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Qiong Wu
,
Hong-Qing Wang
,
Yin-Jing Lin
,
Yi-Zhou Zhuang
, and
Yan Zhang

Abstract

An optical flow algorithm based on polynomial expansion (OFAPE) was used to derive atmospheric motion vectors (AMVs) from geostationary satellite images. In OFAPE, there are two parameters that can affect the AMV results: the sizes of the expansion window and optimization window. They should be determined according to the temporal interval and spatial resolution of satellite images. A helpful experiment was conducted for selecting those sizes. The limitations of window sizes can cause loss of strong wind speed, and an image-pyramid scheme was used to overcome this problem. Determining the heights of AMVs for semitransparent cloud pixels (STCPs) is challenging work in AMV derivation. In this study, two-dimensional histograms (H2Ds) between infrared brightness temperatures (6.7- and 10.8-μm channels) formed from a long time series of cloud images were used to identify the STCPs and to estimate their actual temperatures/heights. The results obtained from H2Ds were contrasted with CloudSat radar reflectivity and CALIPSO cloud-feature mask data. Finally, in order to verify the algorithm adaptability, three-month AMVs (JJA 2013) were calculated and compared with the wind fields of ERA data and the NOAA/ESRL radiosonde observations in three aspects: speed, direction, and vector difference.

Full access
Jinzhuo Cai
,
Haiyuan Yang
,
Bolan Gan
,
Hong Wang
,
Zhaohui Chen
, and
Lixin Wu

Abstract

Subtropical western boundary currents (WBCs) are among the most energetic currents in the global circulation system and play an important role in the oceanic meridional heat transport (OHT). Based on nine high-resolution global coupled climate models, this study investigates the change of OHT by subtropical WBCs (WHT) under global warming. We found that WHT in both hemispheres depicts a weakening trend during 1950–2050, primarily caused by the transport change of WBCs. In the Northern Hemisphere, weakening of the Gulf Stream resulting from the slowing AMOC leads to the hemispheric WHT weakening. In the Southern Hemisphere, the WHT decrease is mainly induced by the sharp decline of Agulhas Current transport, associated with the change in wind field in the southern Indian Ocean and Indonesian Throughflow. Compared to the mean flow, the contribution of mesoscale eddies to OHT change is negligible along with WBCs but is important in their extension regions.

Restricted access
Xiaolan L. Wang
,
Yang Feng
,
Vincent Y. S. Cheng
, and
Hong Xu

Abstract

This study first developed a comprehensive semiautomatic data homogenization procedure to produce gap-infilled and homogenized monthly precipitation data series for 425 long-term/critical stations in Canada, which were then used to assess Canadian historical precipitation trends. Data gaps in the 425 series were infilled by advanced spatial interpolation of a much larger dataset. The homogenization procedure repeatedly used multiple homogeneity tests without and with reference series to identify changepoints/inhomogeneities, the results from which were finalized by manual analysis using metadata and visual inspection of the multiphase regression fits. As a result, 298 out of the 425 data series were found to be inhomogeneous. These series were homogenized using quantile matching adjustments. The homogenized dataset shows better spatial consistency of trends than does the raw dataset. The improved gridding and regional mean trend estimation methods also provide more realistic trend estimates. With these improvements, Canadian historical precipitation trends were found to be dominantly positive and significant, except in central-south Canada where the trends are generally insignificant and small with mixed directions. For annual precipitation, the largest increases are seen in southeastern Canada and along the Pacific coast; however, the largest relative increases (in percent of the 1961–90 mean) are seen in northern Canada. The largest trend difference between northern and southern Canada is seen in winter, in which significant increases in the north were matched with significant decreases in the south.

Significance Statement

This study aims to produce a homogenized long-term monthly precipitation dataset for Canada, which is then used to assess Canadian historical precipitation trends. The work is important because it developed a comprehensive algorithm for homogenization of precipitation data, and the results provide better representation of precipitation climate and more robust estimates of precipitation trends. It also identified the causes for large biases in the published estimates of precipitation trends over Canada.

Open access
Pinya Wang
,
Yang Yang
,
Daokai Xue
,
Yi Qu
,
Jianping Tang
,
L. Ruby Leung
, and
Hong Liao

Abstract

Compound hazards are more destructive than the individual ones. Using observational and reanalysis datasets during 1960–2019, this study shows a remarkable concurrent relationship between extreme heatwaves (HWs) over southeastern coast of China (SECC) and tropical cyclone (TC) activities over western North Pacific (WNP). Overall, 70% of HWs co-occurred with TC activities (TC–HWs) in the past 60 years. Although the total frequency of TCs over WNP exhibited a decreasing trend, the occurrences of TC–HWs over SECC have been increasing, primarily due to the increasing HWs in the warming climate. In addition, TC–HWs are stronger and longer lasting than HWs that occur alone (AHWs). And in the long-term perspective, both AHWs and TC–HWs exhibit increasing trends, especially since the mid-1980s. The enhancement on HWs caused by TC activities is sustained until TCs make their landfalls and then collapse. Based on composite analysis, TC activities enhance HWs by modulating atmospheric circulations and triggering anomalous descending motion over southern China mainland which intensifies the western Pacific subtropical high (WPSH) and favors increased temperatures therein. Given the severe adverse impacts of TC–HWs on coastal populations, more research is needed to assess the future projections of TC–HWs, as HWs are expected to be more frequent and stronger as the climate warms, whereas TCs over WNP may occur less often.

Free access
Joshua-Xiouhua Fu
,
Wanqiu Wang
,
Yuejian Zhu
,
Hong-Li Ren
,
Xiaolong Jia
, and
Toshiaki Shinoda

Abstract

Six sets of hindcasts conducted with the NCEP GFS have been used to study the SST-feedback processes and assess the relative contributions of atmospheric internal dynamics and SST feedback on the October and November MJO events observed during the DYNAMO IOP (Oct- and Nov-MJO). The hindcasts are carried out with three variants of the Arakawa–Shubert cumulus scheme under TMI and climatological SST conditions. The positive intraseasonal SST anomaly along with its convergent Laplacian produces systematic surface disturbances, which include enhanced surface convergence, evaporation, and equivalent potential temperature no matter which cumulus scheme is used. Whether these surface disturbances can grow into a robust response of MJO convection depends on the characteristics of the cumulus schemes used. If the cumulus scheme is able to amplify the SST-initiated surface disturbances through a strong upward–downward feedback, the model is able to produce a robust MJO convection response to the underlying SST anomaly; otherwise, the model will not produce any significant SST feedback. A new method has been developed to quantify the “potential” and “practical” contributions of the atmospheric internal dynamics and SST feedback on the MJOs. The present results suggest that, potentially, the SST feedback could have larger contributions than the atmospheric internal dynamics. Practically, the contributions to the Oct- and Nov-MJO events are, respectively, dominated by atmospheric internal dynamics and SST feedback. Averaged over the entire period, the contributions from the atmospheric internal dynamics and SST feedback are about half and half.

Open access
Haiyuan Yang
,
Ping Chang
,
Bo Qiu
,
Qiuying Zhang
,
Lixin Wu
,
Zhaohui Chen
, and
Hong Wang

ABSTRACT

Using the high-resolution Community Earth System Model (CESM) output, this study investigates air–sea interaction and its role in eddy energy dissipation in the Kuroshio Extension (KE) region. Based on an eddy energetics analysis, it is found that the baroclinic pathway associated with temperature variability is the main eddy energy source in this region. Both the air–sea heat flux and wind stress act as eddy killers that remove energy from oceanic eddies. Heat exchange between atmosphere and oceanic eddies dominates the dissipation of eddy temperature variance within the surface layer and accounts for 36% of the total dissipation in the upper 350-m layer. Compared to the heat exchange, the role of wind power in damping the eddy kinetic energy (EKE) is relatively small. Only 18% of EKE dissipation in the upper 350 m is attributed to eddy wind power. Misrepresentation of the damping role of mesoscale ocean–atmosphere interaction can result in an incorrect vertical structure of eddy energy dissipation, leading to an erroneous representation of vertical mixing in the interior ocean.

Full access
Ming-Yang He
,
Hong-Bo Liu
,
Bin Wang
, and
Da-Lin Zhang

Abstract

In this study, the three-dimensional structures and diurnal evolution of a typical low-level jet (LLJ) with a maximum speed of 24 m s−1 occurring in the 850–800-hPa layer are examined using both large-scale analysis and a high-resolution model simulation. The LLJ occurred on the eastern foothills of the Yun-Gui Plateau in south China from 1400 LST 29 June to 1400 LST 30 June 2003. The effects of surface radiative heating, topography, and latent heat release on the development of the LLJ case are also studied. Results show that a western Pacific Ocean subtropical high and a low pressure system on the respective southeast and northwest sides of the LLJ provide a favorable large-scale mean pressure pattern for the LLJ development. The LLJ reaches its peak intensity at 850 hPa near 0200 LST with wind directions veering from southerly before sunset to southwesterly at midnight. A hodograph at the LLJ core shows a complete diurnal cycle of the horizontal wind with a radius of 5.5 m s−1. It is found that in an LLJ coordinates system the along-LLJ geostrophic component regulates the distribution and 65% of the intensity of LLJ, whereas the ageostrophic component contributes to the clockwise rotation, thus leading to the formation and weakening of the LLJ during night- and daytime, respectively. Numerical sensitivity experiments confirm the surface radiative heating as the key factor in determining the formation of the nocturnal LLJ. The existence of the Yun-Gui Plateau, and the downstream condensational heating along the mei-yu front play secondary roles in the LLJ formation.

Full access
Chi-Cherng Hong
,
Wang-Ling Tseng
,
Huang-Hsiung Hsu
,
Ming-Ying Lee
, and
Chi-Chun Chang

Abstract

The northern extratropics—including regions in northern Europe, northeast Asia, and North America—experienced extremely prolonged heat waves during May–August 2018. Record-breaking surface temperatures, which caused numerous deaths, were observed in several cities. The 2018 heat waves exhibited a circumglobal characteristic owing to a circumpolar perturbation (CCP) in the middle–upper troposphere of the Northern Hemisphere (NH). The CCP had two parts: a wavelike perturbation and a hemispheric perturbation that was almost zonally symmetric. Singular-value decomposition analysis revealed that the zonally symmetric perturbation was coupled to the SST warming trend, whereas the wavelike perturbation was primarily coupled to the interannually varying SST anomaly (SSTA), particularly in the tropical North Pacific, which reached an extreme in 2018. Numerical experiments confirmed that the zonally symmetric component primarily resulted from the SSTA associated with the warming trend, whereas the interannually varying SSTAs in the NH contributed mostly to the wavelike perturbation. The warming trend component of SSTA, especially that in the tropics, compounded by the unusually large SSTAs in 2018, was hypothesized to have contributed to inducing the circumpolar circulation anomaly that caused the record-breaking heat waves in the extratropical NH in 2018.

Full access
Qihua Peng
,
Shang-Ping Xie
,
Dongxiao Wang
,
Youichi Kamae
,
Hong Zhang
,
Shineng Hu
,
Xiao-Tong Zheng
, and
Weiqiang Wang

Abstract

The influence of eastern tropical Pacific (EPAC; 10°S–10°N, 140°–80°W) wind anomalies on El Niño is investigated using observations and model experiments. Extreme and moderate El Niños exhibit contrasting anomalous wind patterns in the EPAC during the peak and decay phases: westerly wind anomalies during extreme El Niño and southeasterly (southwesterly) wind anomalies south (north) of the equator during moderate El Niño. Experiments with an ocean general circulation model indicate that for extreme El Niño, the eastward intrusion of westerly wind anomalies contributes to the prolonged positive sea surface temperature (SST) anomalies in the eastern equatorial Pacific throughout boreal spring by weakened upwelling and horizontal advection. For moderate El Niño, by contrast, both the meridional and zonal anomalous winds over the EPAC are important in the rapid (slow) SST cooling south (north) of the equator through advection and wind–evaporation–SST feedback. Atmospheric model experiments confirm that these EPAC anomalous winds are primarily forced by tropical SST anomalies. The interplay between wind and SST anomalies suggests positive air–sea feedbacks over EPAC during the decay phase of El Niño. Ocean model results show that the frequency of extreme El Niño increases when EPAC wind anomalies are removed, suggesting the importance of EPAC winds for El Niño diversity.

Free access
Zhongkun Hong
,
Zhongying Han
,
Xueying Li
,
Di Long
,
Guoqiang Tang
, and
Jianhua Wang

Abstract

Precipitation over the Tibetan Plateau (TP), known as Asia’s water tower, plays a critical role in regional water and energy cycles, largely affecting water availability for downstream countries. Rain gauges are indispensable in precipitation measurement, but are quite limited in the TP, which features complex terrain and a harsh environment. Satellite and reanalysis precipitation products can provide complementary information for ground-based measurements, particularly over large, poorly gauged areas. Here we optimally merged gauge, satellite, and reanalysis data by determining weights of various data sources using artificial neural networks (ANNs) and environmental variables including elevation, surface pressure, and wind speed. A Multi-Source Precipitation (MSP) dataset was generated at a daily time scale and a spatial resolution of 0.1° across the TP for the 1998–2017 period. The correlation coefficient (CC) of daily precipitation between the MSP and gauge observations was highest (0.74) and the root-mean-square error was the second lowest compared with four other satellite products, indicating the quality of the MSP and the effectiveness of the data merging approach. We further evaluated the hydrological utility of different precipitation products using a distributed hydrological model for the poorly gauged headwaters of the Yangtze and Yellow Rivers in the TP. The MSP achieved the best Nash–Sutcliffe efficiency coefficient (over 0.8) and CC (over 0.9) for daily streamflow simulations during 2004–14. In addition, the MSP performed best over the ungauged western TP based on multiple collocation evaluation. The merging method could be applicable to other data-scarce regions globally to provide high-quality precipitation data for hydrological research.

Full access