Search Results

You are looking at 1 - 10 of 78 items for

  • Author or Editor: Chen Zhao x
  • All content x
Clear All Modify Search
Zhan Zhao and Shu-Hua Chen

Abstract

Identifying pollutant sources that contribute to downstream locations is important for policy making and air-quality control. In this study, a computationally economic signal technique was implemented into a three-dimensional nonhydrostatic atmospheric model to help to identify source–receptor relationships. An idealized supercell case and a semireal air-pollution case in Turkey were used to investigate the potential of the technique. For each pollutant, signals with various frequencies were emitted from different source locations and added into that particular type of emitted pollutants. The time series of pollutant concentration collected at receptors were then projected onto frequency space using the Fourier transform and short-time Fourier transform methods to identify the source locations. During the model integration, a particular tracer was also emitted from each pollutant source location (i.e., a conventional method to study the source–receptor relationship) to validate and evaluate the signal technique. Results show that frequencies could be slightly shifted after signals were transported for some distance and that evident secondary frequencies (i.e., beat frequencies) could be generated as a result of nonlinear effects. Although these could potentially confuse the identification of signals released from source points, signals were still distinguishable in this study. Results from a sensitivity test of the diffusion effect on different frequencies suggest that the effect of diffusion on amplitude damping is stronger for higher frequencies than for lower frequencies.

Full access
Zezong Chen, Longgang Zhang, Chen Zhao, Xi Chen, and Jianbo Zhong

Abstract

Wind sea and swell representing different weather conditions generally coexist in both open waters and coastal areas, which results in bimodal or multipeaked features in directional wave spectrum. Because they make wave parameters such as significant wave height and mean wave period of the mixed sea state less meaningful, the processes of separation and identification of wind sea and swell are crucial. Consistent wind sea and swell results can be obtained by a commonly used method based on wave age (WA) with the directional wave spectrum and wind velocity. However, the subjective dependence of wave age threshold selection and the required wind information restrict the application of this method. In this study, a practical method based on the overshoot phenomenon (OP) in wind-generated waves is proposed to extract wind sea and swell from the directional wave spectrum without any other meteorology information. Directional wave spectra derived from an S-band Doppler radar deployed on the coast of the South China Sea have been utilized as the datasets to investigate the performance of both methods. The proposed OP method is then validated by comparing it with the WA method and the verifying results are presented.

Full access
Haikun Zhao, Philp J. Klotzbach, and Shaohua Chen

Abstract

A conventional empirical orthogonal function (EOF) analysis is performed on summertime (May–October) western North Pacific (WNP) tropical cyclone (TC) track density anomalies during 1970–2012. The first leading EOF mode is characterized by a consistent spatial distribution across the WNP basin, which is closely related to an El Niño–Southern Oscillation (ENSO)-like pattern that prevails on both interannual and interdecadal time scales. The second EOF mode is represented by a tripole pattern with consistent changes in westward and recurving tracks but with an opposite change for west-northwestward TC tracks. This second EOF pattern is dominated by consistent global sea surface temperature anomaly (SSTA) patterns on interannual and interdecadal time scales, along with a long-term increasing global temperature trend. Observed WNP TC tracks have three distinct interdecadal epochs (1970–86, 1987–97, and 1998–2012) based on EOF analyses. The interdecadal change is largely determined by the changing impact of ENSO-like and consistent global SSTA patterns. When global SSTAs are cool (warm) during 1970–86 (1998–2012), these SSTAs exert a dominant impact and generate a tripole track pattern that is similar to the positive (negative) second EOF mode. In contrast, a predominately El Niño–like SSTA pattern during 1987–97 contributed to increasing TC occurrences across most of the WNP during this 11-yr period. These findings are consistent with long-term trends in TC tracks, with a tripole track pattern observed as global SSTs increase. This study reveals the potential large-scale physical mechanisms driving the changes of WNP TC tracks in association with climate change.

Restricted access
Changyan Zhou, Ping Zhao, and Junming Chen

Abstract

In recent decades, long-term changes of the Tibetan Plateau (TP) water vapor and the associated mechanisms have not been investigated fully. This study aims to examine the interdecadal change of summer TP water vapor using the monthly mean European Centre for Medium-Range Weather Forecasts interim reanalysis during 1979–2014. The results show a drier phase in the TP during 1979–94, with a subsequent wetter phase, which suggests an interdecadal variation of summer TP water vapor around the middle of the 1990s. This interdecadal variation is mainly due to a significant change of the water vapor export on the eastern boundary of the TP, which is closely associated with a summer atmospheric circulation anomaly near Lake Baikal. When a cyclonic (an anticyclonic) anomaly occurs near Lake Baikal, there is less (more) water vapor over the TP. On the interdecadal scale, the atmospheric circulation anomaly near Lake Baikal is related to an extratropical large-scale anomalous wave train over the northwestern Atlantic–East Asian region, with an eastward propagation of the anomalous wave energy from the Atlantic to East Asia. Climate model simulations further demonstrate an impact of sea surface temperature (SST) anomalies in the northwestern Atlantic on the anomalous wave train. Both the extratropical tropospheric anomalous wave train and the anomalous atmospheric circulation near Lake Baikal are successfully simulated by changing the summer northwestern Atlantic SST. Therefore the warming northwestern Atlantic is an important factor contributing to the wetting TP in recent decades.

Open access
Xingchao Chen, Fuqing Zhang, and Kun Zhao

Abstract

Convection-permitting numerical experiments using the Weather Research and Forecasting (WRF) Model are performed to explore the influence of monsoonal onshore wind speed and moisture content on the intensity and diurnal variations of coastal rainfall over south China during the mei-yu seasons. The focus of the analyses is on a pair of 10-day WRF simulations with diurnally cyclic-in-time lateral boundary conditions averaged over the high versus low onshore wind speed days of the 2007–09 mei-yu seasons. Despite differences in the rainfall intensity, the spatial distributions and diurnal variations of rainfall in both simulations verified qualitatively well against the mean estimates derived from ground-based radar observations, averaged respectively over either the high-wind or low-wind days.

Sensitivity experiments show that the pattern of coastal rainfall spatial distribution is mostly controlled by the ambient onshore wind speed. During the high-wind days, strong coastal rainfall is concentrated along the coastline and reaches its maximum in the early morning. The coastal lifting induced by the differential surface friction and small hills is the primary cause for the strong coastal rainfall, while land breeze enhances coastal lifting and precipitation from evening to early morning. In the low-wind days, on the other hand, coastal rainfall is mainly induced by the land–sea-breeze fronts, which has apparent diurnal propagation perpendicular to the coastline. With stronger land–sea temperature contrast, the land–sea breeze is stronger during the low-wind days. Both in the high-wind and low-wind days, the coastal rainfall intensity is sensitive to the incoming moisture in the upstream oceanic airflow, especially to the moisture content in the boundary layer.

Full access
Haikun Zhao, Shaohua Chen, and Philip J. Klotzbach

Abstract

This study examines the association between the western North Pacific (WNP) summer monsoon (WNPSM) and WNP tropical cyclone (TC) frequency during June–August from 1979 to 2016. The interannual relationship between the WNPSM and the total number of WNP TCs has strengthened since 1998. There has also been a significant reduction in the number of TCs forming within the WNP monsoon trough (WNPMT)—hereafter called ITCs, for internal or inside TCs—since 1998. These two important features are found to be closely associated with the climate regime shift that occurred around 1998. During 1998–2016, the Pacific decadal oscillation (PDO) tended to be in a cold phase, with an increasing occurrence of central Pacific–type El Niño–Southern Oscillation (ENSO) events, whereas the 1979–97 period tended to be characterized by a warm phase of the PDO and east Pacific–type ENSO events. During 1998–2016, the tropical Pacific was characterized by enhanced easterlies, which led to a westward-retreated WNPMT that caused a significant decrease in ITCs over the WNP basin. However, there was little change in TCs outside of the WNPMT region (hereafter called OTCs) compared to that before 1998. A significant in-phase (out-of-phase) relationship between the WNPSM and the number of ITCs (OTCs) is observed before 1998, thus greatly weakening the WNPSM–TC relationship. The recent enhanced relationship between the WNPSM and TCs is mainly due to a strong in-phase relationship between the WNPSM and ITCs. The interannual change in ITCs is mainly controlled by WNPSM changes since 1998, while OTC changes are mainly modulated by changes in the tropical upper-tropospheric trough.

Full access
Jian Ling, Yuqing Zhao, and Guiwan Chen

ABSTRACT

The simulated Madden–Julian oscillation (MJO) events in 27 general circulation models (GCMs) are identified using an MJO tracking method. The results suggest that the occurrence frequencies of simulated MJO events can represent a model’s ability to simulate several characteristics of the MJO to a certain extent during boreal winter, such as propagation range, strength, and termination longitude. All tracked MJO events are classified into those that propagate through the Maritime Continent (MC) (MJO-C) and those that do not (MJO-B), and the weakening and blocking effects on MJO propagation by the MC in GCMs were quantified. In general, if a GCM shows a stronger weakening effect on MJO strength over the MC, it tends to produce a stronger blocking effect on MJO propagation over the MC during boreal winter. The barrier effect of the MC on MJO propagation is exaggerated in most GCMs, while it can be underestimated in some GCMs, especially the coupled GCMs. Strong lower-tropospheric premoistening is identified ahead of the MJO convection center when it is over the central MC for MJO-C but not for MJO-B in most GCMs. Such strong premoistening is mainly attributed to the zonal gradient of lower-tropospheric easterly anomalies within the front Walker cell, which could be a precursor leading to the eastward propagation of MJO convection. In contrast to the observation, the role of the background sea surface temperature and land–sea precipitation contrast in the barrier effect on MJO propagation by the MC is not well captured by most GCMs.

Full access
Ge Liu, Ping Zhao, and Junming Chen

Abstract

The summer (June–August) Asian–Pacific Oscillation (APO), a large-scale atmospheric teleconnection pattern, is closely associated with climate anomalies over the Northern Hemisphere. Using the NOAA/CIRES twentieth-century reanalysis, the ECMWF twentieth-century atmospheric reanalysis, and the NCEP reanalysis, this study investigates the variability of the summer APO on the interannual time scale and its relationship with the thermal condition over the Tibetan Plateau (TP). The results show that the interannual variability of the APO is steadily related to the summer TP surface air temperature during the last 100 years. Observation and simulation further show that a positive heating anomaly over the TP can increase the upper-tropospheric temperature and upward motion over Asia. This anomalous upward flow moves northward in the upper troposphere, and then turns and moves eastward, before finally descending over the mid- to high latitudes of the central-eastern North Pacific, concurrently accompanied by anomalous upward motion over the lower latitudes of the central-eastern North Pacific. The anomalous downward and upward motions over the central-eastern North Pacific reduce the in situ mid- and upper-tropospheric temperature, mainly through modulating condensation latent heat from precipitation and/or dry adiabatic heat, which ultimately leads to the interannual variability of the summer APO. In this process, the zonal vertical circulation over the extratropical Asian–North Pacific sector plays an important bridging role.

Open access
Xiaomin Chen, Yuqing Wang, and Kun Zhao

Abstract

The typical synoptic flow patterns and environmental factors that favor the rapid intensification (RI) of tropical cyclones (TCs) in the South China Sea (SCS) have been identified based on all TCs formed in the SCS between 1981 and 2011. The quantity RI is defined as the 24-h increase in maximum sustained surface wind speed by 15 m s−1 as in previous studies, which is close to the 95th percentile of 24-h intensity change of all SCS samples excluding those after landfall. There are 4.9% (2.3%) of tropical depressions (tropical storms) that experienced RI. No typhoons satisfied the RI threshold.

Six low-level synoptic flow patterns favoring RI have been identified based on 18 RI cases. In the monsoon season very few TCs experience RI due to large vertical wind shear (VWS). Most RI cases occurred in the postmonsoon season when the midlatitude troughs often penetrated into the SCS whereas the southwesterly monsoon flow is still strong in the southern SCS. Compared with those of non-RI cases, the mean initial conditions of RI cases include weak VWS and relatively strong forcing from midlatitude troughs. Several criteria of significant environmental factors for RI are statistically identified based on all TC samples. It is found that 16 non-RI TCs fitted in the RI flow patterns but only two of them satisfy all the criteria, suggesting that a combination of the synoptic flow pattern and the environmental factors can be used to predict RI in the SCS. In addition, two RI cases involving TC–trough interaction are analyzed.

Full access
Jie Zhang, Haishan Chen, and Siwen Zhao

Abstract

Because of the interactive margin between the East Asian summer monsoon and westerly circulation, summer rainfall in northern China (NC) exhibits high variability. By employing reanalysis data and geostationary satellite data from the Fengyun-2G (FY-2G) satellite and using the linear baroclinic model (LBM) and Hybrid Single-Particle Lagrangian Integrated Trajectory model, this study suggests a tripole pattern in summer rainfall over NC and the Indian subcontinent (IS) that is related to the Indian summer monsoon. The distributions of atmospheric circulation indicate three teleconnections: one is from the IS via the Indo-China Peninsula (ICP) and NC, enhancing the Pacific–Japan (PJ) pattern; another is from the IS via west-central Asia and NC, arousing a Eurasian wave pattern; and the third is an IS–TP–NC pattern via the Tibetan Plateau (TP). Those teleconnections modulate vorticity and atmospheric stability over NC. In addition, along with the circulation distribution related to those teleconnections, two pathways of moisture transport related to the IS rainfall are suggested, except for moisture transport via the Bay of Bengal: one is from the Indo-Pacific to NC due to enhancing cyclones over the Indo-Pacific and a PJ-like pattern; and another is from the IS to NC via the TP within the midtroposphere, which modulates midtroposphere moisture fluxes and atmospheric stability over NC. Both teleconnections and moisture transport result in anomalous rainfall over NC. This study reveals a new mechanism and pathway of the Indian summer monsoon impacting NC rainfall, possibly explaining the reason behind the high variability in NC rainfall.

Open access