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1.
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 eoetfieient between the simulation and the observations confirms the improved results.  相似文献   

2.
l. IntroductiOnThe parameterizat1on of atmospheric convection is one of the most challenging issues inglobal climate modeling. Since convection interacts strongly with clouds and the large--scalecirculation, its representation in GCMs has a tremendous impact on the slmulation of theglobal climate and its variations. For example, in the National Center for Atmospheric Re-search (NCAR) Community Climate Model Version 2 (CCM2), excessive surface 1atent heatflux in the tropics was simulate…  相似文献   

3.

This paper presents the first multi-model ensemble of 10-year, “convection-permitting” kilometer-scale regional climate model (RCM) scenario simulations downscaled from selected CMIP5 GCM projections for historical and end of century time slices. The technique is to first downscale the CMIP5 GCM projections to an intermediate 12–15 km resolution grid using RCMs, and then use these fields to downscale further to the kilometer scale. The aim of the paper is to provide an overview of the representation of the precipitation characteristics and their projected changes over the greater Alpine domain within a Coordinated Regional Climate Downscaling Experiment Flagship Pilot Study and the European Climate Prediction system project, tasked with investigating convective processes at the kilometer scale. An ensemble of 12 simulations performed by different research groups around Europe is analyzed. The simulations are evaluated through comparison with high resolution observations while the complementary ensemble of 12 km resolution driving models is used as a benchmark to evaluate the added value of the convection-permitting ensemble. The results show that the kilometer-scale ensemble is able to improve the representation of fine scale details of mean daily, wet-day/hour frequency, wet-day/hour intensity and heavy precipitation on a seasonal scale, reducing uncertainty over some regions. It also improves the representation of the summer diurnal cycle, showing more realistic onset and peak of convection. The kilometer-scale ensemble refines and enhances the projected patterns of change from the coarser resolution simulations and even modifies the sign of the precipitation intensity change and heavy precipitation over some regions. The convection permitting simulations also show larger changes for all indices over the diurnal cycle, also suggesting a change in the duration of convection over some regions. A larger positive change of frequency of heavy to severe precipitation is found. The results are encouraging towards the use of convection-permitting model ensembles to produce robust assessments of the local impacts of future climate change.

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4.
Based on a decade of research on cloud processes, a new version of the LMDZ atmospheric general circulation model has been developed that corresponds to a complete recasting of the parameterization of turbulence, convection and clouds. This LMDZ5B version includes a mass-flux representation of the thermal plumes or rolls of the convective boundary layer, coupled to a bi-Gaussian statistical cloud scheme, as well as a parameterization of the cold pools generated below cumulonimbus by re-evaporation of convective precipitation. The triggering and closure of deep convection are now controlled by lifting processes in the sub-cloud layer. An available lifting energy and lifting power are provided both by the thermal plumes and by the spread of cold pools. The individual parameterizations were carefully validated against the results of explicit high resolution simulations. Here we present the work done to go from those new concepts and developments to a full 3D atmospheric model, used in particular for climate change projections with the IPSL-CM5B coupled model. Based on a series of sensitivity experiments, we document the differences with the previous LMDZ5A version distinguishing the role of parameterization changes from that of model tuning. Improvements found previously in single-column simulations of case studies are confirmed in the 3D model: (1) the convective boundary layer and cumulus clouds are better represented and (2) the diurnal cycle of convective rainfall over continents is delayed by several hours, solving a longstanding problem in climate modeling. The variability of tropical rainfall is also larger in LMDZ5B at intraseasonal time-scales. Significant biases of the LMDZ5A model however remain, or are even sometimes amplified. The paper emphasizes the importance of parameterization improvements and model tuning in the frame of climate change studies as well as the new paradigm that represents the improvement of 3D climate models under the control of single-column case studies simulations.  相似文献   

5.
A one-dimensional penetrative plume model has been constructed to parameterize the process of deep convection in ocean general circulation models (OGCMs). This research is motivated by the need for OGCMs to better model the production of deep and intermediate water masses. The parameterization scheme takes the temperature and salinity profiles of OGCM grid boxes and simulates the subgrid-scale effects of convection using a one-dimensional parcel model. The model moves water parcels from the surface layer down to their level of neutral buoyancy, simulating the effect of convective plumes. While in transit, the plumes exchange water with the surrounding environment; however, the bulk of the plume water mass is deposited at e level of neutral buoyancy. Weak upwelling around the plumes is included to maintain an overall mass balance. The process continues until the negative buoyant energy of the one-dimensional vertical column is minimized. The parameterized plume entrainment rate, which plays a central role in the parameterization, is calculated using modified equations based on the physics of entraining buoyant plumes. This scheme differs from the convective adjustment techniques currently used in OGCMs, because the parcels penetrate downward with the appropriate degree of mixing until they reach their level of neutral stability.  相似文献   

6.
Based on the non-hydrostatic version of Mesoscale Model version 5 (MMS) and the data sets of four heavy rainfall scenarios occurring in August 2001 in China,this paper investigates the impacts of diabatic physical processes on predictions of dynamic and thermodynamic elements of heavy rainfall in China,deeply analyzes the effects of convective schemes on mesoscale heavy rain simulations and discusses the feasibility of using model physics perturbations in ensemble simulation of heavy rain,The results show that diabatic physical processes have little impact on the short-range prediction of geopotential height.However,planetary boundary layer schemes and convective schemes have significant influence on moisture divergence flux,vertical velocity,and unstable stratification,which are the three basic conditions of torrential rain.The forecast deviations in different convection schemes increase rapidly in the first 12 h time periods of simulation and the deviation structures are well correlated to that of sub-grid scale rainfall,while in the later periods of simulation with less correlation.Diabatie physical processes influence the structure and evolution of the simulations.For the rain storm events with a homogeneous thermal environmental condition in China.the numerical model ensembles could be created by perturbing the planetary parameterization scheme and convective parameterization.  相似文献   

7.
In the context of non-hydrostatic MM5 version we have explored the impact of convective parameterization schemes on uncertainty in mesoscale numerical prediction of South China heavy rain and mesoscale heavy rainfall short-range ensemble simulation by using two kinds of physics perturbation methods through a heavy rain case occurring on June 8, 1998 in Guangdong and Fujian Provinces. The results show the physical process of impacts of convective schemes on heavy rainfall is that different latent heat of convective condensation produced by different convective schemes can make local temperature perturbation, leading to the difference of local vertical speed by the intrinsic dynamic and thermodynamic processes of atmosphere,and therefore, making difference of the timing, locations and strength of mesh scale and subgrid scale precipitation later. New precipitations become the new source of latent heat and temperature perturbation,which finally make the dynamic and thermodynamic structures different in the simulations. Two kinds of methods are used to construct different model version stochastically. The first one is using different convective parameterization and planetary boundary layer schemes, the second is adjusting different parameters of convective trigger functions in Grell scheme. The results indicate that the first ensemble simulations can provide more uncertainty information of location and strength of heavy rainfall than the second. The single determinate predictions of heavy rain are unstable; physics ensemble predictions can reflect the uncertainty of heavy rain, provide more useful guidance and have higher application value.Physics ensembles suggest that model errors should be taken into consideration in the heavy rainfall ensembles. Although the method of using different parameters in Grell scheme could not produce good results, how to construct the perturbation model or adjust the parameter in one scheme according to the physical meaning of the parameter still needs further investigation. The limitation of the current study is that it is based on a single case and more cases will be addressed in the future researches.  相似文献   

8.
Although the parcel method and the convective available potential energy (CAPE) are widely used to predict the strength and height of convection, they ignore the pressure perturbation and fail to explain strong updrafts observed in tropical cyclones and hurricanes without CAPE, or deep, strong warm downdrafts in hurricane eye-walls, tropopause folds, or downslope winds leeward of mountains. Those phenomena can be explained by the Bernoulli equation that conserves the sum of kinetic energy, potential energy and enthalpy in an inviscid fluid. Our analytic and numerical results also show how, in a moist stable environment without CAPE, updrafts and clouds can develop against negative buoyancy. Deep warm downdrafts can also form in cloud-free regions or areas without significant evaporative cooling from precipitation.  相似文献   

9.
物理过程参数化方案对中尺度暴雨数值模拟影响的研究   总被引：48，自引：5，他引：43  
陈静  薛纪善  颜宏 《气象学报》2003,61(2):203-218
利用中尺度非静力MM 5模式和中国 2 0 0 1年 8月的 4个暴雨个例 ,研究了非绝热物理过程对中国暴雨动力和热力场预报的影响 ,深入分析了对流参数化方案在中尺度暴雨预报中的作用 ,讨论了利用模式扰动方法开展中国暴雨集合预报的可行性。结果表明 ,在短期数值预报中 ,非绝热物理过程对高度场预报影响较小 ,但边界层方案和对流参数化方案对产生暴雨的 3个基本条件即水汽通量散度、垂直速度、不稳定层结的影响很明显。不同对流参数化方案所预报的中尺度热力、动力场离差的结构特征与所预报降水的离差特征相似 ,且主要是在模式积分初期迅速增加 ,其后即趋于稳定。对中国热力场较均匀的暴雨过程 ,可以通过扰动模式的边界层和对流参数化方案 ,构造集合预报模式  相似文献   

10.
A good representation of the interaction between the planetary boundary layer(PBL) and the surface layer(SL) in numerical models is of great importance for the prediction of the initiation and development of convection. This study examined an ensemble that consists of the available suites of PBL and SL parameterizations based on a torrential rainfall event over south China. The sensitivity of the simulations was investigated against objective measurements using multiple PBL and SL parameterization schemes. The main causes of the bias from different parameterization schemes were further analysed by comparing the good and bad ensemble members. The results showed that good members tended to underestimate the rainfall amount but presented a decent evolution of mesoscale convective systems that were responsible for the torrential rainfall. Using the total energy mass flux(TEMF) scheme, the bad members overestimated the amount and spatial coverage of rainfall. The failure of the bad member was due to a spurious convection initiation(CI) resulting from the overestimated high-θe elevated air. The spurious CI developed and expanded rapidly, causing intensive and extensive rainfall over south China. Consistent with previous studies, the TEMF scheme tends to produce a warmer and moister PBL environment. The detailed sensitivity analysis of this case may provide reference for the operational forecast of rainfall over south China using multiple PBL and SL parameterizations.  相似文献   

11.
气象预报是影响大气重污染预报精度的关键所在。针对2016年12月16～21日北京市一次重污染过程,开展了中尺度气象模式WRF的参数化方案配置敏感性试验。对微物理过程、长波辐射过程、短波辐射过程、陆面过程、边界层过程、近地面过程以及积云对流参数化过程进行组合优选,共设计51组参数化方案组合,分析不同模拟方案下北京市8个气象站点温度、相对湿度、10 m风速的模拟精度及其敏感性。试验结果表明:温度模拟对长波过程参数化方案最为敏感,集合离散度达2.4～7.4°C,再次是短波过程参数化方案;相对湿度模拟也对长波过程参数化方案最敏感,再次是陆面过程;风速模拟对不同过程参数化方案的敏感性程度差异不大。通过模拟结果与观测的统计对比,优选出模拟误差最小的方案组合为Lin微物理方案、RRTMG长波方案、RRTMG短波方案、Tiedtke积云对流方案、Noah陆面方案、MYNN 3rd边界层方案和MYNN近地面方案,并将其与集合平均、基准方案进行对比。对于集合平均来说,其温度模拟与观测相关系数为0.69,高于基准方案,其模拟偏差与均方根误差比基准方案低25%和11%;集合平均的相对湿度和风速模拟相比基准方案变化较小。与集合平均相比,优选方案能同时改进温度、相对湿度和风速模拟,使温度模拟偏差和均方根误差比基准方案下降35%和17%,使相对湿度模拟偏差和均方根误差下降43%和13%,使风速模拟偏差和均方根误差下降33%和24%。以上结果表明,参数化方案的敏感性试验和优选能显著减小重污染期间气象要素的模拟误差,重污染预报改进需重点关注参数化方案模拟上的不确定性。本研究也发现MYNN3rd边界层方案在这次重污染过程的气象要素模拟上具有良好性能,可为未来重污染预报改进提供参考。  相似文献   

12.
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).  相似文献   

13.
Tongwen Wu 《Climate Dynamics》2012,38(3-4):725-744
A simple mass-flux cumulus parameterization scheme suitable for large-scale atmospheric models is presented. The scheme is based on a bulk-cloud approach and has the following properties: (1) Deep convection is launched at the level of maximum moist static energy above the top of the boundary layer. It is triggered if there is positive convective available potential energy (CAPE) and relative humidity of the air at the lifting level of convection cloud is greater than 75%; (2) Convective updrafts for mass, dry static energy, moisture, cloud liquid water and momentum are parameterized by a one-dimensional entrainment/detrainment bulk-cloud model. The lateral entrainment of the environmental air into the unstable ascending parcel before it rises to the lifting condensation level is considered. The entrainment/detrainment amount for the updraft cloud parcel is separately determined according to the increase/decrease of updraft parcel mass with altitude, and the mass change for the adiabatic ascent cloud parcel with altitude is derived from a total energy conservation equation of the whole adiabatic system in which involves the updraft cloud parcel and the environment; (3) The convective downdraft is assumed saturated and originated from the level of minimum environmental saturated equivalent potential temperature within the updraft cloud; (4) The mass flux at the base of convective cloud is determined by a closure scheme suggested by Zhang (J Geophys Res 107(D14), doi:10.1029/2001JD001005, 2002) in which the increase/decrease of CAPE due to changes of the thermodynamic states in the free troposphere resulting from convection approximately balances the decrease/increase resulting from large-scale processes. Evaluation of the proposed convection scheme is performed by using a single column model (SCM) forced by the Atmospheric Radiation Measurement Program’s (ARM) summer 1995 and 1997 Intensive Observing Period (IOP) observations, and field observations from the Global Atmospheric Research Program’s Atlantic Tropical Experiment (GATE) and the Tropical Ocean and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE). The SCM can generally capture the convective events and produce a realistic timing of most events of intense precipitation although there are some biases in the strength of simulated precipitation.  相似文献   

14.
The NCEP Climate Forecast System (CFS) with the relaxed Arakawa Schubert (RAS, hereafter referred to as CTRL) convection scheme of Moorthi and Suarez exhibits better performance in representing boreal summer tropical intraseasonal variability as compared with a simulation using simplified Arakawa–Schubert scheme. The intraseasonal moist static energy (MSE) budget is analyzed in this version of the CFS model (CTRL), which produces realistic eastward and northward propagation characteristics. The moist and thermodynamic processes involved in the maintenance and propagation of the poleward moving intraseasonal oscillation (ISO) disturbances are examined here. Budget diagnostics show that horizontal MSE advection is the principal component of the budget, contributing to the poleward movement of the convection. The injection of MSE moistens the atmosphere north of the convective area causing the poleward movement of convection by destabilization of the atmosphere. The moistening process is mainly contributed by the climatological wind acting on the anomalous moisture gradient as confirmed from the examination of moisture advection equation. While surface enthalpy fluxes (consisting of radiative and surface turbulent heat fluxes) maintain the ISO anomalies, they oppose the MSE tendency due to horizontal advection thus regulating the poleward propagation characteristics. In addition, the model results show that wind–evaporation feedback dominates over cloud–radiation feedback for ISO propagation; this is in contrast to our estimates using the newly available European Centre for Medium Range Weather Forecasts Interim reanalysis. Sensitivity experiments suggest that intraseasonal variability in the CFS model with the RAS scheme is highly sensitive to the parameterization of both the shallow convection and the convective rain evaporation and downdrafts. Removal of these components adversely affects the propagation characteristics and greatly reduces the amplitude of intraseasonal variability. Our results support the primary importance of the moisture preconditioning ahead of the ISO and the physical relationship between moisture and precipitation. For realistic ISO simulations, models need to represent these features appropriately.  相似文献   

15.
1. IntroductionHeavy rain is a kind of severe natural calamitythat influences South China. After decades of years oftests and theoretical exploration by Chinese scientists,significant progresses have been achieved in its predic-tion and basic theoretical studies (Huang, 1986; Xue,1999; Zhou et al., 2003). Currently, the mesoscale nu-merical model has already been employed as one of themajor tools in the prediction and research on heavyrain in South China, promoting considerably the ac-curac…  相似文献   

16.
文中在综合比较各类积云对流参数化方案优缺点的基础上 ,主要参考陈伯民等修正的ECMWF质量通量积云参数化方案 ,对其进行简化和修改 ,发展了一个质量通量积云对流参数化方案 ,文中表示为 MFS(Mass Flux Scheme)。MFS是一种综合型的方案 ,既考虑了大尺度水汽辐合的重要性 ,又考虑了积云中的上升运动、下沉运动、环境中的补偿下沉运动 ,以及卷入、卷出和蒸发等 ,用总体云模式来描述积云与环境的相互作用 ,同时考虑了深对流和浅对流。将 MFS植入 NCAR区域气候模式 Reg CM2中 ,对 1 991年 5～ 7月江淮地区特大降水过程的夏季风气候特征和变化进行了模拟 ,并与 NCAR Reg CM2选用 Kuo方案 ,在同样初、边值条件和其它物理过程选择下的模拟结果进行了对比分析。分析结果表明 ,植入 MFS后的模式能够模拟这次极端的降水气候事件。在某些方面 ,如地表气温 ,降水的模拟上 ,植入 MFS后的模式的模拟结果要比原模式的结果更合理  相似文献   

17.
This paper describes the effects of convective adjustment time scale (τ) on the simulation of tropical climate. The NCAR-Community Atmosphere Model version 3 (CAM3) has been used for this study. In the default configuration of the model, the prescribed value of τ, a characteristic time scale with which convective available potential energy (CAPE) is removed at an exponential rate by convection, is assumed to be 1?h. However, some recent observational findings suggest that, it is larger by around one order of magnitude. In order to investigate the dependence of tropical climate simulation to this time scale, we conducted two simulations, one with a time scale of 1?h (CTRL) and another with 8?h (EXPT), and examined the differences in simulated climate. For this, we analyzed both the mean as well as transient features, viz., seasonal mean quantities, equatorial waves, and meridional migration of convective disturbances. The spatial distributions of seasonal mean precipitation are found to be better in EXPT. The spatial correlation coefficients of CTRL and EXPT with the observations are 0.79 and 0.83, respectively, for northern hemisphere winter. Similarly, for northern hemisphere summer, the values are 0.67 and 0.79, respectively. In addition, there is also an improvement in the simulation of equatorial waves, specifically, the Kelvin waves, Madden–Julian oscillation, and n?=?1 equatorial Rossby waves become more realistic in EXPT. The characteristics of meridional migration of convective activity over tropics also become more reasonable in EXPT. Thus, it is found that there is a clear improvement in some of the key aspects of the simulated tropical climate with the revised convective adjustment time scale.  相似文献   

18.
Abstract

The influences of surface fluxes and convective precipitation are investigated for two 36‐h periods of cyclogenesis over the northeastern Pacific Ocean. Three methods are tested of specifying the fraction of moisture supply that produces convective precipitation in a modified form of Kuo's (1974) parametrization scheme using an 8‐level primitive equations model.

When convection is included, precipitation amounts are greater and the cyclone deepening is better predicted than when convection is not included. Predicted cyclogenesis is very sensitive to sea temperature. As the low moves over warmer water, the effect of sensible heating is to increase the moisture convergence in the atmospheric boundary layer. This increases the precipitation rates and accelerates deepening. It is concluded that the CISK mechanism plays an important role in extratropical cyclogenesis.  相似文献   

19.
Abstract

The seasonal cycle of the GLAS/U of Maryland GCM is analysed in terms of the behaviour of the monthly and seasonal mean fields and the structure of the annual harmonic. (The stationary and transient eddies are treated in a companion paper.)

Both polar regions at upper levels are much too cold in the annual mean, leading to excessive zonal winds above 200 mb. The problem is present in all seasons, but is most severe in local winter. A compensating belt of warm temperatures at lower latitudes is found. It is argued that the inclusion of gravity wave drag is not necessarily the solution to this problem.

The simulated annual harmonics of Northern Hemisphere sea‐level pressure and 200‐mb heights are realistically intense over the eastern continents and weak over the eastern oceans. Problems in the simulation include the anomalously deep Aleutian low and the low values of the height over Europe, both occurring in winter.

The simulation of the annual harmonic in sea‐level pressure and 200‐mb heights in the Southern Hemisphere is realistic. The GCM fails to show the observed amplitude of the annual harmonic in 200‐mb temperature over Antarctica.

The GCM precipitation is too intense over land, particularly in summer. It is suggested that the problem is related to the parametrizations of moist convection and the boundary layer. The seasonal patterns of precipitation over the western tropical Pacific are generally realistic.

There is no evidence that the GCM systematically underestimates momentum flux convergence.  相似文献   

20.
Turbulent convection forced by a surface heat flux into a stably stratified region is a feature of both the atmospheric and oceanic planetary boundary layers. Of particular interest is the interface between the convective layer and the stable stratification, where the entrainment of fluid into the convective layer by penetrating plumes may lead to a reverse buoyancy flux, and an enhancement of the stable stratification. Whereas in the atmosphere the influence of rotation on this penetrative convection is negligible, oceanic convection may be subjected to lower Rossby numbers and hence greater rotational influence. To isolate the effects of rotation, we present three numerical solutions for turbulent penetrative convection, characterised by different rotation rates, with all other parameters being held constant. Our results indicate that at lower Rossby numbers the lateral scale of the plumes is reduced, whereas the vertical vorticity of the plumes is much enhanced. Vertical transports of buoyancy and kinetic energy across the convective layer are reduced, leading to less efficient penetration at the interface with the stratified layer, and hence less reverse buoyancy flux in this region.  相似文献   

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