Search Results

You are looking at 51 - 60 of 105 items for

  • Author or Editor: Da-Lin Zhang x
  • All content x
Clear All Modify Search
Yueting Gong, Ying Li, and Da-Lin Zhang

Abstract

Tropical cyclones (TCs) tend to change translation direction and speed when moving across Taiwan’s Central Mountain Range (CMR), which makes forecasting of landfalling points a challenging task. This study examines the statistical characteristics of unusual TC tracks around Taiwan Island during the 66-yr period of 1949–2014. Results show that 1) about 10% more TCs were deflected to the right than to the left as they moved across the CMR, but with more occurrences of the latter on Taiwan’s eastern coast and southern strait; 2) TCs around Taiwan Island moved slower than the average speed over the western North Pacific Ocean but then exhibited anomalous acceleration along Taiwan’s eastern coast and anomalous deceleration over the southern Taiwan Strait; 3) about 33% of TCs passing the island were accompanied by terrain-induced secondary low pressure centers (SCs), more favored in the northwestern, southwestern, and southeastern quadrants, with the TC–SC separation distance varying from 33 to 643 km; 4) about 36% of landfalling TCs experienced discontinuous tracks, with an average separation distance of 141 km at the time when TC centers were replaced by SCs, and smaller Froude numbers than those associated with continuous-tracking TCs; and 5) a total of 12 TCs had looping movements near Taiwan Island, most of which were accompanied by SCs on their southern or western sides. Results also indicate that a stronger SC was likely to take place when a stronger TC approached the CMR with a shorter separation distance and that a weaker SC was likely to take place when a weaker TC approached the CMR with a longer separation distance.

Full access
Da-Lin Zhang and Chanh Q. Kieu

Abstract

Although the forced secondary circulations (FSCs) associated with hurricane-like vortices have been previously examined, understanding is still limited to idealized, axisymmetric flows and forcing functions. In this study, the individual contributions of latent heating, frictional, and dry dynamical processes to the FSCs of a hurricane vortex are separated in order to examine how a hurricane can intensify against the destructive action of vertical shear and how a warm-cored eye forms. This is achieved by applying a potential vorticity (PV) inversion and quasi-balanced omega equations system to a cloud-resolving simulation of Hurricane Andrew (1992) during its mature stage with the finest grid size of 6 km.

It is shown that the latent heating FSC, tilting outward with height, acts to oppose the shear-forced vertical tilt of the storm, and part of the upward mass fluxes near the top of the eyewall is detrained inward, causing the convergence aloft and subsidence warming in the hurricane eye. The friction FSC is similar to that of the Ekman pumping with its peak upward motion occurring near the top of the planetary boundary layer (PBL) in the eye. About 40% of the PBL convergence is related to surface friction and the rest to latent heating in the eyewall.

In contrast, the dry dynamical forcing is determined by vertical shear and system-relative flow. When an axisymmetric balanced vortex is subjected to westerly shear, a deep countershear FSC appears across the inner-core region with the rising (sinking) motion downshear (upshear) and easterly sheared horizontal flows in the vertical. The shear FSC is shown to reduce the destructive roles of the large-scale shear imposed, as much as 40%, including its forced vertical tilt. Moreover, the shear FSC intensity is near-linearly proportional to the shear magnitude, and the wavenumber-1 vertical motion asymmetry can be considered as the integrated effects of the shear FSCs from all the tropospheric layers. The shear FSC can be attributed to the Laplacian of thermal advection and the temporal and spatial variations of centrifugal force in the quasi-balanced omega equation, and confirms the previous finding of the development of wavenumber-1 cloud asymmetries in hurricanes.

Hurricane eye dynamics are presented by synthesizing the latent heating FSC with previous studies. The authors propose to separate the eye formation from maintenance processes. The upper-level inward mass detrainment forces the subsidence warming (and the formation of an eye), the surface pressure fall, and increased rotation in the eyewall. This increased rotation will induce an additional vertical pressure gradient force to balance the net buoyancy generated by the subsidence warming for the maintenance of the hurricane eye. In this sense, the negative vertical shear in tangential wind in the eyewall should be considered as being forced by the subsidence warming, and maintained by the rotation in the eyewall.

Full access
Da-Lin Zhang and J. Michael Fritsch

Abstract

The effects of different model physics and different convective and boundary layer parameterization schemes are investigated using an 18-h nested-grid numerical simulation of the mesoscale convective systems (MCSs) that were responsible for the 19-20 July 1977 Johnstown flood. It is found that convective and resolable-scale diabatic process play crucial yet very different roles in the development and evolution of the MCSs. In particular, latent heat release resulted in development of strong vertical circulations, generation of an upper-level jet streak, formation of pronounced mesoβ-scale surface pressure perturbations, and rapid amplification of the traveling mesoα-scale wave that helped initiate the condensation processes. Resolvable-scale condensations appear to be directly responsible for the generation of a warm-core mesovortex and indirectly for a mesoscale convective complex (MCC). Without resolvable-scale heating, the model only reproduces the propagation of a squall line.

Incorporation of moist downdrafts also had a significant impact on the general evolution of the MCSs by producing important surface perturbations such as mesohighs and outflow boundaries. The role of moist down-drafts in the life cycle of the MCSs appears to be twofold. On one hand, the downdrafts vertically stabilized atmospheric columns and removed low-level moisture that otherwise would have been used for mesocyclogenesis and stratiform precipitation. On the other, the downdrafts horizontally destabilized the environment through the formation of horizontal temperature and pressure gradients. Specifically, it was found that the cold outflow boundaries over western and southern Pennsylvania helped the development and organization of continued deep convection during the nighttime hours. However, in central Pennsylvania, the warm-core mesovortex was significantly weakened when moist downdrafts were coupled with the updrafts in the convective parameterition scheme.

Inclusion of radiative heating in the surface energy budget tended to produce a conditionally unstable environment favorable for the development and maintenance of deep convection. In general, inclusion of the radiative heating at the surface improved the prediction of timing, frequency and location of convective precipitation. Omission of radiative heating has roughly the same “breaking” effect on the development of the mesovortex as the introduction of the moist downdrafts. It appears that the pronounced diurnal cycle of MCCs is directly related to the thermal cycle of the boundary layer.

Because of sharp and pronounced inhomogeneities in the horizontal moisture distribution, inclusion of virtual temperature instead of just temperature can considerably increase horizontal gradients of geopotential height. Without the virtual temperature effect in the ideal gas law, the model fails to reproduce the warm-core mesovortex and the MCC.

Use of a bulk boundary layer parameterization scheme appears to have a significant effect over mountainous regions. The scheme tends to overestimate the upward energy transport on the upslope side of a terrain feature and underestimate it on the downslope side.

In general, the results indicate that rigorous treatment of model physics is extremely important for simulating the mesoscale convective weather systems and precipitation associated with the Johnstown flood. The results also indicate that successful prediction of “convective” weather systems not only hinges upon the convective parameterization, but also upon the magnitude and distribution of the resolvable-scale latent heat release, and the concurrent development of the diurnal cycle of the boundary layer.

Full access
Chanh Q. Kieu and Da-Lin Zhang

Abstract

Although tropical cyclogenesis occurs over all tropical warm ocean basins, the eastern Pacific appears to have the highest frequency of tropical cyclogenesis events per unit area. In this study, tropical cyclogenesis from merging mesoscale convective vortices (MCVs) associated with breakdowns of the intertropical convergence zone (ITCZ) is examined. This is achieved through a case study of the processes leading to the genesis of Tropical Storm Eugene (2005) over the eastern Pacific using the National Centers for Environmental Prediction reanalysis, satellite data, and 4-day multinested cloud-resolving simulations with the Weather Research and Forecast (WRF) model at the finest grid size of 1.33 km.

Observational analyses reveal the initiations of two MCVs on the eastern ends of the ITCZ breakdowns that occurred more than 2 days and 1000 km apart. The WRF model reproduces their different movements, intensity and size changes, and vortex–vortex interaction at nearly the right timing and location at 39 h into the integration as well as the subsequent track and intensity of the merger in association with the poleward rollup of the ITCZ. Model results show that the two MCVs are merged in a coalescence and capture mode due to their different larger-scale steering flows and sizes. As the two MCVs are being merged, the low- to midlevel potential vorticity and tangential flows increase substantially; the latter occurs more rapidly in the lower troposphere, helping initiate the wind-induced surface heat exchange process leading to the genesis of Eugene with a diameter of 400 km. Subsequently, the merger moves poleward with characters of both MCVs. The simulated tropical storm exhibits many features that are similar to a hurricane, including the warm-cored “eye” and the rotating “eyewall.” It is also shown that vertical shear associated with a midlevel easterly jet leads to the downshear tilt and the wavenumber-1 rainfall structures during the genesis stage, and the upshear generation of moist downdrafts in the vicinity of the eyewall in the minimum equivalent potential temperature layer. Based on the above results, it is concluded that the ITCZ provides a favorable environment with dynamical instability, high humidity, and background vorticity, but it is the merger of the two MCVs that is critical for the genesis of Eugene. The storm decays as it moves northwestward into an environment with increasing vertical shear, dry intrusion, and colder sea surface temperatures. The results appear to have important implications for the high frequency of development of tropical cyclones in the eastern Pacific.

Full access
Da-Lin Zhang and J. Michael Fritsch

Abstract

The Pennsylvania State University/NCAR mesoscale model, originally developed by Anthes and Warner, is modified to simulate the meso-β scale structure and evolution of convectively driven weather systems. The modifications include: (i) two-way interactive nested-grid procedures, (ii) the Fritsch-Chappell convective parameterization scheme, and (iii) the Blackadar boundary layer package.

An 18-h simulation of the Johnstown flood of July 1977 is conducted. Compared to the documentation of Hoxit et al. and Bosart and Sanders, the simulation reproduced many of the different aspect of the mesoscale convective complex and squall line that were responsible for the heavy rain over western Pennsylvania. In particular, the model predicts the size, propagation rate and orientation of the mesoscale convective components that were observed in the mid-Atlantic states. The simulated evolution of the planetary boundary layer, cool outflow boundaries and surface pressure perturbations, such as meso-β scale lows, highs, ridges and troughs, compare favorably to observations. Other mesoscale features, for example, low-level jets and maximum/minimum horizontal wind couplets associated with the convective systems, were also reproduced reasonably well. Of particular significance is that the model-forecast rainfall amounts and distribution are similar to the observed.

Recognizing that a single case study does not provide a rigorous test of the predictability of a model, the results suggest that it may be possible to forecast the meso-β scale structure and evolution of convective weather systems with useful skill for periods up to about 18 hours. The results also suggest that significant improvements in warm-season quantitative precipitation forecasts might be possible if numerical forecasts of the meso-β scale structure and evolution of convective weather systems became operational.

Full access
Lin Zhu, Da-Lin Zhang, Stefan F. Cecelski, and Xinyong Shen

Abstract

The “bottom up” generation of low-level vortices (LVs) and midlevel vortices (MVs) during the genesis of Tropical Storm Debby (2006) and the roles of a midlevel “marsupial pouch” associated with an African easterly wave (AEW) are examined using an 84-h simulation with the finest grid size of 1.33 km. Results show that several MVs are generated in leading convective bands and then advected rearward into stratiform regions by front-to-rear ascending flows. Because of different Lagrangian storm-scale circulations, MVs and LVs are displaced along different paths during the early genesis stages. MVs propagate cyclonically inward within the AEW pouch while experiencing slow intensification and merging under the influence of converging flows. The MVs’ merging into a mesovortex is accelerated as they come closer to each other in the core region. In contrast, the low-level Lagrangian circulation is opened as a wave trough prior to tropical depression (TD) stage, so the LVs tend to “escape” from the pouch region. Only after the low-level flows become closed do some LVs congregate and contribute directly to Debby’s genesis. The TD stage is reached when the midlevel mesovortex and an LV are collocated with a convective zone having intense low-level convergence. Results also show the roles of upper-level warming in hydrostatically maintaining the midlevel pouch and producing mesoscale surface pressure falls. It is found that the vertically tilted AEW with a cold dome below is transformed to a deep warm-core TD vortex by subsiding motion. A conceptual model describing the key elements in the genesis of Debby is also provided.

Full access
Da-Lin Zhang, Shunli Zhang, and Scott J. Weaver

Abstract

Although considerable research has been conducted to study the characteristics of the low-level jets (LLJs) over the Great Plains states, little is known about the development of LLJs over the Mid-Atlantic states. In this study, the Mid-Atlantic LLJ and its associated characteristics during the warm seasons of 2001 and 2002 are documented with both the wind profiler data and the daily real-time model forecast products. A case study with three model sensitivity simulations is performed to gain insight into the three-dimensional structures and evolution of an LLJ and the mechanisms by which it developed. It is found that the Mid-Atlantic LLJ, ranging from 8 to 23 m s−1, appeared at an average altitude of 670 m and on 15–25 days of each month. About 90% of the 160 observed LLJ events occurred between 0000 and 0600 LST, and about 60% had southerly to westerly directions. Statistically, the real-time forecasts capture most of the LLJ events with nearly the right timing, intensity, and altitude, although individual forecasts may not correspond to those observed. For a selected southwesterly LLJ case, both the observations and the control simulation exhibit a pronounced diurnal cycle of horizontal winds in the lowest 1.5 km. The simulation shows that the Appalachian Mountains tend to produce a sloping mixed layer with northeasterly thermal winds during the daytime and reversed thermal winds after midnight. With additional thermal contrast effects associated with the Chesapeake Bay and the Atlantic Ocean, the daytime low-level winds vary significantly from the east coast to the mountainous regions. The LLJ after midnight tends to be peaked preferentially around 77.5°W near the middle portion of the sloping terrain, and it decreases eastward as a result of the opposite thermal gradient across the coastline from the mountain-generated thermal gradient. Although the Mid-Atlantic LLJ is much weaker and less extensive than that over the Great Plains states, it has a width of 300–400 km (to its half-peak value) and a length scale of more than 1500 km, following closely the orientation of the Appalachians. Sensitivity simulations show that eliminating the surface heat fluxes produces the most significant impact on the development of the LLJ, then topography and the land–sea contrast, with its area-averaged intensity reduced from 12 m s−1 to about 6, 9, and 10 m s−1, respectively.

Full access
Xuerong Zhang, Ying Li, Da-Lin Zhang, and Lianshou Chen

Abstract

Despite steady improvements in tropical cyclone (TC) track forecasts, it still remains challenging to predict unusual TC tracks (UNTKs), such as the tracks of sharp turning or looping TCs, especially after they move close to coastal waters. In this study 1059 UNTK events associated with 564 TCs are identified from a total of 1320 TCs, occurring in the vicinity of China’s coastal waters, during the 65-yr period of 1949–2013, using the best-track data archived at the China Meteorological Administration’s Shanghai Typhoon Institute. These UNTK events are then categorized into seven types of tracks—sharp westward turning (169), sharp eastward turning (86), sharp northward turning (223), sharp southward turning (46), looping (153), rotating (199), and zigzagging (183)—on the basis of an improved UNTK classification scheme. Results show significant annual variability of unusual tracking TCs, ranging between 2 and 18 per year, many of which experience more than one UNTK event in the same or different UNTK types during their life spans. The monthly distribution of the UNTK events resembles that of TCs, with more occurring in June–November. An analysis of their spatial distributions reveals that all of the UNTK events tend to take place in the areas to the south of 30°N, most frequently in the South China Sea and to the east of the Philippines. The results suggest that more attention be paid to the improved understanding and prediction of UNTK events so that the current positive trend in TC track forecast accuracy can continue for many years to come.

Full access
Mingxin Li, Da-Lin Zhang, Jisong Sun, and Qinghong Zhang

Abstract

An 8-yr (i.e., 2008–15) climatology of the spatiotemporal characteristics of hail events in China and their associated environmental conditions are examined using hail observations, L-band rawinsondes, and global reanalysis data. A total of 1003 hail events with maximum hail diameter (MHD) of greater than 5 mm are selected and then sorted into three hail-size bins. Hail events with the largest MHD bin correspond to the median vertical wind shear in the lowest 6-km layer (SHR6) of 21.6 m s−1, precipitable water (PW) of 34.8 mm, and convective available potential energy (CAPE) of 2192 J kg−1. Hail with different MHD bins share similar freezing-level heights (FLHs) of about 4000 m. The thickness of the hail growth zone is thinner for hail events with the largest MHD bin. Hail events with different MHD bins display seasonal variations associated with the summer monsoon; that is, the hail season starts in South China in spring and then shifts to North China in summer. Larger hail is mainly observed during the spring in South China before monsoon onset in the presence of an upper-level jet and a low-level southwesterly flow accounting for large SHR6 and PW. In contrast, smaller-MHD hailstorms occur mainly during the summer in North China when surface heating is high and the low-level southerly flow shifts northward with pronounced baroclinicity providing large CAPE and PW, moderate SHR6, and low FLH. Environmental CAPE and SHR6 for large hailstones in China are comparable in magnitude to those in the United States but larger than those in some European countries.

Full access
Yali Luo, Weimiao Qian, Renhe Zhang, and Da-Lin Zhang

Abstract

Heavy rainfall hit the Yangtze–Huai Rivers basin (YHRB) of east China several times during the prolonged 2007 mei-yu season, causing the worst flood since 1954. There has been an urgent need for attaining and processing high-quality, kilometer-scale, hourly rainfall data in order to understand the mei-yu precipitation processes, especially at the mesoβ and smaller scales. In this paper, the authors describe the construction of the 0.07°-resolution gridded hourly rainfall analysis over the YHRB region during the 2007 mei-yu season that is based on surface reports at 555 national and 6572 regional automated weather stations with an average resolution of about 7 km. The gridded hourly analysis is obtained using a modified Cressman-type objective analysis after applying strict quality control, including not only the commonly used internal temporal and spatial consistency and extreme value checks, but also verifications against mosaic radar reflectivity data. This analysis reveals many convectively generated finescale precipitation structures that could not be seen from the national station reports. A comprehensive quantitative assessment ensures the quality of the gridded hourly precipitation data. A comparison of this dataset with the U.S. Climate Prediction Center morphing technique (CMORPH) dataset on the same resolution suggests the dependence of the latter's performance on different rainfall intensity categories, with substantial underestimation of the magnitude and width of the mei-yu rainband as well as the nocturnal and morning peak rainfall amounts, due mainly to its underestimating the occurrences of heavy rainfall (i.e., >10 mm h−1).

Restricted access