Search Results

You are looking at 11 - 20 of 42 items for

  • Author or Editor: Yang Lu x
  • All content x
Clear All Modify Search
Jun Yang, Weitao Lu, Ying Ma, and Wen Yao

Abstract

Cloud detection is a basic research for achieving cloud-cover state and other cloud characteristics. Because of the influence of sunlight, the brightness of sky background on the ground-based cloud image is usually nonuniform, which increases the difficulty for cirrus cloud detection, and few detection methods perform well for thin cirrus clouds. This paper presents an effective background estimation method to eliminate the influence of variable illumination conditions and proposes a background subtraction adaptive threshold method (BSAT) to detect cirrus clouds in visible images for the small field of view and mixed clear–cloud scenes. The BSAT algorithm consists of red-to-blue band operation, background subtraction, adaptive threshold selection, and binarization. The experimental results show that the BSAT algorithm is robust for all types of cirrus clouds, and the quantitative evaluation results demonstrate that the BSAT algorithm outperforms the fixed threshold (FT) and adaptive threshold (AT) methods in cirrus cloud detection.

Full access
Shan He, Song Yang, Mengmeng Lu, and Zhenning Li

Abstract

The Afro-Eurasian intermediate-frequency atmospheric teleconnection conveys meteorological signals zonally, leads to various atmospheric variations, and causes extreme events along its path. This study, aimed at demonstrating the characteristics of the teleconnection, reveals that the teleconnection accounts for nearly half of the atmospheric variability and significantly influences different meteorological fields. With the propagation of signals associated with the teleconnection, local weather varies from prolonged dry and warm days to extended wet and cold days. El Niño–Southern Oscillation (ENSO) modulates the interannual variation of the teleconnection: it becomes more active and its downstream pattern shifts southward during El Niño events. Two responsible mechanisms are proposed for the ENSO modulation: the eddy-to-eddy interaction that leads to the change in the activeness of the teleconnection and the waveguide effect that accounts for the shift of the teleconnection. First, the El Niño–related Atlantic anomalies of the Rossby wave train and storm track amplify the Atlantic disturbances of the intermediate frequency and thus the activeness of the teleconnection. Second, during El Niño years, the East Asian jet stream shifts southward, resulting in the southward shifts of the downstream waveguide effect and thus the downstream pattern. This study also demonstrates that when investigating an atmospheric mode or its impacts, the signals of different time scales should be separated and the cross-frequency interactive systems necessitate examinations.

Open access
Qin Wen, Kristofer Döös, Zhengyao Lu, Zixuan Han, and Haijun Yang

ABSTRACT

The role of the Tibetan Plateau (TP) in El Niño–Southern Oscillation (ENSO) variability is investigated using coupled model experiments with different topography setups. Removing the TP results in weakened trade winds in the tropical Pacific, an eastward shift of atmospheric convection center, a shallower mixed layer in the equatorial Pacific, and a flattened equatorial thermocline, which leads to an El Niño–like sea surface temperature (SST) response. In association with these mean climate changes in the tropical atmosphere–ocean system, the ENSO variability exhibits a much stronger amplitude in the world without the TP. Detailed diagnoses reveal that in the absence of the TP, both thermocline feedback in the eastern equatorial Pacific and Ekman pumping feedback in the central-eastern equatorial Pacific are enhanced substantially, leading to stronger ENSO variability. The changes of these two feedbacks are caused by the eastward shift of the atmospheric convection center and enhanced ocean sensitivity; the latter is due to the shallower mixed layer and flattened thermocline. This study suggests that the presence of the TP may be of fundamental importance for modern-day tropical climate variability; namely, the TP may have played a role in suppressing ENSO variability.

Open access
Hui Li, Panmao Zhai, Yang Chen, and Er Lu

Abstract

In this study, cases of the East Asia–Pacific (EAP) teleconnection pattern not responsible for persistent precipitation processes in the Yangtze River valley (YRV) have been investigated. The results suggest that such a type of EAP pattern has some linkage with persistent precipitation processes in south China (SC) with the following properties: 1) in response to the negative SSTAs and anticyclone near the Philippines, the meridional energy propagates from the low latitudes over the north of the Philippines; 2) the western Pacific subtropical high (WPSH) then intensifies and extends westward; 3) a meridional triple structure of the EAP teleconnection pattern is established; 4) at the same time, the cyclonic circulation over northeastern China introduces cold and dry air to the lower latitudes, merging with the water vapor into SC and leading to heavy precipitation from the fringe of the WPSH, the South China Sea, and the Bay of Bengal and the combination of systems persists for at least 3 days, leading to the persistent precipitation processes in SC; and 5) compared with the EAP teleconnection responsible for the precipitation in YRV, the positions of the three centers in the mid- and low latitudes are more southerly located than the YRV EAP centers. Further study indicates that the ocean surface heat conditions in the areas near the Philippines seem to be important in affecting the EAP teleconnection pattern for persistent precipitation processes in SC. Finally, all of the cases with persistent precipitation in SC during 1961–2010 linked with the EAP pattern have been investigated; the results are consistent with the above conclusions.

Full access
Kaiqiang Deng, Song Yang, Mingfang Ting, Chundi Hu, and Mengmeng Lu

Abstract

The mid-Pacific trough (MPT), occurring in the upper troposphere during boreal summer, acts as an atmospheric bridge connecting the climate variations over Asia, the Pacific, and North America. The first (second) mode of empirical orthogonal function analysis of the MPT, which accounts for 20.3% (13.4%) of the total variance, reflects a change in its intensity on the southwestern (northeastern) portion of the trough. Both modes are significantly correlated with the variability of tropical Pacific sea surface temperature (SST). Moreover, the first mode is affected by Atlantic SST via planetary waves that originate from the North Atlantic and propagate eastward across the Eurasian continent, and the second mode is influenced by the Arctic sea ice near the Bering Strait by triggering an equatorward wave train over the northeast Pacific.

A stronger MPT shown in the first mode is significantly linked to drier and warmer conditions in the Yangtze River basin, southern Japan, and the northern United States and wetter conditions in South Asia and northern China, while a stronger MPT shown in the second mode is associated with a drier and warmer southwestern United States. In addition, an intensified MPT (no matter whether in the southwestern or the northeastern portion) corresponds to more tropical cyclones (TCs) over the western North Pacific (WNP) and fewer TCs over the eastern Pacific (EP) in summer, which is associated with the MPT-induced ascending and descending motions over the WNP and the EP, respectively.

Open access
MENGMENG LU, SONG YANG, JUNBIN WANG, YUTING WU, and XIAOLONG JIA

Abstract

The thermal effect of entire Tibetan Plateau (TP) tends to strengthen the South Asian summer monsoon (SASM); however, how does this monsoon component respond to the thermal conditions of different TP domains? How do the thermal condition of entire TP influences other monsoons including the East Asian summer monsoon (EASM) and the Southeast Asian summer monsoon (SEASM)? These questions are addressed by conducting an experiment with the CESM, which is forced by reducing the surface albedo over the plateau by half, from a TP-averaged 0.20 to 0.10, from May to September, and similar experiments for different TP domains. Both observation and model results show that the entire-TP heating intensifies the large-scale Asian monsoon, the SASM, and the EASM, but surprisingly weakens the SEASM. It is also surprising that the TP heating exerts a stronger effect on the EASM than on the SASM. The southern TP (south of 35°N) does not show the strongest impact on the SASM compared to other TP domains and it exerts a weakest impact on the EASM, which is most strongly influenced by the thermal effect of eastern (east of 90°E) and northern TP. The western TP weakens the SEASM as the other domains, while it strengthens other monsoon components. The thermal condition of southern and eastern TP are accompanied by signals of tropical atmospheric response at relatively broader spatial scales, while that of northern TP more apparently leads to a significant wave train extending eastward from the TP to western Eurasia over the higher latitudes.

Restricted access
Sijia Lou, Yang Yang, Hailong Wang, Jian Lu, Steven J. Smith, Fukai Liu, and Philip J. Rasch

ABSTRACT

El Niño–Southern Oscillation (ENSO) is the leading mode of Earth’s climate variability at interannual time scales with profound ecological and societal impacts, and it is projected to intensify in many climate models as the climate warms under the forcing of increasing CO2 concentration. Since the preindustrial era, black carbon (BC) emissions have substantially increased in the Northern Hemisphere. But how BC aerosol forcing may influence the occurrence of the extreme ENSO events has rarely been investigated. In this study, using simulations of a global climate model, we show that increases in BC emissions from both the midlatitudes and Arctic weaken latitudinal temperature gradients and northward heat transport, decrease tropical energy divergence, and increase sea surface temperature over the tropical oceans, with a surprising consequential increase in the frequency of extreme ENSO events. A corollary of this study is that reducing BC emissions might serve to mitigate the possible increasing frequency of extreme ENSO events under greenhouse warming, if the modeling result can be translated into the climate in reality.

Open access
Chang Cao, Yichen Yang, Yang Lu, Natalie Schultze, Pingyue Gu, Qi Zhou, Jiaping Xu, and Xuhui Lee

Abstract

Heat stress caused by high air temperature and high humidity is a serious health concern for urban residents. Mobile measurement of these two parameters can complement weather station observations because of its ability to capture data at fine spatial scales and in places where people live and work. In this paper, we describe a smart temperature and humidity sensor (Smart-T) for use on bicycles to characterize intracity variations in human thermal conditions. The sensor has several key characteristics of internet of things (IoT) technology, including lightweight, low cost, low power consumption, ability to communicate and geolocate the data (via the cyclist’s smartphone), and the potential to be deployed in large quantities. The sensor has a reproducibility of 0.03°–0.05°C for temperature and of 0.18%–0.33% for relative humidity (one standard deviation of variation among multiple units). The time constant with a complete radiation shelter and moving at a normal cycling speed is 9.7 and 18.5 s for temperature and humidity, respectively, corresponding to a spatial resolution of 40 and 70 m. Measurements were made with the sensor on street transects in Nanjing, China. Results show that increasing vegetation fraction causes reduction in both air temperature and absolute humidity and that increasing impervious surface fraction has the opposite effect.

Restricted access
Lu Yang, Mingxuan Chen, Xiaoli Wang, Linye Song, Meilin Yang, Rui Qin, Conglan Cheng, and Siteng Li

Abstract

The ability to forecast thermodynamic conditions aloft and near the surface is critical to the accurate forecasting of precipitation type at the surface. This paper presents an experimental version of a new scheme for diagnosing precipitation type. The method considers the optimum surface temperature threshold associated with each precipitation type and combines model-based explicit fields of hydrometeors with higher-resolution modified thermodynamic and topographic information to determine precipitation types in North China. Based on over 60 years of precipitation-type samples from North China, this study explores the climatological characteristics of the five precipitation types—snow, rain, ice pellets (IP), rain/snow mix (R/S MIX), and freezing rain (FZ)—as well as the suitable air temperature T a and wet-bulb temperature T w thresholds for distinguishing different precipitation types. Direct output from numerical weather prediction (NWP) models, such as temperature and humidity, was modified by downscaling and bias correction, as well as by incorporating the latest surface observational data and high-resolution topographic data. Validation of the precipitation-type forecasts from this scheme was performed against observations from the 2016 to 2019 winter seasons and two case studies were also analyzed. Compared with the similar diagnostic routine in the High-Resolution Rapid Refresh (HRRR) forecasting system used to predict precipitation type over North China, the skill of the method proposed here is similar for rain and better for snow, R/S MIX, and FZ. Furthermore, depiction of the diagnosed boundary between R/S MIX and snow is good in most areas. However, the number of misclassifications for R/S MIX is significantly larger than for rain and snow.

Restricted access
Ji-Qin Zhong, Bing Lu, Wei Wang, Cheng-Cheng Huang, and Yang Yang

Abstract

In this study, the causes of the underestimated diurnal 2-m temperature range and the overestimated 2-m specific humidity in the winter of northern China in the Rapid-Refresh Multiscale Analysis and Prediction System–Short Term (RMAPS-ST) are investigated. Three simulations based on RMAPS-ST are conducted from 1 November 2016 to 28 February 2017. Further analyses show that the partitioning of surface upward sensible heat fluxes and downward ground heat fluxes might be the main contributing factor to the 2-m temperature forecast bias. In this study, two simulations are conducted to examine the effect of soil moisture initialization and soil hydraulic property on the 2-m temperature and 2-m specific humidity forecasts. First, the High-Resolution Land Data Assimilation System (HRLDAS) is used to provide an alternative soil moisture initialization. The results show that the drier soil moisture could lead to noticeable change in energy partitioning at the land surface, which in turn results in improved prediction of the diurnal 2-m temperature range, although it also enlarges the 2-m specific humidity bias in some parts of the domain. Second, a soil texture dataset developed by Beijing Normal University and the revised hydraulic parameters are applied to provide a more detailed description of soil properties, which could further improve the 2-m specific humidity bias. In summary, the combination of using optimized soil moisture initialization, an updated soil map, and revised soil hydraulic parameters can help improve the 2-m temperature and 2-m specific humidity prediction in RMAPS-ST.

Free access