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Xuelong Chen, Zhongbo Su, Yaoming Ma, Kun Yang, Jun Wen, and Yu Zhang

Abstract

Roughness height for heat transfer is a crucial parameter in the estimation of sensible heat flux. In this study, the performance of the Surface Energy Balance System (SEBS) has been tested and evaluated for typical land surfaces on the Tibetan Plateau on the basis of time series of observations at four sites with bare soil, sparse canopy, dense canopy, and snow surface, respectively. Both under- and overestimation at low and high sensible heat fluxes by SEBS was discovered. Through sensitivity analyses, it was identified that these biases are related to the SEBS parameterization of bare soil’s excess resistance to heat transfer (kB −1, where k is the von Kármán constant and B −1 is the Stanton number). The kB −1 of bare soil in SEBS was replaced. The results show that the revised model performs better than the original model.

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Tianjiao Ma, Wen Chen, Hans-F. Graf, Shuoyi Ding, Peiqiang Xu, Lei Song, and Xiaoqing Lan

Abstract

The present study investigates different impacts of the East Asian winter monsoon (EAWM) on surface air temperature (Ts) in North America (NA) during ENSO and neutral ENSO episodes. In neutral ENSO years, the EAWM shows a direct impact on the Ts anomalies in NA on an interannual time scale. Two Rossby wave packets appear over the Eurasian–western Pacific (upstream) and North Pacific–NA (downstream) regions associated with a strong EAWM. Further analysis suggests that the downstream wave packet is caused by reflection of the upstream wave packet over the subtropical western Pacific and amplified over the North Pacific. Also, the East Asian subtropical westerly jet stream (EAJS) is intensified in the central and downstream region over the central North Pacific. Hence, increased barotropic kinetic energy conversion and the interaction between transient eddies and the EAJS tend to maintain the circulation anomaly over the North Pacific. Therefore, a strong EAWM tends to result in warm Ts anomalies in northwestern NA via the downstream wave packet emanating from the central North Pacific toward NA. A weak EAWM tends to induce cold Ts anomalies in western-central NA with a smaller magnitude. However, in ENSO years, an anomalous EAJS is mainly confined over East Asia and does not extend into the central North Pacific. The results confirm that the EAWM has an indirect impact on the Ts anomalies in NA via a modulation of the tropical convection anomalies associated with ENSO. Our results indicate that, for seasonal prediction of Ts anomalies in NA, the influence of the EAWM should be taken into account. It produces different responses in neutral ENSO and in ENSO years.

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Weixin Xu, Edward J. Zipser, Yi-Leng Chen, Chuntao Liu, Yu-Chieng Liou, Wen-Chau Lee, and Ben Jong-Dao Jou

Abstract

This study investigates a long-duration mesoscale system with extremely heavy rainfall over southwest Taiwan during the Terrain-influenced Monsoon Rainfall Experiment (TiMREX). This mesoscale convective system develops offshore and stays quasi-stationary over the upstream ocean and southwest coast of Taiwan. New convection keeps developing upstream offshore but decays or dies after moving into the island, dropping the heaviest rain over the upstream ocean and coastal regions. Warm, moist, unstable conditions and a low-level jet (LLJ) are found only over the upstream ocean, while the island of Taiwan is under the control of a weak cold pool. The LLJ is lifted upward at the boundary between the cold pool and LLJ. Most convective clusters supporting the long-lived rainy mesoscale system are initiated and develop along that boundary. The initiation and maintenance is thought to be a “back-building–quasi-stationary” process. The cold pool forms from previous persistent precipitation with a temperature depression of 2°–4°C in the lowest 500 m, while the high terrain in Taiwan is thought to trap the cold pool from spreading or moving. As a result, the orography of Taiwan is “extended” to the upstream ocean and plays an indirect effect on the long-duration mesoscale system.

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Wei-Chyung Wang, Wei Gong, Wen-Shung Kau, Cheng-Ta Chen, Huang-Hsiung Hsu, and Chia-Hsiu Tu

Abstract

Observations indicate that the East Asian summer monsoon (EASM) exhibits distinctive characteristics of large cloud amounts with associated heavy and persistent rainfall, although short breaks for clear sky usually occur. Consequently, the effects of cloud–radiation interactions can play an important role in the general circulation of the atmosphere and, thus, the evolution of the EASM. In this note, as a first step toward studying the topic, the 5-yr (January 1985–December 1989) Earth Radiation Budget Experiment (ERBE) dataset is used to show the spatial and temporal patterns of both shortwave (SW) and longwave (LW) cloud radiative forcing (CRF) at the top of the atmosphere over east China, and to compare the observed features with Atmospheric Model Intercomparison Project-II (AMIP-II) simulations with the University at Albany, State University of New York (SUNYA) Community Climate Model 3 (CCM3) and the ECHAM4 general circulation models.

The observations indicate that the net CRF provides a cooling effect to the atmosphere–surface climate system, dominated by the SW CRF cooling (albedo effect) with partial compensation from the LW CRF warming (greenhouse effect). The SW CRF shows a strong seasonal cycle, and its peak magnitude is particularly large, ∼110 W m−2, for south China and the Yangtze–Huai River valley (YHRV) during May and June, while the LW CRF is about 50 W m−2 for the same months with a weak dependence on the latitudes and seasons. These characteristics are in sharp contrast to the Northern Hemispheric zonal means of the same latitude bands and seasons, thus implying a unique role for cloud–radiation interaction in east China. Both model simulations show similar observed characteristics, although biases exist. For example, in May, the ECHAM4 underestimates the SW CRF while the SUNYA CCM3 simulates a significantly larger value, both attributed to the respective biases in the simulated total cloud cover. Model-to-observation comparisons of the association between total cloud cover and SW CRF, and between high cloud cover and LW CRF, are also presented and their differences are discussed. Finally, the SUNYA CCM3 biases in the CRF and its relevance to the model cloud biases are discussed in the context of model cold and dry biases in climate simulations.

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Hao Huang, Kun Zhao, Guifu Zhang, Qing Lin, Long Wen, Gang Chen, Zhengwei Yang, Mingjun Wang, and Dongming Hu

Abstract

Quantitative precipitation estimation (QPE) with polarimetric radar measurements suffers from different sources of uncertainty. The variational approach appears to be a promising way to optimize the radar QPE statistically. In this study a variational approach is developed to quantitatively estimate the rainfall rate (R) from the differential phase (ΦDP). A spline filter is utilized in the optimization procedures to eliminate the impact of the random errors in ΦDP, which can be a major source of error in the specific differential phase (K DP)-based QPE. In addition, R estimated from the horizontal reflectivity factor (Z H) is used in the a priori with the error covariance matrix statistically determined. The approach is evaluated by an idealized case and multiple real rainfall cases observed by an operational S-band polarimetric radar in southern China. The comparative results demonstrate that with a proper range filter, the proposed variational radar QPE with the a priori included agrees well with the rain gauge measurements and proves to have better performance than the other three approaches, that is, the proposed variational approach without the a priori included, the variational approach proposed by Hogan, and the conventional power-law estimator-based approach.

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Donghai Zheng, Rogier van der Velde, Zhongbo Su, Xin Wang, Jun Wen, Martijn J. Booij, Arjen Y. Hoekstra, and Yingying Chen

Abstract

This is the first part of a study focusing on evaluating the performance of the Noah land surface model (LSM) in simulating surface water and energy budgets for the high-elevation source region of the Yellow River (SRYR). A comprehensive dataset is utilized that includes in situ micrometeorological and profile soil moisture and temperature measurements as well as laboratory soil property measurements of samples collected across the SRYR. Here, the simulation of soil water flow is investigated, while Part II concentrates on the surface heat flux and soil temperature simulations. Three augmentations are proposed: 1) to include the effect of organic matter on soil hydraulic parameterization via the additivity hypothesis, 2) to implement the saturated hydraulic conductivity as an exponentially decaying function with soil depth, and 3) to modify the vertical root distribution to represent the Tibetan conditions characterized by an abundance of roots in the topsoil. The diffusivity form of Richards’ equation is further revised to allow for the simulation of soil water flow across soil layers with different hydraulic properties. Usage of organic matter for calculating the porosity and soil suction improves the agreement between the estimates and laboratory measurements, and the exponential function together with the Kozeny–Carman equation best describes the in situ . Through implementation of the modified hydraulic parameterization alone, the soil moisture underestimation in the upper soil layer under wet conditions is resolved, while the soil moisture profile dynamics are better captured by also including the modified root distribution.

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Wen Li Zhao, Guo Yu Qiu, Yu Jiu Xiong, Kyaw Tha Paw U, Pierre Gentine, and Bao Yu Chen

Abstract

Quantifying the uncertainties caused by resistance parameterizations is fundamental for understanding, improving, and developing terrestrial evapotranspiration (ET) models. Using high-density eddy covariance (EC) tower observations in a heterogeneous oasis in northwest China, this study evaluates the impacts of resistances on the estimation of latent heat flux (LE), the energy equivalent of ET, by comparing resistance parameterizations with different complexities under one- and two-source Penman–Monteith (PM) equations. The results showed that the mean absolute percent error (MAPE) for the LE estimates from the one- and two-source PM equations varied from 32% to 53%, and the uncertainties were caused mainly by the resistance parameterizations. Calibrating the parameters required in the resistance estimations could improve the performance of the PM equations; specifically, the MAPEs for the one-source PM equations were approximately 16%, whereas they were 38% for the two-source PM equations, emphasizing that multiple resistances result in increased uncertainties. The following conclusions were reached: 1) the empirical and biophysical parameters required in resistance estimations were responsible for the uncertainty; 2) increasingly complex resistance parameterizations resulted in greater uncertainties in LE estimates; and 3) models without resistance parameterizations exhibited reduced uncertainties in LE estimates.

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Gang Chen, Kun Zhao, Guifu Zhang, Hao Huang, Su Liu, Long Wen, Zhonglin Yang, Zhengwei Yang, Lili Xu, and Wenjian Zhu

Abstract

In this study, the capability of using a C-band polarimetric Doppler radar and a two-dimensional video disdrometer (2DVD) to estimate monsoon-influenced summer rainfall during the Observation, Prediction and Analysis of Severe Convection of China (OPACC) field campaign in 2014 and 2015 in eastern China is investigated. Three different rainfall R estimators, for reflectivity at horizontal polarization [R(Z h)], for reflectivity at horizontal polarization and differential reflectivity factor [R(Z h, Z dr)], and for specific differential phase [R(K DP)], are derived from 2-yr 2DVD observations of summer precipitation systems. The radar-estimated rainfall is compared to gauge observations from eight rainfall episodes. Results show that the two polarimetric estimators, R(Z h, Z dr) and R(K DP), perform better than the traditional Z hR relation [i.e., R(Z h)]. The K DP-based estimator [i.e., R(K DP)] produces the best rainfall accumulations. The radar rainfall estimators perform differently across the three organized convective systems (mei-yu rainband, typhoon rainband, and squall line). Estimator R(Z h) overestimates rainfall in the mei-yu rainband and squall line, and R(Z h, Z dr) mitigates the overestimation in the mei-yu rainband but has a large bias in the squall line. QPE from R(K DP) is the most accurate among the three estimators, but it possesses a relatively large bias for the squall line compared to the mei-yu case. The high variability of drop size distribution (DSD) related to the precipitation microphysics in different types of rain is largely responsible for the case-dependent QPE performance using any single radar rainfall estimator. The squall line has a distinct ice-phase process with a large mean size of raindrops, while the mei-yu rainband and typhoon rainband are composed of smaller raindrops. Based on the statistical QPE error in the Z HZ DR space, a new composite rainfall estimator is constructed by combining R(Z h), R(Z h, Z dr), and R(K DP) and is proven to outperform any single rainfall estimator.

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Donghai Zheng, Rogier van der Velde, Zhongbo Su, Xin Wang, Jun Wen, Martijn J. Booij, Arjen Y. Hoekstra, and Yingying Chen

Abstract

This is the second part of a study on the assessment of the Noah land surface model (LSM) in simulating surface water and energy budgets in the high-elevation source region of the Yellow River. Here, there is a focus on turbulent heat fluxes and heat transport through the soil column during the monsoon season, whereas the first part of this study deals with the soil water flow. Four augmentations are studied for mitigating the overestimation of turbulent heat flux and underestimation of soil temperature measurements: 1) the muting effect of vegetation on the thermal heat conductivity is removed from the transport of heat from the first to the second soil layer, 2) the exponential decay factor imposed on is calculated using the ratio of the leaf area index (LAI) over the green vegetation fraction (GVF), 3) Zilitinkevich’s empirical coefficient for turbulent heat transport is computed as a function of the momentum roughness length , and 4) the impact of organic matter is considered in the parameterization of the thermal heat properties. Although usage of organic matter for calculating improves the correspondence between the estimates and laboratory measurements of heat conductivities, it is shown to have a relatively small impact on the Noah LSM performance even for large organic matter contents. In contrast, the removal of the muting effect of vegetation on and the parameterization of greatly enhances the soil temperature profile simulations, whereas turbulent heat flux and surface temperature computations mostly benefit from the modified formulation. Further, the nighttime surface temperature overestimation is resolved from a coupled land–atmosphere perspective.

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Kun Zhao, Mingjun Wang, Ming Xue, Peiling Fu, Zhonglin Yang, Xiaomin Chen, Yi Zhang, Wen-Chau Lee, Fuqing Zhang, Qing Lin, and Zhaohui Li

Abstract

On 4 October 2015, a miniature supercell embedded in an outer rainband of Typhoon Mujigae produced a major tornado in Guangdong province of China, leading to 4 deaths and up to 80 injuries. This study documents the structure and evolution of the tornadic miniature supercell using coastal Doppler radars, a sounding, videos, and a damage survey. This tornado is rated at least EF3 on the enhanced Fujita scale. It is by far the strongest typhoon rainband tornado yet documented in China, and possessed double funnels near its peak intensity.

Radar analysis indicates that this tornadic miniature supercell exhibited characteristics similar to those found in United States landfalling hurricanes, including a hook echo, low-level inf low notches, an echo top below 10 km, a small and shallow mesocyclone, and a long lifespan (3 h). The environmental conditions—which consisted of moderate convective available potential energy (CAPE), a low lifting condensation level, a small surface dewpoint depression, a large veering low-level vertical wind shear, and a large cell-relative helicity—are favorable for producing miniature supercells. The mesocyclone, with its maximum intensity at 2 km above ground level (AGL), formed an hour before tornadogenesis. A tornado vortex signature (TVS) was identified between 1 and 3 km AGL, when the parent mesocyclone reached its peak radar-indicated intensity of 30 m s−1. The TVS was located between the updraft and forward-flank downdraft, near the center of the mesocyclone. Dual-Doppler wind analysis reveals that tilting of the low-level vorticity into the vertical direction and subsequent stretching by a strong updraft were the main contributors to the mesocyclone intensification.

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