Numerical simulation of January 28 cold air outbreak during GALE part II: The mesoscale circulation and marine boundary layer

A two-dimensional (2-D) mesoscale numerical model is applied to simulate the January 28 cold-air outbreak over the Gulf Stream region during the Intensive Observation Period-2 (IOP-2) of the 1986 Genesis of Atlantic Lows Experiment (GALE). The model utilizes a turbulence closure which involves the turbulent kinetic energy (TKE) and dissipation () equations and combines the level 2.5 formulations of Mellor and Yamada (1982) for better determination of the eddy Prandtl number.The modeled marine boundary layer (MBL) is in good agreement with the observations (Wayland and Raman, 1989) showing a low-level jet west of the Gulf Stream warm core and a constrained boundary layer due to the middle-level (2–4.5 km) stable layer. The MBL-induced single cloud and rain band first appears east of the Gulf Stream boundary, and then moves offshore at the speed of the circulation front. The front, however, moves slightly slower than the ambient flow. Removal of the tropopause does not influence the low-level circulation and the movement of the front. The speed of the front is slightly larger in the baroclinic downshear flow than in the barotropic flow. The results also indicate that the observed high cloud streets propagating downwind of the Gulf Stream may be related to upper-level baroclinic lee waves triggered by an elevated density mountain. The density mountain waves, however, become evanescent as the baroclinity (which gives a larger Scorer parameter) is removed.The modeled 2-D circulation systems are found to be sensitive to differing eddy Prandtl numbers, in contrast to the 1-D model results presented in Part I. Sensitivities become increasingly important as the clouds begin to interact with the MBL. A constant eddy Prandtl number of unity produces a more slantwise convection compared to that by the level 2.5 case. Cloud development is stronger in slantwise convection than in upright convection. The fastest development of clouds can be explained in terms of the conditional symmetric instability (CSI), which begins as the MBL baroclinity becomes sufficiently large.     

Numerical simulation of vortex roll development during a cold air outbreak

Cold air outbreaks can be identified by the formation of cloud streets downwind from a land-sea boundary, as can be seen in numerous satellite pictures. These cloud streets are caused by horizontal roll vortices which in turn are due to dynamic and convective instability of the planetary boundary layer over sea. The development of these roll vortices is simulated with a numerical model and compared to observations obtained over the Bering Sea. Vertical heat transport is found to be due to turbulent diffusion in the initial stage of a cold air outbreak before organized roll vortices contribute to the heat flux in the higher levels of the boundary layer. The influence of a capping inversion on the dynamic and convective instability is also elucidated.     

Numerical simulation of the development of the convective boundary layer during a cold air outbreak

During cold air outbreaks, cold and stably stratified air masses are advected from land or ice surfaces over a warmer sea surface. Due to the heating from below, a convective boundary layer develops. For small fetches, convection is organized in the form of horizontal roll vortices, which at greater distances join in a zone with open or closed cells. The formation of the convective boundary layer, and the associated roll vortices, is simulated with a numerical model and results are compared with observations obtained during the MASEX experiment off the east coast of the United States. To validate the model, a comparison with a one-dimensional mixed-layer model is also made, with special attention given to the exact representation of the observed initial and boundary conditions. Comparisons between model results and observations show good qualitative and quantitative correspondence in mean temperature and heat flux profiles respectively at different distances from the coast. Maximum values of vertical velocity are well reproduced. Turbulent kinetic energy is found to be concentrated in the small updraft regions of the rolls, which is in accordance with observations from the MASEX-experiment.     

Mean and turbulent structure of a baroclinic marine boundary layer during the 28 January 1986 cold-air outbreak (GALE 86)

Aircraft (NCAR Electra), ship (R/V Cape Hatteras), buoy (NCSU Buoy 2) and satellite (NOAA-7 and 9) measurements have been used to observe the structure of the Marine Boundary Layer (MBL) offshore of Wilmington, North Carolina, during the intense cold-air outbreak of 28 January, 1986, as part of the Genesis of Atlantic Lows Experiment (GALE). Air mass modification processes, driven primarily by the surface turbulent latent and sensible heat fluxes, caused the overlying air mass to warm and moisten as it advected over the warmer waters of the eastern United States continental shelf. Maximum observed total (latent + sensible) heat flux was 1045W/m² (at a height of 49 m) over the core of the Gulf Stream. Heat flux values decreased both east and west of this region, primarily in response to changes in the air-sea temperature difference.MBL height increased steadily in the offshore direction in response to increasing convection. The turbulent structure showed a buoyancy-dominated MBL between 0.1 z/h and 0.8 z/h; whereas shear was important above and below this level, vertical transport of kinetic energy (KE) was dominant as a source term only above 0.8 z/h. The normalized turbulent kinetic energy (TKE) budgets observed at different offshore locations showed general agreement at different flight levels. Thus the findings support the validity of the similarity relations under intense convective conditions.     

Structure of the marine atmospheric boundary layer during two cold air outbreaks of varying intensities: GALE 86

Aircraft, surface, upper air and satellite measurements have been used to observe the evolution and growth of the convective Marine Atmospheric Boundary Layer (MABL) offshore of North Carolina in close proximity to the Gulf Stream, during the intense cold air outbreak of 28 January 1986 and the moderate event of 12 February 1986, as part of the Genesis of Atlantic Lows Experiment (GALE). Air mass modification processes, driven primarily by the ocean-atmosphere exchanges of surface turbulent sensible and latent heat fluxes, caused the overlying air mass to warm and moisten as it advected over the warmer waters of the eastern United States continental shelf. Maximum observed near-surface total heat fluxes were 1045 and 811 W·m^–2 over the core of the Gulf Stream, for 28 January and 12 February 1986, respectively. The observed changes in the overlying air mass occurred almost instantaneously as the ambient flow traversed different underlying SST conditions.The turbulent structure showed a buoyancy-dominated MABL below approximately 0.8z/h. However, shear was also observed to be an important production term above 0.8z/h and below 0.1z/h for the 28 January 1986 event. Dissipation of turbulent kinetic energy was the dominant destruction term in the budgets, but vertical transport of energy was a strong contributor below 0.5z/h, above which this term became a source of turbulent energy. Additionally, the normalized standard deviations of the horizontal velocity components showed a near-equal contribution to the turbulence, while the vertical velocity components displayed the characteristic mid-layer maximum profile observed for a convective, well-mixed boundary layer.     

A comparison of local and non-local turbulence closure methods for the case of a cold air outbreak

Numerical experiments have shown that large-eddy-simulation models (LES) are able to reproduce the common features of convective boundary layers (CBL) quite well. Models which cannot resolve the convective motions due to their grid structure (1D-models or models with coarse horizontal and/or vertical resolution) have to take into account the effects of large eddies within their subgrid diffusion terms. Turbulent fluxes are frequently parameterized through first-order-closure methods (K-theory). Recently, non-local closure schemes have also been developed. In this paper we compare 1D-and 2D-models using different local and non-local first-order closure methods. The analysis is carried out for the case of an idealized cold air outbreak (CAO). One of the non-local closures is based on the so-called transilient turbulence theory. The reference states are given by a bulk-model and a 2D-model which resolves the large eddies explicitly. A comparison of the results is presented for characteristic quantities such as evolution of boundary-layer height and surface heat flux as well as mean wind and temperature profiles. It is found that simple local first-order closure does not give good agreement with the reference models. The results of the transilient turbulence model shows that a non-local closure is able to parameterize the effects of the large eddies. Comparable results are produced by a local closure where eddy diffusivities are parameterized by dimensionless gradient-functions.     

Cold air outbreak during MASEX: Lidar observations and boundary-layer model test

Lidar observations of boundary-layer development during a cold air outbreak over the Atlantic Ocean were examined. Very rapid rise rates were measured in the first 20 km off the coast. A large region of partial cloudiness was found to exist between the totally clear region near shore and the overcast region far from the coast. As the layer became overcast, rise rate of the boundary layer tripled, suggesting a direct relation between cloudiness and entrainment. Boundary-layer evolution was reasonably well simulated by a simple slab model. The model was not capable of predicting the area of partial cloudiness, nor the region of rapid entrainment near the coast.     

A new large-eddy simulation model for simulating air flow and warm clouds above highly complex terrain. Part I: The dry model

This paper presents the dry version of a new large-eddy simulation (LES) model, which is designed to simulate air flow and clouds above highly complex terrain. The model is three-dimensional and nonhydrostatic, and the governing equations are sound filtered by use of the anelastic approximation. A fractional step method is applied to solve the equations on a staggered Cartesian grid. Arbitrarily steep and complex orography can be accounted for through the method of viscous topography. The dynamical model core is validated by comparing the results for a spreading density current against a benchmark solution. The model accuracy is further assessed through the simulation of turbulent flow across a quasi two-dimensional ridge. The results are compared with wind-tunnel data. The method of viscous topography is not restricted to moderately sloped terrain. Compared to models using curvilinear grids, it allows this model to be applied to a much wider range of flows. This is illustrated through the simulation of an atmospheric boundary-layer flow over a surface mounted cube. The results show that the dry model version is able to accurately represent the complex flow in the vicinity of three-dimensional obstacles. It is concluded that the method of viscous topography was successfully implemented into a micrometeorological LES model. As will be shown in Part II, this allows the detailed study of clouds in highly complex terrain.     

Snowdrift suspension and atmospheric turbulence. Part I: Theoretical background and model description

Snowdrift is one of the manymanifestations of two-phase flow, in which theinteraction between suspended particles and theambient fluid brings about some interesting features.Specifically, the drag required to keep particles insuspension against the downward gravitational pullrequires expenditure of turbulent kinetic energy(TKE). Other effects include the increased density of theair-snow mixture and the stable thermal stratificationcaused by the snowdrift sublimation-induced cooling.An atmospheric surface-layer model that includes snowdriftsuspension is described that includes the effects ofupward diffusion, gravitational settling andsublimation of snow particles in 48 size classes, theeffects of snowdrift sublimation on the heat andmoisture budget of the surface layer and the dampingof turbulence in the presence of suspended particles. Thewell-known E- closure model is applied toevaluate the eddy exchange coefficient, with a newterm representing buoyancy reduction induced by thestably stratified suspended particle profile includedin the prognostic equation for TKE.     

The vertical transport of air pollutants by convective clouds Part II: Transport of soluble gases and sensitivity tests

A two-dimensional, non-reactive convective cloud transport model is used to simulate in detail the vertical transport and wet scavenging of soluble pollutant gases by a deep thunderstorm system, Simulations show that for gases with not very high solubility, a deep and intense thunderstorm can still rapidly and efficiently transport them from boundary layer (PBL) up to mid and upper troposphere, resulting in a local significant increase of concentration in the upper layer and a reduction in PBL. Dissolution effects decrease both the incloud gas concentration and the upward net fluxes. The higher the solubility is, the more remarkable the decrease is. However, for very low soluble gases (H < 102 M atm-1), the influences are very slight. In addition, the effects of irreversible dissolution and aqueous reactions in drops on the vertical transport of gaseous pollutants are estimated in extreme.     

Numerical studies on cold fronts Part I: Gravity flows in a neutral and stratified atmosphere

Summary We have used a two dimensional version of a nonhydrostatic mesoscale model to simulate atmospheric gravity currents for different thermal stratification. The horizontal and vertical grid increments are chosen such, that the major features of a current like head and elevated nose are resolvable.When the density current propagates into a neutral stratified environment it was found, that frontspeed agrees well with an empirical formula. Also characteristic length scales like depth of the head or height of the following cold air body agree well with observations found in water tank experiments.When a stable atmosphere is adopted, the front moves faster and the generated gravity waves have a significant influence on the atmospheric variables ahead of the current. This results especially in a pressure increase before the front arrives, an effect, which was found in observations, too.Finally, it is shown, that an elevated inversion, embedded in a stable layer, intensifies the vertical velocities and therefore the mesoscale heat flux, which results in a stronger entrainment. For this case a remarkable decrease of front speed is simulated with time.With 7 Figures     

Numerical simulation of particle trajectories in inhomogeneous turbulence,I: Systems with constant turbulent velocity scale

A means of numerical simulation of particle trajectories in inhomogeneous turbulence is described. The method employs a simple coordinate transformation which allows a trajectory in inhomogeneous turbulence to be converted to a corresponding trajectory in homogeneous turbulence. Concentration distributions predicted by the trajectory-simulation method agree precisely with analytical solutions in the special cases of homogeneous turbulence, turbulence with power-law wind and eddy diffusivity profiles, and the neutral atmospheric surface layer.     

Numerical simulation of plume-advection over a hill with coupled models. Part I: Numerical solution of the transport-equation

Summary Four numerical methods suitable for the calculation of transport are compared on a linear two-dimensional advection equation. It is found that a mass-compensation algorithm which removes negative values has also a beneficial effect on the accuracy of schemes which are of higher order but are not positive. It is shown by numerical experiments, that the upstream spline advection scheme combined with a mass compensation is prefereable for some applications rather than other high accurate positive schemes like the flux-corrected transport scheme.With 3 Figures     

The Vertical Transport of Air Pollutants by Convective Clouds. Part I: A Non-Reactive Cloud Transport Model

A convective cloud transport model, without chemical processes, is developed by joining a set of concentration conservative equations into a two-dimensional, slab-symmetric and fully elastic numerical cloud model, and a numerical experiment is completed to simulate the vertical transport of ground-borne, inert gaseous pollutant by deepthunderstorm. The simulation shows that deep convective storm can very effectively transport high concentrated pollutant gas from PBL upward to the upper troposphere in 30 to 40 minutes, where the pollutant spreads laterally outward with strong anvil outflow, forming an extensive high concentration area. Meanwhile, relatively low concentration areas are formed in PBL both below and beside the cloud, mainly caused by dynamic pumping effect and sub-cloud downdraft flow. About 80% of the pollutant gas transported to the upper troposphere is from the layer below 1.5 km AGL (above ground level).     

汾河谷地大气环境容量与大气污染过程的数值模拟与敏感性分析

对汾河谷地及太原市在2015年1月的一次重污染过程运用WRF-Chem模式进行污染过程的数值模拟、观测验证和地形敏感性实验,分析了河谷地形对区域污染过程和大气环境容量的影响机制。结果表明:太原市及周边汾河谷地大气边界层环流和大气污染传输受天气系统、地形和城市化共同影响;地形环流强度明显强于太原城市热岛环流,且对其发展存在明显抑制作用,从而限制了城市大气环境容量,加剧城市近地面大气污染物的堆积和空气质量恶化;研究区域大气环境容量受气象主导风向影响:当天气主导风向偏南北向,也即与狭长的汾河谷地走向一致时,有利于该区域大气污染物的扩散清除,而当天气主导风向偏东西向时,则该区域大气环境容量明显减小,且近地面大气污染物浓度与大气环境容量之间呈强相关性,相关系数达到0.74;当地形敏感组实验中取消太原市周边河谷地形特征时,上述相关系数降低到0.21。     

Development and Validation of an Evaporation Duct Model.Part Ⅰ:Model Establishment and Sensitivity Experiments

Based on the Coupled Ocean-Atmospheric Response Experiment（COARE）bulk algorithm and the Naval Postgraduate School（NPS）model,a universal evaporation duct（UED）model that can flexibly accommodate the latest improvements in component（such as stability function,velocity roughness,and scalar roughness）schemes for different stratification and wind conditions,is proposed in this paper.With the UED model,the sensitivity of the model-derived evaporation duct height（EDH）to stability function（Ψ）,ocean wave effect under moderate to high wind speeds,and scalar roughness length parameterization,is investigated,and relative contributions of these factors are compared.The results show that the stability function is a key factor influencing the simulated EDH values.Under unstable conditions,the EDH values from stability functions of Fairall et al.（1996）and Hu and Zhang（1992）are generally higher than those from others;while under stable conditions,unreasonably high EDHs can be avoided by use of the stability functions of Hu and Zhang（1992）and Grachev et al.（2007）.Under moderate to high wind speeds,the increase in velocity roughness length z₀ due to consideration of the true ocean wave effect acts to reduce modeled EDH values;this trend is more pronounced under stable conditions.Although the scalar roughness length parameterization has a minor effect on the model-derived EDH,a positive correlation is found between the scalar roughness length z_0qand the model-derived EDH.     

Structure and evolution of a tropopause fold during GALE IOP-1: An eta model study

Summary A fine-mesh regional model simulation of upper-level cyclogenesis is carried out to examine the structure and evolution of the accompanying tropopause fold and its relationship to the surface and upper-level cyclones. The initial state for the simulation, conducted using the 80-km, 16-level version of the National Meteorological Center Eta model, uses the Level III-b gridded dataset for 1200 UTC, 18 January 1986, during the First Intensive Observing Period (IOP-1) of the Genesis of Atlantic Lows Experiment (GALE) project.Results are presented from a 48 hour integration of the model. The emphasis is on the examination of the synoptic scale evolution and structure of the upper-level cyclone and tropopause fold, both of which were successfully simulated in the model. The potential vorticity structure associated with a propagating jet-streak displayed distinctive structure, with its tilt reversing as the jet-streak moved around the base of an amplifying upper-level trough. In addition, the model simulates the intrusion of dry, stratospheric air containing high potential vorticity anomalies into the lower troposphere as well as subsidence warming when the folding of the tropopause occurs. the model also predicts upper-level frontogenesis as a result of a thermally indirect secondary circulation in the exit region of the jet-streak. The success of the model simulation is most likely the result of comprehensive physics and the fine grid resolution employed and, more importantly, the excellent distribution of subsynoptic scale initial data during the GALE project.With 16 Figures     

太平洋大尺度环流数值模拟 I:数学模式及其性能

本工作基于曾庆存的海洋环流模式设计思想和大气物理研究所海洋环流模式(IAPOGCM)基本动力框架,设计并实现了四层太平洋环流模式;同时引进稳定有效的时间积分方案和海表热通量参数化方法,对太平洋大尺度环流进行了较为系统的数值模拟. 本文首先导出消除“刚盖”近似、扣除海洋标准层结、保持总有效能量守恒的OGCM基本方程组;给出空间差分格式和时间积分方案,引入海表热通量计算方法;然后,在太平洋区域对所设计的四层OGCM的一些模式性能进行了分析、表明,时间积分方案采用基于“灵活性系数”技术的加速收敛方法和适应过程中流场正压斜压进一步分解算法,同时采取一些技术处理,可确保模式稳定而有效地进行长时间数值积分、有效能量分析表明,与海面起伏相联系的有效表面位能是总有效能量的重要组成部分,它与动能同量级甚至更大些,因此在OGCM中须考虑有效表面位能与其它能量间的转换机制,这在物理上更加合理,而采用“刚盖”近似则是不合理的;同时,扣除海洋标准层结,缩小了位能与动能之间的量级差别,有利于长期数值积分的稳定进行.此外,这个模式能直接计算海面起伏和分析有效能量,使得表面流场和上层海洋热储率的模拟比已有文献的结果有明显的改进.除模式设计外,本部分只给出模式的基本性能,系统的模拟结果将在第Ⅱ,Ⅲ两部分发表.