A Parameterization of Dry Thermals and Shallow Cumuli for Mesoscale Numerical Weather Prediction

For numerical weather prediction models and models resolving deep convection, shallow convective ascents are subgrid processes that are not parameterized by classical local turbulent schemes. The mass flux formulation of convective mixing is now largely accepted as an efficient approach for parameterizing the contribution of larger plumes in convective dry and cloudy boundary layers. We propose a new formulation of the EDMF scheme (for Eddy Diffusivity\Mass Flux) based on a single updraft that improves the representation of dry thermals and shallow convective clouds and conserves a correct representation of stratocumulus in mesoscale models. The definition of entrainment and detrainment in the dry part of the updraft is original, and is specified as proportional to the ratio of buoyancy to vertical velocity. In the cloudy part of the updraft, the classical buoyancy sorting approach is chosen. The main closure of the scheme is based on the mass flux near the surface, which is proportional to the sub-cloud layer convective velocity scale w _*. The link with the prognostic grid-scale cloud content and cloud cover and the projection on the non- conservative variables is processed by the cloud scheme. The validation of this new formulation using large-eddy simulations focused on showing the robustness of the scheme to represent three different boundary layer regimes. For dry convective cases, this parameterization enables a correct representation of the countergradient zone where the mass flux part represents the top entrainment (IHOP case). It can also handle the diurnal cycle of boundary-layer cumulus clouds (EUROCS\ARM) and conserve a realistic evolution of stratocumulus (EUROCS\FIRE).     

Hydrostatic and non-hydrostatic simulations of moist convection: Review and further study

Summary Quantitatively comparative experiments of moist convection using hydrostatic and non-hydrostatic models are reviewed and a further study is made of the suitability of the hydrostatic approximation for a high-resolution model when the grid size falls below 20 km. Idealized moist convection is treated, and then the torrential rain that occurred on 6 August 1993 in Kagoshima, southern Kyushu, Japan is simulated by each model. An explicit warm-rain process predicting cloud water and rainwater and the scheme of moist convective adjustment are individually or conjunctively employed in the model. The effect of hydrostatic water loading is also examined in detall.For the simulation of idealized convection, the hydrostatic simulation tends to overestimate and overexpand precipitation in comparison with the non-hydrostatic counterpart, and the drag effect of hydrostatic water loading is more significant for convective development than the non-hydrostatic effect. In the 20-km simulations, however, the hydrostatic simulation with hydrostatic water loading produces results that are comparable to the nonhydrostatic counterpart. For the simulation with real data, the comparative results well correspond to those of idealized convection. Furthermore, the 5 km hydrostatic simulation overestimates total precipitation more than that of dealized convection. On the basis of these results, when developing 1020 km numerical weather prediction (NWP) models, hydrostatic water loading should be evaluated in preference to adopting non-hydrostatic models, and a non-hydrostatic model with hydrostatic water loading is thought to be recommendable for a high-resolution NWP model.With 7 Figures     

积云参数化方案对热带降水年循环模态模拟的影响

本文利用中国科学院大气物理研究所大气科学和地球流体力学数值模拟国家重点实验室( LASG)发展的大气环流模式(SAMIL),采用Zhang-McFarlane (ZM)和Tiedtke (TDK)两种积云对流参数化方案,讨论了积云对流参数化方案对热带降水年循环模态模拟的影响.结果表明,两种积云对流参数化方案均能合理再现...     

NWP产品在强对流天气诊断分析中的应用

利用ECMWF、T213、T106数值预报产品及各物理量场,对已知中尺度系统发生发展的大尺度条件,预报中尺度环流出现的统计概率进行探讨:首先将武汉中心气象台过去总结的强对流天气模型与数值预报产品中的环流形势进行对比分析,当所预报的环流形势满足强对流天气模型时,认为大尺度条件将会促进中尺度天气的发生发展。再对数值预报产品中的有关物理要素场和值进行诊断分析,当所诊断的结果反映出有中尺度次级环流出现或有利中尺度对流天气发生时,最后对强对流天气落区、落点及降水性质进行诊断分析。同时,还利用AREM中尺度数值模式对2002年7月21~24日过程进行诊断分析,预报效果较好。,通过诊断分析,得出了强对流天气落区、落点及降水性质与各要素之间预报场和值的关系,同时也为今后精细化预报提供了启示和参考。     

中国冬夏季模式降水方案的应用分析研究

通过对中国冬夏季各一次降水过程的模拟分析 ,研究了不同云降水方案在中国区域的应用特征。结果指出 ,夏季 ,Kain Fritsch(KF)和Betts Miller(BM)积云方案模拟产生的积云降水是有差异的 ,KF方案对本次连续降水过程的预报要优于BM方案 ;KF方案和BM方案可以使周围环境大气状况发生不同的变化 ,KF方案可以使周围环境大气变得更湿、上升气流更加深厚 ,更有利于网格尺度降水的产生 ,即不同积云对流方案对网格尺度降水具有重要的影响 ;冬季 ,中国北方大陆基本没有对流降水产生 ,不同积云对流方案对网格尺度降水预报的影响基本可以不予考虑 ,中国北方的冬季降水主要是由网格尺度降水构成的。     

WRF模式对青藏高原南坡夏季降水的模拟分析

利用中尺度数值模式WRF研究积云对流参数化方案、网格嵌套技术和模式分辨率对陡峭的青藏高原南坡夏季降水模拟的影响。对2006年7月青藏高原南坡地区降水的模拟分析表明:降水对积云对流参数化方案的选择很敏感,不同方案模拟的结果差异显著,采用Grell-Devenyi质量通量方案时的模拟效果优于其他方案。在此基础上,通过5种试验方案比较发现,使用积云对流参数化方案、提高模式分辨率和应用网格嵌套技术能改善降水强度和空间分布的模拟,组合使用时模拟的降水与观测资料更接近。它们均能改进风场,使得水汽的输送和辐合过程的模拟更加准确;还能影响大气的垂直加热状态,导致不同的对流发生,使垂直速度的分布趋于合理。未使用积云对流参数化方案时,大气湿度偏小,而模式分辨率和网格嵌套技术对大气湿度的影响不大。      

Application and Study of Precipitation Schemes in Weather Simulation in Summer and Winter over China

Through simulation of summer and winter precipitation cases in China, the cloud precipitation schemes of model were examined. Results indicate that it is discrepant between convective precipitation simulated by the Kain-Fritsch (KF) scheme and Betts-Miller (BM) scheme in summer, the former scheme is better than the latter in this case. The ambient atmosphere may be varied by different convective schemes. The air is wetter and the updraft is stronger in the KF scheme than in the BM scheme, which can induce the more grid scale precipitation in the KF scheme, i.e., the different cumulus schemes may have the different and important effect on the grid scale precipitation. However, there is almost no convective rain in winter in northern China, so the effect of cumulus precipitation on the grid scale precipitation can be disregarded. Therefore, the gird scale precipitation is primary in the winter of northern China.     

青藏高原一次强对流过程对水汽垂直输送的数值模拟

本文采用中尺度天气研究预测模式(WRF)模拟了青藏高原那曲地区的一次强对流过程,分析了强对流对水汽的垂直输送量及对模式不同云微物理参数化方案的敏感性.通过与实测资料的比较,发现此次模拟在对流发生时间、地点、降水时间等方面均与实际接近.敏感性试验表明:当对流发生时,对流区域向上的水汽通量随海拔高度呈先增大后减小的趋势,该...     

两种对流参数化方案对辐射能量收支的影响研究

利用中国科学院大气物理研究所大气科学和地球流体力学数值模拟国家重点实验室的格点大气环流模式(GAMIL)1.0版设计了两组数值模拟实验来研究两种不同的对流参数化方案对辐射能量收支的影响.这两种对流参数化方案分别是:Zhang and McFarlance/Hack方案(简称ZM)和Tiedtke/Nordeng方案(简称 TN).对应的数值模拟实验分别取名为EX-ZM和EX-TN.通过对实验结果的分析表明:在对流过程中,EX-ZM允许深对流和浅对流同时发生,因此两种对流同时在模式低层消耗了更多的水汽,释放了更多的潜热,引起了更大的增温;EX_TN每次只允许一种对流发生,也就避免了不同类型的对流在同一层同时消耗水汽的现象.因此对流过后,EX-ZM的环境空气相对湿度较小,而EX-TN周围空气的相对湿度较大,有利于低云云量的生成和大尺度的凝结,因此EX-TN模拟的低云云量偏多,低层的云水含量偏高,模式低层的云光学厚度偏大,这就使得EX_TN中更多的太阳短波辐射通量被云反射掉,严重低估了模式对短波波段的辐射通量的模拟.此外,不同的对流参数化方案通过改变云的长波发射率和降水,进而影响了模式对长波波段的辐射通量、感热和潜热通量的模拟.     

尺度适应的对流参数化方案对一次华南飑线模拟的影响

尺度适应(scale-aware)的物理过程是现代数值预报发展的一种趋势,本文针对GRAPES_Meso(Global/Regional Assimilation and Prediction Enhanced System)模式没有考虑物理过程尺度适应的现状,首先在KFeta(Kain-Fritsch Eta)积云对流参数化方案中引进了尺度适应过程,对该方案的对流时间尺度、格点垂直速度以及夹卷率进行了基于尺度适应参数化的改进。为研究尺度适应KFeta方案与原KFeta方案对不同分辨率模式模拟结果的影响,选取了一次华南飑线过程进行数值模拟和影响分析。结果表明:在3 km、5 km、10 km、20 km水平分辨率的GRAPES_Meso模式中,尺度适应KFeta方案相比原方案,对降水强度及落区分布的模拟有一定的正效果,随着模式水平分辨率提高,次网格降水减少、格点降水增多、对流层中低层夹卷略有增强,对原来存在的对流层高层及低层偏冷的偏差有一定的改进,对流活跃区域的上升气流强度、云中水凝物含量更符合真实的天气系统演变。综合来看,改进后的方案更适用于高分辨率数值预报模式,该研究结果可以为尺度适应对流参数化方案的应用及数值模式强降水预报性能的优化提供有益的参考。     

Spatial patterns and trends of daily rainfall regime in Peninsular Malaysia during the southwest and northeast monsoons: 1975–2004

Model precipitation can be produced implicitly through convective parameterization schemes or explicitly through cloud microphysics schemes. These two precipitation production schemes control the spatial and temporal distribution of precipitation and consequently can yield distinct vertical profiles of heating and moistening in the atmosphere. The partition between implicit and explicit precipitation can be different as the model changes resolutions. Within the range of mesoscale resolutions (about 20 km) and cumulus scale, hybrid solutions are suggested, in which cumulus convection parameterization is acting together with the explicit form of representation. In this work, it is proposed that, as resolution increases, the convective scheme should convert less condensed water into precipitation. Part of the condensed water is made available to the cloud microphysics scheme and another part evaporates. At grid sizes smaller than 3 km, the convective scheme is still active in removing convective instability, but precipitation is produced by cloud microphysics. The Eta model version using KF cumulus parameterization was applied in this study. To evaluate the quantitative precipitation forecast, the Eta model with the KF scheme was used to simulate precipitation associated with the South Atlantic Convergence Zone (SACZ) and Cold Front (CF) events. Integrations with increasing horizontal resolutions were carried out for up to 5 days for the SACZ cases and up to 2 days for the CF cases. The precipitation partition showed that most of precipitation was generated by the implicit scheme. As the grid size decreased, the implicit precipitation increased and the explicit decreased. However, as model horizontal resolution increases, it is expected that precipitation be represented more explicitly. In the KF scheme, the fraction of liquid water or ice, generated by the scheme, which is converted into rain or snow is controlled by a parameter S ₁. An additional parameter was introduced into KF scheme and the parameter acts to evaporate a fraction of liquid water or ice left in the model grid by S ₁ and return moisture to the resolved scale. An F parameter was introduced to combine the effects of S ₁ and S ₂ parameters. The F parameter gives a measure of the conversion of cloud liquid water or ice to convective precipitation. A function dependent on the horizontal resolution was introduced into the KF scheme to influence the implicit and explicit precipitation partition. The explicit precipitation increased with model resolution. This function reduced the positive precipitation bias at all thresholds and for the studied weather systems. With increased horizontal resolution, the maximum precipitation area was better positioned and the total precipitation became closer to observations. Skill scores for all events at different forecast ranges showed precipitation forecast improvement with the inclusion of the function F.     

雷暴与强对流临近天气预报技术进展

临近预报指0—6h(0—2h为重点)的高时空分辨率的天气预报,预报对象是该时段内出现明显变化的天气现象,主要包括雷暴、强对流、降水、冬季暴风雪、冻雨、沙尘暴、低能见度(雾)、天空云量等,其中,以雷暴和强对流天气的临近预报最具挑战性。综述了针对雷暴和强对流天气的以主观预报为主、结合客观算法的临近预报技术,同时讨论了高分辨率数值预报模式在临近预报中的应用。主观临近预报技术包括基于多普勒天气雷达观测数据并结合其他资料(常规高空和地面观测、气象卫星云图、快速同化循环的数值预报产品等)对雷暴生成、发展和衰减,特别是对强对流天气(包括强冰雹、龙卷、雷暴大风和对流性暴雨)的临近预报,客观算法包括几种应用最广的雷达回波或云图外推算法和强对流天气识别技术。高分辨率数值预报模式的应用包括与雷达回波外推融合延长临近预报时效,与各种观测资料融合得到快速更新的三维格点资料为雷暴和强对流近风暴环境的判断提供重要参考。     

THE LIMITATION OF CLOUD-BASE MASS FLUX IN CUMULUS PARAMETERIZATION AND ITS APPLICATION IN A HIGH-RESOLUTION MODEL

A large area of unrealized precipitation is produced with the standard convective parameterization scheme in a high-resolution model, while subgrid-scale convection that cannot be explicitly resolved is omitted without convective parameterization. A modified version of the convection scheme with limited mass flux at cloud base is introduced into a south-China regional high-resolution model to alleviate these problems. A strong convection case and a weak convection case are selected to analyze the influence of limited cloud-base mass flux on precipitation forecast. The sensitivity of different limitation on mass flux at cloud base is also discussed. It is found that using instability energy closure for Simplified Arakawa- Schubert Scheme will produce better precipitation forecast than the primary closure based on quasi-equilibrium assumption. The influence of the convection scheme is dependent on the upper limit of mass flux at cloud base. The total rain amount is not so sensitive to the limitation of mass flux in the strong convection case as in the weak one. From the comparison of two different methods for limiting the cloud-base mass flux, it is found that shutting down the cumulus parameterization scheme completely when the cloud-base mass flux exceeds a given limitation is more suitable for the forecast of precipitation.     

不同云降水方案对一次登陆台风的降水模拟

利用美国最新发展的新一代中尺度数值预报模式(WRF),研究了不同云降水方案对一次登陆台风的降水模拟问题,结果表明,在网格距适当小的情况下,同时采用积云对流参数化方案和云微物理方案(Kessler方案)时,其降水预报优于只使用积云对流参数化方案时的预报;在台风降水模拟的初期,Kain-Fritsch方案比Betts-Miller方案产生降水更快,更接近实况降水;在台风登陆后随时间的延长,对流降水重要性逐步下降,网格尺度降水逐渐增强。     

Evaluating the impact of lightning data assimilation on mesoscale model simulations of a flash flood inducing storm

Flash floods are associated with highly localized convective storms producing heavy rainfall. Quantitative precipitation forecasting of such storms will potentially benefit from explicit representations of deep moist convection in numerical weather prediction models. However, explicit representation of moist convection is still not viable in operational mesoscale models, which rely on convective parameterizations for issuing short to medium-range forecasts. In this study we evaluate a technique that uses regional Cloud-to-Ground (CG) lightning observations to define areas of deep moist convection in thunderstorms and adjust the model-generated precipitation fields in those regions. The study focuses on a major flash flood inducing storm in central Europe (23 August 2005) that was simulated with the aid of an operational weather forecasting system (POSEIDON system based on Eta/NCEP model). The performance of the technique is assessed using as reference distributed rainfall estimates from a network of radar observations. The results indicate that CG lightning data can offer sufficient information to increase the mesoscale model skill in reproducing local convective precipitation that leads to flash floods. The model error correction is shown to be proportional to the density of lightning occurrence, making the technique potentially suitable for operational forecasting of flash flood inducing thunderstorms.     

日间对流边界层中的非局地动量混合

日间对流边界层最显著的结构特征是在热力作用下所形成的组织化对流。与小尺度湍涡不同的是，组织化对流具有边界层尺度的垂直相干性，可实现垂直贯穿边界层的非局地物质和能量传输。本文针对对流边界层中的动量混合，探究组织化对流对动量输送的贡献。以高精度大涡模拟数据为研究资料，通过傅里叶变换、本征正交分解和经验模态分解3种滤波方法，分离组织化对流和背景湍涡，计算与两者相关的非局地和局地动量通量，发现与组织化对流相关的非局地动量通量是总通量的重要组成部分，并主导混合层中的垂直动量输送。而后，基于协谱和相位谱分析，探究组织化对流的空间结构对动量传输的影响，发现在热力主导的不稳定环境中，单体型环流结构对动量的传输效率较低。而在风切较强的近中性环境中，滚涡型组织化结构可使垂直和水平流向扰动速度的相位差减小，从而提升动量传输效率。研究结果表明，边界层方案需要包含非局地动量通量项，其参数化应考虑整体稳定度对传输效率的影响。     

不同微物理方案对一次梅雨锋暴雨过程模拟的影响

中尺度模式中描述湿物理过程的方案主要有对流参数化方案和云微物理方案,当网格距达到可以分辨积云对流尺度时,云微物理方案对描述云和降水物理过程的作用将变得更为重要.利用GRAPES高分辨率中尺度数值模式对2007年7月7-9日中国梅雨锋暴雨过程进行了数值模拟,从降水量、雷达回波、水成物分布方面结合观测资料,分析了NCFP简...     

Variability of precipitation intensity: sensitivity to treatment of moist convection in an RCM and a GCM

The present study investigates the sensitivity of the frequency distribution of precipitation rates to the closure employed in the penetrative mass flux cumulus parameterization of Zhang and McFarlane in the Canadian regional climate model (CRCM) and in the Canadian Centre for Climate Modelling and Analysis third generation global atmospheric general circulation model (AGCM3). The effects of an alternative prognostic closure for mass flux cumulus parameterization in place of the original diagnostic closure are investigated. A set of experiments is performed in which changes in the frequency distribution of precipitation rates and cloud base mass-flux are examined as a function of the parameters that define each closure scheme. The relationship between the frequency distribution of precipitation and cloud base mass flux is examined and a self-consistent relation is found when the depth of convection is taken into account. Experiments performed with the prognostic closure favor relatively strong cloud base mass-flux and deep penetrative convection with relatively more intense convective precipitation. The mean of the frequency distribution of convective precipitation is larger and the heavier events become more intense. Also, experiments performed with the prognostic closure favor less frequent convective activity. However these changes in the distribution of convective component of precipitation are generally offset by opposite changes in the distribution of the resolved large-scale component of precipitation, resulting in relatively smaller changes in total precipitation. The altered partition of precipitation between convective and large-scale components is found to alter the energy balance and the thermodynamic equilibrium structure of the troposphere. The robustness found in the CRCM results regarding the sensitivity of the frequency distribution of precipitation to changes in the closure of the deep convection parameterization is investigated by performing a similar analysis of AGCM3 simulations. A remarkable similarity of AGCM3 and CRCM results is found suggesting that the closure sensitivity identified in this study is robust.     

边界层参数化方案在“灰色区域”尺度下的适用性评估

随着数值预报模式分辨率的提高,当模式网格距与含能湍涡的长度尺度相当时,模式动力过程可解析一部分湍流运动,而剩余的湍流运动仍需参数化,此时便产生了湍流参数化的“灰色区域”问题。对传统的PBL（Planetary Boundary Layer）方案在“灰色区域”下的适用性评估,是改进PBL方案以使其能够适应分辨率变化的前提和基础。本研究基于干对流边界层的大涡模拟试验,比较了WRF（Weather Research and Forecast Model）模式中四种常用的边界层参数化方案[YSU（Yonsei University）、MYJ（Mellor-Yamada-Janjic）、MYNN2.5（Mellor-Yamada-Nakanishi-Niino Level 2.5）、MYNN3）]在“灰色区域”尺度下的表现。研究表明,混合层内总热通量对所使用的参数化方案和水平分辨率均不敏感。不同参数化方案中次网格与网格通量的比例表现出对水平网格距不同的依赖性。局地PBL方案（MYJ、MYNN2.5）在混合层内的平均位温随网格距减小而增大,次网格通量随网格距减小而减小,较参考湍流场对次网格通量有所低估。YSU方案的非局地项几乎不随水平格距改变而变化,对次网格通量的表征并未表现出较强的分辨率依赖性,且过强的非局地次网格输送使混合层内温度层结呈弱稳定,抑制了可分辨湍流输送,不易于激发次级环流。MYNN3方案的非局地次网格通量（负梯度输送项）随网格距减小而减小,使其对次网格通量的表征具有较好的分辨率依赖性。PBL方案在“灰色区域”尺度下的适用性与具体分辨率有关。以分辨率500 m为例,四种PBL方案中不存在一种最佳方案,能对边界层的热力结构和湍流统计特征均有准确的描述。     

中国夏季模式降水方案的应用分析研究

通过对我国夏季一次连续降水过程的模拟分析。研究了不同云降水方案在中国区域的应用特征。结果指出,使用不同的积云对流参数化方案。模拟产生的积云降水是有差异的。并且由于积云对流参数化方案的不同,也会引起网格尺度降水产生差异。