Based on the Coupled Ocean-Atmospheric Response Experiment （COARE） bulk algorithm and the Naval Postgraduate School （NPS） model, a universal evaporation duct （UED） model that can flexibly accommodate the latest improvements in component （such as stability function, velocity roughness, and scalar roughness） schemes for different stratification and wind conditions, is proposed in this paper. With the UED model, the sensitivity of the model-derived evaporation duct height （EDH） to stability function （ψ）, ocean wave effect under moderate to high wind speeds, and scalar roughness length parameterization, is investigated, and relative contributions of these factors are compared. The results show that the stability function is a key factor influencing the simulated EDH values. Under unstable conditions, the EDH values from stability functions of Fairall et al. （1996） and Hu and Zhang （1992） are generally higher than those from others; while under stable conditions, unreasonably high EDHs can be avoided by use of the stability functions of Hu and Zhang （1992） and Grachev et al. （2007）. Under moderate to high wind speeds, the increase in velocity roughness length z0 due to consideration of the true ocean wave effect acts to reduce modeled EDH values; this trend is more pronounced under stable conditions. Although the scalar roughness length parameterization has a minor effect on the model-derived EDH, a positive correlation is found between the scalar roughness length z0q and the model-derived EDH.
This study aims to validate and improve the universal evaporation duct （UED） model through a further analysis of the stability function （ψ）. A large number of hydrometeorological observations obtained from a tower platform near Xisha Island of the South China Sea are employed, together with the latest variations inψ function. Applicability of different ψ functions for specific sea areas and stratification conditions is investigated based on three objective criteria. The results show that, under unstable conditions, ψfunction of Fairall et al. （1996） （i.e., Fairal196, similar for abbreviations of other function names） in general offers the best performance. However, strictly speaking, this holds true only for the stability （represented by bulk Richardson number RiB） range -2.6 ≤ RiB 〈 -0.1; when conditions become weakly unstable （-0.1 ≤ RiB 〈 --0.01）, Fairal196 offers the second best performance after Hu and Zhang （1992） （HYQ92）. Conversely, for near-neutral but slightly unstable conditions （-0.01≤ RiB 〈 0.0）, the effects of Edson04, Fairall03, Grachev00, and Fairal196 are similar, with Edson04 being the best function but offering only a weak advan- tage. Under stable conditions, HYQ92 is the optimal and offers a pronounced advantage, followed by the newly introduced SHEBA07 （by Grachev et al., 2007） function. Accordingly, the most favorable functions, i.e., Fairal196 and HYQ92, are incorporated into the UED model to obtain an improved version of the model. With the new functions, the mean root-mean-square （rms） errors of the modified refractivity （M）, 0-5-m M slope, 5-40-m M slope, and the rms errors of evaporation duct height （EDH） are reduced by 21.65%, 9.12%, 38.79%, and 59.06%, respectively, compared to the classical Naval Postgraduate School model.
基于中国台湾地区主持的侵台台风之飞机侦察及下投式探空仪观测实验(Dropwindsonde Observations for Typhoon Surveillance near the Taiwan Region,DOTSTAR)获得的高分辨率下投式探空仪探测资料,分析了2003年9月—2012年8月所有发生在中国台湾地区附近海域的台风型大气波导事件,遴选出一次由0920号超强台风"卢碧"引起的强台风型海上大气波导过程作为研究对象。利用欧洲中期数值预报中心(ECMWF)再分析资料(水平分辨率0.125°×0.125°),对此次波导的生成原因进行了分析;基于WRF模式比较了不同初始化方法对台风强度、尺度和周围台风型大气波导的模拟能力。结果表明,此次强台风型大气波导发生在台风环流西北侧外围的弱下沉运动区,其形成与850 hPa高度附近北方强干空气平流导致湿度随高度锐减密切相关。在数值模拟中运用台风动力初始化方法,可以有效改进台风强度、路径和尺度的模拟效果,进而有利于改善台风型大气波导尤其是波导层所在高度的模拟效果。台风外围出现的大气波导通常以悬空波导为主,模拟效果与台风螺旋雨带和内核尺度的模拟关系密切,而与台风强度和眼墙结构关系不大。中尺度数值模式WRF具有模拟台风型大气波导的能力,是研究台风型大气波导的有力手段。
The Weather Research and Forecasting model version 3.2 （WRF v3.2） was used with the bogus data assimilation （BDA） scheme and sea spray parameterization （SSP）, and experiments were conducted to assess the impacts of the BDA and SSP on prediction of the typhoon ducting process induced by Typhoon Mindule （2004）. The global positioning system （GPS） dropsonde observations were used for comparison, The results show that typhoon ducts are likely to form in every direction around the typhoon center, with the main type of ducts being elevated duct. With the BDA scheme included in the model initialization, the model has a better performance in predicting the existence, distribution, and strength of typhoon ducts. This improvement is attributed to the positive effect of the BDA scheme on the typhoon＇s ambient boundary layer structure. Sea spray affects typhoon ducts mainly by changing the latent heat （LH） flux at the air-sea interface beyond 270 km from the typhoon center. The strength of the typhoon duct is enhanced when the boundary layer under this duct is cooled and moistened by the sea spray; otherwise, the typhoon duct is weakened. The sea spray induced changes in the air-sea sensible heat （SH） flux and LH flux are concentrated in the maximum wind speed area near the typhoon center, and the changes are significantly weakened with the increase of the radial range.
