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Peng Hu, Wen Chen, Shangfeng Chen, Yuyun Liu, and Ruping Huang

Abstract

The El Niño–Southern Oscillation (ENSO) is regarded as one of the most important factors for onset of the South China Sea summer monsoon (SCSSM). Previous studies generally indicated that an El Niño event tends to result in a late onset of the SCSSM monsoon. However, this relationship has not been true in recent years, particularly when an extremely early SCSSM onset (1 May 2019) occurred following the 2018/19 El Niño event in the preceding winter. The processes of the second earliest SCSSM onset in the past 41 years were investigated using NCEP–DOE reanalysis, OLR data, and ERSST. A negative sea surface temperature and associated anticyclonic anomalies were absent over the western North Pacific in the late spring of 2019 following an El Niño event in the preceding winter. Thus, the mean circulation in the late spring of 2019 does not prevent SCSSM onset, which is in sharp contrast to the composited spring of the El Niño decaying years. The convective active and westerly phases of a 30–60-day oscillation originating from the Indian Ocean provided a favorable background for the SCSSM onset in 2019. In addition, the monsoon onset vortex over the Bay of Bengal and the cold front associated with a midlatitude trough over East Asia also played important roles in triggering the early onset of the SCSSM in 2019. No tropical cyclone appeared over the western North Pacific during April and May, and the enhancement of quasi-biweekly oscillation mainly occurs after the SCSSM onset; thus, these two factors contribute little to the SCSSM onset in 2019.

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Shangfeng Chen, Renguang Wu, Wen Chen, and Kai Li

Abstract

This study reveals a pronounced out-of-phase relationship between surface air temperature (SAT) anomalies over northeast Eurasia in boreal winter and the following summer during 1980–2017. A colder (warmer) winter over northeast Eurasia tends to be followed by a warmer (cooler) summer of next year. The processes for the out-of-phase relation of winter and summer SAT involve the Arctic Oscillation (AO), the air–sea interaction in the North Atlantic Ocean, and a Eurasian anomalous atmospheric circulation pattern induced by the North Atlantic sea surface temperature (SST) anomalies. Winter negative AO/North Atlantic Oscillation (NAO)-like atmospheric circulation anomalies lead to continental cooling over Eurasia via anomalous advection and a tripolar SST anomaly pattern in the North Atlantic. The North Atlantic SST anomaly pattern switches to a dipolar pattern in the following summer via air–sea interaction processes and associated surface heat flux changes. The summer North Atlantic dipolar SST anomaly pattern induces a downstream atmospheric wave train, including large-scale positive geopotential height anomalies over northeast Eurasia, which contributes to positive SAT anomalies there via enhancement of downward surface shortwave radiation and anomalous advection. Barotropic model experiments verify the role of the summer North Atlantic SST anomalies in triggering the atmospheric wave train over Eurasia. Through the above processes, a colder winter is followed by a warmer summer over northeast Eurasia. The above processes apply to the years when warmer winters are followed by cooler summers except for opposite signs of SAT, atmospheric circulation, and SST anomalies.

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Peiqiang Xu, Lin Wang, Wen Chen, Juan Feng, and Yuyun Liu

Abstract

The Pacific–Japan (PJ) pattern, also known as the East Asia–Pacific pattern, is a teleconnection that significantly influences the East Asian summer climate on various time scales. Based on several reanalysis and observational datasets, this study suggests that the PJ pattern has experienced a distinct three-dimensional structural change in the late 1990s. Compared with those during 1979–98, the PJ pattern shifts eastward by approximately 20° during 1999–2015, and the intensity of its barotropic structure in the extratropics weakens significantly. As a result, its influences on the summer rainfall along the mei-yu band are weakened after the late 1990s. These observed changes can be attributed to three reasons. First, the location where the PJ pattern is excited shifts eastward. Second, the easterly shear of the background wind is very weak around the source region of the PJ pattern after the late 1990s, which prevents the convection-induced baroclinic mode from converting into barotropic mode and thereby from propagating into the extratropics. Third, the PJ pattern–induced rainfall anomalies are weak along the mei-yu band after the late 1990s. As a result, their feedbacks to the PJ pattern become weak and play a considerably reduced role in maintaining the structure of the PJ pattern in the midlatitudes. In contrast, the eddy energy conversion from the basic flow efficiently maintains the PJ pattern before and after the late 1990s and thereby contributes little to the observed change.

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Wenxin Zeng, Guixing Chen, Yu Du, and Zhiping Wen

Abstract

A succession of MCSs developed during the last week of October 2016 and produced extreme heavy rainfall in central China. The event underwent an evident shift from a mei-yu-like warm scenario to an autumn cold scenario. Diurnal cycles of rainfall and low-level winds may be modulated by the shifting of large-scale atmospheric conditions. We conducted observational analyses and numerical experiments to examine how large-scale circulations influenced rainfall systems through diurnally varying processes. The results show that, in the first half (warm) period of the event, intense rainfall mostly occurred in eastern-central China with an early morning peak. It was closely related to a nocturnal southwesterly low-level jet (NLLJ) on the flank of the western Pacific subtropical high. The NLLJ formed near midnight in southern China where ageostrophic wind rotated clockwise due to Blackadar’s inertial oscillation. The NLLJ extended downstream to central China during the predawn hours due to the horizontal advection of momentum. Both the formation and extension of the NLLJ were supported by an enhanced subtropical high that provided relatively warm conditions with surface heating for boundary layer inertial oscillation and strong background southwesterly winds for momentum transport. The NLLJ induced MCSs at its northern terminus where the low-level ascent, moisture flux convergence, and convective instability were enhanced during the predawn hours. In the second half period with an intrusion of cold air, the diurnal amplitude of low-level winds became small under relatively cold and cloudy conditions. Moderate rainfall tended to occur in western-central China with a peak after midnight, most likely due to frontogenetic processes, upslope lifting, and nighttime cloud-top cooling.

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Long Wen, Kun Zhao, Guifu Zhang, Su Liu, and Gang Chen

Abstract

Instrumentation limitations on measured raindrop size distributions (DSDs) and their derived relations and physical parameters are studied through a comparison of the DSD measurements during mei-yu season in east China by four collocated instruments, that is, a two-dimensional video disdrometer (2DVD), a vertically pointing Micro Rain Radar (MRR), and two laser-optical OTT Particle Size Velocity (PARSIVEL) disdrometers (first generation: OTT-1; second generation: OTT-2). Among the four instruments, the 2DVD provides the most accurate DSD and drop velocity measurements, so its measured rainfall amount has the best agreement with the reference rain gauge. Other instruments tend to miss more small drops (D < 1 mm), leading to inaccurate DSDs and a lower rainfall amount. The low rainfall estimation becomes significant during heavy rainfall. The impacts of instrument limitations on the microphysical processes (e.g., evaporation and accretion rates) and convective storm morphology are evaluated. This is important especially for mei-yu precipitation, which is dominated by a high concentration of small drops. Hence, the instrument limitations need to be taken into account in both QPE and microphysics parameterization.

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Jun Li, Yi-Leng Chen, and Wen-Chau Lee

Abstract

A heavy rainfall event during the Taiwan Area Mesoscale Experiment intensive observing period 13 has been studied using upper-air, surface mesonet, and dual-Doppler radar data. The heavy rainfall (≥231 mm day−1) occurred over northwestern Taiwan with the maximum rainfall along the northwestern coast and was caused by a long-lived, convective rainband in the prefrontal atmosphere. It occurred in an upper-level divergence region and along the axis of the maximum equivalent potential temperature at the 850-hPa level.

As a Mei-Yu front advanced southeastward, the postfrontal cold air in the lowest levels was retarded by the hilly terrain along the southeastern China coast. As a result, a low-level wind-shift line associated with a pressure trough at the 850-hPa level moved over the Taiwan Strait before the arrival of the surface front. The westerly flow behind the trough interacted with a barrier jet along the northwestern coast of Taiwan. The barrier jet is caused by the interaction between the prefrontal southwest monsoon flow and the island obstacle. A low-level convergence zone (∼3 km deep) was observed along the wind-shift line between the westerly flow coming off the southeastern China coast and the barrier jet. A long-lived rainband developed within the low-level convergence zone and moved southeastward toward the northwestern Taiwan coast with the wind-shift line.

There were several long-lived (>2 h) reflectivity maxima embedded in the rainband. They often had several individual cells with a much shorter lifetime. The reflectivity maxima formed on the southwestern tip of the rainband and along the low-level wind-shift line. They intensified during their movement from the southwest to the northeast along the rainband. The continuous generation of the reflectivity maxima along the wind-shift line and the intensification of them over the low-level convergence zone maintained the long lifetime of the rainband and produced persistent heavy rainfall along the northwestern coast as these reflectivity maxima moved toward the coast. During the early stage of their lifetime, the reflectivity maxima were observed along the wind-shift line with upward motion in the lower troposphere. As they moved toward the northeastern part of the rainband and matured, the reflectivity maxima were observed southeast of the convergence zone with sinking motion in the lower troposphere. The upward motion was rooted along the wind-shift line and tilted southeastward with height. The reflectivity maxima dissipated as they moved inland. During the early stage of the rainband, the reflectivity maxima on the northeastern part of the rainband also merged with the convective line associated with the land-breeze front offshore of the northwestern coast.

The Mei-Yu front was shallow (<1 km) and moved slowly southward along the western coast. Convection associated with the front was weak with echo tops (∼10 dBZ) below 6 km.

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Yuanyuan Guo, Zhiping Wen, Renguang Wu, Riyu Lu, and Zesheng Chen

Abstract

The leading mode of boreal winter precipitation variability over the tropical Pacific for the period 1980–2010 shows a west–east dipole pattern with one center over the western North Pacific (WNP) and Maritime Continent and the other center over the equatorial central Pacific (CP). Observational evidence shows that the variability of the East Asian upper-tropospheric subtropical westerly jet (EAJ) has a significant correlation with precipitation anomalies over the WNP and CP and that tropical precipitation anomalies over WNP and CP have a distinct influence on the variation of the EAJ. A series of numerical experiments based on a linear baroclinic model are performed to confirm the influence of the heating anomalies associated with precipitation perturbations over the WNP and CP on the EAJ. The results of numerical experiments indicate that a heat source over the WNP can excite a northward-propagating Rossby wave train in the upper troposphere over East Asia and facilitate a poleward eddy momentum flux. It results in the acceleration of the westerlies between 30° and 45°N, which favors a northward displacement of the EAJ. The response induced by a heat sink over the CP features a zonal easterly band between 25° and 40°N, suggesting that the response to heat sink associated with negative precipitation anomalies over the CP may weaken the EAJ. A strengthened relationship was found between tropical Pacific precipitation and the EAJ since 1979. The modeling results suggest that the shift of mean states might be responsible for the strengthened EAJ–rainfall relationship after 1979.

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Wen Wang, Wei Cui, Xiaoju Wang, and Xi Chen

Abstract

The Global Land Data Assimilation System (GLDAS) is an important data source for global water cycle research. Using ground-based measurements over continental China, the monthly scale forcing data (precipitation and air temperature) during 1979–2010 and model outputs (runoff, water storage, and evapotranspiration) during 2002–10 of GLDAS models [focusing on GLDAS, version 1 (GLDAS-1)/Noah and GLDAS, version 2 (GLDAS-2)/Noah] are evaluated. Results show that GLDAS-1 has serious discontinuity issues in its forcing data, with large precipitation errors in 1996 and large temperature errors during 2000–05. While the bias correction of the GLDAS-2 precipitation data greatly improves temporal continuity and reduces the biases, it makes GLDAS-2 precipitation less correlated with observed precipitation and makes it have larger mean absolute errors than GLDAS-1 precipitation for most months over the year. GLDAS-2 temperature data are superior to GLDAS-1 temperature data temporally and spatially. The results also show that the change rates of terrestrial water storage (TWS) data by GLDAS and the Gravity Recovery and Climate Experiment (GRACE) do not match well in most areas of China, and both GLDAS-1 and GLDAS-2 are not very capable of capturing the seasonal variation in monthly TWS change observed by GRACE. Runoff is underestimated in the exorheic basins over China, and runoff simulations of GLDAS-2 are much more accurate than those of GLDAS-1 for two of the three major river basins of China investigated in this study. Evapotranspiration is overestimated in the exorheic basins in China by both GLDAS-1 and GLDAS-2, whereas the overestimation of evapotranspiration by GLDAS-2 is less than that by GLDAS-1.

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Xu Zhang, Jian-Wen Bao, Baode Chen, and Evelyn D. Grell

Abstract

A new three-dimensional (3D) turbulent kinetic energy (TKE) subgrid mixing scheme is developed using the Advanced Research version of the Weather Research and Forecasting (WRF) Model (WRF-ARW) to address the gray-zone problem in the parameterization of subgrid turbulent mixing. The new scheme combines the horizontal and vertical subgrid turbulent mixing into a single energetically consistent framework, in contrast to the conventionally separate treatment of the vertical and horizontal mixing. The new scheme is self-adaptive to the grid-size change between the large-eddy simulation (LES) and mesoscale limits. A series of dry convective boundary layer (CBL) idealized simulations are carried out to compare the performance of the new scheme and the conventional treatment of subgrid mixing to the WRF-ARW LES dataset. The importance of including the nonlocal component in the vertical buoyancy specification in the newly developed general TKE-based scheme is illustrated in the comparison. The improvements of the new scheme with the conventional treatment of subgrid mixing across the gray-zone model resolutions are demonstrated through the partitioning of the total vertical flux profiles. Results from real-case simulations show the feasibility of using the new scheme in the WRF Model in lieu of the conventional treatment of subgrid mixing.

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Sihua Huang, Bin Wang, Zhiping Wen, and Zesheng Chen

Abstract

Previous studies found a tight connection between the tropical easterly jet (TEJ) and Indian summer monsoon rainfall (ISMR). Here we show that the TEJ–ISMR relationship is nonstationary and breaks down from 1994 to 2003 (epoch P2), in contrast to the significant positive correlation during epochs P1 (1979–93) and P3 (2004–16). The breakdown of the TEJ–ISMR relationship concurs with the increased rainfall variability over the tropical eastern Indian Ocean (TEIO). The enhanced TEIO rainfall anomalies excite a significant lower-level cyclonic circulation that reduces the ISMR and meanwhile strengthen the upper-level divergence and excite a pair of upper-level anticyclones to the west of the TEIO as Rossby wave responses, both accelerating the TEJ. Thus, the TEIO rainfall plays a more important role than the ISMR in TEJ variability during P2, causing the breakdown of the TEJ–ISMR relationship. In contrast, a relatively weak amplitude of the TEIO rainfall during P1 and P3 was unable to change the positive TEJ–ISMR relationship. The changes in the TEIO rainfall variability are mainly attributed to the increased SST variability over the tropical southeastern Indian Ocean, but their cause remains elusive.

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