Search Results

You are looking at 91 - 100 of 105 items for

  • Author or Editor: Da-Lin Zhang x
  • All content x
Clear All Modify Search
Fan Zhang, Ming Li, Andrew C. Ross, Serena Blyth Lee, and Da-Lin Zhang

Abstract

Through a case study of Hurricane Arthur (2014), the Weather Research and Forecasting (WRF) Model and the Finite Volume Coastal Ocean Model (FVCOM) are used to investigate the sensitivity of storm surge forecasts to physics parameterizations and configurations of the initial and boundary conditions in WRF. The turbulence closure scheme in the planetary boundary layer affects the prediction of the storm intensity: the local closure scheme produces lower equivalent potential temperature than the nonlocal closure schemes, leading to significant reductions in the maximum surface wind speed and surge heights. On the other hand, higher-class cloud microphysics schemes overpredict the wind speed, resulting in large overpredictions of storm surge at some coastal locations. Without cumulus parameterization in the outermost domain, both the wind speed and storm surge are grossly underpredicted as a result of large precipitation decreases in the storm center. None of the choices for the WRF physics parameterization schemes significantly affect the prediction of Arthur’s track. Sea surface temperature affects the latent heat release from the ocean surface and thus storm intensity and storm surge predictions. The large-scale atmospheric circulation models provide the initial and boundary conditions for WRF, and influence both the track and intensity predictions, thereby changing the spatial distribution of storm surge along the coastline. These sensitivity analyses underline the need to use an ensemble modeling approach to improve the storm surge forecasts.

Full access
Da-Lin Zhang, John S. Kain, J. Michael Fritsch, and Kun Gao

Abstract

No abstract available

Full access
Fan Wu, Xiaopeng Cui, Da-Lin Zhang, Dongxia Liu, and Dong Zheng

Abstract

In this study, the spatiotemporal characteristics of cloud-to-ground (CG) and intracloud (IC) lightning flashes observed by Surveillance et Alerte Foudre par Interférometrie Radioélectrique (SAFIR)-3000 over the Beijing metropolitan region (BMR) during 2005–07 were investigated. The results showed the presence of 299 lightning days with 241 688 flashes, most of which were IC lightning flashes. Only 19% of the total flashes were CG lightning flashes; 14% of these CG flashes were positive. Most lightning activity occurred during the summer months (June–August), with a major diurnal peak around 1900 Beijing standard time (BST) and a secondary peak around 2300 BST. Spatial variations in flash density and lightning days both exhibited an obvious southeastwardly increasing pattern, with higher flash densities or more lightning days occurring in the southeastern plains and lower values distributed on the northwestern mountains. The Z ratio (IC/CG lightning flashes) exhibited a similar spatial pattern, but the percentage of positive CG lightning flashes showed an almost opposite pattern. The results also showed significant topographic effects on the spatiotemporal variations in lightning activity. That is, flash counts on the northeastern and southwestern mountains peaked in the afternoon, whereas those on the southeastern plains peaked in the late night to early morning, which could be attributed to the propagation of thunderstorms from the mountains to the plains. The results showed that the SAFIR-3000 lightning data are more useful than CG lightning data alone for forecasting the development and propagation of thunderstorms over the BMR.

Full access
Da-Lin Zhang, Hai-Ru Chang, Nelson L. Seaman, Thomas T. Warner, and J. Michael Fritsch

Abstract

A two-way interactive, nested-grid system tested with The Pennsylvania Slate University/INCAR three-dimensional mesoscale model is described. A mesh structure, designed to minimize numerical noise, together with a procedure for obtaining compatible coarse grid mesh (COM) and fine grid mesh (FOM) terrain conditions, is presented. Also, a method to initialize the nested-grid meshes is proposed. The nested-grid system has been tested with real data and raw terrain under different severe conditions. A 12-h simulation of a propagating jet streak over complex terrain is presented; the results indicate relatively noise-free solutions on both the OGM and FGM domains.

Full access
Na Wei, Ying Li, Da-Lin Zhang, Zi Mai, and Shi-Qi Yang

Abstract

The geographical and temporal characteristics of upper-tropospheric cold low (UTCL) and their relationship to tropical cyclone (TC) track and intensity change over the western North Pacific (WNP) during 2000–12 are examined using the TC best track and global meteorological reanalysis data. An analysis of the two datasets shows that 73% of 346 TCs coexist with 345 UTCLs, and 21% of the latter coexist with TCs within an initial cutoff distance of 15°. By selecting those coexisted systems within this distance, the possible influences of UTCL on TC track and intensity change are found, depending on their relative distance and on the sectors of UTCLs where TCs are located. Results show that the impact of UTCLs on TC directional changes are statistically insignificant when averaged within the 15° radius. However, left-turning TCs within 5° distance from the UTCL center exhibit large deviated directional changes from the WNP climatology, due to the presence of highly frequent abrupt left turnings in the eastern semicircle of UTCL. The abrupt turnings of TCs are often accompanied by their slow-down movements. Results also show that TCs seem more (less) prone to intensify at early (late) development stages when interacting with UTCLs compared to the WNP climatology. Intensifying (weakening) TCs are more distributed in the southern (northern) sectors of UTCLs, with less hostile conditions for weakening within 9°–13° radial range. In addition, rapid intensifying TCs take place in the south-southwest and east-southeast sectors of UTCLs, whereas rapid weakening cases appear in the western semicircle of UTCLs due to their frequent proximity to mainland coastal regions.

Full access
Da-Lin Zhang, Yi-Xuan Shou, Russell R. Dickerson, and Fei Chen

Abstract

Although there has been considerable research on urban heat island (UHI) effects, most of the previous studies have attributed UHI effects to localized, surface processes. In this study, the impact of upstream urbanization on enhanced UHI effects is examined using surface observations and numerical simulations of an extreme UHI event that occurred on 9 July 2007 over Baltimore, Maryland. Under southwesterly wind, Baltimore experienced higher peak surface temperatures and higher pollution concentrations than did the larger urban area of Washington, D.C. Results from a coupled ultrahigh-resolution mesoscale–urban canopy model with 2001 National Land Cover Data show an advective contribution from upstream urbanization to the UHI event. This dynamical process is demonstrated by replacing Baltimore or its upstream urban areas by natural vegetation (in the model), indicating that the UHI effects could be reduced by as much as 25%. An analysis of the urban–bay interaction reveals the importance of horizontal wind direction in determining the intensity of bay breezes and the urban boundary layer structures. In addition, the vertical growth and structures of UHI effects are shown as layered “hot plumes” in the mixed layer with pronounced rising motions, and these plumes can be advected many kilometers downstream. These findings suggest that judicious land use and urban planning, especially in rapidly developing countries, could help to alleviate UHI consequences, including heat stress and smog. They also have important implications for improving the prediction of urban weather, including the initiation of moist convection, air quality, and other environment-related problems.

Full access
William A. Gallus Jr., Brian Colle, Da-Lin Zhang, and Phil Schumacher
Full access
Fumin Ren, Chenchen Ding, Da-Lin Zhang, Deliang Chen, Hong-li Ren, and Wenyu Qiu

Abstract

Combining dynamical models with statistical algorithms is an important way to improve weather and climate prediction. In this study, a concept of a perfect model, whose solutions are from observations, is introduced, and a dynamical-statistical-analog ensemble forecast (DSAEF) model is developed as an initial-value problem of the perfect model. This new analog-based forecast model consists of the following three steps: (i) construct generalized initial value (GIV), (ii) identify analogs from historical observations, and (iii) produce an ensemble of predictands. The first step includes all appropriate variables, not only at an instant state but also during their temporal evolution, that play an important role in determining the accuracy of each predictand. An application of the DSAEF model is illustrated through the prediction of accumulated rainfall associated with 21 landfalling typhoons occurring over South China during the years of 2012–16. Assuming a reliable forecast of landfalling typhoon track, two different experiments are conducted, in which the GIV is constructed by including (i) typhoon track only; and (ii) both typhoon track and landfall season. Results show overall better performance of the second experiment than the first one in predicting heavy accumulated rainfall in the training sample tests. In addition, the forecast performance of both experiments is comparable to the operational numerical weather prediction models currently used in China, the United States, and Europe. Some limitations and future improvements as well as comparisons with some existing analog ensemble models are also discussed.

Open access
Ming-Yang He, Hong-Bo Liu, Bin Wang, and Da-Lin Zhang

Abstract

In this study, the three-dimensional structures and diurnal evolution of a typical low-level jet (LLJ) with a maximum speed of 24 m s−1 occurring in the 850–800-hPa layer are examined using both large-scale analysis and a high-resolution model simulation. The LLJ occurred on the eastern foothills of the Yun-Gui Plateau in south China from 1400 LST 29 June to 1400 LST 30 June 2003. The effects of surface radiative heating, topography, and latent heat release on the development of the LLJ case are also studied. Results show that a western Pacific Ocean subtropical high and a low pressure system on the respective southeast and northwest sides of the LLJ provide a favorable large-scale mean pressure pattern for the LLJ development. The LLJ reaches its peak intensity at 850 hPa near 0200 LST with wind directions veering from southerly before sunset to southwesterly at midnight. A hodograph at the LLJ core shows a complete diurnal cycle of the horizontal wind with a radius of 5.5 m s−1. It is found that in an LLJ coordinates system the along-LLJ geostrophic component regulates the distribution and 65% of the intensity of LLJ, whereas the ageostrophic component contributes to the clockwise rotation, thus leading to the formation and weakening of the LLJ during night- and daytime, respectively. Numerical sensitivity experiments confirm the surface radiative heating as the key factor in determining the formation of the nocturnal LLJ. The existence of the Yun-Gui Plateau, and the downstream condensational heating along the mei-yu front play secondary roles in the LLJ formation.

Full access
Kun Gao, Da-Lin Zhang, Mitchell W. Moncrieff, and Han-Ru Cho

Abstract

A mesoβ-scale momentum budget and its effect on larger-scale mean flow in a midlatitude mesoscale convective system are investigated using a numerical simulation of an intense squall line that occurred during 10–11 June 1985 PRE-STORM. It is found that the momentum generation normal to the line associated with the latent heating and cooling contributes most significantly to the momentum budget and determines the mesoβ-scale internal structure and evolution of the squall line. The momentum generation along the line contributes to the initial development of a mesovortex but has little effect on the final vertical structure of the along-line flow. Both vertical and horizontal momentum advection have significant contributions, particularly to the vertical mixing of the along-line flow; and this component of the horizontal momentum is locally transported down-gradient. It is also found that for midlatitude convective systems, convectively generated downdrafts can play as prominent a role as updrafts in vertically transporting horizontal momentum within both convective and stratiform regions.

The momentum flux associated with the mesoβ-scale circulations of the simulated squall line is found to agree with previous observational investigations, namely, normal to the line the squall system transports horizontal momentum in a countergradient sense while parallel to the line the transport is downgradient. Implications with respect to the convective momentum parameterization are discussed in the context of the mesoβ-scale momentum budget.

Full access