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Lin Liu, Jianping Guo, Wen Chen, Renguang Wu, Lin Wang, Hainan Gong, Bo Liu, Dandan Chen, and Jian Li

Abstract

The present study applies the empirical orthogonal function (EOF) method to investigate the interannual covariations of East Asian–Australian land precipitation (EAALP) during boreal winter based on observational and reanalysis datasets. The first mode of EAALP variations is characterized by opposite-sign anomalies between East Asia (EA) and Australia (AUS). The second mode features an anomaly pattern over EA similar to the first mode, but with a southwest–northeast dipole structure over AUS. El Niño–Southern Oscillation (ENSO) is found to be a primary factor in modulating the interannual variations of land precipitation over EA and western AUS. By comparison, the Indian Ocean subtropical dipole mode (IOSD) plays an important role in the formation of precipitation anomalies over northeastern AUS, mainly through a zonal vertical circulation spanning from the southern Indian Ocean (SIO) to northern AUS. In addition, the ENSO-independent cold sea surface temperature (SST) anomalies in the western North Pacific (WNP) impact the formation of the second mode. Using the atmospheric general circulation model ECHAM5, three 40-yr numerical simulation experiments differing in specified SST forcings verify the impacts of the IOSD and WNP SST anomalies. Further composite analyses indicate that the dominant patterns of EAALP variability are largely determined by the out-of-phase and in-phase combinations of ENSO and IOSD. These results suggest that in addition to ENSO, IOSD should be considered as another crucial factor influencing the EAALP variability during the boreal winter, which has large implications for improved prediction of EAALP land precipitation on the interannual time scale.

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Yu-Chieng Liou, Tai-Chi Chen Wang, Wen-Chau Lee, and Ya-Ju Chang

Abstract

The ground-based velocity track display (GBVTD) technique is extended to two Doppler radars to retrieve the structure of a tropical cyclone’s (TC’s) circulation. With this extension, it is found that the asymmetric part of the TC radial wind component can be derived up to its angular wavenumber-1 structure, and the accuracy of the retrieved TC tangential wind component can be further improved. Although two radar systems are used, a comparison with the traditional dual-Doppler synthesis indicates that this extended GBVTD (EGBVTD) approach is able to estimate more of the TC circulation when there are missing data. Previous research along with this study reveals that the existence of strong asymmetric radial flows can degrade the quality of the GBVTD-derived wind fields. When a TC is observed by one radar, it is suggested that the GBVTD method be applied to TCs over a flat surface (e.g., the ocean) where the assumption of relatively smaller asymmetric radial winds than asymmetric tangential winds is more likely to be true. However, when a TC is observed by two radar systems, especially when the topographic effects are expected to be significant, the EGBVTD rather than the traditional dual-Doppler synthesis should be used.

The feasibility of the proposed EGBVTD method is demonstrated by applying it to an idealized TC circulation model as well as a real case study. Finally, the possibility of combining EGBVTD with other observational instruments, such as dropsonde or wind profilers, to recover the asymmetric TC radial flow structures with even higher wavenumbers is discussed.

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Eugene M. Rasmusson, Phillip A. Arkin, Wen-Yuan Chen, and John B. Jalickee

Abstract

Surface temperature variations over the contiguous United States during the period 1931–75 are examined using mean monthly averages for the 344 climate divisions. This data matrix is decomposed into orthogonal components using the method of singular decomposition.

The third empirical orthogonal function, which accounts for nine percent of the nonseasonal variance, exhibits a significant quasi-biennial oscillation (QBO). The phase and amplitude of the QBO implied by this analysis were further studied using an extension of the singular decomposition method which we call Hilbert Singular Decomposition (USD). HSD uses the Hilbert Transformer to augment the data matrix and transform the real elements into complex elements so that coherent “wavelike” variations can be represented in terms of a complex singular decomposition. Additional cross-spectral analyses were performed for selected climate division aggregates.

Two areas of maximum QBO amplitude are indicated; one over the northeastern United States, and a less distinct area over the southwestern United States. Variations in the two areas appear to be almost 180° out of phase.

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Xianglei Huang, Hui-Wen Chuang, Andrew Dessler, Xiuhong Chen, Kenneth Minschwaner, Yi Ming, and V. Ramaswamy

Abstract

Both observational analysis and GCM simulations indicate that the tropical Walker circulation is becoming weaker and may continue to weaken as a consequence of climate change. Here, the authors use a conceptual radiative–convective equilibrium (RCE) framework to interpret the weakening of the Walker circulation as simulated by the GFDL coupled GCM. Based on the modeled lapse rate and clear-sky cooling rate profiles, the RCE framework can directly compute the change of vertical velocity in the descending branch of the Walker circulation, which agrees with the counterpart simulated by the GFDL model. The results show that the vertical structure of clear-sky radiative cooling rate QR will change in response to the increased water vapor as the globe warms. The authors explain why the change of QR is positive in the uppermost part of the troposphere (<300 hPa) and is negative for the rest of the troposphere. As a result, both the change of clear-sky cooling rate and the change of tropospheric lapse rate contribute to the weakening of circulation. The vertical velocity changes due to the two factors are comparable to each other from the top of the planetary boundary layer to 600 hPa. From 600 to 300 hPa lapse rate changes are the dominant cause of the weakening circulation. Above 300 hPa, the change due to QR is opposite to the change due to lapse rate, which forces a slight increase in vertical velocity that is seen in the model simulation.

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Hao Huang, Guifu Zhang, Kun Zhao, Su Liu, Long Wen, Gang Chen, and Zhengwei Yang

Abstract

Drop size distribution (DSD) is a fundamental parameter in rain microphysics. Retrieving DSDs from polarimetric radar measurements extends the capabilities of rain microphysics research and quantitative precipitation estimation. In this study, issues in rain DSD retrieval were studied with simulated and measured data. It was found that a three-parameter gamma distribution model was not suitable for directly retrieving DSD from polarimetric radar measurements. A statistical constraint, such as the shape–slope relation used in the constrained-gamma (C-G) distribution model, helped to reduce the uncertainties and errors in the retrieval. The inclusion of specific differential phase (K DP) measurements resulted in more accurate DSD retrieval and rain physical parameter estimation if the measurement errors were properly characterized in the error minimization analysis (EMA), which was verified using two real precipitation events. The study demonstrated the potential of using full polarimetric radar measurements to improve rain DSD retrieval.

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Ping Zhao, Song Yang, Renguang Wu, Zhiping Wen, Junming Chen, and Huijun Wang

Abstract

The authors have identified an interannual relationship between Asian tropospheric temperature and the North Atlantic Ocean sea surface temperature (SST) during summer (May–September) and discussed the associated features of atmospheric circulation over the Atlantic–Eurasian region. When tropospheric temperature is high (low) over Asia, positive (negative) SST anomalies appear in the extratropical North Atlantic. This relationship is well supported by the changes in background atmospheric circulation and ocean–atmosphere–land thermodynamic processes. When heat transfer from the land surface to the atmosphere over Asia strengthens, local tropospheric temperature increases and positive temperature anomalies propagate westward from Asia to the North Atlantic, leading to an increase in summer tropospheric temperature over the Atlantic–Eurasian region. Accordingly, a deep anomalous ridge occurs over the extratropical North Atlantic Ocean, with low-level southerly anomalies over the western portion of the ocean. Sensitivity experiments with climate models show that the interannual variations of the North Atlantic–Eurasian atmospheric circulation may not be forced by the extratropical Atlantic SST. Instead, experiments with changing Asian land surface heating capture the above observed features of atmospheric circulation anomalies, westward propagation of tropospheric anomalies, and Atlantic SST anomalies. The consistency between the observational and model results indicates a possible impact of Asian land heating on the development of atmospheric circulation and SST anomalies over the Atlantic–Eurasian region.

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FengJiao Chen, ShaoXue Sheng, ZhengQing Bao, HuaYang Wen, LianSheng Hua, Ngarukiyimana Jean Paul, and YunFei Fu

Abstract

Utilizing the cloud parameters derived from the Tropical Rainfall Measuring Mission (TRMM) Visible and Infrared Scanner and the near-surface rainfall detected by the TRMM Precipitation Radar, the differences of cloud parameters for precipitating clouds (PCs) and nonprecipitating clouds (NPCs) are examined in tropical cyclones (TCs) during daytime from June to September 1998–2010. A precipitation delineation scheme that is based on cloud parameter thresholds is proposed and validated using the independent TC datasets in 2011 and observational datasets from Terra/MODIS. Statistical analysis of these results shows that the differences in the effective radius of cloud particles R e are small for PCs and NPCs, while thick clouds with large cloud optical thickness (COT) and liquid water path (LWP) can be considered as candidates for PCs. The probability of precipitation increases rapidly as the LWP and COT increase, reaching ~90%, whereas the probability of precipitation reaches a peak value of only 30% as R e increases. The combined threshold of a brightness temperature at 10.8 μm (BT4) of 270 K and an LWP of 750 g m−2 shows the best performance for precipitation discrimination at the pixel levels, with the probability of detection (POD) reaching 68.2% and false-alarm ratio (FAR) reaching 31.54%. From MODIS observations, the composite scheme utilizing BT4 and LWP also proves to be a good index, with POD reaching 77.39% and FAR reaching 24.2%. The results from this study demonstrate a potential application of real-time precipitation monitoring in TCs utilizing cloud parameters from visible and infrared measurements on board geostationary weather satellites.

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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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Anne Ru Cheng, Tim Hau Lee, Hsin I. Ku, and Yi Wen Chen

Abstract

This paper introduces a quality control (QC) program for the real-time hourly land surface temperature observation developed by the Central Weather Bureau in Taiwan. There are three strategies involved. The first strategy is a range check scheme that inspects whether the observation falls inside the climatological limits of the station to screen out the obvious outliers. Limits are adjusted according to the station’s elevation. The second strategy is a spatial check scheme that scrutinizes whether the observation falls inside the derived confidence interval, according to the data from the reference stations and the correlations among the stations, to judge the reliability of the data. The scheme is specialized, as it employs the theorems of unbiased and minimum error estimators to determine the weights. The performance evaluation results show that the new method is in theory superior to the spatial regression test (You et al.). The third strategy is a temporal check scheme that examines whether the temperature difference of two successive observations exceeds the temperature variation threshold for judging the rationality of the data. Different thresholds are applied for the data observed in different times under different rainfall conditions. Procedurally, the observation must pass the range check first and then go through the spatial or the temporal check. The temporal check is applied only when the spatial check is unavailable. Post-examinations of the data from 2014 show that the QC program is able to filter out most of the significant errors.

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Zesheng Chen, Yan Du, Zhiping Wen, Renguang Wu, and Shang-Ping Xie

Abstract

The south tropical Indian Ocean (TIO) warms following a strong El Niño, affecting Indo-Pacific climate in early boreal summer. While much attention has been given to the southwest TIO where the mean thermocline is shallow, this study focuses on the subsequent warming in the southeast TIO, where the mean sea surface temperature (SST) is high and deep convection is strong in early summer. The southeast TIO warming induces an anomalous meridional circulation with descending (ascending) motion over the northeast (southeast) TIO. It further anchors a “C-shaped” surface wind anomaly pattern with easterlies (westerlies) in the northeast (southeast) TIO, causing a persistent northeast TIO warming via wind–evaporation–SST feedback. The southeast TIO warming lags the southwest TIO warming by about one season. Ocean wave dynamics play a key role in linking the southwest and southeast TIO warming. South of the equator, the El Niño–forced oceanic Rossby waves, which contribute to the southwest TIO warming, are reflected as eastward-propagating oceanic Kelvin waves along the equator on the western boundary. The Kelvin waves subsequently depress the thermocline and develop the southeast TIO warming.

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