背景场云凝结核浓度对理想热带气旋强度的影响

Numerical Study of Background Cloud Condensation Nuclei Effects on the Intensity of Idealized Tropical Cyclone

采用中尺度模式WRF中"Aerosol-aware"Thompson云微物理方案,分别进行了干净、污染、重度污染大气情景下的3个试验,研究了背景场凝结核浓度对理想热带气旋(Tropical Cyclone,TC)强度的影响。结果表明,凝结核增加导致TC强度减弱,而最大风速半径和外围风圈半径变化不明显。不同试验眼墙降水的时空分布差异较大,但降水总量的变化较小。重度污染大气下的云滴数量远大于干净大气下,不同试验中水物质变化及其形态间转换的分布存在较大差异。正是这种差异导致了不同的潜热加热分布特征,与热力动力过程作用,对TC强度产生了不同影响。如重度污染下发展的TC外围潜热增加,减少了和TC中心的气压梯度,导致水平方向气压梯度力减弱,TC强度减弱。TC减弱和外雨带的发展也密切相关,外雨带降水过程中,形成地面冷池流入TC眼墙,可导致TC强度减弱。这表明"Aerosol-aware"Thompson方案对气溶胶影响描述较为合理。

By making use of the Weather Research and Forecasting(WRF) model in which the newly updated Thompson microphysics scheme was implemented,three idealized numerical experiments were conducted under different atmospheric background conditions(clear air,polluted air and severe polluted air) to examine the impact of background aerosol concentration on tropical cyclone(TC) intensity.The new Thompson microphysics scheme took into account the influence of aerosol acting as CCN and IN,which is called 'Aerosol-aware'scheme.The results showed that,during the stage of early development,the simulated TC intensity was the weakest,middle and strongest for server polluted,polluted and clear air conditions,respectively.While with forecasting time increasing,the differences of TC intensities among three experiment tended to be indistinctive.The increased aerosol had little influence on total precipitation amount,rather than delayed the timing of rainfall occurrence in TC area,nevertheless,a significant impact on the spatial and temporal distribution of the simulated precipitation was found.The cloud droplet number concentration in the TC developed under server polluted conditions are much higher than that under clean conditions,which,combined with dynamical and thermal process,can result in a large difference in the hydrometers and their conversion for different simulations.More latent heat release in the outside of TC under server polluted conditions led to a weaker horizontal pressure gradient,and as a result,a reduced intensity and increased size.Moreover the weakening of the TC was also related to its spiral rain band development.With development of the spiral rain band,a cold pool formed and invaded into the eye wall,resulting in a reduced intensity.The experiment results would suggest that aerosol-aware'Thompson microphysics scheme should be reasonably persuasive in the description of 'indirect effect' of aerosols on precipitation process.