Analytic solutions to long's model: A comparison of nonhydrostatic and hydrostatic cases

Summary In this paper, analytic solutions of the nonhydrostatic and hydrostatic forms of Long's model were obtained under two different sets of vertical boundary conditions: The first uses a sumusoidal obstacle at the lower boundary and a rigid-lid top for the upper boundary. The second set applies an isolated obstacle of the Witch of Agnesi type at the lower boundary, while still using a rigid lid at the top. Following the solution evaluations, comparisons between the nonhydrostatic and hydrostatic solutions were processed in order to describe several influences introduced by using the hydrostatic assumption in this model.Through comparisons we have found that, in the case of a sinusoidal lower boundary condition, the hydrostatic solution is obtained as the zero mode of the nonhydrostatic solution. The influence of the hydrostatic assumption on the model solution is trivial in this case. When an isolated lower boundary condition is applied, however, the solutions illustrate dramatic differences, showing the significance of the effect of hydrostatic assumption on this model's solutions. These effects vary considerably with the model parameters as well. The comparison results also reveal that the realization of the hydrostatic assumption in this model's solutions is accomplished through the vertical boundary conditions used in the model evaluations.With 5 Figures     

A nonhydrostatic model for simulation of airflow over mesoscale bell-shaped ridges

This paper describes a nonhydrostatic and incompressible mesoscale model formulation using a terrain-following coordinate system. A tensor transformation procedure is used to derive a diagnostic equation for the nonhydrostatic pressure field. The model features a simplified second-order turbulence closure scheme. The two-dimensional version of the nonhydrostatic model, as well as the corresponding hydrostatic model, are applied to simulate stably stratified airflow over mesoscale bell-shaped mountain ridges. The results show that the nonhydrostatic model is capable of simulating nonhydrostatic dynamics of mesoscale lee wave systems such as the trapped wave phenomenon.     

Stochastic representation of dynamic model tendency : Formulation and preliminary results

Stochastic representation of forecast uncertainties has been taken into account to improve dynamical seasonal prediction. In this study, perturbing the dynamic tendency by a random number is introduced to account for inherent uncertainties associated with computational representations of the underlying partial differential equations that govern the atmospheric motion. Compared to the traditional approach to perturb the physical tendency, the sensitivity of fluctuations in forecast variables to the magnitude of random forcing is found to be greater in the case of perturbing the dynamical tendency. Realizing that the major advantage of stochastic tendency in traditional approaches lies in the increase in ensemble spread, our approach manifests a greater potential in the field of dynamical ensemble prediction. An evaluation of a simulated climate for a boreal summer demonstrates a significant enhancement in forecast skill in terms of the large-scale features and precipitation, when both the dynamical and physical tendencies are simultaneously perturbed. This finding implies that model uncertainties could be addressed in terms of not only the physical parameterization but also the dynamical portion that used to be regarded as deterministically solved.     

Development and Test of Hydrostatic Extraction Scheme in Spectral Model

The introduction of “hydrostatic extraction” scheme, or “standard stratification approximation”, into spectral model gained some advantages compared with commonly used schemes. However, computational instability may oc-cur for high vertical resolution versions if the stratification parameter C0 taken as a constant. In this paper, the pos-sible cause leading to the instability is discussed and an improved scheme presented where C0 is generalized to be a function of both height and latitudes. Hence the reference atmosphere gets closer to the real atmosphere and the tem-perature deviation field to be expanded becomes smoother everywhere. Test by real case forecasts shows good computational stability of the new scheme and better prediction performance than usual schemes of spectral model.     

Formulation of the thermal internal boundary layer in a mesoscale model

Mesoscale models using a non-local K-scheme for parameterization of boundary-layer processes require an estimate of the planetary boundary layer (PBL) height z _i at all times. In this paper, two-dimensional sea-breeze experiments are carried out to evaluate three different formulations for the advective contribution in the z _i prognostic equation of Deardorff (1974).Poor representation of the thermal internal boundary layer in the sea breeze is obtained when z _i is advected by the wind at level z _i . However, significantly better results are produced if the mean PBL wind is used for the advecting velocity, or if z _i is determined simply by checking for the first sufficiently stable layer above the ground.A Lagrangian particle model is used to demonstrate the effect of each formulation on plume dispersion by the sea breeze.     

Synoptic and mesoscale study of a severe convective outbreak with the nonhydrostatic Global Environmental Multiscale (GEM) model

Summary ?A nonhydrostatic 4-km version of the Global Environmental Multiscale (GEM) model, with detailed microphysics included, was used to forecast the initiation, development, and structure of a tornado-producing supercell storm that occurred near Pine Lake (Alberta, Canada) on 15 July 2000. Examination of observations and comparison with conceptual models indicate that this storm is a good example of supercell storms that regularly produce summertime severe weather over Alberta. It was found that the high-resolution model was able to reproduce the early initiation of convective activity along the Rocky Mountains foothills, as well as the rapid northeastward propagation towards the Pine Lake area and the subsequent intensification into a supercell storm. The general structures of the forecasted convective system correspond well with conceptual representations of such events. Overall, this high-resolution forecast of the Pine Lake supercell storm was a significant improvement over the current operational version of the GEM model (24 km), which was not able to intensify the foothills’ convection into a supercell storm. Finally, it was found that the nonhydrostatic version of the model produces better trajectory and propagation speed of the convective system, as compared with the hydrostatic one. Received March 20, 2001; revised August 24, 2001     

A high resolution nonhydrostatic tropical atmospheric model and its performance

A high resolution nonhydrostatic tropical atmospheric model is developed by using a ready-made regional atmospheric modeling system. The motivation is to investigate the convective activities associated with the tropical intraseasonal oscillation (ISO) through a cloud resolving calculation. Due to limitations in computing resources, a 2000 kmx2000 km region covering the forefront of an ISO-related westerly is selected as the model domain, in which a cloud-resolving integration with a 5-km horizontal resolution is conducted. The results indicate the importance of stratus-cumulus interactions in the organization of the cloud clusters embedded in the ISO. In addition, comparative integrations with 2-km and 5-km grid sizes are conducted, which suggest no distinctive differences between the two cases although some finer structures of convections are discernible in the 2-km case. The significance of this study resides in supplying a powerful tool for investigating tropical cloud activities without the controversy of cloud parameterizations. The parallel computing method applied in this model allows sufficient usage of computer memory, which is different from the usual method used when parallelizing regional model. Further simulation for the global tropics with a resolution around 5 km is being prepared.     

高分辨中尺度雷电预报模式GRAPES_LM的建立及其初步应用试验

为了探索雷电天气的数值预报新方法,在高分辨GRAPES_Meso中尺度预报模式中添加了各种水成物电荷密度预报方程组,同时从传统基于云模式的三维雷电模式中剥离出起电过程和放电过程参数化方案,作为中尺度模式的一种物理过程参数化方案嵌套进中尺度模式,使得整套中尺度模式包含了大气动力、雷电过程及各种物理过程,建立了大气动力-物理过程和雷电过程一体化的数值预报模式GRAPES_LM,建立的GRAPES_LM模式包含了多种复杂的微物理过程参数化方案,其中,包括中国气象科学研究院开发研制的双参数对流云参数化方案;包含了霰/雹与冰晶/雪晶碰撞回弹的非感应起电、霰/雹与云滴碰撞的感应起电及霰碰撞大云滴造成冰晶繁生的次生冰晶起电3种起电参数化方案以及最新的随机双向放电参数化。并利用美国CCOPE观测试验中的个例观测资料(1981年7月19日),进行了初步应用模拟试验,对模式的雷电数值预报的可行性进行了验证分析。结果表明,高分辨率GRAPES_LM模式对1981年7月19日的雷电过程个例做出了成功的模拟预报,较好地给出了雷暴云的动力、微物理和电过程的时空演变,雷电天气数值预报新方法的可行性得到了初步验证。     

A new general circulation model: formulation and preliminary results in a single- and multi-processor environment

This article describes a new general circulation model (GCM) developed jointly by The University of New South Wales (UNSW) and the University of Hamburg. The model is versatile in that it can be run as a medium-range (1 to 15 days) global numerical weather prediction (NWP) model; as an extended range (15 to 30 days) NWP model; and as a GCM for periods extending from seasons, through annual and decadal periods, and beyond. The model can be coupled with ocean models that vary in complexity from simple "swamp" oceans to complex ocean GCMs. The atmospheric GCM also has a number of novel features, particularly in the numerical integration scheme which is a high-order, mass-conserving, semi-implicit semi-Lagrangian scheme, thereby removing the stability restriction on the time-step and allowing efficient long-term integrations. The emphasis here will be on demonstrating that the new model performs effectively on the usual measures of skill (statistics such as mean errors, root-mean-square errors and anomaly correlations) in several standard applications upon which new models usually are assessed. These applications include medium range weather forecasts out to 10 days on a daily basis over a one year period; a limited 10-year simulation climatology, prediction of atmospheric anomalies using SST anomalies in an El Nino year; and an alternative two-way approach to regional modelling (the "down-scaling problem") made possible because the unconditional stability of the semi-implicit, semi-Lagrangian formulation permits large variations in grid spacing without changing the time step size. Finally, the model is run on a variety of parallel computing platforms and it is shown that near-linear speed-up can be attained. This is significant for both medium range NWP and very long-term GCM integrations. Received: 28 February 1996 / Accepted: 30 July 1996     

The effects of precipitation schemes and horizontal resolution on the major rainband in typhoon Flo (1990) predicted by the MRI mesoscale nonhydrostatic model

Summary ?This paper describes a numerical study of the major spiral rainband in typhoon Flo (1990) using the Meteorological Research Institute Mesoscale Nonhydrostatic Model (MRI-NHM). The effects of precipitation schemes and horizontal resolution on the representation of the simulated rainband are discussed. Dynamic and thermodynamic structures of the simulated major rainband to the north of the storm center are well represented in the model with a 5 km horizontal resolution. The structures are consistent with observational results reported for other tropical cyclones. Among the realistic features are: a cold pool and convergence on the inner side of the band; convergence above low-level inflow layers; and the outward slope of the updraft with height. The band is caused by the motion of the storm through its surroundings where horizontal wind has vertical shear. The simulation of the structure and precipitation pattern associated with the major rainband depends on the precipitation scheme rather than the horizontal resolution. The band appears more realistic when using explicit cloud microphysics as a precipitation scheme, rather than moist convective adjustment. This result is attributable to the difference in scheme triggering. In the simulation with moist convective adjustment, the elimination of vertical instability in low-level atmosphere is excessive, suppressing band formation. The overall structure of the band is also more realistic in the simulation using explicit cloud microphysics, because a cold pool exists in the lower layers and the vertical axis of upward flow tilts outward. This result suggests that prediction will partly depend on variables associated with cloud microphysics, such as the mixing ratio of cloud water. The horizontal grid distance, which varied between 5 and 20 km, quantitatively influenced the rainfall amount, although the large-scale band structure remained unchanged. The rainfall amount increased as the grid interval was reduced from 20 to 10-km but decreased as the interval was further reduced from 10 to 5 km. Received March 20, 2001; revised August 20, 2001     

中尺度模式质量控制源程序的改进

介绍NCAR／PSU中尺度大气数值模式质量控制的基本原理和技术方法，分析MM4质量控制源程序中存在的问题和不足，并进行优化改进，提高程序的质量控制功能。     

Formulation of the thermal internal boundary layer in a mesoscale model. II. Simulations with a level-2.5 turbulence closure

Physick et al. (1989) have discussed some difficulties associated with simulation of the sea-breeze thermal internal boundary layer (TIBL), using a numerical model containing a profile exchange coefficient formulation developed by O'Brien (1970). They suggested that a closure scheme based on a prediction of turbulent kinetic energy (TKE) would be able to resolve the TIBL better than a profile scheme. This Research Note reports simulations of the case discussed in I, using a numerical model with a TKE-based turbulence closure.     

A land surface model (IAP94) for climate studies Part II: Implementation and preliminary results of coupled model with IAP GCM

The Institute of Atmospheric Physics Land Surface Model (IAP94) has been incorporated into the IAP two-level atmospheric general circulation model (IAP GCM). Global and regional climatology averaged over the last 25 years of 100 year integrations from the IAP GCM with and without IAP94 (“bucket” scheme) is compared. The simulated results are also compared with the reanalysis data. Major findings are:(1) The IAP GCM simulation without IAP94 has extensive regions of warmer than observed surface air tempera?tures, while the simulation with IAP94 very much improves the surface air temperature.(2) The IAP GCM simulation with IAP94 gives improvement of the simulated precipitation pattern and intensity, especially the precipitation of East Asian summer monsoon and its intraseasonal migration of the rainbelts.(3) In five selected typical regions, for most of the surface variables such as surface air temperature, precipitation, precipitation minus evaporation, net radiation, latent heat flux and sensible heat flux, the IAP GCM with IAP94 pro?vides better simulations.     

大气数值预报模式中高度地形追随坐标的设计研究：理论分析与理想试验

采用一种统一的地形追随坐标的形式,对Gal-Chen & Somerville(简称Gal.C.S坐标)、平缓坐标(smoothed level vertical coordinate,简称SLEVE坐标)等几种典型的高度地形追随坐标进行了气压梯度力计算误差影响和二维质量平流试验的理论分析,并与一种新提出的高度地形追随坐标——三角函数平缓坐标(简称COS坐标)进行比较.气压梯度力计算误差分析结果显示,与Gal.C.S坐标相比,单尺度平缓坐标(简称SLEVE1坐标)、双尺度平缓坐标(简称SLEVE2坐标)和COS坐标在减小气压梯度力计算误差上有不同程度的改进,SLEVE2坐标和COS坐标比其他两种坐标更具优势,衰减系数b和坐标转换的雅可比项对减小误差起决定性作用.二维质量平流试验也有类似的结果,与无地形的参考试验结果相比,COS坐标的质量输送计算误差最小,且经优化的COS坐标的质量输送计算误差几乎和参考计算误差完全重合,在4种坐标中最优.     

A Global Spectral Model and Test of Its Performance

A brief introduction is given of a global spectral model, its dynamical framework and diabatic physical processes involved. A number of real forecasts are shown to illustrate the forecasting capability of the model for various weather processes. It can even manage to predict some of those typical weather processes in summer which used to be difficult to forecasters.     

Shear stress results from a mixed spectral finite-difference model: Application to the Askervein Hill project data

Beljaars et al. (1987) developed a model for neutrally stratified boundary-layer flow over roughness changes and topography. It has been discovered that a constant parameter, , was missing in the algebraic-stress closure equations of their paper. This omission also occurred in the MSFD model code but only affects the Askervein Hill shear-stress results for the E-- turbulence closure in Beljaars et al. It also changes the stress results of Karpik (1988), but not his conclusions regarding the robustness of his improved numerical scheme. The present paper demonstrates the effect of the omission of the parameter, , and tests the sensitivity of the model to variations in its value. The new results are compared with the data and model results of Zeman and Jensen (1987).     

Numerical simulation by a stretched-coordinate Ocean General Circulation Model: A preliminary results

Summary ?A three-dimensional Ocean General Circulation Model has been developed in stretched coordinate from scratch. The same model has been used to perform some numerical experiments to simulate the basic circulation pattern and the model variability to atmospheric forcing. For numerical simulations 72 × 25 grid points in the horizontal directions and nine (10, 30, 75, 250, 500, 1000, 1500, 2000 and 3000 m) vertical levels are considered. The lateral boundaries are set at 60° N and 60° S. The basic focus of the paper is on the demonstration of the performance of the model and its assessment by employing appropriate forcing from the outputs of an atmospheric general circulation model. Hence, the model was forced with the forcing (wind and thermodynamic) derived from the ECMWF runs from the AMIP archives. The preliminary results show the realistic simulation of basic pattern of different fields. The model simulations show that the model is able to reproduce some of the general features of the ocean, such as surface currents, surface temperature and salinity, mass transport and meridional heat transport. It is also to be noted that the model is capable to capture the El-Ni?o and La-Ni?a type events. Received April 3, 2002; revised June 6, 2002; accepted July 24, 2002 Published online: February 20, 2003