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Weiqing Zhang, William Perrie, and Weibiao Li

Abstract

A coupled atmosphere–wave–sea spray model system is used to evaluate the combined impacts of spray evaporation and wave drag on midlatitude storms. The focus of this paper is on the role of air–sea fluxes on storm intensity and development, and related impacts on the structure of the atmospheric boundary layer. The composite model system consists of the Canadian Mesoscale Compressible Community atmospheric model coupled to the operational wave model WAVEWATCH III, and a recent bulk parameterization for heat fluxes due to sea spray. The case studies are extratropical Hurricane Earl (in 1998) and two intense winter storms from 2000 and 2002, hereafter denoted “superbomb” and “bomb,” respectively. The results show that sea spray tends to intensify storms, whereas wave-related drag tends to weaken storms. The mechanisms by which spray and wave-related drag can influence storm intensity are quite different. When wind speeds are high and sea surface temperatures warm, spray can significantly increase the surface heat fluxes. By comparison, momentum fluxes related to wave drag are important over regions of the storm where young, newly generated waves are prevalent, for example during the rapid development phase of the storm. These momentum fluxes decrease in areas where the storm waves reach maturity. The collective influence of spray and waves on storm intensity depends on their occurrence in the early stages of a storm’s rapid intensification phase, and their spatial distribution with respect to the storm center. Moreover, for the case of the superbomb, a potential vorticity framework is used to show the relative importance of these surface flux impacts compared with baroclinic processes.

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Guixing Chen, Ruoyu Lan, Wenxin Zeng, He Pan, and Weibiao Li

Abstract

The complex features of rainfall diurnal cycles at the south China coast are examined using hourly rain gauge data and satellite products (CMORPH and TRMM 3B42) during 1998–2014. It is shown that morning rainfall is pronounced near the coasts and windward mountains, with high rainfall in the summer monsoon season, while afternoon rainfall is dominant on land, and nocturnal rainfall occurs at northern inland sites. Both satellite products report less morning rainfall and more afternoon rainfall than the rain gauge data, and they also miss the midnight rainfall minimum. These errors are mainly attributable to an underestimation of morning moderate and intense rains at coasts and an overestimation of afternoon–evening light rains on land. With a correction of the systematic bias, satellite products faithfully resolve the spatial patterns of normalized rainfall diurnal cycles related to land–sea contrast and terrains, suggesting an improved data application for regional climate studies. In particular, they are comparable to the rain gauge data in showing the linear reduction of morning rainfall from coasts to inland regions. TRMM is marginally better than CMORPH in revealing the overall features of diurnal cycles, while higher-resolution CMORPH captures more local details. All three datasets also present that morning rainfall decreases from May–June to July–August, especially on land; it exhibits pronounced interannual variations and a decadal increase in 1998–2008 at coasts. Such long-term variations of morning rainfall are induced by the coastal convergence and mountain liftings of monsoon shear flow interacting with land breeze, which is mainly regulated by monsoon southwesterly winds in the northern part of the South China Sea.

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Will Perrie, Weiqing Zhang, Edgar L Andreas, Weibiao Li, John Gyakum, and Ron McTaggart-Cowan

Abstract

Air–sea transfer processes over the ocean strongly affect how hurricanes develop. High winds generate large amounts of sea spray, which can modify the transfer of momentum, heat, and moisture across the air–sea interface. However, the extent to which sea spray can modify extratropical or midlatitude hurricanes and intense cyclones has not been resolved. This paper reports simulations of extratropical Hurricanes Earl (1998) and Danielle (1998) and an intense winter cyclone from January 2000 using a mesoscale atmospheric model and a recent sea spray parameterization. These simulations show that sea spray can increase the sea surface heat flux, especially the latent heat flux, in a midlatitude cyclone and that sea spray’s impact on cyclone intensity depends on the storm structure and development and is strongest for cyclones with high winds.

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Chong Shen, Xiaoyang Chen, Wei Dai, Xiaohui Li, Jie Wu, Qi Fan, Xuemei Wang, Liye Zhu, Pakwai Chan, Jian Hang, Shaojia Fan, and Weibiao Li

Abstract

On urban scales, the detailed characteristics of land-use information and building properties are vital to improving the meteorological model. The WRF Model with high-spatial-resolution urban fraction (UF) and urban morphology (UM) is used to study the impacts of these urban canopy parameters (UCPs) on dynamical and thermal meteorological fields in two representative seasons in Guangzhou. The results of two seasons are similar and as follows. 1) The impacts of updated UF and UM are obvious on wind speed but minor on temperature and humidity. In the urban environment, the results with updated UF and UM are more consistent with observations compared with the default UCPs, which means the performance of the model has been improved. 2) The dynamical factors associated with wind speed are analyzed. Turbulent kinetic energy (TKE) is significantly affected by UM but little by UF. And both UF and UM are found to influence friction velocity U*. The UM and greater UF attained larger U*. 3) In addition, the thermal fields are analyzed. The UM and increased UF induce higher surface skin temperature (TSK) and ground heat flux in the daytime, indicating that more heat is transported from the surface to the soil. At night, more heat is transported from the soil to the surface, producing higher TSK. For sensible heat flux (HFX), greater UF induces larger HFX during the daytime. But the effects of UM are complex, which makes HFX decrease during the daytime and increase at night. Finally, larger UF attains lower latent heat in the daytime.

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