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2022 Vol. 39, No. 3

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Original Paper
Seasonal Prediction of Summer Precipitation over East Africa Using NUIST-CFS1.0
Temesgen Gebremariam ASFAW, Jing-Jia LUO
2022, 39(3): 355-372. doi: 10.1007/s00376-021-1180-1
East Africa is particularly vulnerable to precipitation variability, as the livelihood of much of the population depends on rainfed agriculture. Seasonal forecasts of the precipitation anomalies, when skillful, can therefore improve implementation of coping mechanisms with respect to food security and water management. This study assesses the performance of Nanjing University of Information Science and Technology Climate Forecast System version 1.0 (NUIST-CFS1.0) on forecasting June–September (JJAS) seasonal precipitation anomalies over East Africa. The skill in predicting the JJAS mean precipitation initiated from 1 May for the period of 1982–2019 is evaluated using both deterministic and probabilistic verification metrics on grid cell and over six distinct clusters. The results show that NUIST-CFS1.0 captures the spatial pattern of observed seasonal precipitation climatology, albeit with dry and wet biases in a few parts of the region. The model has positive skill across a majority of Ethiopia, Kenya, Uganda, and Tanzania, whereas it doesn’t exceed the skill of climatological forecasts in parts of Sudan and southeastern Ethiopia. Positive forecast skill is found over regions where the model shows better performance in reproducing teleconnections related to oceanic SST. The prediction performance of NUIST-CFS1.0 is found to be on a level that is potentially useful over a majority of East Africa.
Another Record: Ocean Warming Continues through 2021 despite La Niña Conditions
Lijing CHENG, John ABRAHAM, Kevin E. TRENBERTH, John FASULLO, Tim BOYER, Michael E. MANN, Jiang ZHU, Fan WANG, Ricardo LOCARNINI, Yuanlong LI, Bin ZHANG, Zhetao TAN, Fujiang YU, Liying WAN, Xingrong CHEN, Xiangzhou SONG, Yulong LIU, Franco RESEGHETTI, Simona SIMONCELLI, Viktor GOURETSKI, Gengxin CHEN, Alexey MISHONOV, Jim REAGAN
2022, 39(3): 373-385. doi: 10.1007/s00376-022-1461-3
The increased concentration of greenhouse gases in the atmosphere from human activities traps heat within the climate system and increases ocean heat content (OHC). Here, we provide the first analysis of recent OHC changes through 2021 from two international groups. The world ocean, in 2021, was the hottest ever recorded by humans, and the 2021 annual OHC value is even higher than last year’s record value by 14 ± 11 ZJ (1 zetta J = 1021 J) using the IAP/CAS dataset and by 16 ± 10 ZJ using NCEI/NOAA dataset. The long-term ocean warming is larger in the Atlantic and Southern Oceans than in other regions and is mainly attributed, via climate model simulations, to an increase in anthropogenic greenhouse gas concentrations. The year-to-year variation of OHC is primarily tied to the El Niño-Southern Oscillation (ENSO). In the seven maritime domains of the Indian, Tropical Atlantic, North Atlantic, Northwest Pacific, North Pacific, Southern oceans, and the Mediterranean Sea, robust warming is observed but with distinct inter-annual to decadal variability. Four out of seven domains showed record-high heat content in 2021. The anomalous global and regional ocean warming established in this study should be incorporated into climate risk assessments, adaptation, and mitigation.
A New X-band Weather Radar System with Distributed Phased-Array Front-ends: Development and Preliminary Observation Results
Xiaoqiong ZHEN, Shuqing MA, Hongbin CHEN, Guorong WANG, Xiaoping XU, Siteng LI
2022, 39(3): 386-402. doi: 10.1007/s00376-021-1114-y
A novel weather radar system with distributed phased-array front-ends was developed. The specifications and preliminary data synthesis of this system are presented, which comprises one back-end and three or more front-ends. Each front-end, which utilizes a phased-array digital beamforming technology, sequentially transmits four 22.5°-width beams to cover the 0°–90° elevational scan within about 0.05 s. The azimuthal detection is completed by one mechanical scan of 0°–360° azimuths within about 12 s volume-scan update time. In the case of three front-ends, they are deployed according to an acute triangle to form a fine detection area (FDA). Because of the triangular deployment of multiple phased-array front-ends and a unique synchronized azimuthal scanning (SAS) rule, this new radar system is named Array Weather Radar (AWR). The back-end controls the front-ends to scan strictly in accordance with the SAS rule that assures the data time differences (DTD) among the three front-ends are less than 2 s for the same detection point in the FDA. The SAS can maintain DTD < 2 s for an expanded seven-front-end AWR. With the smallest DTD, gridded wind fields are derived from AWR data, by sampling of the interpolated grid, onto a rectangular grid of 100 m ×100 m ×100 m at a 12 s temporal resolution in the FDA. The first X-band single-polarized three-front-end AWR was deployed in field experiments in 2018 at Huanghua International Airport, China. Having completed the data synthesis and processing, the preliminary observation results of the first AWR are described herein.
A New Index Developed for Fast Diagnosis of Meteorological Roles in Ground-Level Ozone Variations
Weihua CHEN, Weiwen WANG, Shiguo JIA, Jingying MAO, Fenghua YAN, Lianming ZHENG, Yongkang WU, Xingteng ZHANG, Yutong DONG, Lingbin KONG, Buqing ZHONG, Ming CHANG, Min SHAO, Xuemei WANG
2022, 39(3): 403-414. doi: 10.1007/s00376-021-1257-x
China experienced worsening ground-level ozone (O3) pollution from 2013 to 2019. In this study, meteorological parameters, including surface temperature (T2), solar radiation (SW), and wind speed (WS), were classified into two aspects, (1) Photochemical Reaction Condition (PRC = T2 × SW) and (2) Physical Dispersion Capacity (PDC = WS). In this way, a Meteorology Synthetic Index (MSI = PRC/PDC) was developed for the quantification of meteorology-induced ground-level O3 pollution. The positive linear relationship between the 90th percentile of MDA8 (maximum daily 8-h average) O3 concentration and MSI determined that the contribution of meteorological changes to ground-level O­3 varied on a latitudinal gradient, decreasing from ~40% in southern China to 10%–20% in northern China. Favorable photochemical reaction conditions were more important for ground-level O3 pollution. This study proposes a universally applicable index for fast diagnosis of meteorological roles in ground-level O3 variability, which enables the assessment of the observed effects of precursor emissions reductions that can be used for designing future control policies.
Hydro-climatic Characteristics of Yarlung Zangbo River Basin since the Last Glacial Maximum
Shuang LIU, Kaiheng HU, Weiming LIU, Paul A. CARLING
2022, 39(3): 415-426. doi: 10.1007/s00376-021-1150-7
Global climate changes significantly impact the water condition of big rivers in glacierized high mountains. However, there is a lack of studies on hydrological changes within river basins caused by climate changes over a geological timescale due to the impossibility of direct observations. In this study, we examine the hydro-climatic variation of the Yarlung Zangbo River Basin in the Tibet Plateau since the Last Glacial Maximum (LGM) by combining δ18O proxy records in Indian and Omani caves with the simulated Indian summer monsoon, surface temperature, precipitation, evapotranspiration and runoff via the Community Climate System Model and the reconstructed glacier coverage via the Parallel Ice Sheet Model. The mean river runoff was kept at a low level of 145 billion cubic meters per year until an abrupt increase at a rate of 8.7 million cubic meters per year in the Bølling-Allerød interval (BA). The annual runoff reached a maximum of 250 billion cubic meters in the early Holocene and then reduced to the current value of 180 billion cubic meters at a rate of 6.4 million cubic meters per year. The low runoff in the LGM and Heinrich Stadial 1 (HS1) is likely attributed to such a small contribution of precipitation to runoff and the large glacier cover. The percentage of precipitation to runoff was only 20% during the LGM and HS1. Comparison of glacier area among different periods indicates that the fastest deglaciation occurred during the late HS1, when nearly 60% of glacier area disappeared in the middle reach, 50% in the upper reach, and 30% in the lower reach. The rapid deglaciation and increasing runoff between the late HS1 and BA may have accelerated widespread ice-dam breaches and led to extreme outburst flood events. Combining local geological proxy records and regional simulations could be a useful approach for the study of paleo-hydrologic variations in big river basins.
Diagnosing SST Error Growth during ENSO Developing Phase in the BCC_CSM1.1(m) Prediction System
Ben TIAN, Hong-Li REN
2022, 39(3): 427-442. doi: 10.1007/s00376-021-1189-5
In this study, the predictability of the El Niño-South Oscillation (ENSO) in an operational prediction model from the perspective of initial errors is diagnosed using the seasonal hindcasts of the Beijing Climate Center System Model, BCC_CSM1.1(m). Forecast skills during the different ENSO phases are analyzed and it is shown that the ENSO forecasts appear to be more challenging during the developing phase, compared to the decay phase. During ENSO development, the SST prediction errors are significantly negative and cover a large area in the central and eastern tropical Pacific, thus limiting the model skill in predicting the intensity of El Niño. The large-scale SST errors, at their early stage, are generated gradually in terms of negative anomalies in the subsurface ocean temperature over the central-western equatorial Pacific, featuring an error evolutionary process similar to that of El Niño decay and the transition to the La Niña growth phase. Meanwhile, for short lead-time ENSO predictions, the initial wind errors begin to play an increasing role, particularly in linking with the subsurface heat content errors in the central-western Pacific. By comparing the multiple samples of initial fields in the model, it is clearly found that poor SST predictions of the Niño-3.4 region are largely due to contributions of the initial errors in certain specific locations in the tropical Pacific. This demonstrates that those sensitive areas for initial fields in ENSO prediction are fairly consistent in both previous ideal experiments and our operational predictions, indicating the need for targeted observations to further improve operational forecasts of ENSO.
The Weakening Relationship between ENSO and the South China Sea Summer Monsoon Onset in Recent Decades
Peng HU, Wen CHEN, Shangfeng CHEN, Lin WANG, Yuyun LIU
2022, 39(3): 443-455. doi: 10.1007/s00376-021-1208-6
The El Niño-Southern Oscillation (ENSO) is traditionally regarded as the most important factor modulating the interannual variation of the South China Sea summer monsoon (SCSSM) onset. A preceding El Niño (La Niña) usually tends to be followed by a delayed (an advanced) monsoon onset. However, the close relationship between ENSO and SCSSM onset breaks down after the early-2000s, making seasonal prediction very difficult in recent years. Three possible perspectives have been proposed to explain the weakening linkage between ENSO and SCSSM onset, including interdecadal change of the ENSO teleconnection (i.e., the Walker circulation), interferences of other interannual variability (i.e., the Victoria mode), and disturbances on intraseasonal time scales (i.e., the quasi-biweekly oscillation). By comparing the epochs of 1979–2001 and 2002–19, it is found that the anomalous tropical Walker circulation generated by ENSO is much weaker in the latter epoch and thus cannot deliver the ENSO signal to the SCSSM onset. Besides, in recent years, the SCSSM onset is more closely linked to extratropical factors like the Victoria mode, and thus its linkage with ENSO becomes weaker. In addition to these interannual variabilities, the intraseasonal oscillations like the quasi-biweekly oscillation can disrupt the slow-varying seasonal march modulated by ENSO. Thus, the amplified quasi-biweekly oscillation may also contribute to the weakening relationship after the early-2000s. Given the broken relationship between ENSO and SCSSM onset, the extratropical factors should be considered in order to make skillful seasonal predictions of SCSSM onset, and more attention should be paid to the extended-range forecast based on intraseasonal oscillations.
Detection and Attribution of Changes in Thermal Discomfort over China during 1961−2014 and Future Projections
Wanling LI, Xin HAO, Li WANG, Yuqing LI, Jiandong LI, Huixin LI, Tingting HAN
2022, 39(3): 456-470. doi: 10.1007/s00376-021-1168-x
In this study, variation in the frequency of thermal discomfort days over China during the period of 1961−2014, including heat discomfort days (HDDs) and cold discomfort days (CDDs), and the influence of external forcings on it are discussed. HDDs are the conditions of overheating and overhumidity (represented by humidity index), and CDDs reflect the dangers from cold temperatures and winds (represented by wind chill index). Observations show significant increases (decreases) in the frequency of HDDs (CDDs) over China from 1961 to 2014, with clear regional distinctions. The historical ALL and greenhouse gas (GHG) simulations can sufficiently reproduce the spatial patterns of the observational trend in the frequency of both HDDs and CDDs over China. Further, the impacts of GHG and anthropogenic forcings on the HDDs (CDDs) are detectable over China, except for central and eastern China, based on the optimal fingerprinting method. GHG forcing is identified as a dominant factor for the observational changes in the frequency of HDDs over southern China; GHG and anthropogenic forcings have dominant effects on the variation in the frequency of CDDs over southwestern China. Although trends in the frequency of HDDs and CDDs in historical aerosol forcing simulations seems to be opposite to observations, an aerosol signal fails to be detected. Natural forcing contributes to the observational variation in the frequency of HDDs over northwestern China. In addition, the future projections of thermal discomfort days indicate that Chinese residents will face more threats of heat discomfort and fewer threats of cold discomfort in the future under global warming.
One-Dimensional Variational Retrieval of Temperature and Humidity Profiles from the FY4A GIIRS
Qiumeng XUE, Li GUAN, Xiaoning SHI
2022, 39(3): 471-486. doi: 10.1007/s00376-021-1032-z
A physical retrieval approach based on the one-dimensional variational (1D-Var) algorithm is applied in this paper to simultaneously retrieve atmospheric temperature and humidity profiles under both clear-sky and partly cloudy conditions from FY-4A GIIRS (geostationary interferometric infrared sounder) observations. Radiosonde observations from upper-air stations in China and level-2 operational products from the Chinese National Satellite Meteorological Center (NSMC) during the periods from December 2019 to January 2020 (winter) and from July 2020 to August 2020 (summer) are used to validate the accuracies of the retrieved temperature and humidity profiles. Comparing the 1D-Var-retrieved profiles to radiosonde data, the accuracy of the temperature retrievals at each vertical level of the troposphere is characterized by a root mean square error (RMSE) within 2 K, except for at the bottom level of the atmosphere under clear conditions. The RMSE increases slightly for the higher atmospheric layers, owing to the lack of temperature sounding channels there. Under partly cloudy conditions, the temperature at each vertical level can be obtained, while the level-2 operational products obtain values only at altitudes above the cloud top. In addition, the accuracy of the retrieved temperature profiles is greatly improved compared with the accuracies of the operational products. For the humidity retrievals, the mean RMSEs in the troposphere in winter and summer are both within 2 g kg–1. Moreover, the retrievals performed better compared with the ERA5 reanalysis data between 800 hPa and 300 hPa both in summer and winter in terms of RMSE.
An Adaptive Nonhydrostatic Atmospheric Dynamical Core Using a Multi-Moment Constrained Finite Volume Method
Pei HUANG, Chungang CHEN, Xingliang LI, Xueshun SHEN, Feng XIAO
2022, 39(3): 487-501. doi: 10.1007/s00376-021-1185-9
An adaptive 2D nonhydrostatic dynamical core is proposed by using the multi-moment constrained finite-volume (MCV) scheme and the Berger-Oliger adaptive mesh refinement (AMR) algorithm. The MCV scheme takes several point-wise values within each computational cell as the predicted variables to build high-order schemes based on single-cell reconstruction. Two types of moments, such as the volume-integrated average (VIA) and point value (PV), are defined as constraint conditions to derive the updating formulations of the unknowns, and the constraint condition on VIA guarantees the rigorous conservation of the proposed model. In this study, the MCV scheme is implemented on a height-based, terrain-following grid with variable resolution to solve the nonhydrostatic governing equations of atmospheric dynamics. The AMR grid of Berger-Oliger consists of several groups of blocks with different resolutions, where the MCV model developed on a fixed structured mesh can be used directly. Numerical formulations are designed to implement the coarse-fine interpolation and the flux correction for properly exchanging the solution information among different blocks. Widely used benchmark tests are carried out to evaluate the proposed model. The numerical experiments on uniform and AMR grids indicate that the adaptive model has promising potential for improving computational efficiency without losing accuracy.
The Linkage between Midwinter Suppression of the North Pacific Storm Track and Atmospheric Circulation Features in the Northern Hemisphere
Minghao YANG, Chongyin LI, Xin LI, Xiong CHEN, Lifeng LI
2022, 39(3): 502-518. doi: 10.1007/s00376-021-1145-4
The midwinter suppression (MWS) of the North Pacific storm track (NPST) has been an active research topic for decades. Based on the daily-mean NCEP/NCAR reanalysis from 1948 to 2018, this study investigates the MWS-related atmospheric circulation characteristics in the Northern Hemisphere by regression analysis with respect to a new MWS index, which may shed more light on this difficult issue. The occurrence frequency of the MWS of the upper-tropospheric NPST is more than 0.8 after the mid-1980s. The MWS is accompanied by significantly positive sea-level pressure anomalies in Eurasia and negative anomalies over the North Pacific, which correspond to a strengthened East Asian winter monsoon. The intensified East Asian trough and atmospheric blocking in the North Pacific as well as the significantly negative low-level air temperature anomalies, lying upstream of the MNPST, are expected to be distinctly associated with the MWS. However, the relationship between the MWS and low-level atmospheric baroclinicity is somewhat puzzling. From the diagnostics of the eddy energy budget, it is identified that the inefficiency of the barotropic energy conversion related to the barotropic governor mechanism does not favor the occurrence of the MWS. In contrast, weakened baroclinic energy conversion, buoyancy conversion, and generation of eddy available potential energy by diabatic heating are conducive to the occurrence of the MWS. In addition, Ural blocking in the upstream region of the MNPST may be another candidate mechanism associated with the MWS.
Notes & Letters
Observations of Dynamic Turbulence in the Lower Stratosphere over Inner Mongolia Using a High-resolution Balloon Sensor Constant Temperature Anemometer
Xiaoyu REN, Yi LIU, Zhaonan CAI, Yuli ZHANG
2022, 39(3): 519-528. doi: 10.1007/s00376-021-1233-5
We present characterizations of the dynamic turbulence in the lower stratosphere measured by a new balloon-based system designed for detecting finer scale dynamic turbulence. The balloon-based system included a constant temperature anemometer (CTA) operating at a sampling rate of 2 kHz at an ascent speed of 5 m s−1 (corresponding to a vertical resolution of 2.5 mm), an industrial personal computer, batteries, sensors for ambient temperature and humidity, an A/D converter, and others. The system was successfully launched to 24 km altitude over Bayannur City, Inner Mongolia Province. Results show the spatial intermittence of the turbulence layers, with clear boundaries between turbulent and non-turbulent regions. This is the first time that the dynamic turbulence spectrum down to the viscous sub-range has been obtained throughout the lower stratosphere over China. With that, the energy dissipation rates of dynamic turbulence could be calculated with high precision. The profile of the dissipation rates varied from 7.37 × 10−7 to 4.23 W kg−1 and increased with altitude in the stratosphere.
A New Method of Significance Testing for Correlation-Coefficient Fields and Its Application
Xiaojuan SUN, Siyan LI, Julian X. L WANG, Panxing WANG, Dong GUO
2022, 39(3): 529-535. doi: 10.1007/s00376-021-1196-6
Correlation-coefficient fields are widely used in short-term climate prediction research. The most frequently used significance test method for the correlation-coefficient field was proposed by Livezey, in which the number of significant-correlation lattice (station) points on the correlation coherence map is used as the statistic. However, the method is based on two assumptions: (1) the spatial distribution of the lattice (station) points is uniform; and (2) there is no correlation between the physical quantities in the correlation-coefficient field. However, in reality, the above two assumptions are not valid. Therefore, we designed a more reasonable method for significance testing of the correlation-coefficient field. Specifically, a new statistic, the significant-correlation area, is introduced to eliminate the inhomogeneity of the grid (station)-point distribution, and an empirical Monte Carlo method is employed to eliminate the spatial correlation of the matrix. Subsequently, the new significance test was used for simultaneous correlation-coefficient fields between intensities of the atmospheric activity center in the Northern Hemisphere and temperature/precipitation in China. The results show that the new method is more reasonable than the Livezey method.
Meeting Summary
The 16th Workshop on Antarctic Meteorology and Climate and 6th Year of Polar Prediction in the Southern Hemisphere Meeting
David H. BROMWICH, Matthew A. LAZZARA, Arthur M. CAYETTE, Jordan G. POWERS, Kirstin WERNER, John J. CASSANO, Steven R. COLWELL, Scott CARPENTIER, Xun ZOU
2022, 39(3): 536-542. doi: 10.1007/s00376-021-1384-4