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Li Li and Yaocun Zhang

Abstract

Observational analysis indicates that the East Asian jet stream consists of two separate branches: the East Asian subtropical jet (EASJ) and the East Asian polar front jet (EAPJ). The impacts of different intensity configurations of the EASJ and EAPJ on precipitation during the mei-yu season are investigated using the NCEP–NCAR Reanalysis Project (NNRP) dataset and daily gauge observations in East China. The intensity and location of precipitation are associated with different configurations of the EASJ and EAPJ. Precipitation intensity increases with intensification of the EASJ and EAPJ. The rainband is located to the north of the mei-yu region when the EASJ intensifies and the EAPJ weakens. Further analyses indicate that the intensity changes of the EASJ and EAPJ are linked to the cold and warm airmass activities. For cases with strong EASJ and EAPJ, both the warm-moist and cold air masses are active. When the warm-moist and cold air masses meet near 30°N, abundant precipitation occurs in the Yangtze-Huai River basin (YHRB). For cases with weak EASJ and EAPJ, both the cold and warm-moist air masses are inactive, and no significant precipitation occurs in the YHRB. For cases with strong EASJ and weak EAPJ, the warm-moist air mass moves northward while the cold air mass is weak. Precipitation concentrates to the north of YHRB. For cases with weak EASJ and strong EAPJ, cold air extends farther south while the warm-moist air mass is inactive. Precipitation occurs to the south of YHRB.

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Feng Zhang and Jiangnan Li

Abstract

Though the single-layer solutions have been found for the δ-four-stream spherical harmonic expansion method (SHM) in radiative transfer, there is lack of a corresponding doubling–adding method (4SDA), which enables the calculation of radiative transfer through a vertically inhomogeneous atmosphere with multilayers. The doubling–adding method is based on Chandrasekhar's invariance principle, which was originally developed for discrete ordinates approximation. It is shown that the invariance principle can also be applied to SHM and δ-four-stream spherical harmonic expansion doubling–adding method (δ-4SDA) is proposed in this paper. The δ-4SDA method has been systematically compared to the δ-Eddington doubling–adding method (δ-2SDA), the δ-two-stream discrete ordinates doubling–adding method (δ-2DDA), and δ-four-stream discrete ordinates doubling–adding method (δ-4DDA). By applying δ-4SDA to a realistic atmospheric profile with gaseous transmission considered, it is found that the accuracy of δ-4SDA is superior to δ-2SDA or δ-2DDA, especially for the cloudy/aerosol conditions. It is shown that the relative errors of δ-4SDA are generally less than 1% in both heating rate and flux, while the relative errors of both δ-2SDA and δ-2DDA can be over 6%. Though δ-4DDA is slightly more accurate than δ-4SDA in heating rates, both of them are accurate enough to obtain the cloud-top solar heating. Here δ-4SDA is superior to δ-4DDA in computational efficiency. It is found that the error of aerosol radiative forcing can be up to 3 W m−2 by using δ-2SDA at the top of the atmosphere (TOA); such error is substantially reduced by applying δ-4SDA. In view of the overall accuracy and computational efficiency, δ-4SDA is suitable for application in climate models.

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Xiaosong Li and Jin Zhang

Abstract

The green vegetation fraction Fg, which represents the horizontal density of live vegetation, is an important parameter for the study of global energy, carbon, hydrological, and biogeochemical cycling. A common method of calculating Fg is to create a simple linear mixing model between two NDVI endmembers: bare soil NDVI, , and full vegetation NDVI, . However, many uncertainties exist for the determination of these parameters at large scales. The present study investigates how and determination can impact Fg calculations for all of China, based on different land-cover datasets, hyperspectral data, and soil type classification maps. The results show the following: 1) The regional ChinaCover dataset, with higher accuracy and more detailed classification, is preferable for calculating Fg in China, compared with the most commonly used MOD12Q1 dataset, although it would not lead to too much difference in values. 2) The soil NDVI from Hyperion datasets shows that soils have highly variable NDVI values (0.006–0.2), and 79.36% of the area studied has a much larger NDVI value than the commonly used value of 0.05. Therefore, the dynamic values with different soil types are much better for Fg calculation than the invariant value (0.05), which would yield a significant overestimation of Fg, especially for areas with low vegetation coverage. 3) A high-quality Fg dataset for China from 2000 to 2010 was established with and parameters based on MOD13Q1 250-m NDVI data.

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Lei Zhang and Tim Li

Abstract

How sea surface temperature (SST) changes under global warming is critical for future climate projection because SST change affects atmospheric circulation and rainfall. Robust features derived from 17 models of phase 5 of the Coupled Model Intercomparison Project (CMIP5) include a much greater warming in high latitudes than in the tropics, an El Niño–like warming over the tropical Pacific and Atlantic, and a dipole pattern in the Indian Ocean. However, the physical mechanism responsible for formation of such warming patterns remains open.

A simple theoretical model is constructed to reveal the cause of the future warming patterns. The result shows that a much greater polar, rather than tropical, warming depends primarily on present-day mean SST and surface latent heat flux fields, and atmospheric longwave radiation feedback associated with cloud change further enhances this warming contrast. In the tropics, an El Niño–like warming over the Pacific and Atlantic arises from a similar process, while cloud feedback resulting from different cloud regimes between east and west ocean basins also plays a role. A dipole warming over the equatorial Indian Ocean is a response to weakened Walker circulation in the tropical Pacific.

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Yunying Li and Minghua Zhang

Abstract

Cumulus (Cu) from shallow convection is one of the dominant cloud types over the Tibetan Plateau (TP) in the summer according to CloudSatCALIPSO observations. Its thermodynamic effects on the atmospheric environment and impacts on the large-scale atmospheric circulation are studied in this paper using the Community Atmospheric Model, version 5.3 (CAM5.3). It is found that the model can reasonably simulate the unique distribution of diabatic heating and Cu over the TP. Shallow convection provides the dominant diabatic heating and drying to the lower and middle atmosphere over the TP. A sensitivity experiment indicates that without Cu over the TP, large-scale condensation and stratiform clouds would increase dramatically, which induces enhanced low-level wind and moisture convergence toward the TP, resulting in significantly enhanced monsoon circulation with remote impact on the areas far beyond the TP. Cu therefore acts as a safety valve to modulate the atmospheric environment that prevents the formation of superclusters of stratiform clouds and precipitation over the TP.

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Yunying Li and Minghua Zhang

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Cumulus (Cu) can transport heat and water vapor from the boundary layer to the free atmosphere, leading to the redistribution of heat and moist energy in the lower atmosphere. This paper uses the fine-resolution CloudSat–CALIPSO product to characterize Cu over the Tibetan Plateau (TP). It is found that Cu is one of the dominant cloud types over the TP in the northern summer. The Cu event frequency, defined as Cu occurring within 50-km segments, is 54% over the TP in the summer, which is much larger over the TP than in its surrounding regions. The surface wind vector converging at the central TP and the topographic forcing provide the necessary moisture and dynamical lifting of convection over the TP. The structure of the atmospheric moist static energy shows that the thermodynamical environment over the northern TP can be characterized as having weak instability, a shallow layer of instability, and lower altitudes for the level of free convection. The diurnal variation of Cu with frequency peaks during the daytime confirms the surface thermodynamic control on Cu formation over the TP. This study offers insights into how surface heat is transported to the free troposphere over the TP and provides an observational test of climate models in simulating shallow convection over the TP.

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Mingxin Li, Qinghong Zhang, and Fuqing Zhang

Abstract

Based on a comprehensive collection of hail observations and the NCEP–NCAR reanalyses from 1960 to 2012, the long-term trends of hail day frequency in mainland China and the associated changes in atmospheric circulation patterns were analyzed. There was no detectable trend in hail frequency from 1960 to the early 1980s, but a significant decreasing trend was apparent in later periods throughout most of China and in particular over the Tibetan Plateau from the early 1980s and over northern and northwestern China from the early 1990s. Hail frequency in southern China did not decrease as significantly as in other regions over the last couple of decades. An objective classification method, the obliquely rotated T-mode principal component technique, was used to investigate atmospheric circulation patterns. It was found that 51.85% of the hail days occurred during two major circulation types, both of which were associated with cold frontal systems in northern China. More specifically, the synoptic trough in East Asia, signified by the meridional circulation at 850 hPa, became considerably weaker after 1990. This change in the synoptic pattern is consistent with a weakening trend in the East Asian summer monsoon, the primary dynamic forcing of moisture transport that contributes to the generation of severe convection in northern China. The long-term variability of hail day frequency over the Tibetan Plateau was more strongly correlated with the change in mean freezing-level height (FLH) than the strength of the East Asian monsoon.

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Min Li, Ting Zhang, Jianzhu Li, and Ping Feng

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Many drought forecasting methods have been proposed, but only a few have considered the changing environment. The main purpose of this study is to improve the accuracy of drought forecasting models under changing environments by considering the influence of large-scale climate patterns and human activities on hydrological drought. To select the most significant large-scale climatic index that influences drought events in the Luanhe River basin, Spearman’s rho correlation test was applied to detect the relationship between large-scale oceanic–atmospheric circulation patterns and the standardized runoff index (SRI). We also proposed a human activity index (HI) to represent the effect of human activities on hydrological drought. Based on a multivariate normal distribution, we included the above indices in a probabilistic forecasting model, which forecasted the probabilities of transition from the current to a future SRI value. Using the Liying hydrological station as an example, the impacts of a controlled large-scale climatic index (Niño-3.4) and the HI on the transition probabilities were illustrated, and the results showed that the turning point of the Niño-3.4 effect on the transition probabilities occurred within the range from 25.91 to 26.90. Finally, a scoring method was applied to compare the forecasting model performances. The results showed that the inclusion of the large-scale climatic index and HI improved the forecasting accuracy.

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Quanjia Zhong, Jianping Li, Lifeng Zhang, Ruiqiang Ding, and Baosheng Li

Abstract

The predictability limits of tropical cyclone (TC) intensity over the western North Pacific (WNP) are investigated using TC best track data. The results show that the predictability limit of the TC minimum central pressure (MCP) is ~102 h, comparable to that of the TC maximum sustained wind (MSW). The spatial distribution of the predictability limit of the TC MCP over the WNP is similar to that of the TC MSW, and both gradually decrease from the eastern WNP (EWNP) to the South China Sea (SCS). The predictability limits of the TC MCP and MSW are relatively high over the southeastern WNP where the modified accumulated cyclone energy (MACE) is relatively large, whereas they are relatively low over the SCS where the MACE is relatively small. The spatial patterns of the TC lifetime and the lifetime maximum intensity (LMI) are similar to that of the TC MACE. Strong and long-lived TCs, which have relatively long predictability, mainly form in the southwestern WNP. In contrast, weak and short-lived TCs, which have relatively short predictability, mainly form in the SCS. In addition to the dependence of the predictability limit on genesis location, the predictability limits of TC intensity also evolve in the TC life cycle. The predictability limit of the TC MCP (MSW) gradually decreases from 102 (108) h at genesis time (00 h) to 54 (84) h 4 days after TC genesis.

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Qiuyun Wang, Jianping Li, Yanjie Li, Jingwen Zhang, and Jiayu Zheng

Abstract

The influence of the intraseasonal Indo–western Pacific convection oscillation (IPCO) on tropical cyclone (TC) genesis location and frequency over the Indo–western North Pacific (WNP) during the boreal extended summer (May–October) is explored. Observational analysis shows that the impacts of the intraseasonal IPCO on TCs over the Indo–WNP include an evident “phase lock of TC genesis location” and distinct differences in TC frequency. In the WNP, in the positive intraseasonal IPCO phase, the atmosphere gains heat through the release of latent heat in cumulus convective condensation, and the anomalous cyclonic circulation weakens the western Pacific subtropical high (WPSH) and enhances TC genesis, thereby tending to produce many more TCs. Moreover, the diminished WPSH and the westward shift of the centers of anomalous cyclonic circulations lock TC genesis locations to the west WNP and lower latitudes (around 5°–20°N), especially in the South China Sea. The almost opposite situation occurs in a negative phase. In the north Indian Ocean, the total TC genesis frequencies in the two intraseasonal IPCO phases are approximate. However, in the positive intraseasonal IPCO phase, the environmental conditions to the north of 13°N are similar to those in the WNP except without the WPSH control, whereas south of 13°N the situation is reversed, leading to a northward shift of the TC genesis location (around 13°–20°N). The negative phase reflects an opposite situation.

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