在EBEX-2000实验资料中湍流谱和局地各向同性特征

采用 2 0 0 0年 8月在美国加州棉花地两个高度上应用超声三分量仪、快速响应温度和湿度仪进行的EBEX 2 0 0 0(InternationalEnergyBalanceExperiment,2 0 0 0 ,EBEX 2 0 0 0 )风速三分量、温度和湿度湍流实验观测数据 ,计算分析了湍流速度、温度和湿度谱在不同稳定度下的特征。对湍流的局地各向同性进行了讨论并与Kansas和长白山原始森林湍流实验得到的结果进行了比较 ,得到了一些湍流特征量在不同下垫面情况下的一些有意义的特征。

在EBEX-2000实验资料中的湍流宏观量特征

采用2000年8月在美国加州棉花地两个高度上应用超声三分量仪、快速响应温度和湿度仪进行的EBEX2000(InternationalEnergyBalanceExperiment,2000)风速三分量、温度和湿度湍流实验观测数据,计算分析了湍流宏观量(即u,T,σu/u,σv/u,σw/u和σT/T等),湍流动能和感热通量等的特征,并与其他湍流实验得到的结果进行了比较。

在EBEX-2000实验资料中湍流耗散率、长度尺度和结构参数特征

采用2000年8月在美国加州棉花地两个高度上应用超声三分量仪、快速响应温度和湿度仪进行的EBEX2000(InternationalEnergyBalanceExperiment,2000,简称EBEX2000)风速三分量、温度和湿度湍流实验观测数据,计算分析了在不同稳定度下的湍流能量和热量耗散率和湍流结构参数特征。并与Kansas和长白山原始森林湍流实验得到的结果进行了比较,得到了一些湍流特征量在不同下垫面情况下的一些有意义的特征。

Sensitivity of the Simulation of Tropical Cyclone Size to Microphysics Schemes

The sensitivity of the simulation of tropical cyclone （TC） size to microphysics schemes is studied using the Advanced Hurricane Weather Research and Forecasting Model （WRF）. Six TCs during the 2013 western North Pacific typhoon season and three mainstream microphysics schemes-Ferrier （FER）, WRF Single-Moment 5-class （WSM5） and WRF Single-Moment 6-class （WSM6）-are investigated. The results consistently show that the simulated TC track is not sensitive to the choice of microphysics scheme in the early simulation, especially in the open ocean. However, the sensitivity is much greater for TC intensity and inner-core size. The TC intensity and size simulated using the WSM5 and WSM6 schemes are respectively higher and larger than those using the FER scheme in general, which likely results from more diabatic heating being generated outside the eyewall in rainbands. More diabatic heating in rainbands gives higher inflow in the lower troposphere and higher outflow in the upper troposphere, with higher upward motion outside the eyewall. The lower-tropospheric inflow would transport absolute angular momentum inward to spin up tangential wind predominantly near the eyewall, leading to the increment in TC intensity and size （the inner-core size, especially）. In addition, the inclusion of graupel microphysics processes （as in WSM6） may not have a significant impact on the simulation of TC track, intensity and size.

Structural Changes of a Tropical Cyclone during Landfall: β-plane Simulations

This study investigated the effects of landfall on the structure of a tropical cyclone （TC）. Numerical simulations were performed using the Weather Research and Forecasting Model on a β-plane. Two landfall experiments, one with an east–west and another with a north–south oriented coastline, were performed. Similar to previous studies on an f-plane, large-scale flows in the low-to-middle troposphere were modified due to friction. A pair of counter rotating gyres was found, which was shown to be consistent with the slight deffection of the TC relative to the control experiment without land. Compared to previous f-plane simulations, because of the inherent asymmetries due to the β-gyres, the large-scale pattern of flows and convergences/divergences related to friction were found to depend on coastline orientations. On the other hand, regardless of the coastline orientation, convergences were found to be stronger to the left for both cases near landfall, as in previous f-plane simulations. Such a convergence pattern subsequently induced a change in convection and rainfall at the eyewall.

Influences of Structures on Urban Ventilation: A Numerical Experiment

Numerical simulations are carried out for the disturbed flow caused by three six-story buildings and a twenty-story tall tower respectively, as well as the distribution of automobile exhaust gas from a nearby road, based on the Peking University Model of Atmospheric Environment. The results show that the ventilation is better around the tall tower than around the three six-story residential buildings for the same number of households in the same urban region.

Recent Decrease in the Difference in Tropical Cyclone Occurrence between the Atlantic and the Western North Pacific

Climatologically,among all ocean basins,the western North Pacific(WNP)has the largest annual number of tropical cyclones(TCs)of around 26 while the Atlantic has around 13,yielding a difference of 13.However,the difference is-7 in 2020,with 30 TCs in the Atlantic and 23 in the WNP,which is the most negative difference within the last 46 years.In fact,during the last 26 years,the difference in TC number is below 10 in ten years,with four years being negative.Such a decreasing difference in TC number can be attributed to the natural multidecadal variation of the Atlantic Multidecadal Oscillation and Interdecadal Pacific Oscillation,as well as other external forcings such as anthropogenic aerosol forcing and increased greenhouse gases,with the additional impact from the La Niña condition.This result has significant implications on climate model projections of future TC activity in the two ocean basins.

Growing Threat of Rapidly-Intensifying Tropical Cyclones in East Asia

This study examines the long-term change in the threat of landfalling tropical cyclones(TCs) in East Asia over the period 1975–2020 with a focus on rapidly intensifying(RI) TCs. The increase in the annual number of RI-TCs over the western North Pacific and the northwestward shift of their genesis location lead to an increasing trend in the annual number of landfalling RI-TCs along the coast of East Asia. The annual power dissipation index(PDI), a measure of the destructive potential of RI-TCs at landfall, also shows a significant increasing trend due to increases in the annual frequency and mean landfall intensity of landfalling RI-TCs. The increase in mean landfall intensity is related to a higher lifetime maximum intensity(LMI) and the LMI location of the landfalling RI-TCs being closer to the coast. The increase in the annual PDI of East Asia is mainly associated with landfalling TCs in the southern(the Philippines, South China, and Vietnam) and northern parts(Japan and the Korean Peninsula) of East Asia due to long-term changes in vertical wind shear and TC heat potential. The former leads to a northwestward shift of favorable environments for TC genesis and intensification, resulting in the northwestward shift in the genesis, RI, and LMI locations of RI-TCs. The latter provides more heat energy from the ocean for TC intensification, increasing its chances to undergo RI.

Importance of Air-Sea Coupling in Simulating Tropical Cyclone Intensity at Landfall

An atmosphere-only model system for making seasonal prediction and projecting future intensities of landfalling tropical cyclones(TCs)along the South China coast is upgraded by including ocean and wave models.A total of 642 TCs have been re-simulated using the new system to produce a climatology of TC intensity in the South China Sea.Detailed comparisons of the simulations from the atmosphere-only and the fully coupled systems reveal that the inclusion of the additional ocean and wave models enable differential sea surface temperature responses to various TC characteristics such as translational speed and size.In particular,interaction with the ocean does not necessarily imply a weakening of the TC,with the coastal bathymetry possibly playing a role in causing a near-shore intensification of the TC.These results suggest that to simulate the evolution of TC structure more accurately,it is essential to use an air-sea coupled model instead of an atmosphere-only model.

Tropical cyclone activities in the Western North Pacific in 2022

Based on the best-track dataset from the Shanghai Typhoon Institute/China Meteorological Administration,the paper provides a compre-hensive summary and analysis of tropical cyclone(TC)activities in the Western North Pacific(WNP)and the South China Sea(SCS)for 2022.Using the historical climatology from 1951 to 2020,the anomalous conditions during 2022 in TC frequency,origin locations,tracks,intensity,and duration for the entire ocean basin as well as landfall events in China are examined.Results show that the overall TC frequency is slightly lower than normal,but the multiple TC events have a very high frequency of occurrence.Origin locations of TCs,which mark the starting points of their paths,show a large westward and northward deviation from climatology.Around 40%of the named TCs exhibit a shift in their direction of movement from westerly to easterly.Additionally,comparisons of the means,medians,upper and lower quartiles all indicate that the intensity of TCs in 2022 is generally lower than the climatology,with the duration of TCs at tropical storm intensity or above being shorter than usual.A notable observation is the fewer incidence of TC landfalls in China,but with a geographical concentration in Guangdong Province.These anomalous annual TC activities are influenced by related atmospheric and oceanic environmental conditions modulated by multi-scale climate variability.Thefindings provide useful information for enhancing disaster mitigation strategies in the Asia-Pacific region.

SEASONAL TROPICAL CYCLONE FORECASTING

This paper summarizes the forecast methods,outputs and skill offered by twelve agencies for seasonal tropical cyclone(TC)activity around the world.These agencies use a variety of techniques ranging from statistical models to dynamical models to predict basinwide activity and regional activity.In addition,several dynamical and hybrid statistical/dynamical models now predict TC track density as well as landfall likelihood.Realtime Atlantic seasonal hurricane forecasts have shown low skill in April modest skill in June and good skill in August at predicting basinwide TC activity when evaluated over 2003-2018.Real-time western North Pacific seasonal TC forecasts have shown good skill by July for basinwide intense typhoon numbers and the ACE index when evaluated for 2003-2018.Both hindcasts and real-time forecasts have shown skill for other TC basins.A summary of recent research into forecasting TC activity beyond seasonal(e.g.,multi-year)timescales is included.Recommendations for future areas of research are also discussed.

Sensitivity of precipitation statistics to urban growth in a subtropical coastal megacity cluster

This short paper presents an investigation on how human activities may or may not affect precipitation based on numerical simulations of precipitation in a benchmark case with modified lower boundary conditions, representing different stages of urban development in the model. The results indicate that certain degrees of urbanization affect the likelihood of heavy precipitation significantly, while less urbanized or smaller cities are much less prone to these effects. Such a result can be explained based on our previous work where the sensitivity of precipitation statistics to surface anthropogenic heat sources lies in the generation of buoyancy and turbulence in the planetary boundary layer and dissipation through triggering of convection. Thus only mega cities of sufficient size, and hence human-activity-related anthropogenic heat emission, can expect to experience such effects. In other words, as cities grow, their effects upon precipitation appear to grow as well.

ADVANCES IN UNDERSTANDING DIFFICULT CASES OF TROPICAL CYCLONE TRACK FORECASTS

Although tropical cyclone track forecast errors have substantially decreased in recent decades,there are still cases each season with large uncertainties in the forecasts and/or very large track errors.As such cases are challenging for forecasters,it is important to understand the mechanisms behind the low predictability.For this purpose the research community has developed a number of tools.These tools include ensemble and adjoint sensitivity models,ensemble perturbation experiments and nudging experiments.In this report we discuss definitions of difficult cases for tropical cyclone track forecasts,diagnostic techniques to understand sources of errors,lessons learnt in recent years and recommendations for future work.

Large tropical cyclone track forecast errors of global numerical weather prediction models in western North Pacific basin

Although tropical cyclone(TC)track forecast errors(TFEs)of operational warning centres have substantially decreased in recent decades,there are still many cases with large TFEs.The International Grand Global Ensemble(TIGGE)data are used to study the possible reasons for the large TFE cases and to compare the performance of different numerical weather prediction(NWP)models.Forty-four TCs in the western North Pacific during the period 2007-2014 with TFEs(+24 to+120 h)larger than the 75 th percentile of the annual error distribution(with a total of 93 cases)are identified.Four categories of situations are found to be associated with large TFEs.These include the interaction of the outer structure of the TC with tropical weather systems,the intensity of the TC,the extension of the subtropical high(SH)and the interaction with the westerly trough.The crucial factor of each category attributed to the large TFE is discussed.Among the TIGGE model predictions,the models of the European Centre for Medium-Range Weather Forecasts and the UK Met Office generally have a smaller TFE.The performance of different models in different situations is discussed.