Numerical simulations of a large-amplitude mesoscale gravity wave event

Summary Numerical simulations with the NCAR/PSU Mesoscale Model 5 (MM5) were performed to study a large-amplitude gravity wave event that occurred on 4 January 1994 along the East Coast of the United States. Results from the MM5 control simulation using a 12-km mesh resolution compared well with the synoptic and mesoscale observational analysis. The simulated gravity waves displayed timing, location, wavelength, and propagation speed similar to those observed in a synoptic-scale environment described by the Uccellini and Koch (1987) conceptual model. Additional features existing upstream of the wave generation region not contained within their conceptual model were a warm occlusion and tropopause fold prior to and during the gravity wave generation. Wave ducting criteria were nearly satisfied along the path of the gravity waves.Several sensitivity tests were performed. In a simulation in which the Appalachian Mountains were removed, the model still produced similar cyclone development and mesoscale gravity waves. Thus topography was not directly responsible for the gravity wave genesis. Also, three different fake dry sensitivity tests were performed with the latent heating related to changes of water substance turned off in the model at different stages of the simulation. The results from these simulations suggest that diabatic heating played an important role in both jet/cyclone development and in gravity wave amplification and maintenance, though not wave generation. The simulation with grid resolution increased to 4km, which included fully explicit microphysics produced gravity wave characteristics similar to those in the control simulation, though the higher resolution resolved much shorter waves (though unverifiable) closely associated with convection. This 4-km sensitivity experiment with no cumulus parameterization also confirmed that the dominant gravity wave was not an artifact of the particular cumulus parameterization scheme used for the control simulation. The reliability of the simulated gravity waves is further confirmed with another sensitivity experiment initialized 20 hours before the observed wave generation in which qualitatively-similar gravity waves were produced.Received August 28, 2000; revised May 2002; accepted October 8, 2002 Published online: April 10, 2003     

大尺度环境中中尺度对流系统生成的数值模拟试验

对1983年6月11日20时(北京时)实测资料进行低通滤波处理,取其大尺度部分作为初值,用七层原始方程模式对江淮气旋内中尺度对流系统的生成过程进行数值模拟试验.结果表明,从大尺度背景场出发,可以模拟出中尺度对流系统的生成.在西南低空急流十分潮湿的情况下,中尺度对流系统可以在江淮气旋暖区、低空急流大风速中心的前部、位势稳定度倾向维持负值的地区生成和发展.它的动力、热力结构,降水分布,以及发生、发展过程等,都与暖季(3—9月)在美国中部频繁出现的中尺度对流复合体(MCC)十分相似.     

NUMERICAL SIMULATION OF THE GENERATION OF MESOSCALE CONVECTTVE SYSTEMS IN LARGE－SCALE ENVIRONMENT

The generation of mesoscale convective systems is simulated by a 7-level primitive equation model. The large-scale parts of observed, data at 1200 Z June 11, 1983, which are obtained by low-pass filter, are used as the initial data. The results show that the generation of mesoscale convective systems can be simulated from fields of meteorological variables on the large-scale background. When the low-level south-west jet stream is very moist, mesoscale convective systems can develop ahead of the wind speed maximum in the warm sector of Jiang-Huai (Changjiang-Huaihe Rivers) cyclone, where the potential stability tends to remain negative. Furthermore, they are similar to the mesoscale convective complex (MCC), which appears frequently in the central part of the United States during the warm season (March to September), in dynamical and thermal structure, distribution of precipitation and the process of generation and development.     

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.     

Numerical study of mesoscale cellular convection

Mesoscale cellular convections over the East China Sea during cold air outbreaks are simulated with a high-resolution numerical model. The model incorporates important physical processes involved in shallow convection, such as the exchange of heat and moisture between water and air; condensation; evaporation; and vertical turbulent transfer of heat, moisture, and momentum.The results show that open cells develop with aspect ratios as large as 14. The structure of the convection is examined in detail. The organized mesoscale circulation is responsible for breaking up the initial stratus cloud deck and enhancing turbulence in the upward-moving area (especially inside cumulus clouds). However, it is found that the heat flux contributed by MCC's themselves is much smaller than the eddy heat flux.     

Model simulations of mesoscale eddies and deep convection in the Labrador Sea

Deep convection in the Labrador Sea is confined within a small region in the southwest part of the basin.The strength of deep convection in this region is related to the local atmospheric and ocean characteristics,which favor processes of deep convection preconditioning and intense air-sea exchange during the winter season.In this study,we explored the effect of eddy-induced flux transport on the stratification of the Labrador Sea and the properties of deep convection.Simulations from an eddy-resolving ocean model are presented for the Labrador Sea.The general circulation was well simulated by the model,including the seasonal cycle of the deep Labrador Current.The simulated distribution of the surface eddy kinetic energy was also close to that derived from Topex-Poseidon satellite altimeter data,but with smaller magnitude.The energy transfer diagnostics indicated that Irminger rings are generated by both baroclinic and barotropic processes; however,when they propagate into the interior basin,the barotropic process also disperses them by converting the eddy energy to the mean flow.In contrast to eddy-permitting simulations,deep convection in the Labrador Sea was better represented in the eddyresolving model regarding their lateral position.Further analysis indicated that the improvement might be due to the lateral eddy flux associated with the resolved Irminger rings in the eddy-resolving model,which contributes to a realistic position of the isopycnal dome in the Labrador Sea and correspondingly a realistic site of deep convection.     

Effects of land-surface heterogeneity on numerical simulations of mesoscale atmospheric boundary layer processes

Landscape heterogeneity that causes surface flux variability plays a very important role in triggering mesoscale atmospheric circulations and convective weather processes. In most mesoscale numerical models, however, subgrid-scale heterogeneity is somewhat smoothed or not adequately accounted for, leading to artificial changes in heterogeneity patterns (e.g., patterns of land cover, land use, terrain, and soil types and soil moisture). At the domain-wide scale, the combination of losses in subgrid-scale heterogeneity from many adjacent grids may artificially produce larger-scale, more homogeneous landscapes. Therefore, increased grid spacing in models may result in increased losses in landscape heterogeneity. Using the Weather Research and Forecasting model in this paper, we design a number of experiments to examine the effects of such artificial changes in heterogeneity patterns on numerical simulations of surface flux exchanges, near-surface meteorological fields, atmospheric planetary boundary layer (PBL) processes, mesoscale circulations, and mesoscale fluxes. Our results indicate that the increased heterogeneity losses in the model lead to substantial, nonlinear changes in temporal evaluations and spatial patterns of PBL dynamic and thermodynamic processes. The decreased heterogeneity favor developments of more organized mesoscale circulations, leading to enhanced mesoscale fluxes and, in turn, the vertical transport of heat and moisture. This effect is more pronounced in the areas with greater surface heterogeneity. Since more homogeneous land-surface characteristics are created in regional models with greater surface grid scales, these artificial mesoscale fluxes may have significant impacts on simulations of larger-scale atmospheric processes.     

Application of the E-ε closure model to simulations of mesoscale topographic effects

A mesoscale Planetary Boundary Layer (PBL) model with a simple turbulence closure scheme based on the turbulence kinetic energy (TKE) equation and the dissipation () equation is used to simulate atmospheric flow over mesoscale topography. Comparative studies with different parameterizations suggest that with a proper closure assumption for turbulence dissipation, the E-model can simulate the circulation induced by the mesoscale topography with results similar to those obtained using the E- model. On the other hand, the first-order closure using O'Brien's cubic interpolation for eddy diffusivities (K) generally produces much larger K profiles in the stable and the unstable regions, which is believed to be due to the overprediction of the height of the PBL. All models with the TKE equation yield quite similar ensemble mean fields, which are found to be little sensitive to the closure assumption for turbulence dissipation, though their predicted magnitudes of TKE and K may differ appreciably. A discussion on the diurnal evolution of the mesoscale topography-induced circulation and the spatial variations of the turbulence fluxes in the surface layer is also given based on the E- model results.     

弱垂直风切变下台前飑线不同发展阶段的热、动力特征分析

利用多普勒雷达、风廓线雷达、FY-4A黑体亮温(TBB)、地基微波辐射计、地面自动站等多种观测资料以及ERA5再分析资料,对“利奇马”(2019)台前飑线的过程演变及其发生位置、移动方向和长生命史的异常特征进行分析,结果表明：(1) 台前飑线过程经历了发展形成、加强成熟和减弱消亡三个阶段,其中第二阶段影响范围最广、强度最强,过境造成的气象要素变化具有极端性。地面中尺度雷暴高压、飑前热低压、冷池、暖中心特征明显。(2) 台前飑线发生在增强的大陆高压与台风之间的湿区带上;台风为台前飑线过程提供了充沛水汽,强的不稳定环境产生大的对流有效位能(CAPE)和东北气流辐合;上干下湿的热力不稳定层结、低空强的垂直风切变是此次台前飑线生成的大气环境条件。(3) 台前飑线进入福建后,大气层结不稳定持续显著增强、低空垂直风切变异常增强且时间长维持以及台前飑线与前缘激发生成的对流风暴合并促使台前飑线发展增强,是其生命史延长的重要原因。(4) 东北气流增强引导台前飑线向西南方向移动,飑线与引导气流夹角大,二者移动方向一致,移速加快,有利地面大风增幅。(5) 地基微波辐射计资料计算的热力对流参数(K、TT和A指数)的峰值与大风极值有着很好的对应关系,K≥37 ℃和A≥11 ℃指数对强对流天气预警有一定的参考价值。

     

山洪防治中的环境问题

山洪灾害是指由于降雨在山丘区引发的洪水及山洪诱发的泥石流、滑坡等对国民经济和人民生命财产造成损失的灾害. 黑龙江省山洪灾害的特点是突发性强,多由集中或局地暴雨造成的,受灾面积小,人员伤亡与固定资产损失在各种灾害中较为严重,危害很大.黑龙江省水害异常气候多发生在小兴安岭南部伊春、铁力、庆安区域,黑龙江省南部山区宾县、尚志、五常区域,以及大兴安岭南部龙江、甘南区域,这三大区域均在全省大暴雨出现次数密度与频度较大的区域内.龙江、甘南区域,已拥有许多大型水库调控洪水,不易发生山洪;宾县、尚志、五常区域,流域积水面积不大,且主沟长度短,坡度不是很大,灾害性的洪水频度不高;而伊春、铁力、庆安区域,异常气候、暴雨量与频度为全省最敏感区域.呼兰河上游区域、汤旺河中游区域、牡丹江中游区域、穆棱河上中游区域,发生洪水灾害频度较大.     

Assessment of upper-ocean variability and the Madden-Julian Oscillation in extended-range air-ocean coupled mesoscale simulations

Atmosphere-ocean interaction, particular the ocean response to strong atmospheric forcing, is a fundamental component of the Madden-Julian Oscillation (MJO). In this paper, we examine how model errors in previous Madden-Julian Oscillation (MJO) events can affect the simulation of subsequent MJO events due to increased errors that develop in the upper-ocean before the MJO initiation stage. Two fully coupled numerical simulations with 45-km and 27-km horizontal resolutions were integrated for a two-month period from November to December 2011 using the Navy’s limited area Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS^®). There are three MJO events that occurred subsequently in early November, mid-November, and mid-December during the simulations. The 45-km simulation shows an excessive warming of the SSTs during the suppressed phase that occurs before the initiation of the second MJO event due to erroneously strong surface net heat fluxes. The simulated second MJO event stalls over the Maritime Continent which prevents the recovery of the deep mixed layer and associated barrier layer. Cross-wavelet analysis of solar radiation and SSTs reveals that the diurnal warming is absent during the second suppressed phase after the second MJO event. The mixed layer heat budget indicates that the cooling is primarily caused by horizontal advection associated with the stalling of the second MJO event and the cool SSTs fail to initiate the third MJO event. When the horizontal resolution is increased to 27-km, three MJOs are simulated and compare well with observations on multi-month timescales. The higher-resolution simulation of the second MJO event and more-realistic upper-ocean response promote the onset of the third MJO event. Simulations performed with analyzed SSTs indicate that the stalling of the second MJO in the 45-km run is a robust feature, regardless of ocean forcing, while the diurnal cycle analysis indicates that both 45-km and 27-km ocean resolutions respond realistically when provided with realistic atmospheric forcing. Thus, the problem in the 45-km simulation appears to originate in the atmosphere. Additional simulations show that while the details of the simulations are sensitive to small changes in the initial integration time, the large differences between the 45-km and 27-km runs during the suppressed phase in early December are robust.     

中尺度低压系统形成和维持的数值实验

使用细网格的多层原始方程模式对中尺度低压进行了数值模拟。发现:(1)在中尺度低压形成和维持过程中风场扰动的动力学过程比气压场扰动的作用更为重要。(2)对五种不同垂直结构的风场扰动进行了实验,其中以中低层的风场扰动在中低压形成过程中的作用更显著。(3)对湿模式和干模式进行的对比实验表明,两者重力波传播的相速度有明显的差别。前者将近为零,而后者达44米/秒。     

Altai-Sayan lee cyclogenesis: Numerical simulations

Summary In this paper we compare and analyse two cases of lee cyclogenesis observed near the Altai-Sayan massif (14–16 April 1988; 3–5 March 1986). The interpretation of the events is based on numerical simulations with an isentropic primitive equation model, where runs without orography are compared to those with realistic orography. The runs with mountains are successful in depicting the observed developments, so that the no-mountain experiments can be assumed to provide further information on the dynamics of both cases.The case of 14–16 April 1988 is dominated by rapid cyclogenesis and cut-off low formation aloft linked to the southward motion of a potential vorticity maximum towards the Altai-Sayan massif. Cyclone formation would have occurred without mountains as well. However, cyclogenesis at low levels is enhanced by the presence of the mountains through blocking of the cold air and corresponding deformation of the surface cold front. So this is a case of orographically modified cyclogenesis whereas the cyclogenesis of 3–5 March 1986 is induced by the mountain. In that case a broad and almost steady trough moves eastward over the Altai-Sayan region. Cyclone formation is absent in the no-mountain run. However, both cases exhibit some similarities with respect to the low-level developments induced by the mountains. A conceptual model is presented in order to further elucidate the key features of both cases.With 11 Figures     

Numerical simulations of Cross-Appalachian transport and diffusion

Numerical studies of short-term transport and diffusion over regional distances have been made with a particle-in-cell model and compared with observations from the Cross-Appalachian Tracer Experiment. The plume configurations of all seven simulations show general agreement with the observed patterns. Frequency distributions of observed and predicted values are broadly similar, although correlation coefficients of points paired in space and time are poor; this results from small dislocations in predicted versus observed plume orientations.     

Comparison of the DRAIS and EURAD model simulations of air pollution in a mesoscale area

Summary In this paper the results of simulations of air pollution carried out with the mesoscale model system KAMM/DRAIS are presented. They are compared with results of the European scale model EURAD which have been provided by the EURAD-Group, Cologne. With this comparison it is intended to analyse to what extent better resolution of topography and emission data used by the mesoscale model effects the model results. The simulations have been carried out for July 15, 1986, a typical summer day. The model domain contains south-west Germany and part of Alsace with a resolution of 5 km. The emissions for this resolution have been derived by a combination of the coarse EURAD emission data with the data of the TULLA experiment which are available on a much finer grid. The initial and boundary conditions for the species concentrations are determined from the results of the EURAD model. This coupling introduces the long range transport of pollutants into the mesoscale simulation.The meteorological and concentration data of the EURAD model are compared with the corresponding DRAIS model results. The mesoscale flow field is characterized by the channeling along the Upper Rhine Valley, which is not resolved in the EURAD model. The concentration distributions of both models are similar during midday, because of the strong vertical mixing. In the night and especially, in the morning and evening hours the spatial distribution is much better represented by the DRAIS model results. The better resolution of the emissions and the topography in the DRAIS model compared with the EURAD model (80 km grid size) becomes really noticeable. The difference of the ozone concentrations between cities and the, surrounding areas and between the Rhine Valley and the limiting mountains are in the order of 30 ppb as compared to a few ppb in the EURAD simulation. In the morning NO concentrations of about 200 ppb are simulated in the area between Heilbronn and Stuttgart. The EURAD model provides only about 5 ppb. Comparisons with measurements show that the DRAIS simulations are more realistic than the EURAD model results. The features mentioned are also found in an evaluation of the concentration variations in areas corresponding to a grid cell of the EURAD model. Two completely different areas are selected to demonstrate the possible range of the concentration variation. In the area around the City of Stuttgart the ozone concentration in the morning and the evening varies between zero ppb and 50 ppb, approximately. The mean value is nearly the same in both simulations.List of Abbreviations DRAIS Dreidimensionales Regionales Ausbreitungsund Immissions-Simulationsmodell - EMEP European co-operative program for Monitoring and Evaluation of the long-range transmission of air Pollutants - EUMAC European Modelling of Atmospheric Constituents - EURAD European Acid Deposition Model - EUROTRAC EUROpean experiment on TRAnsport and transformation of environmentally relevant trace Constituents in the troposphere over Europe - JDR Joint Dry Case - KAMM Karlsruher Atmosphärisches Mesoskaliges Modell - MM4 Mesoscale Model 4 - NCAR National Center for Atmospheric Research - RADM Regional Acid Deposition Model - TADAP Transport and Deposition of Acidifying Pollutants - TULLA Transport und Umwandlung von Luftschadstoffen im Lande Baden-Württemberg und aus Anrainerstaaten; in English: Transport and Transformation of Air Pollutants in the State of Baden-Württemberg and from neighbouring countries With 17 Figures     

Numerical simulations of fractional vegetation coverage influences on the convective environment over the source region of the Yellow River

The spatial distribution of fractional vegetation coverage (FVC) over the source region of the Yellow River (SRYR) shows heterogeneity and has changed much for vegetation degradation during the past 30 years. In this paper, three numerical tests were conducted using the Weather Research and Forecasting (WRF) model for a fair weather case and a rainy case over the SRYR. The first test used FVC derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) data, the second test took a spatial constant FVC (=50 %), and the last one deployed the model default FVC. The results showed that simulated 2-m height potential temperature decreased and specific humidity increased as FVC increased for both cases. The magnitudes of the sensible and latent heat fluxes were different over the wetland at the fair day and the rainy day. There was a divergence center near the Zoige wetland in the fair weather case but a convergent center in the rainy case. As a conclusion, when the default FVC in the WRF model were replaced by the new MODIS-derived FVC data, the root-mean-square errors (RMSEs) between measurements and the simulated 2-m height air temperature and relative humidity decreased for both cases, the mean RMSEs between measurements and the simulated 2-m height air temperature and relative humidity declined by 0.3 K and 3.0 % in the fair day, 0.4 K and 3.4 % in the rainy case. The mean differences of simulated precipitation for seven ground stations were 1.1 mm for the fair day and 4.2 mm for the rainy case. Therefore, the updated MODIS FVC has influences on the convective environment over the SRYR.     

热带气旋生成过程的中尺度涡旋活动数值模拟

热带气旋生成过程中包含不同尺度环流及其相互作用。为此,本文将热带气旋生成数值模拟的起点提前到模拟中尺度涡旋(MCV)的生成,从而利用高分辨率数值试验结果,对热带气旋过程中的不同尺度涡旋活动进行分析。模式首先模拟了季风涡旋的东南侧增强的西南气流中出现低形变旋转性扰动,随着扰动的旋转性增强,中层出现水平尺度为200 km左右的MCV。在扰动区内的不同高度上还发现10~20 km尺度不等的中γ气旋性涡旋扰动,其中部分涡旋扰动具有热塔的特征,中γ气旋性涡旋扰动在MCV的旋转环境内不断组织化,低层气旋性涡旋扰动的分布比中层更加集中。模拟表明这些较小尺度的气旋性中尺度涡旋扰动对热带气旋的生成有重要作用。     

Numerical study of the impact of cloud droplet spectral change on mesoscale precipitation

The change of cloud droplet spectra can modify the microphysics and radiative processes of the atmosphere, which in turn affects surface precipitation. In this paper, the impact of cloud droplet spectral change (CDSC) on mesoscale precipitation is studied by simulating two cases, a South China Storm on June 8, 1998 and a Yangtze River Storm on July 22, 2002, and employing the MM5V3 with newly developed, dual-parameterized explicit moisture scheme coupled to. The results show that CDSC has a slight influence on rainfall distribution/pattern, but can significantly change the precipitation intensity, especially the position and intensity of the precipitation centers. The effects of CDSC are more distinct in the daytime and have an obvious diurnal cycle on rain rate. Precipitation increase (decrease) due to CDSC is ascribed to the relative upper radiative cooling (heating) and lower radiative heating (cooling) in the daytime atmosphere.     

A regression-based statistical correction of mesoscale simulations for near-surface wind speed using remotely sensed surface observations

Wind speed is an important meteorological variable for various scientific communities. In this study, numerical mesoscale simulations were performed over the Republic of Korea in 2006, to produce wind information distributed homogeneously with space. Then, an attempt was made to statistically correct the simulated nearsurface wind speed using remotely sensed surface observations. The weak wind season (WWS, from May to October) and strong wind season (SWS, from November to April) were classified on the basis of the annual mean wind speed. Although the spatial features and monthly variation pattern of the near-surface wind speed were reasonably simulated in the Weather Research and Forecasting (WRF) model, the simulations overestimated the observed values. To correct the simulated wind speeds, a regression-based statistical algorithm with different constants and coefficients for WWS and SWS was developed using match-up datasets of wind observations and satellitederived variables (land surface temperature and normalized difference water index). The corrected wind speeds showed reasonable performance for both WWS and SWS with respect to observed values. The monthly variation in the corrected wind speeds over the Republic of Korea also matched better with observations throughout the year, within a monthly bias range of approximately ± 0.2 m s^?1. The proposed algorithm using remotely sensed surface observations may be useful for correcting simulated near-surface wind speeds and improving the accuracy of wind assessments over the Republic of Korea.