Search Results

You are looking at 1 - 10 of 156 items for

  • Author or Editor: Zhong Zhong x
  • Refine by Access: All Content x
Clear All Modify Search
Yao Ha and Zhong Zhong

Abstract

This study investigates the decadal change in tropical cyclone (TC) activity over the South China Sea (SCS) in the boreal summer (June–August) since the early 1990s and explores possible causes behind it. Results show that the SCS TC activity experienced an abrupt decadal decrease at around 2003/03. Compared to the TC activities from the early 1990s to 2002, the number of TCs formed in the SCS markedly decreased from 2003 through the early 2010s. Moreover, most of the TCs were primarily confined within the SCS basin during this period. The TCs that formed during the period of 2003–11 usually moved west-northwestward and rapidly weakened after making landfall. It is found that a significant decadal-scale sea surface temperature (SST) warming occurred in the northern Indian Ocean and the western Pacific Ocean after 2002 while convection intensified over the tropical regions between 60° and 80°E and around 150°E, respectively. The warm SST anomalies induced an anomalous subsiding flow over the SCS basin via the Walker-like (zonal) circulation. Meanwhile, anomalously dry, sinking air around 5°–20°N derived from local Hadley (meridional) circulation reinforced the subsiding flow of the zonal circulation. The above circulation patterns suppressed TC genesis over the northern SCS, leading to the decadal decrease in TC activity that occurred around 2002/03. In addition, in conjunction with the local anomalous easterly flow, the intraseasonal atmospheric variability over the SCS has decreased since the early 2000s. This is unfavorable for the development of synoptic-scale disturbances and may also contribute to the decadal decrease in TC activity.

Full access
Zhong Liu

Abstract

Launched on 27 February 2014, the Global Precipitation Measurement (GPM) mission comprises an international constellation of satellites to provide the next generation of global observations of precipitation. Built upon the success of the widely used TRMM Multisatellite Precipitation Analysis (TMPA) products, the Integrated Multisatellite Retrievals for GPM (IMERG) products continue to make improvements in areas such as spatial and temporal resolutions and snowfall estimates, etc., which will be valuable for research and applications. During the transition from TMPA to IMERG, characterizing the differences between these two product suites is important in order for users to make adjustments in research and applications accordingly. In this study, the newly released IMERG Final Run monthly product is compared with the TMPA monthly product (3B43) in the boreal summer of 2014 and the boreal winter of 2014/15 on a global scale. The results show the IMERG monthly product can capture major heavy precipitation regions in the Northern and Southern Hemispheres reasonably well. Differences between IMERG and 3B43 vary with surface types and precipitation rates in both seasons. Over land, systematic differences are much smaller compared to those over ocean because of the similar gauge adjustment used in the two monthly products. Positive relative differences (IMERG > 3B43) are primarily found at low precipitation rates and negative differences (IMERG < 3B43) at high precipitation rates. Over ocean, negative systematic differences (IMERG < 3B43) prevail at all precipitation rates. Analysis of the passive microwave (PMW) and infrared (IR) monthly products from TMPA and IMERG shows the large systematic differences in the tropical oceans are closely associated with the differences in the PMW products.

Full access
Xian Chen, Zhong Zhong, and Wei Lu

Abstract

The NCEP–NCAR reanalysis dataset and the tropical cyclone (TC) best-track dataset from the Regional Specialized Meteorological Center (RSMC) Tokyo Typhoon Center were employed in the present study to investigate the possible linkage of the meridional displacement of the East Asian subtropical upper-level jet (EASJ) with the TC activity over the western North Pacific (WNP). Results indicate that summertime frequent TC activities would create the poleward shift of the EASJ through a stimulated Pacific–Japan (PJ) teleconnection pattern as well as the changed large-scale meridional temperature gradient. On the contrary, in the inactive TC years, the EASJ is often located more southward than normal with an enhanced intensity. Therefore, TC activities over the WNP are closely related to the location and intensity of the EASJ in summer at the interannual time scale.

Full access
Yuan Sun, Zhong Zhong, and Wei Lu

Abstract

The Advanced Research version of Weather Research and Forecasting (WRF-ARW) Model is used to examine the sensitivity of a simulated tropical cyclone (TC) track and the associated intensity of the western Pacific subtropical high (WPSH) to microphysical parameterization (MP) schemes. It is found that the simulated WPSH is sensitive to MP schemes only when TCs are active over the western North Pacific. WRF fails to capture TC tracks because of errors in the simulation of the WPSH intensity. The failed simulation of WPSH intensity and TC track can be attributed to the overestimated convection in the TC eyewall region, which is caused by inappropriate MP schemes. In other words, the MP affects the simulation of the TC activity, which influences the simulation of WPSH intensity and, thus, TC track. The feedback of the TC to WPSH plays a critical role in the model behavior of the simulation. Further analysis suggests that the overestimated convection in the TC eyewall results in excessive anvil clouds and showers in the middle and upper troposphere. As the simulated TC approaches the WPSH, the excessive anvil clouds extend far away from the TC center and reach the area of the WPSH. Because of the condensation of the anvil clouds’ outflows and showers, a huge amount of latent heat is released into the atmosphere and warms the air above the freezing level at about 500 hPa. Meanwhile, the evaporative (melting) process of hydrometers in the descending flow takes place below the freezing level and cools the air in the lower and middle troposphere. As a result, the simulated WPSH intensity is weakened, and the TC turns northward earlier than in observations.

Full access
Yafang Zhong and Zhengyu Liu

Abstract

Atmospheric response to North Pacific oceanic variability is assessed in Community Climate System Model, version 3 (CCSM3) using two statistical methods and one dynamical method. All methods identify an equivalent barotropic low response to a warmer sea surface temperature (SST) anomaly in the Kuroshio Extension region (KOE) during early–midwinter. While all three methods capture the major features of the response, the generalized equilibrium feedback assessment method (GEFA) isolates the impact of KOE SST from a complex context, and thus makes itself an excellent choice for similar practice.

Full access
Lejiang Yu and Shiyuan Zhong

Abstract

Strong wind events (SWEs) over Antarctica and its surrounding oceans are investigated using gridded surface wind data from the ERA-Interim for the 1979–2017 period. Throughout the year, SWEs are more prevalent over the coastal region of East Antarctica where mean surface wind speeds are also higher. The occurrences of SWEs appear to be accompanied by positive anomalies in surface temperature and negative (positive) anomalies in mean sea level pressure related to cyclone (anticyclone) activity over the Ronne and Ross Ice Shelves and coastal regions (the inland areas of East Antarctica). The interannual variability of the SWE occurrences appears to be related to the southern annular mode (SAM) and, to a lesser degree, ENSO. The trends of SWE in the recent four decades exhibit considerable regional variations that are consistent with the trends in seasonal mean wind speed and surface air temperature, and can be largely explained by the variations in the sea level pressure trends across the region.

Full access
Shijie Zhong and Michael Gurnis

Abstract

Mantle convection models have been formulated to investigate the relation between plate kinematics and mantle dynamics. The cylindrical geometry models incorporate mobile, faulted plate margins, a phase change at 670 km depth, non-Newtonian rheology, and tectonic plates. Models with a variety of parameters indicate that a relatively stationary trench is more likely to be associated with a subducted slab that penetrates into the lower mantle with a steep dip angle. However, a subducted slab that is deflected above the 670-km phase change with a shallow dip is more likely to be associated with a margin that has undergone rapid retrograde trench migration. This relation between slab morphology and plate kinematics is consistent with seismic tomography and plate reconstruction of western Pacific subduction zones. The efficiency of slab penetration through the 670-km phase change is controlled by both the buoyancy of the subducting plate and the mobility of the overriding plate. While older subducting plates have a greater propensity for slab penetration, trench mobility reduces the propensity for slab penetration. Smaller overriding plates have a greater mobility. When subducted slabs approach the bottom thermal boundary layer, hot fluid is pushed aside, and plumes form on the periphery of slab accumulations. There are sharp temperature contrasts between the subducted slab and the thermal boundary layer at the core mantle boundary (CMB). Old subducted slabs and a thermal boundary layer lead to large-scale lateral structure near the CMB.

Full access
Marwan Katurji and Shiyuan Zhong

Abstract

A high-resolution numerical investigation of a cold-air pooling process (under quiescent conditions) is carried out that systematically highlights the relations between the characteristics of the cold-air pools (e.g., slope winds, vertical temperature and wind structure, and cooling rate) and the characteristics of the topography (e.g., basin size and slope angle) under different ambient stabilities. The Advanced Regional Prediction System model is used to simulate 40 different scenarios at 100-m (10 m) horizontal (vertical) resolution. Results are within the range of similar observed phenomena. The main physical process governing the cooling process near the basin floor (<200 m in height) was found to be longwave radiative flux divergence, whereas vertical advection of temperature dominated the cooling process for the upper-basin areas. The maximum downslope wind speed is linearly correlated with both basin size and slope angle, with stronger wind corresponding to larger basin and lower slope angle. As the basin size increases, the influence of slope angle on maximum downslope wind decreases and the maximum is located farther down the slope. These relationships do not appear to be sensitive to stability, but weaker stability produces more cooling in the basin atmosphere by allowing stronger rising motion and adiabatic cooling. Insight gained from this study helps to improve the understanding of the cold-air pooling process within the investigated settings.

Full access
Wenli Zhong and Jinping Zhao

Abstract

In 2004, a cold mode of Atlantic Water (AW) entered the western Canada basin, replacing the anomalously warm AW that resided in the basin since the 1990s. This slightly colder AW was denser than the 1990s warm mode; it gradually filled most of the western basin by 2009. The enhanced surface stress curl led to the spinup of the Beaufort Gyre and convergence of freshwater. The spinup also resulted in a deepening of the AW core at the center of the gyre and in shoaling of the AW core at the margins of the gyre. The density versus depth relationship revealed in this study shows that the depth of the maximum AW temperature was mainly controlled by the density structure before 2007; thus, it is the case when the denser water was deeper and the case when the lighter water was shallower around the basin. However, this relationship was reversed to become the case when the denser water was shallower and the case when the lighter water was deeper since 2008 inside the Beaufort Gyre. The combined effect of density and sea ice retreat that enhanced surface stress curl determined the depth of the AW inside the Beaufort Gyre since 2008. The deepening of the AW core and expanding of the area where the AW deepening occurred had a profound effect on the large-scale circulation in the Arctic Ocean.

Full access
Shiyuan Zhong and Jerome Fast

Abstract

This study presents what is, to the authors' knowledge, the first intercomparison and evaluation of three state-of-the-art mesoscale numerical models, the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MMS), the Regional Atmospheric Modeling System (RAMS), and the NCEP Meso-Eta, at horizontal resolution finer than 1 km. Simulations were carried out for both weak and strong synoptic forcing cases during the Vertical Transport and Mixing (VTMX) field campaign conducted in the Salt Lake valley in October of 2000. Both upper-air and surface observations at high spatial and temporal resolution were used to evaluate the simulations with a focus on boundary layer structures and thermally driven circulations that developed in the valley. Despite differences in the coordinate systems, numerical algorithms, and physical parameterizations used by the three models, the types of forecast errors were surprisingly similar. The common errors in predicted valley temperature structure include a cold bias extending from the surface to the top of the valley atmosphere, lower than observed mixed-layer depths when the observed mixed layers were relatively high, and much weaker nocturnal inversion strengths over the valley floor. Relatively large wind forecast errors existed at times in the midvalley atmosphere even in the case of strong synoptic winds. The development of valley, slope, and canyon flows and their diurnal reversals under weak synoptic forcing were captured better by RAMS and MM5 than by Meso-Eta. Meso-Eta consistently underpredicted the strengths of these terrain-induced circulations and the associated convergence and divergence over the valley floor. As operational mesoscale modeling moves toward subkilometer resolution in the near future, more detailed forecasts of the circulation patterns and boundary layer structure can be produced for local-scale applications. However, this study shows that relatively large forecast errors can still exist even with sufficiently fine spatial resolution, indicating that the future for accurate local forecasting still lies in improved model parameterization of longwave radiation and turbulent mixing.

Full access