Performance of Convective Parameterization Schemes in Asia Using RegCM:Simulations in Three Typical Regions for the Period 1998–2002

This study discusses the sensitivity of convective parameterization schemes（CPSs） in the Regional Climate Model（version 4.3）（Reg CM4.3） over East/South Asia. The simulations using different CPSs in Reg CM are compared to discover a suitable scheme for this region, as the performance of different schemes is greatly influenced by region and seasonality. Over Southeast China and the Bay of Bengal, the Grell scheme exhibits the lowest RMSEs of summer precipitation compared to observed data. Moreover, the Emanuel over land and Grell over ocean（ELGO） scheme enhances the simulation, in comparison with any single CPS（Grell/Emanuel） over Western Ghats, Sri Lanka, and Southeast India. Over the Huang–Huai–Hai Plain（3H） and Tibetan Plateau（TP） regions of China, the Tiedtke scheme simulates the more reasonable summer precipitation with high correlation coefficient and comparable amplitude. Especially, it reproduces a minimum convective precipitation bias of 8 mm d^-1and the lowest RMSEs throughout the year over East/South Asia. Furthermore, for seasonal variation of precipitation, the Tiedtke scheme results are closer to the observed data over the 3H and TP regions. However, none of the CPSs is able to simulate the seasonal variation over North Pakistan（NP）. In comparison with previous research, the results of this study support the Grell scheme over South Asia. However, the Tiedtke scheme shows superiority for the 3H, TP and NP regions. The thicker PBL, less surface latent heat flux, the unique ability of deep convection and the entrainment process in the Tiedtke scheme are responsible for reducing the wet bias.

Impact of Convective Parameterization Schemes on the Quality of Rainfall Forecast over Tanzania Using WRF-Model

To describe the evolution of atmospheric processes and rainfall forecast in Tanzania, the Advanced Weather Research and Forecasting (WRF-ARW) model was used. The principal objectives of this study were 1) the understanding of mesoscale WRF model and adapting the model for Tanzania;2) to conduct numerical experiments using WRF model with different convective parameterization schemes (CP’s) and investigate the impact of each scheme on the quality of rainfall forecast;and 3) the investigation of the capability of WRF model to successfully simulate rainfall amount during strong downpour. The impact on the quality of rainfall forecast of six CP’s was investigated. Two rainy seasons, short season “Vuli” from October to December (OND) and long season “Masika” from March to May (MAM) were targeted. The results of numerical experiments showed that for rainfall prediction in Dar es Salaam and (the entire coast of the Indian Ocean), GD scheme performed better during OND and BMJ scheme during MAM. Results also showed that NC scheme should not be used, which is in agreement to the fact that in tropics rainfall is from convective activities. WRF model to some extent performs better in the cases of extreme rainfall.

Parameterization for the Depth of the Entrainment Zone above the Convectively Mixed Layer

It has been noted that when the convective Richardson number Ri* is used to characterize the depth of the entrainment zone, various parameterization schemes can be obtained. This situation is often attributed to the invalidity of parcel theory. However, evidence shows that the convective Richardson number Ri* might be an improper characteristic scaling parameter for the entrainment process. An attempt to use an innovative parameter to parameterize the entrainment-zone thickness has been made in this paper. Based on the examination of the data of water-tank experiments and atmospheric measurements, it is found that the total lapse rate of potential temperature across the entrainment zone is proportional to that of the capping inversion layer. Inserting this relationship into the so-called parcel theory, it thus gives a new parameterization scheme for the depth of the entrainment zone. This scheme includes the lapse rate of the capping inversion layer that plays an important role in the entrainment process. Its physical representation is reasonable. The new scheme gives a better ordering of the data measured in both water-tank and atmosphere as compared with the traditional method using Ri*. These indicate that the parcel theory can describe the entrainment process suitably and that the new parameter is better than Ri*.

An Improvement of the Mass Flux Convection Parameterization Scheme and its Sensitivity Tests for Seasonal Prediction over China

A modified cumulus parameterization scheme, suitable for use in a seasonal forecast model, is presented. This parameterization scheme is an improvement of the mass flux convection scheme developed by Gregory and Rowntree (1989; 1990). This convection scheme uses a 'bulk' cloud model to present an ensemble of convective clouds, and aims to represent shallow, deep, and mid-level convection. At present, this convection scheme is employed in the NCC T63L20 model (National Climate Center, China Meteorological Administration). Simulation results with this scheme have revealed some deficiencies in the scheme, although to some extent, it improves the accuracy of the simulation. In order to alleviate the deficiencies and reflect the effect of cumulus convection in the actual atmosphere, the scheme is modified and improved. The improvements include (i) the full estimation of the effects of the large-scale convergence in the lower layer upon cumulus convection, (ii) the revision of the initial convective mass flux, and (iii) the regulation of convective-scale downdrafts. A comparison of the results obtained by using the original model and the modified one shows that the improvement and modification of the original convection scheme is successful in simulating the precipitation and general circulation field, because the modified scheme provides a good simulation of the main features of seasonal precipitation in China, and an analysis of the anomaly correlation coefficient between the simulation and the observations confirms the improved results.

Comparative Studies of Different Mesoscale Convection Parameterization Schemes in the Simulation of Mei-Yu Front Heavy Rain

The mei-yu front heavy rainstorms occurred over Nanjing on 3 5 and 8 9 July 2003 and were simulated in this paper using the Weather Research and Forecasting Model (WRFv3.1) with various mesoscale convection parameterization schemes (MCPSs). The simulations show that the temporal and spatial evolution and distribution of rainstorms can be modeled; however, there was incongruity between the comparative simulations of four different MCPSs and the observed data. These disparities were exhibited in the simulations of both the 24-hour surface rainfall total and the hourly precipitation rate. Further analysis revealed that the discrepancies of vertical velocity and the convective vorticity vector (CVV) between the four simulations were attributed to the deviation of rainfall values. In addition, the simulations show that the mid-scale convection, particularly the mesoscale convection system (MCS) formation, can be well simulated with the proper mesoscale convection parameterization schemes and may be a crucial factor of the mei-yu front heavy rainstorm. These results suggest that, in an effort to enhance simulation and prediction of heavy rainfall and rainstorms, subsequent studies should focus on the development and improvement of MCPS.

The Sensitivity of Numerical Simulation of the East Asian Monsoon to Different Cumulus Parameterization Schemes

In this paper, a 5-level spectral AGCM is used to examine the sensitivity of simulated East Asian summer monsoon circulation and rainfall to cumulus parameterization schemes. From the simulated results of East Asian monsoon circulations and rainfalls during the summers of 1987 and 1995, it is shown that the Kuo′s convective parameterization scheme is more suitable for the numerical simulation of East Asian summer monsoon rainfall and circulation. This may be due to that the cumulus in the rainfall system is not strong in the East Asian monsoon region.

Validation for a tropical belt version of WRF: sensitivity tests on radiation and cumulus convection parameterizations

Version 3.9 of WRF-ARW is run with a tropical belt configuration for a period from 2012 to 2016 in this study. The domain covers the entire tropics between 45°S and 45°N with a spatial resolution of about 45 km. In order to verify two radiation schemes and four cumulus convection schemes, eight experiments are performed with different combinations of physics parameterization schemes. The results show that eight experiments present reasonable spatial patterns of surface air temperature and precipitation in boreal summer, with the spatial correlation coefficient (COR) between simulated and observed temperature exceeding 0.95, and that between simulated and observed precipitation ranges from 0.65 to 0.82. The four experiments with the RRTMG radiation scheme show a better performance than the other four experiments with the CAM radiation scheme. In the four experiments with the RRTMG radiation scheme, the COR between simulated and observed surface air temperature is about 0.98, and that between simulated and observed precipitation ranges from 0.76 to 0.82. Comparatively, the two experiments using the new Tiedtke cumulus parameterization scheme can simulate better diurnal variation of precipitation in boreal summer than the other six experiments. In particular, for the diurnal cycle of precipitation over land and ocean, the experiment using the RRTMG radiation scheme and the new Tiedtke cumulus convection scheme shows that the peaks of precipitation rate appear at 0400 LST and 1600 LST, in agreement with observation.

A MODIFIED CUMULUS PARAMETERIZATION SCHEME AND ITS APPLICATION IN THE SIMULATIONS OF THE HEAVY PRECIPITATION CASES

According to the characteristics of organized cumulus convective precipitation in China,a cumulus parameterization scheme suitable for describing the organized convective precipitation in East Asia is presented and modified.The Kain-Fristch scheme is chosen as the scheme to be modified based on analyses and comparisons of simulated precipitation in East Asia by several commonly-used mesoscale parameterization schemes.A key dynamic parameter to dynamically control the cumulus parameterization is then proposed to improve the Kain-Fristch scheme.Numerical simulations of a typhoon case and a Mei-yu front rainfall case are carried out with the improved scheme,and the results show that the improved version performs better than the original in simulating the track and intensity of the typhoons,as well as the distribution of Mei-yu front precipitation.

A Comparative Study of the Numerical Simulation of the 1998 Summer Flood in China by Two Kinds of Cumulus Convective Parameterized Methods

The NCC T63L20 model of the National Climate Center, China Meteorological Administration is employed to simulate the 1998 summer flood, which mainly occurred in the region of the Yangtze River and Northeast China. For this study, two kinds of cumulus convection parameterized schemes are employed in this model respectively. The simulations show that the Gregory parameterized scheme, which is still used in the United Kingdom Meteorological Office routine model, simulates more reasonable rainfall amount and distribution compared to the Kuo-type scheme. Moreover, the Gregory scheme better simulates the tendency of general circulation than the Kuo-type scheme. On the whole, the Gregory scheme provides a good simulation of the main features of the seasonal precipitation and general circulation in China, although the simulated result still exhibits some departures from the observations.

区域气候模拟中Betts-Miller对流参数化方案的敏感性试验

利用NCARRegCM2气候模式 ,对Betts Miller对流参数化方案中影响参考态的几个关键性参数 :稳定度参数α、云底和冻结层参考态的饱和气压Δp 和对流调整时间τ进行了敏感性试验 ,分析这些参数的不同取值对区域气候模拟的影响及意义 ,并由此提出适合我国东部地区汛期区域气候模拟的Betts Miller对流参数化方案中较优的关键性参数取值。

Development of an Integrated Vertical-Slantwise Convective Parameterization Scheme and Its Associated Numerical Experiments

An integrated vertical-slantwise convective parameterization scheme, based on the vertical Kuo-Anthes and the slantwise Nordeng convective parameterization schemes, is introduced into the MM5 model. By employing the MM5 model with the proposed scheme, numerical simulations of a snowstorm event that occurred over southern China on 28-29 January 2008 and of Typhoon Haitang （2005） are conducted. The results indicate that during the snowstorm event, the atmosphere was convectively stable in the vertical direction but with conditional symmetric instability （CSI） in the lower troposphere, and when the area of CSI developed and extended to upper levels, strong rising motion occurred and triggered the release of large amount of energy, producing enhanced convective precipitation with the total precipitation much closer to the observation. The development and strengthening of CSI corresponded to changes in the intensity of snowfall, convergence, and ascending motions of air, revealing that CSI was responsible for the initiation and growth of the snowstorm. The results from a 72-h explicit simulation of Typhoon Haitang indicate that CSI occurred mainly at lower levels with a well-defined spiral structure, and it tended to have a larger impact on the intensity of typhoon than on its track. The minimum pressure at the typhoon center for the 72-h runs with the integrated vertical-slantwise convective parameterization scheme was on average 3 hPa （maximum 8 hPa） lower than that from the runs with only the vertical cumulus parameterization scheme. Introducing the influence of CSI into the model has improved the warm core structure at the middle and upper levels of the typhoon, with stronger and persistent upward motions causing increased precipitation, and the latent heat released through convection in turn made the typhoon develop further.

边界层湍流垂直混合强度对局地热对流模拟影响的个例研究

针对弱环境场下局地对流性降水难于准确预报问题,本文以长江下游地区两次局地对流性降水过程为例,通过调整WRF模式中两类边界层参数化方案(YSU和ACM2)的湍流垂直混合强度,探究改善降水预报准确度的一种可行途径。结果表明:在模式默认的垂直混合强度下,YSU方案模拟的对流发展较缓,对流触发时间略晚;ACM2方案则由于垂直混合过强,模拟的对流弱于YSU方案,对流触发时间晚于观测1~2 h。无论是YSU还是ACM2方案,减弱边界层内垂直混合强度能够更准确模拟对流触发及其发展演变。不同垂直混合主要通过影响边界层内位温、水汽混合比、风的垂直分布和能量输送来影响对流过程模拟:减弱垂直混合后,对流前期边界层内更湿冷,风速和垂直风切变增大,同时对流有效位能(Convective Available Potential Energy,CAPE)增加,这些因素利于更早触发对流,模拟的对流强度也更强。

深对流方案闭合条件变化对全球-区域一体化模式降水模拟的影响

针对全球-区域一体化模式(GRIST)在热带对流降水过多的问题,将Zhang-Mcfarlane(ZM)闭合假设中对流在固定时间内消耗全部对流有效位能(CAPE)改为消耗一定比例的对流有效位能,从而减少对流降水。选取对流消耗95%、90%、80%、75%CAPE进行敏感实验,分析闭合假设变化对降水、云及其辐射效应的影响。结果表明,降水对CAPE消耗比例最敏感。不完全消耗CAPE会减少热带降水,主要表现为减小了赤道东太平洋、亚马逊以及印度尼西亚降水正偏差,其中CAPE95和CAPE90的降水模拟结果与观测更为接近。相比于其他实验,CAPE90消除了秋季的双赤道辐合带,夏季的双赤道辐合带也有所缓解。对流消耗CAPE量减少时,热带太平洋弱降水发生的频率减小且强度增强。相比于降水,云及其辐射效应对对流消耗CAPE量无明显敏感性。

GRAPES中尺度模式中不同对流参数化方案模拟对流激发的研究

针对GRAPES中尺度数值模式的三种积云对流参数化方案,对华南2005年6月17～25日的降水过程进行模拟,研究了不同参数化方案中对流激发的时间和空间特征,讨论对流的激发状态和预报降水量的关系以及模式预报降水偏少的可能原因。21日的24小时降水量模拟结果显示,总体上雨带与实况接近,但中尺度暴雨中心位置不同。各方案的初始对流激发的状态及演变存在显著不同,并与各自24小时模拟降水量的分布和雨量紧密相关;同时预报对流激发的时间也不同。综合来看,SAS方案能较好地模拟对流的触发,但激发的降水在实况出现小雨和大雨的时段没有太大的区别。用KFETA方案探讨模拟降水偏少的可能原因之一是,对流层中层垂直速度偏小。

积云对流参数化方案对大气含水量及降水的影响

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所发展的高分辨率全球大气环流谱模式SAMIL,自从发展成R42L26新版本之后,显示出对全球气候基本态的模拟能力,但模式对流层低层以及热带地区整层偏干,且对热带地区的降水模拟存在"双赤道辐合带"这一普遍误差,在赤道地区以及北半球中纬度降水偏少,而在拉丁美洲降水偏多。敏感性试验表明这些误差是相互联系的。通过将700 hPa以下的相对湿度趋近于"观测资料"的试验显示模拟的降水误差减小了。据此,对Tiedtke积云对流参数化方案中的浅对流部分进行修改,增加了浅对流的侧向混合的卷入以及卷出率,并减小了对流方案中的云水-雨水的转换率,将其耦合到模式中。积分结果表明,修改后的方案明显改进了湿度和温度场的模拟,对降水的模拟能力有了很大的提高,基本消除了"双赤道辐合带"现象。

华南中尺度暴雨数值预报的不确定性与集合预报试验

利用非静力MM5模式,分析了不同积云对流参数化方案对华南暖区暴雨数值预报的不确定性影响,进行了中尺度暴雨模式扰动集合预报试验。不同对流参数化方案的对流凝结加热引起不同的局地温度扰动,通过大气内部的热力动力过程,导致垂直速度的差异,进而影响网格尺度和次网格尺度降水时间、地点和强度。后续降水再通过凝结潜热释放形成新的扰动源。不同积云对流参数化方案还可引起扰动源能量传播方式不同,最终使模拟大气的动力和热力结构有差异。针对物理过程的不确定性,使用两种模式扰动方法构造集合预报扰动模式,第一种方法是随机组合不同积云对流参数化方案和边界层方案,第二种方法是扰动Grell积云对流参数化方案中主要参数振幅。集合预报结果表明,第一种方法的集合预报效果优于第二种方法,仅扰动参数振幅值似乎还不足以反映华南暴雨预报的不确定性。单一的确定性预报在暴雨落区和强度方面的可信度不稳定,集合产品能给华南暴雨过程提供更有用价值的指导预报,具有较高的应用价值。

季风槽环境中暴雨中尺度对流系统的分析与数值预报试验

应用地面降水观测资料、卫星云图、雷达回波以及NCEP再分析资料,对华南沿海受季风槽影响下发生的一次持续性暴雨的中尺度对流系统(MCS)进行分析,并探讨采用数值模式对中尺度对流系统降水进行预报的可能性。分析表明,暴雨由多个相继发展的中尺度对流系统造成。在相似环境中,不同中尺度对流系统发展形态和水平尺度有较大差异,最大可组织发展成α中尺度对流复合体(MCC),但一般为β中尺度线状或带状对流系统。对其中发展形态分别表现为椭圆形中尺度对流复合体(MCS-2)和带状β中尺度对流系统(MCS-4)的对比分析发现,对流的起始发展均发生在夜间,与季风槽中低空急流的南风脉动有良好对应关系。基于临近探空资料的诊断发现,被认为对中尺度对流系统组织发展有指示作用的关键物理量如对流有效位能(CAPE)和风垂直切变难以区分不同中尺度对流系统的发展形态和趋势,探空资料的代表性将影响诸如"配料法"等暴雨客观预报方法的建立和应用。利用华南区域中心GRAPES(GRAPES_GZ)数值模式对两个中尺度对流系统进行的模拟预报结果表明,采用数值模式对中尺度对流系统降水进行显式预报已成为可能。比较而言,3 km水平分辨率模式可以更好地预报出暴雨的发生,但结果对是否调用对流参数化(CP)方案敏感。尽管不依靠对流参数化方案模式能够较好地预报出中尺度对流系统初始降水的发生,但会过度预报发展成熟后的降水。模式中如何描述中尺度对流系统对流的组织发展机制、如何处理对流参数化方案的"灰色区分辨率"问题需要仔细考虑。

中尺度模式中各种积云参数化方案的对比试验

利用非静力中尺度MM5数值模式,选择Anthes-Kuo、Grell、Kain-Fritsch和Betts-M iller 4种积云对流参数化方案,对2003年7月25—26日台风登陆减弱为中尺度低压系统后影响云南产生的强降水过程进行模拟试验,重点分析了4种参数化方案模拟的降水分布、降水强度和中尺度低压的流场特征。结果表明:4种积云参数化方案对这次强降水过程均有一定的模拟能力,能够很好地模拟过程强降水中心的位置,但Grell、Kain-Fritsch和Betts-M iller 3种方案模拟的大雨范围比实况大雨范围明显偏小,Betts-M iller方案模拟的降水强度比实况偏大,Anthes-Kuo方案的模拟结果与实况比较接近,它不仅能够很好地模拟强降水过程的降水区范围、降水强度和降水中心位置,还能很好地再现低压环流系统的一些中尺度特征。

积云参数化方案研究的现状

对现有的广泛使用的１１种积云参数化方案进行综述，侧重于它们的热力学特点和它们的优缺点，并对目前积云参数化的研究进行讨论。

对流边界层顶部夹卷速度参数化的分析研究

应用 1979年 7月美国CIRCE试验边界层观测资料对两个夹卷速度参数化方案进行了对比验证 ,结果表明 :基于简单夹卷层结构模型的参数化方案we/w =A·Ri- 1 只适用于自由对流边界层 ,当对流边界层中存在较强机械湍流时 ,该方案变得不再适用 ;而基于“侵吞模型”的参数化方案we/w =B·θ /(Γzi)则不仅适用于自由对流边界层 ,对于含有较强机械湍流的对流边界层也能较好地符合 ,因而具有较强的适用性 .当对流边界层受机械湍流和浮力湍流共同驱动时 ,仅以浮力对流速度w 表征混合层垂直湍流速度尺度是不恰当的 ,还应该包含机械湍流的贡献 .对比验证表明 ,这一改进非常有效 ,不仅使物理意义更加合理 。