAO, Gang HUANG, Jian LIU, Deliang CHEN
2020, 37(7): 663-670. doi: 10.1007/s00376-020-2041-z
Original Paper
Estimation of PM2.5 Mass Concentration from Visibility
Denghui JI, Zhaoze DENG, Xiaoyu SUN, Liang RAN, Xiangao XIA, Disong FU, Zijue SONG, Pucai WANG, Yunfei WU, Ping TIAN, Mengyu HUANG
2020, 37(7): 671-678. doi: 10.1007/s00376-020-0009-7
Aerosols in the atmosphere not only degrade visibility, but are also detrimental to human health and transportation. In order to develop a method to estimate PM2.5 mass concentration from the widely measured visibility, a field campaign was conducted in Southwest China in January 2019. Visibility, ambient relative humidity (RH), PM2.5 mass concentrations and scattering coefficients of dry particles were measured. During the campaign, two pollution episodes, i.e., from 4−9 January and from 10−16 January, were encountered. Each of the two episodes could be divided into two periods. High aerosol hygroscopicity was found during the first period, when RH was higher than 80% at most of the time, and sometimes even approached 100%. The second period experienced a relatively dry but more polluted condition and aerosol hygroscopicity was lower than that during the first period. An empirical relationship between PM2.5 mass concentration and visibility (ambient aerosol extinction) under different RH conditions could thus be established. Based on the empirical relationship, PM2.5 mass concentration could be well estimated from visibility and RH. This method will be useful for remote sensing of PM2.5 mass concentration.
Modeling the Impacts of Nitrogen Dynamics on Regional Terrestrial Carbon and Water Cycles over China with Noah-MP-CN
Jingjing LIANG, Zong-Liang YANG, Xitian CAI, Peirong LIN, Hui ZHENG, Qingyun BIAN
2020, 37(7): 679-695. doi: 10.1007/s00376-020-9231-6
As an important part of biogeochemical cycling, the nitrogen cycle modulates terrestrial ecosystem carbon storage, water consumption, and environmental quality. Modeling the complex interactions between nitrogen, carbon and water at a regional scale remains challenging. Using China as a testbed, this study presents the first application of the nitrogen-augmented community Noah land surface model with multi-parameterization options (Noah-MP-CN) at the regional scale. Noah-MP-CN parameterizes the constraints of nitrogen availability on photosynthesis based on the Fixation and Uptake of Nitrogen plant nitrogen model and the Soil and Water Assessment Tool soil nitrogen model. The impacts of nitrogen dynamics on the terrestrial carbon and water cycles are investigated by comparing the simulations with those from the original Noah-MP. The results show that incorporating nitrogen dynamics improves the carbon cycle simulations. Noah-MP-CN outperforms Noah-MP in reproducing leaf area index (LAI) and gross primary productivity (GPP) for most of China, especially in the southern warm and humid regions, while the hydrological simulations only exhibit slight improvements in soil moisture and evapotranspiration. The impacts of fertilizer application over cropland on carbon fixation, water consumption and nitrogen leaching are investigated through a trade-off analysis. Compared to halved fertilizer use, the actual quantity of application increases GPP and water consumption by only 1.97% and 0.43%, respectively; however, the nitrogen leaching is increased by 5.35%. This indicates that the current level of fertilizer use is a potential concern for degrading the environment.
Impacts of Urbanization on the Precipitation Characteristics in Guangdong Province, China
Meng YAN, Johnny C. L. CHAN, Kun ZHAO
2020, 37(7): 696-706. doi: 10.1007/s00376-020-9218-3
With the development of urbanization, whether precipitation characteristics in Guangdong Province, China, from 1981 to 2015 have changed are investigated using rain gauge data from 76 stations. These characteristics include annual precipitation, rainfall frequency, intense rainfall (defined as hourly precipitation ≥ 20 mm), light precipitation (defined as hourly precipitation ≤ 2.5 mm), and extreme rainfall (defined as hourly rainfall exceeding the 99.9th percentile of the hourly rainfall distribution). During these 35 years, the annual precipitation shows an increasing trend in the urban areas. While rainfall frequency and light precipitation have a decreasing trend, intense rainfall frequency shows an increasing trend. The heavy and extreme rainfall frequency both exhibit an increasing trend in the Pearl River Delta region, where urbanization is the most significant. These trends in both the warm seasons (May−October) and during the pre-flood season (April−June) appear to be more significant. On the contrary, the annual precipitation amount in rural areas has a decreasing trend. Although the heavy and extreme precipitation also show an increasing trend, it is not as strong and significant as that in the urban areas. During periods in which a tropical cyclone makes landfall along the South China Coast, the rainfall in urban areas has been consistently more than that in surrounding areas. The precipitation in the urban areas and to their west is higher after 1995, when the urbanization accelerated. These results suggest that urbanization has a significant impact on the precipitation characteristics of Guangdong Province.
Sideswiping Tropical Cyclones and Their Associated Precipitation over China
Tian FENG, Fumin REN, Da-Lin ZHANG, Guoping LI, Wenyu QIU, Hui YANG
2020, 37(7): 707-717. doi: 10.1007/s00376-020-9224-5
Tropical cyclone (TC) precipitation (TCP) has attracted considerable attention in recent decades because of its adverse socioeconomic impacts. In particular, considerable effort has been devoted to quantifying TCP and investigating the precipitation of TCs that make landfall. However, precipitation over land induced by TCs that do not make landfall (i.e., offshore), the so-called “sideswiping” TCs (STCs), is an important component of TCP but has attracted little attention from the research community to date. Here, best-track and daily precipitation data from the China Meteorological Administration during the 59 years of 1960−2018 are used to study STC precipitation (STP). Results show that: (i) the annual number of STCs fluctuates significantly from 3 to 17, with a mean frequency of 8.8 STCs per year; (ii) there are decreasing trends in STC frequency and STP amount over the past 59 years; (iii) both STC frequency and STP are high from July to October, with maxima in August; (iv) the distribution of STP, covering most of China’s coastal regions, is dominated by intense STCs, and the annual STP decreases from southeast coastal regions to northwest inland areas, with a maximum value over the islands of Taiwan and Hainan; and (v) extreme STP events could appear not only over the island and coastal areas, but also over inland areas such as Zhumadian of Henan Province due to the influences of local orography and favorable large-scale forcing.
Isentropic Analysis of Regional Cold Events over Northern China
Qingyi LIU, Qian LIU, Guixing CHEN
2020, 37(7): 718-734. doi: 10.1007/s00376-020-9226-3
From the perspective of cold air mass (CAM) analysis, we examine the characteristics and mechanisms of regional cold events (RCEs) over northwestern and northeastern China in the past 58 years (1958/59−2015/16). The RCEs in northwestern (northeastern) China are shown to have an average duration of 6.8 (4.7) days with a moderate (sharp) temperature drop. We quantitatively estimate the RCE-related CAM, for the first time, using an isentropic analysis method. Before an RCE in northwestern China, CAM is accumulated in western Siberia with convergent CAM flux under a blocking pattern in the Urals region. During RCE outbreak, CAM penetrates the valleys of the Tianshan−Altay Mountains to the Tarim Basin and Hexi Corridor. The CAM moves slowly because of the blocking pattern and orographic effect, which explains the relatively long duration of RCEs. Comparatively, during RCEs in northeastern China, the CAM depth anomaly originates more to the east and quickly passes the Mongolian Plateau guided by an eastward-moving trough. Diagnostic analyses further show that adiabatic processes play a crucial role in regulating the local change of CAM depth during the two kinds of RCEs. The advection term of adiabatic processes mainly increases the CAM depth during RCE outbreak, while the convergence term increases (reduces) CAM depth before (after) RCE outbreak. Both terms are relatively strong during RCEs in northeastern China, resulting in the rate of change in CAM depth being ~50% larger than for those in northwestern China. Therefore, the variations of RCEs in duration and intensity can be well explained by the different evolution of CAM depth and flux.
A New Temperature Channel Selection Method Based on Singular Spectrum Analysis for Retrieving Atmospheric Temperature Profiles from FY-4A/GIIRS
Peipei YU, Chunxiang SHI, Ling YANG, Shuai SHAN
2020, 37(7): 735-750. doi: 10.1007/s00376-020-9249-9
Hyperspectral data have important research and application value in the fields of meteorology and remote sensing. With the goal of improving retrievals of atmospheric temperature profiles, this paper outlines a novel temperature channel selection method based on singular spectrum analysis (SSA) for the Geostationary Interferometric Infrared Sounder (GIIRS), which is the first infrared sounder operating in geostationary orbit. The method possesses not only the simplicity and rapidity of the principal component analysis method, but also the interpretability of the conventional channel selection method. The novel SSA method is used to decompose the GIIRS observed infrared brightness temperature spectrum (700−1130 cm−1), and the reconstructed grouped components can be obtained to reflect the energy variations in the temperature-sensitive waveband of the respective sequence. At 700−780 cm−1, the channels selected using our method perform better than IASI (Infrared Atmospheric Sounding Interferometer) and CrIS (Cross-track Infrared Sounder) temperature channels when used as inputs to the neural network retrieval model.
Evolution of the Mid-tropospheric Vortex during the Formation of Super Typhoon Megi (2010)
Li ZHUO, Dexian FANG, Zheng WU, Juan FANG
2020, 37(7): 751-765. doi: 10.1007/s00376-020-9178-7
As a follow-up of a previously published article on the contribution of tropical waves, this study explores the evolution of the mid-tropospheric mesoscale cyclonic vortex (MV) during the formation of Typhoon Megi (2010) with a successful cloud-resolving simulation. It is found that the formation and intensification of the MV were related to the deep convection and subsequent stratiform precipitation, while the weakening of the MV was related to the shallow convection. Both the upward transport of vorticity related to the deep convection and the horizontal convergence associated with the stratiform precipitation contributed to the formation and intensification of the MV. Even though the latter was dominant, the former could not be ignored, especially in the early stage of the MV. The MV played dual roles in the formation of Megi. On the one hand, the formation and intensification of MV were primarily associated with the stratiform precipitation, which induced the low-level divergence inhibiting the spin-up of the near-surface cyclonic circulation. On the other hand, the coupled low-level cold core under the MV benefited the accumulation of the convective available potential energy (CAPE), which was favorable for the convective activity. A sensitivity experiment with the evaporative cooling turned off indicated that the development of the MV retarded the genesis process of Megi.
The Forced Secondary Circulation of the Mei-yu Front
Zipeng YUAN, Xiaoyong ZHUGE, Yuan WANG
2020, 37(7): 766-780. doi: 10.1007/s00376-020-9177-8
Using National Centers for Environmental Prediction reanalysis data for the period 28 June to 12 July during 2001 to 2013, the secondary circulation (SC) associated with the mei-yu front was quantitatively diagnosed by numerically solving a primitive version of the Sawyer−Eliassen equation. Results demonstrate that a direct SC exists near the mei-yu front zone during mid-summer and the synoptic-scale geostrophic deformations are the main factors determining SC structures. About 94% of the sinking strength and 61% of the ascending strength in the SC are induced by the geostrophic deformations. Other terms, such as diabatic heating, ageostrophic dynamical forcing, and frictional forcing, mainly influence the fine flow pattern of the SC. The forced SC produces a frontogenesis area tilting to the north with altitude. Further diagnosis clarifies the positive feedback involving the geostrophic shear forcing and vorticity frontogenesis in the upper-level mei-yu front zone. Furthermore, statistical results indicate that all 34 deep convection cases that occurred in the warm region of the mei-yu front over the period 2004−2013 experienced high-level frontogenesis associated with along-jet cold advection. The cyclonic shear forcing “moved” the monsoon SC’s subsidence branch to the warm side of the mei-yu front and caused the subsidence branch to extend downwards to the lower troposphere, conducive to the initiation of deep convection in the warm region of the mei-yu front.
Simulation of Extreme Updrafts in the Tropical Cyclone Eyewall
Yan ZHENG, Liguang WU, Haikun ZHAO, Xingyang ZHOU, Qingyuan LIU
2020, 37(7): 781-792. doi: 10.1007/s00376-020-9197-4
Strong vertical motion (>10 m s−1) has profound implications for tropical cyclone (TC) structure changes and intensity. While extreme updrafts in the TC are occasionally observed in real TCs, the associated small-scale features remain unclear. Based on an analysis of the extreme eyewall updrafts in two numerical experiments conducted with the Advanced Research version of the Weather Research and Forecasting (WRF) model, in which the large-eddy simulation (LES) technique was used with the finest grid spacings of 37 and 111 m, for the first time this study demonstrates that the simulated extreme updrafts that occur mainly in the enhanced eyewall convection on the down-shear left side are comparable to available observations. The simulated extreme updraft exhibits relatively high frequencies in the lower (750 m), middle (6.5 km) and upper (13 km) troposphere, which are associated with different types of small-scale structures. While the lower-level extreme updraft is mainly related to the tornado-scale vortex, the extreme updraft at upper levels is closely associated with a pair of counter-rotating horizontal rolls oriented generally along the TC tangential flow, which are closely associated with the enhanced eyewall convection. The extreme updraft at middle levels is related to relatively complicated small-scale structures. The study suggests that extreme updrafts can be simulated when the grid spacing is about 100 m or less in the WRF-LES framework, although the simulated small-scale features need further verification in both observation and simulation.