Modeling Arctic Boundary Layer Cloud Streets at Grey-zone Resolutions

To better understand how model resolution affects the formation of Arctic boundary layer clouds,we investigated the influence of grid spacing on simulating cloud streets that occurred near Utqiaġvik(formerly Barrow),Alaska,on 2 May 2013 and were observed by MODIS(the Moderate Resolution Imaging Spectroradiometer).The Weather Research and Forecasting model was used to simulate the clouds using nested domains with increasingly fine resolution ranging from a horizontal grid spacing of 27 km in the boundary-layer-parameterized mesoscale domain to a grid spacing of 0.111 km in the large-eddy-permitting domain.We investigated the model-simulated mesoscale environment,horizontal and vertical cloud structures,boundary layer stability,and cloud properties,all of which were subsequently used to interpret the observed roll-cloud case.Increasing model resolution led to a transition from a more buoyant boundary layer to a more shear-driven turbulent boundary layer.The clouds were stratiform-like in the mesoscale domain,but as the model resolution increased,roll-like structures,aligned along the wind field,appeared with ever smaller wavelengths.A stronger vertical water vapor gradient occurred above the cloud layers with decreasing grid spacing.With fixed model grid spacing at 0.333 km,changing the model configuration from a boundary layer parameterization to a large-eddy-permitting scheme produced a more shear-driven and less unstable environment,a stronger vertical water vapor gradient below the cloud layers,and the wavelengths of the rolls decreased slightly.In this study,only the large-eddy-permitting simulation with gird spacing of 0.111 km was sufficient to model the observed roll clouds.     

GRAPES气象-水文模式在一次洪水预报中的应用

尝试将GRAPES (Global-Regional Assimilation and PrEdiction System) 模式与水文模型结合,构建GRAPES气象-水文单向耦合模式,进行洪水预报。气象模式选取GRAPES_Meso模式,分别采用15 km×15 km和5 km×5 km水平分辨率,15 km×15 km的GRAPES模式由NCEP全球预报场提供初始场和侧边界条件;5 km×5 km的GRAPES模式由15 km×15 km GRAPES模式提供初始场和侧边界条件,将GRAPES_Meso模式的定量降水预报分辨率统一降尺度到5 km×5 km分辨率,用于驱动水文模式。水文模型选取新安江模型与分布式新安江模型。以淮河王家坝站以上流域和息县流域为试验流域,将GRAPES降水预报场驱动水文模型进行单向耦合,构建GRAPES气象-水文单向耦合模式,选择2009年8月28日08:00(北京时,下同)—9月9日14:00汛期一次洪水过程,进行实际预报试验。结果表明:15 km×15 km和5 km×5 km的GRAPES模式预报降水与实况降水分布相一致;与水文站观测降水驱动水文模型洪水模拟结果相比,GRAPES气象-水文模式对洪水预报的预见期延长效果明显,对洪水模拟精度也较高,与水文模型输入场分辨率要求相匹配的降水产品对洪水模拟的精度更高。     

Comparison of Measured and Numerically Simulated Turbulence Statistics in a Convective Boundary Layer Over Complex Terrain

The Weather Research and Forecasting (WRF) model can be used to simulate atmospheric processes ranging from quasi-global to tens of m in scale. Here we employ large-eddy simulation (LES) using the WRF model, with the LES-domain nested within a mesoscale WRF model domain with grid spacing decreasing from 12.15 km (mesoscale) to 0.03 km (LES). We simulate real-world conditions in the convective planetary boundary layer over an area of complex terrain. The WRF-LES model results are evaluated against observations collected during the US Department of Energy-supported Columbia Basin Wind Energy Study. Comparison of the first- and second-order moments, turbulence spectrum, and probability density function of wind speed shows good agreement between the simulations and observations. One key result is to demonstrate that a systematic methodology needs to be applied to select the grid spacing and refinement ratio used between domains, to avoid having a grid resolution that falls in the grey zone and to minimize artefacts in the WRF-LES model solutions. Furthermore, the WRF-LES model variables show large variability in space and time caused by the complex topography in the LES domain. Analyses of WRF-LES model results show that the flow structures, such as roll vortices and convective cells, vary depending on both the location and time of day as well as the distance from the inflow boundaries.     

一个三重嵌套的中尺度模式及暴雨模拟试验

该文介绍了新近发展的一个高分辨率有限区模式(HLAM)。它是通过三重嵌套在现有计算机(CYBER-992)条件下实现的，其范围、地理位置及侧边界宽度均设计为灵活可变，尤其水平分辨率可按任意倍数提高。利用水平分辨率为50km左右的模式版本进行了实际资料的降水个例实验。结果是令人鼓舞的:(1)与嵌套的低分辨率(格距增大4倍)模式预报比较，不论低压中心位置、强降水落区、雨带走向及降水中心位置的预报都更接近实况;(2)高分辨率模式积分区域虽然缩小，但并未影响预报效果，其48小时预报仍有较高精度。初步试验表明，进一步完善和改进后的HLAM完全可以发展为一个用于中尺度研究及业务使用的高分辨率区域模式。     

基于不用分辨率和参数化方案的新疆区域数值模式性能初步评估

基于中尺度数值模式WRF,选取新疆两次强降水过程,设计三个试验方案,其中试验1为控制试验,试验2提高分辨率,试验3提高分辨率并调整物理参数化方案,初步评估不同分辨率和参数化方案对新疆区域2米温度、10米风速、降水预报的影响。结果表明：（1）提高分辨率对2米温度、10米风速模拟精度均有提高,2米温度预报精度提高约0.5℃,降低了日间温度模拟冷偏差;10米风速预报精度提高约0.5 m/s,降低了风速模拟正偏差;但提高分辨率后,模式出现虚假降水预报的情况。（2）提高分辨率并调整物理参数化方案后,2米温度模拟误差略有减小,模拟偏差减小约0.2℃;10米风速模拟误差增大约0.5 m/s,模拟偏差增大超过0.5 m/s;对降水落区、量级的模拟精度显著提高,减小了降水中心的模拟强度,对虚假降水预报有一定修正。     

Modelling Extreme Storm-Induced Currents over the Grand Banks

Abstract

The wind climate of the mountainous terrain in the southern Yukon is simulated using the Wind Energy Simulation Toolkit (WEST) developed by the Recherche en Prévision Numérique (RPN) group of Environment Canada and is compared to measurements in the field. WEST combines two models that operate at different spatial scales. The Mesoscale Compressible Community (MC2) model is a mesoscale numerical weather prediction model that produces simulations over large domains of the order of a thousand kilometres. The MC2 model uses long‐term synoptic scale wind climate data from the analysis of radiosonde and other observations to simulate mean wind fields at tens of metres above the ground using a horizontal resolution of a few kilometres. The mesoscale results are used as input to MS‐Micro/3 (Mason and Sykes (1979) version of the Jackson and Hunt (1975) model version for microcomputers/3‐dimensional; MS‐Micro hereafter), a more computer‐efficient, microscale model with simpler linearized momentum equations and a domain restricted to a few tens of kilometres with horizontal grid sizes of tens or hundreds of metres. MS‐Micro provides wind field results at specific wind generator hub heights (typically 30 to 50 m above ground level (AGL)) which are of interest to researchers and developers of wind farms.

WEST shows relatively strong correlations between its simulated long‐term mean wind speed and the measurements from ten wind energy monitoring stations. However, in the mountainous terrain of the Yukon, WEST tends to predict wind speeds which are about 40% too high. The model also produces erroneous wind directions and some were perpendicular to valley orientations. The most likely cause of the wind speed and direction errors is the substantially modified 5‐km grid‐spaced mesoscale terrain used in MC2. The WEST simulation was also found to double the wind speeds observed at airport stations and there was poor correlation between the simulated and observed wind speeds.

The bias in the model could be attributed to a number of factors, including the use of smoothed topography by the model, the discrepancy between the neutral atmosphere assumed in MS‐Micro and the normally observed stable atmosphere, the application of MS‐Micro to every third grid point of the MC2 output, abnormally high sea level wind speeds in the input climate data for MC2, and a certain degree of disagreement between the land surface characteristics used in the model and those found in the field.

At comparatively low computer cost, WEST predicts a wind climate map that compares favourably to the wind measurements made in several locations in the Yukon. However, the problem of the modified terrain in the mountainous regions is the most pressing problem and needs to be addressed before WEST is used in the mountainous regions of Canada.     

国家气象中心准业务有限区客观分析方案

介绍了国家气象中心准业务有限区客观分析、预报系统的分析方案。采用三维最优插值方法，水平方向是1.875°×1.875°经纬度网格，在1000 hPa至50 hPa 12层标准等压面进行高度和风分量的多变量分析，在最低6层标准等压面进行相对湿度的单变量分析。检验表明，该方案为有限区预报模式提供了较好的初值分析场。     

次网格对流参数化效果对水平分辨率的敏感生试验

使用单向嵌套的五层有限域模式进行了次网格对流参数化效果对水平分辨率的敏感性试验.使最细网格覆盖主要降水区,并依次嵌套.设计了四种水平分辨率不同的网格,其水平格距分别为381,190.5,95.25及47.625km.对同一降水个例使用上述各种网格分别作了24小时累计降水量预报(积分48小时)试验.结果表明:(1)总降水量(降水最大值及等面积平均最大值)随网格距的减小而增大.当水平格距减小为50km左右时,便可以很好地预报出一般天气尺度降水的落区及雨量分布;(2)随水平分辨率的提高,次网格对流凝结对总降水的     

超强台风桑美（2006）登陆前后低层风廓线数值模拟分析

利用ARW-WRF模式,以垂直方向40个模式层（对低层加密）、水平方向最高1 km的分辨率,对台风桑美（2006）进行数值模拟,模拟结果与实况基本一致。基于台风桑美（2006）1 km分辨率的模拟结果,对台风低层（海面或地表以上1500 m以下）风场结构进行了分析。结果表明,在台风登陆前,其最大风速半径附近存在水平风速在垂直方向有很强变化的风廓线,该类型风廓线的最大风速高度有明显变化,表现出类似急流的特征;而台风登陆后,其水平风速垂直变化明显减弱,即风廓线类型发生较大变化;另有一种水平风速在高层少变的风廓线类型在台风中是普遍存在的。还根据高层和低层两个切变因子,将台风登陆前的风廓线分为急流型、普通型和过渡型,并进一步分析各类风廓线在台风中出现的位置和急流高度。对急流型风廓线的形成原因也进行了初步探讨,结果表明,超/次梯度风在垂直方向上的变化是形成急流型风廓线的原因,而外围绝对角动量的输送在其中起关键作用。     

Sensitivity studies of high-resolution RegCM3 simulations of precipitation over the European Alps: the effect of lateral boundary conditions and domain size

A suite of high-resolution (10 km) simulations were performed with the International Centre for Theoretical Physics (ICTP) Regional Climate Model (RegCM3) to study the effect of various lateral boundary conditions (LBCs), domain size, and intermediate domains on simulated precipitation over the Great Alpine Region. The boundary conditions used were ECMWF ERA-Interim Reanalysis with grid spacing 0.75^°, the ECMWF ERA-40 Reanalysis with grid spacing 1.125 and 2.5^°, and finally the 2.5^° NCEP/DOE AMIP-II Reanalysis. The model was run in one-way nesting mode with direct nesting of the high-resolution RCM (horizontal grid spacing Δx = 10 km) with driving reanalysis, with one intermediate resolution nest (Δx = 30 km) between high-resolution RCM and reanalysis forcings, and also with two intermediate resolution nests (Δx = 90 km and Δx = 30 km) for simulations forced with LBC of resolution 2.5^°. Additionally, the impact of domain size was investigated. The results of multiple simulations were evaluated using different analysis techniques, e.g., Taylor diagram and a newly defined useful statistical parameter, called Skill-Score, for evaluation of daily precipitation simulated by the model. It has been found that domain size has the major impact on the results, while different resolution and versions of LBCs, e.g., 1.125^° ERA40 and 0.7^° ERA-Interim, do not produce significantly different results. It is also noticed that direct nesting with reasonable domain size, seems to be the most adequate method for reproducing precipitation over complex terrain, while introducing intermediate resolution nests seems to deteriorate the results.     

A Prediction System for Local Wind Variations in Mountainous Terrain

A three-level model system for the prediction of local flows in mountainous terrain is described. The system is based upon an operational weather prediction model with a horizontal grid spacing of about 10 km. The large-scale flow is transformed to a more detailed terrain, first by a mesoscale model with grid spacing of about 1 km, and then by a local-scale model with a grid spacing of about 0.2 km. The weather prediction model is hydrostatic, while the two other models are non-hydrostatic. As a case study the model system has been applied to estimate wind and turbulence over Várnes airport, Norway, where data on turbulent flight conditions were provided near the runway. The actual case was chosen due to previous experiences, which indicate that south-easterly winds may cause severe turbulence in a region close to the airport. Local terrain induced turbulence seems to be the main reason for these effects. The predicted local flow in the actual region is characterized by narrow secondary vortices along the flow, and large turbulent intensity associated with these vortices. A similar pattern is indicated by the sparse observations, although there seems to be a difference in mean wind direction between data and predictions. Due to fairly coarse data for sea surface temperature, errors could be induced in the turbulence damping via the Richardson number. An adjustment for this data problem improved the predictions.     

四川地形谱特征及中尺度模式水平网格分辨率选取

四川地形复杂多样,暴雨频发,常诱发山洪、泥石流等灾害。在利用中尺度模式对复杂地形区域的暴雨进行研究时,模式水平分辨率的选取缺乏定量依据。为了揭示四川地形的复杂特征和给中尺度模式水平网格分辨率的选取提供定量依据,利用二维离散余弦变换,对四川地形高度场和暴雨分布场进行谱分解,根据暴雨分布特征分区讨论了四川盆地地形特征,同时利用地形谱方差和数值试验定量讨论了数值模式水平分辨率的选取问题,得到主要结论有:(1)二维离散余弦变换能较好地表现出研究区域各向异性的复杂特征;(2)雅安地区和四川盆地西北部的地形谱与降水谱有较好的同相关系,盆地东北部和盆地中部的地形谱与降水谱在波长较大处出现反相关系;(3)针对某个区域的地形特征,可以通过计算模式能分辨的地形方差与总地形方差的比值来确定合适的中尺度模式水平网格分辨率。     

A Cluster Space Calibration Scheme for Synthetic Wind Statistics Calculated with a Mesoscale Model and its Application to Regional Wind Climate Studies

Summary ?A scheme for calibrating an ensemble of wind fields computed by a mesoscale model in order to generate synthetic wind statistics is described. It is based on two main points. The first is to exploit the power of a mesoscale model to determine wind fields over complex terrain for different weather situations classified by cluster analysis. The second is to use all the information in the cluster analysis, i.e., the centroid values as well as the internal standard deviations of the clusters, to determine a cluster space distribution at each grid point in the model domain. The latter makes calibration possible if reliable measurements are available at the position of one of the grid points. The accuracy of the calibration is increased by splitting the cluster spaces into several parts. Combining both the modelled mesoscale wind fields and the method of split cluster spaces leads to a spatial transformation of the calibration from the calibration point to each grid point in the model domain. A validation of the scheme is carried out with measurements at grid points other than the calibration point and reveals remarkable improvements in the accuracy of the model wind statistics, especially with regard to wind speed distributions. Received October 8, 1998/Revised March 19, 1999     

The effect of spatial resolution on the simulation of upper-tropospheric frontogenesis using a sigma-coordinate primitive equation model

Summary The effects of varying horizontal and vertical grid resolution on the numerical simulation of upper-tropospheric frontal structures are examined using a, two-dimensional, dry, hydrostatic sigma-coordinate primitive equation model. These effects are illustrated with the results of 72 h model integrations in which frontogenesis is forced solely by confluence. Four different horizontal grid spacings, ranging from 100 km to 12.5 km, and four different vertical, grid resolutions, varying from 10 to 70 layers (90 mb to 13 mb), are considered.The intensity and integrity of the frontogenesis as a function of grid resolution are diagnosed in terms of time histories and spatial distributions of frontal parameters, such as the speed of the along-front jet, maxima of absolute vorticity and potential temperature gradient, and ageostrophic motions. The time histories show that, provided the vertical resolution is sufficient, increasing horizontal resolution leads to better-defined frontal structure due to the decrease in cross-frontal scale. They also indicate that for a given horizontal resolution there exists an optimal vertical resolution beyond which frontal parameters change only slightly. This optimal vertical resolution increases with increasing horizontal resolution, and apparently is related to the horizontal resolution through the slope of the frontal zone.The time histories for simulations combining low vertical resolution with high horizontal resolution exhibit substantial high-frequency variability. Cross sections show that this temporal variability appears to be manifested spatially in the form of gravity waves characterized by wavelengths on the order of 200 km and periods of 12 h, over the range of horizontal and vertical resolution that is considered. Although the source mechanism for these waves cannot be established definitively, it likely involves a grid-induced ageostrophic component of the along-front wind which disrupts thermal wind balance. This results thus demonstrates the potential risk of flawed frontal simulations, when incompatible combinations of horizontal and vertical resolution are used.With 7 Figures     

上海区域降水集合预报系统的建立与运行结果的检验

以MM5模式为基础, 从预报模式的不确定性出发形成8个集合成员, 建立了上海区域降水集合预报系统。该系统实现从资料收集、资料处理、模式预报到预报结果处理与产品输出的全自动化, 于2005年8月1日开始业务运行, 运行稳定可靠。对系统8—10月的运行结果进行检验, 结果表明:集合预报系统对降水的总体预报效果尚可, 其中对量级小的降水的总体预报效果更好, 集合预报产品尤其是概率预报产品具有一定的参考价值, 但系统还存在发散度偏小的问题, 有待进一步改进。     

A Multiple-Cell Flat-Level Model for Atmospheric Tracer Dispersion over Complex Terrain

A multiple-cell flat-level tracer dispersion model is developed for atmospheric pollution study. The horizontal domain may be constructed with multiple-sized cells for varied resolution. The sequence of cells is arbitrary, as in unstructured grids, as long as no holes are left in the horizontal domain, which may be tailored in shape according to local orography. The vertical levels are truly flat and the level spacing may vary from level to level. The surface orography is included by removing cells from the bottom of the three-dimensional cell block. The arrangement of wind velocity and tracer concentration is similar to the Arakawa C grid. Advection and horizontal diffusion are formulated on each cell face, using tracer concentrations in the two cells that share the cell face. Pointer-orientated numerical loops are used to facilitate the arbitrary horizontal cell arrangement and orographic variation of vertical levels. A second-order upstream upper limiter advection scheme is developed for this model and numerically tested to be positive-definite and mass conserving. Vertical diffusion is solved with an implicit scheme and simplified vertical diffusivity, which is parameterised as a function of the mixing layer depth. The model is fast, compact, easy to implement and highly portable. It is suitable for studies ofmesoscale and small-scale atmospheric tracer dispersion over complex terrain, including steep slopes. The model is used to simulate traffic pollution in London, UK, and is compared with available observations.     

Typhoon Winnie （1997） with a Very Large Eye： High Resolution Numerical Simulation

Typhoon Winnie (1997) was one of the hurricanes that had extremely large eyewall ever recorded with a diameter of eyewall reaching 370 km. Using the Penn State University/National Center for Atmospheric Research mesoscale model MM5 with 3-km grid horizontal spacing on the finest nested mesh, Winnie was successfully simulated in terms of track, intensity, eye and concentric eyewalls. The dynamic and thermal structures of concentric eyewalls were studied based on the model output. It was found that the concentric eyewalls and their surrounding wind fields were asymmetric in observation as well as in simulation. Winnie's outer eyewall was associated with a maximum wind ring, a warm moist ring, and a high vorticity ring. The inner eyewall was associated with a secondary maximum wind ring and a warm moist ring. Upward motion dominated the whole layer of inner eyewall and the area above 2-km altitude of the outer eyewall. Downward motion was found inside the eye and the moat. Radial inflow happened in the boundary layer of the outer eyewall and the moat, but radial outflow dominated the middle and upper levels of the outer eyewall.     

Present climate simulation over Korea with a regional climate model using a one-way double-nested system

Summary This study investigates the capability of the regional climate model RegCM3 to simulate surface air temperature and precipitation over the Korean Peninsula. The model is run in one-way double nested mode, with a 60 km grid point spacing “mother” domain encompassing the eastern regions of Asia and a 20 km grid point spacing nested domain covering the Korean Peninsula. The simulation spans the three-year period of 1 October 2000 through 30 September 2003 and the boundary conditions needed to run the mother domain experiment are provided from the NCEP reanalysis of observations. The model results are compared with a high density station observation dataset to examine the fine scale structure of the surface climate signal. The model shows a good performance in capturing both the sign and magnitude of the seasonal and inter-annual variations of the surface variables both over East Asia as a whole and over the Korean Peninsula in the nested system. Some persistent biases are however present. Surface temperature is systematically underestimated, especially over mountainous regions in the warm season. This feature may be due to the relatively coarse representation of the Korean topography. The simulated precipitation over the mother domain successfully reproduces the broad spatial pattern of observed precipitation over East Asia along with its seasonal evolution. On the other hand, fine scale details from the nested results show a varying level of quality for the different individual years. Because of the better resolved topographic forcing, the increased resolution of the nested model improves the spatial agreement with the fine scale observation fields for temperature and cold season precipitation. For summer monsoon precipitation the simulation of individual monsoon convective events and tropical storms is however more important than the topographic forcing, and therefore the performance of the nested system is more case-dependent.     

复杂地形风场的精细数值模拟

风能是一种重要气候资源,随着我国风电规模的迅速增大,发展风能资源评估系统和风功率预测系统已成为一项重要的研究内容。国内外对复杂地形风场结构的数值模拟有大量研究,随着计算机能力增强,以往用于空气动力学精细流场计算的计算流体力学(Computational Fluid Dynamics,CFD)模式越来越多地在气象领域得到应用,人们开始研究用中尺度预报模式和CFD模式结合进行复杂地形风场的数值模拟。本文的耦合模式系统采用中尺度气象模式(WRF),通过嵌套网格到内层尺度(一般是几公里),然后通过耦合CFD模式Fluent软件获得高分辨率(水平30~100 m,垂直150 m高度以下10 m)的风速分布资料,得到精细化的风场信息。通过对鄱阳湖北部区域和云南杨梅山复杂地形的风场模拟,提供了风能评估和预报的一种可行的方法。     

雷达估测降雨水平分辨率对径流模拟的影响——以西苕溪流域为例

天气雷达估测降雨是径流模拟和洪水预报的重要信息之一。由于雷达网格降雨存在误差,且误差随着网格水平尺度的增大而减小,因此对于径流模拟,高分辨率的雷达降雨数据并不意味着径流模拟的精度更高。采用FSS(Fractions Skill Score)方法和HEC-HMS模型(Hydrologic Engineering Center's Hydrologic Modeling System)分析江苏省西苕溪流域雷达估测降雨水平分辨率对径流模拟的影响。在2010年和2011年夏季两场降雨实例中,雷达估测降雨在不同降雨阈值情况下,FSS达到目标精度值对应的最小有效水平尺度为2~8 km,分别以2、4、6、8 km水平分辨率的雷达估测降雨和雨量站测雨作为HEC-HMS模型输入进行径流模拟,结果表明:基于不同水平分辨率的雷达估测降雨的径流模拟结果与实测径流资料基本吻合,雷达估测降雨2、4、6、8 km水平分辨率的变化对径流模拟效果的影响不明显。