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Chia-Chi Wang, Wei-Liang Lee, and Chia Chou

ABSTRACT

Aerosols are one of the key factors influencing the hydrological cycle and radiation balance of the climate system. Although most aerosols deposit near their sources, the induced cooling effect is on a global scale and can influence the tropical atmosphere through slow processes, such as air–sea interactions. This study analyzes several simulations of fully coupled atmosphere–ocean climate models under the influence of anthropogenic aerosols, with the concentrations of greenhouse gases kept constant. In the cooling simulations, precipitation is reduced in deep convective areas but increased around the edges of convective areas, which is opposite to the “rich-get-richer” phenomenon in global warming scenarios in the first-order approximation. Tropical convection is intensified with a shallower depth, and tropical circulations are enhanced. The anomalous gross moist stability (M′) mechanism and the upped-ante mechanism can be used to explain the dynamic and thermodynamic processes in the changes in tropical precipitation and convection. There is a northward cross-equatorial energy transport due to the cooler Northern Hemisphere in most of the simulations, together with the southward shift of the intertropical convergence zone (ITCZ) and the enhancement of the Hadley circulation. The enhancement of the Hadley circulation is more consistent between models than the changes of the Walker circulation. The change in the Hadley circulation is not as negligible as in the warming cases in previous studies, which supports the consistency of the ITCZ shift in cooling simulations.

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Xin-Zhong Liang, Wei-Chyung Wang, and Michael P. Dudek

Abstract

Observed and general circulation climate model (GCM) simulated interannual teleconnection patterns in the Northern Hemisphere are compared on a monthly basis. The study was based on 1946–1991 observations and two separate 100-year simulations corresponding to the present climate and a greenhouse warming climate. The teleconnection patterns are characterized by action centers and composite extreme anomaly (CEA) distributions. The definition and comparison of observed and simulated patterns include examination of time persistence, spatial coherence as well as consistent signatures between 500-mb height, sea level pressure, and surface air temperature.

For the present climate simulation, the GCM reproduces observed spatial and temporal variations of the action centers of four principal teleconnection patterns: the North Atlantic oscillation, the North Pacific oscillation, the Pacific/North American pattern, and the Eurasian pattern. Substantial model biases exist in the magnitude, regional structure as well as monthly transition of anomalies. The CEA regional characteristics are better simulated over land than over the oceans. For example, the model most accurately simulates the Eurasian pattern, which has its dominant action centers over Eurasia. In addition, all three climate variables exhibit substantial anomalies for each land-based action center. In contrast, over the oceans, the model systematically underestimates sea level pressure and 500-mb height CEAs, while it produces small surface temperature responses. It is suggested that atmospheric dynamics associated with flow instability is likely to be the dominant mechanism that generates these teleconnections, while the lack of interactive ocean dynamics may be responsible for small responses over the oceans.

In the greenhouse warming climate, the GCM continues to simulate the four interannual teleconnection patterns. Systematic changes, however, are found for the Pacific/North American and Eurasian patterns in winter, where the action centers shift to the east and the CEAs weaken over land. These results must be considered to be exploratory because of the use of a mixed layer ocean that does not include oceanic dynamics.

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David A. Portman, Wei-Chyung Wang, and Thomas R. Karl

Abstract

Validation of general circulation model (GCM) current climate simulations is important for further GCM development and application to climate change studies. So far, studies that compare GCM output with observations have focused primarily on large-scale spatial averages of the surface climate variables. Here we discuss two approaches to compare output of individual GCM grid boxes with local station observations near the surface and in the free troposphere. The first approach, proposed by Chervin, involves the application of standard parametric statistical analysis and hypothesis testing procedures. The second approach is nonparametric in the sense that no ideal distributions are postulated a priori to ascertain significance of the difference of mean temperature or the ratio of the temperature variance between model grid boxes and local stations. Instead, station observations are first subjected to a bootstrap technique and then used to define a unique set of distributions and confidence limits for each GCM grid box.

To demonstrate the usefulness of the two approaches, we compare daily and seasonal gridbox temperatures simulated by the National Center for Atmospheric Research (NCAR) Community Climate Model (CCM1) with station temperatures at the surface, 850-mb, 500-mb, and 300-mb levels for three different areas in the United States. We find that although CCM1 gridbox temperatures are mostly cooler than station temperatures, they are equally variable. For all grid boxes, gridbox-to-station differences decrease with height and vary with time of year. We conclude that the techniques presented here can provide useful comparisons of GCM regional and local observed temperatures. Application to other variables and GCMs is also discussed.

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Rui Wang, Xin Yan, Zhenguo Niu, and Wei Chen

Abstract

Water surface temperature is a direct indication of climate change. However, it is not clear how China’s inland waters have responded to climate change in the past using a consistent method on a national scale. In this study, we used Moderate Resolution Imaging Spectroradiometer (MODIS) data from 2000 to 2015 to study the temporal and spatial variation characteristics of water surface temperature in China using the wavelet transform method. The results showed the following: 1) the freezing date of China inland water has shown a significant delaying trend during the past 16 years with an average rate of −1.5 days yr−1; 2) the shift of the 0°C isotherm position of surface water across China has clear seasonal changes, which first moved eastward about 25° and northward about 15°, and then gradually moved back after the year 2009; 3) during the past 16 years, the 0°C isotherm of China’s surface water has gradually moved north by about 0.09° in the latitude direction and east by about 1° in the longitude direction; and 4) the interannual variation of water surface temperature in 17 lakes of China showed a similar fluctuation trend that increased before 2010, and then decreased. The El Niño and La Niña around 2010 could have impacts on the turning point of the annual variation of water surface temperature. This study validated the response of China’s inland surface water to global climate change and improved the understanding of the wetland environment’s response to climate change.

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Zhou Shenghui, Wei Ming, Wang Lijun, Zhao Chang, and Zhang Mingxu

Abstract

The sensitivity of the ill-conditioned coefficient matrix (CM) and the size of the analysis volume on the retrieval accuracy in the volume velocity processing (VVP) method are analyzed. By estimating the upper limit of the retrieval error and analyzing the effects of neglected parameters on retrieval accuracy, the simplified wind model is found to decrease the difficulty in solving and stabilizing the retrieval results, even though model errors would be induced by neglecting partial parameters. Strong linear correlation among CM vectors would cause an ill-conditioned matrix when more parameters are selected. By using exact coordinate data and changing the size of the analysis volume, the variation of the condition number indicates that a large volume size decreases the condition number, and the decrease caused by increasing the number of volume gates is larger than that caused by increasing the sector width. Using the spread of errors in the solution, a demonstration using mathematical deduction is provided to explain how a large analysis volume can improve retrieval accuracy. A test with a uniform wind field is used to demonstrate these conclusions.

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Youbing Peng, Caiming Shen, Wei-Chyung Wang, and Ying Xu

Abstract

Studies of the effects of large volcanic eruptions on regional climate so far have focused mostly on temperature responses. Previous studies using proxy data suggested that coherent droughts over eastern China are associated with explosive low-latitude volcanic eruptions. Here, the authors present an investigation of the responses of summer precipitation over eastern China to large volcanic eruptions through analyzing a 1000-yr global climate model simulation driven by natural and anthropogenic forcing. Superposed epoch analyses of 18 cases of large volcanic eruption indicate that summer precipitation over eastern China significantly decreases in the eruption year and the year after. Model simulation suggests that this reduction of summer precipitation over eastern China can be attributed to a weakening of summer monsoon and a decrease of moisture vapor over tropical oceans caused by large volcanic eruptions.

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Changlin Chen, Guihua Wang, Shang-Ping Xie, and Wei Liu

ABSTRACT

The Kuroshio and Gulf Stream, the subtropical western boundary currents of the North Pacific and North Atlantic, play important roles in meridional heat transport and ocean–atmosphere interaction processes. Using a multimodel ensemble of future projections, we show that a warmer climate intensifies the upper-layer Kuroshio, in contrast to the previously documented slowdown of the Gulf Stream. Our ocean general circulation model experiments show that the sea surface warming, not the wind change, is the dominant forcing that causes the upper-layer Kuroshio to intensify in a warming climate. Forced by the sea surface warming, ocean subduction and advection processes result in a stronger warming to the east of the Kuroshio than to the west, which increases the isopycnal slope across the Kuroshio, and hence intensifies the Kuroshio. In the North Atlantic, the Gulf Stream slows down as part of the Atlantic meridional overturning circulation (AMOC) response to surface salinity decrease in the high latitudes under global warming. The distinct responses of the Gulf Stream and Kuroshio to climate warming are accompanied by different regional patterns of sea level rise. While the sea level rise accelerates along the northeastern U.S. coast as the AMOC weakens, it remains close to the global mean rate along the East Asian coast as the intensifying Kuroshio is associated with the enhanced sea level rise offshore in the North Pacific subtropical gyre.

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Wei Wang, Ying-Hwa Kuo, and Thomas T. Warner

Abstract

An analysis of a diabatically driven and long-lived midtropospheric vortex in the lee of the Tibetan Plateau during 24–27 June 1987 is presented. The large-scale conditions were characterized by the westward expansion of the 500-mb western Pacific subtropical high and the amplification of a trough in the lee of the plateau. Embedded within the lee trough, three mesoscale convective systems (MCSs) developed. A vortex emerged following the dissipation of one MCS, with its strongest circulation located in the 400–500-mb layer. Low-level warm advection, and surface sensible and latent heating contributed to the convective initiation. Weak wind and weak ambient vorticity conditions inside the lee trough provided a favorable environment for these MCSs and the vortex to develop and evolve. The organized vortex circulation featured a coherent core of cyclonic vorticity extending from near the surface to 300 mb, with virtually no vertical tilt. The air in the vicinity of the vortex was very moist, and the temperature profile was nearly moist adiabatic, with moderate convective available potential energy. The wind near the vortex center was weak, with little vertical shear. These characteristics are similar to those of mesoscale convectively generated vortices found in the United States. The vortex circulation persisted in the same area for 3 days. The steadiness of large-scale circulation in the region, that is, the presence of the stationary lee trough and a geopotential ridge that developed to the east of the trough, likely contributed to the persistence of the vortex over the same area.

Potential vorticity (PV) diagnosis suggests that the significant increase in the relative vorticity associated with the vortex development was largely a result of diabatic heating associated with the MCS. An elevated PV anomaly was found near 400 mb in situ after the dissipation of the MCS. The PV anomaly was distinctly separated from those associated with baroclinic disturbances located to the north of the Tibetan Plateau, and the region of the PV anomaly was nearly saturated (with relative humidity exceeding 80%). Further support for this hypothesis was provided by the estimated heating profile and the rate of PV generation due to diabatic heating. The heating peaked at 300 mb, while the diabatic generation of PV reached its maximum at 500 mb. The preexisting ambient vorticity contributed about 20% to the total PV generation near the mature stage of the MCS.

The vortex was also associated with heavy precipitation over the western Sichuan Basin of China. The persistent, heavy rainfall took place in the southeasterly flow associated with the vortex circulation, about 300 km north of the vortex center.

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Wei-Yu Chang, Tai-Chi Chen Wang, and Pay-Liam Lin

Abstract

The drop size distribution (DSD) and drop shape relation (DSR) characteristics that were observed by a ground-based 2D video disdrometer and retrieved from a C-band polarimetric radar in the typhoon systems during landfall in the western Pacific, near northern Taiwan, were analyzed. The evolution of the DSD and its relation with the vertical development of the reflectivity of two rainband cases are fully illustrated. Three different types of precipitation systems were classified—weak stratiform, stratiform, and convective—according to characteristics of the mass-weighted diameter Dm, the maximum diameter, and the vertical structure of reflectivity. Further study of the relationship between the height H of the 15-dBZ contour of the vertical reflectivity profile, surface reflectivity Z, and the mass-weighted diameter Dm showed that Dm increased with a corresponding increase in the system depth H and reflectivity Z.

An analysis of DSDs retrieved from the National Central University (NCU) C-band polarimetric radar and disdrometer in typhoon cases indicates that the DSDs from the typhoon systems on the ocean were mainly a maritime convective type. However, the DSDs collected over land tended to uniquely locate in between the continental and maritime clusters. The average mass-weighted diameter Dm was about 2 mm and the average logarithmic normalized intercept Nw was about 3.8 log10 mm−1 m−3 in typhoon cases. The unique terrain-influenced deep convective systems embedded in typhoons in northern Taiwan might be the reason for these characteristics.

The “effective DSR” of typhoon systems had an axis ratio similar to that found by E. A. Brandes et al. when the raindrops were less than 1.5 mm. Nevertheless, the axis ratio tended to be more spherical with drops greater than 1.5 mm and under higher horizontal winds (maximum wind speed less than 8 m s−1). A fourth-order fitting DSR was derived for typhoon systems and the value was also very close to the estimated DSR from the polarimetric measurements in Typhoon Saomai (2006).

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Bin Deng, Shoudong Liu, Wei Xiao, Wei Wang, Jiming Jin, and Xuhui Lee

Abstract

Models of lake physical processes provide the lower flux boundary conditions for numerical predictions of weather and climate in lake basins. So far, there have been few studies on evaluating lake model performance at the diurnal time scale and against flux observations. The goal of this paper is to evaluate the National Center for Atmospheric Research Community Land Model version 4–Lake, Ice, Snow and Sediment Simulator using the eddy covariance and water temperature data obtained at a subtropical freshwater lake, Lake Taihu, in China. Both observations and model simulations reveal that convective overturning was commonplace at night and timed when water switched from being statically stable to being unstable. By reducing the water thermal diffusivity to 2% of the value calculated with the Henderson–Sellers parameterization, the model was able to reproduce the observed diurnal variations in water surface temperature and in sensible and latent heat fluxes. The small diffusivity suggests that the drag force of the sediment layer in this large (2500 km2) and shallow (2-m depth) lake may be strong, preventing unresolved vertical motions and suppressing wind-induced turbulence. Model results show that a large fraction of the incoming solar radiation energy was stored in the water during the daytime, and the stored energy was diffused upward at night to sustain sensible and latent heat fluxes to the atmosphere. Such a lake–atmosphere energy exchange modulated the local climate at the daily scale in this shallow lake, which is not seen in deep lakes where dominant lake–atmosphere interactions often occur at the seasonal scale.

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