Numerical Modelling of Katabatic Flow Over a Melting Outflow Glacier

A realistic simulation of katabatic flows is not a straightforward task for numerical models. One complicating factor is that katabatic flows develop within a stably stratified boundary layer, which is poorly resolved and described in many numerical models. To capture the jet-shaped shallow flow a model set-up with high vertical resolution is also required. In this study, ‘a state of the art’ mesoscale numerical model is applied in a simulation of katabatic flow over a melting glacier. A basic agreement between observations and model results is found. From scale analysis, it is concluded that the simulated flow can be classified as katabatic. Although the background flow varies in strength and direction, the simulated katabatic flow over Breidamerkurjökull is persistent. Two factors vital for this persistence are identified. First, the melting snow maintains the surface temperature close to 0 °C while the air temperature warms adiabatically as it descends the slope. This provides a ‘self enhanced’ negative buoyancy that drives the flow to a balance with local friction. Second, the jet-like shape of the resulting flow gives rise to a large ‘curvature term’ in the Scorer parameter, which becomes negative in the upper jet. This prevents vertical wave propagation and isolates the katabatic layer of the influence from the free troposphere aloft. Our results suggest that the formation of local microclimates dominated by katabatic flow is a general feature over melting glaciers. The modelled turbulence structure illustrates the importance of non-local processes. Neglecting the vertical transport of turbulence in katabatic flows is not a valid assumption. It is also found that the local friction velocity remains larger than zero through the katabatic jet, due to directional shear where the scalar wind speed approaches its maximum.     

Post-Onset Behaviour of the Pure Katabatic Flow

An analytic improvement of the classic Prandtl model for pure katabatic flows is obtained. The one-dimensional unsteady problem employs gradually-varying eddy diffusivity K(z) fixed in time. A new solution is found for thefourth-order governing equation that couples the momentum and heat transfer in an approximate but still systematic way. The solution for wind and temperature perturbations is a generalization of the Prandtl solution allowing for: (1) Local andcumulative K(z) effects, (2) gradual evolution from the initial, discontinuous- towardthe steady-state profiles and (3) the given two-dimensional background potential temperature gradient, the surface slope and its potential temperature deficit atthe surface. The solution that is based on a relaxation equation compares adequatelywith its numerical counterpart soon after the estimated flow onset. It is a product ofthe steady-state solution with a spatio-temporal transfer function. The results can beuseful for data analyses, especially for the scale estimations of inclined stable boundarylayers and for surface flux calculations.     

Structure and Dynamics of Katabatic Flow Jumps: Idealised Simulations

For the first time, results from a high-resolution numerical simulation (with horizontal grid spacing of 35m) were used to reveal the detailed structure near an atmospheric katabatic jump over an idealized slope. The simulation represents flow over the slopes of Coats Land, Antarctica for austral winter conditions. The katabatic jump is characterised by an updraft with vertical velocities of order 1ms⁻¹ and serves as a possible forcing mechanism for the gravity waves frequently observed over the ice shelves around the Antarctic. Results also indicate that strong turbulence is generally confined within a mixing zone near the top of the katabatic layer upstream of the jump and extends downstream through the top of the strong updraft associated with the jump. Detailed analyses of momentum and heat budgets across the katabatic jump indicate that, upstream of the jump, turbulent mixing is important in decelerating the upper part of the katabatic layer, while within the jump the upslope pressure gradient force associated with the pool of cold air plays a role in decelerating the flow near the surface. The heat budget near the jump reveals a simple two-term balance: the turbulent heat flux divergence is balanced by the advection. A comparison of model results with available theories indicates that mixing between layers of different potential temperature structure indeed plays some role in the development of katabatic flow jumps, especially for strong jumps. Theories used to study katabatic jumps should include this mixing process, of which the amount depends on the intensity of the jump. A conceptual model of a katabatic jump, including the main dynamical processes, is constructed from these detailed analyses.     

Numerical Simulation of Flow over Two-Dimensional Hills Using a Second-Order Turbulence Closure Model

We study turbulent flow over two-dimensional hills. The Reynolds stresses are represented by a second-order closure model, where advection, diffusion, production and dissipation processes are all accounted for. We solve a full set of primitive non-hydrostatic dynamic equations for mean flow quantities using a finite-difference numerical method. The model predictions for the mean velocity and Reynolds stresses are compared with the measured data from a wind-tunnel experiment that simulates the atmospheric boundary layer. The agreement is good. The performance of the second-order closure model is also compared withthat of lower level turbulence models, including the eddy-viscositymodel and algebraic Reynolds stress models. It is concluded that thepresent closure is a considerable improvement over the other modelsin representing various physical effects in flow over hills. Thefeasibility of running a finite-difference numerical simulationincorporating a full second-order closure model on an IBM workstationis also demonstrated.     

Katabatic Flow Structures Indicative of the Flux Dissimilarity for Stable Stratification

Boundary-Layer Meteorology - Based on observations over an alpine glacier, we investigate the turbulent flux dissimilarity between momentum and sensible heat transfer in a stably stratified...     

非均匀下垫面局地气候效应的数值模拟

使用美国NCAR非静力平衡模式MM 5V3,模拟了黑河地区非均匀下垫面局地小气候效应 ,再现了山谷风环流及夏季“绿洲效应”、沙漠上的“逆湿”和由于地表热力分布不均匀引起的绿洲—沙漠垂直环流等绿洲—沙漠相互作用的典型特征 ,较全面地揭示了非均匀地表大气边界层内的风、温、湿度场与陆面相互作用的物理机理 ,验证了野外实验的结果     

Direct Numerical Simulation of Turbulent Katabatic Slope Flows with an Immersed-Boundary Method

We investigate a Cartesian-mesh immersed-boundary formulation within an incompressible flow solver to simulate laminar and turbulent katabatic slope flows. As a proof-of-concept study, we consider four different immersed-boundary reconstruction schemes for imposing a Neumann-type boundary condition on the buoyancy field. Prandtl’s laminar solution is used to demonstrate the second-order accuracy of the numerical solutions globally. Direct numerical simulation of a turbulent katabatic flow is then performed to investigate the applicability of the proposed schemes in the turbulent regime by analyzing both first- and second-order statistics of turbulence. First-order statistics show that turbulent katabatic flow simulations are noticeably sensitive to the specifics of the immersed-boundary formulation. We find that reconstruction schemes that work well in the laminar regime may not perform as well when applied to a turbulent regime. Our proposed immersed-boundary reconstruction scheme agrees closely with the terrain-fitted reference solutions in both flow regimes.     

Numerical Simulation Experiment of Land Surface Physical Processes and Local Climate Effect in Forest Underlying Surface

Based on the basic principles of atmospheric boundary layer and plant canopy micrometeorology, a forest underlying surface land surface physical process model and a two-dimensional atmospheric boundary layer numerical model are developed and numerical simulation experiments of biosphere and physiological processes of vegetation and soil volumetric water content have been done on land surface processes with local climate effect. The numerical simulation results are in good agreement with realistic observations, which can be used to obtain reasonable simulations for diurnal variations of canopy temperature, air temperature in canopy, ground surface temperature, and temporal and spatial distributions of potential temperature and vertical wind velocity as well as relative humidity and turbulence exchange coefficient over non-homogeneous underlying surfaces. It indicates that the model developed can be used to study the interaction between land surface process and atmospheric boundary layer over various underlying surfaces and can be extended to local climate studies. This work will settle a solid foundation for coupling climate models with the biosphere.     

Evening and Morning Transition of Katabatic Flows

An experimental investigation of the evening and morning transition phases of katabatic slope flows has been conducted to identify the mechanisms for their development and destruction over an isolated slope. The momentum and energy equations of the flow have been used to describe these mechanisms for the particular topographic features of the studied slope, and to outline the differences from the dynamics of well-developed simple slope flows. In the lowest portion of the slope, frontal characteristics have been identified in early evening periods when the local pre-existing near-surface thermal structure does not impose a katabatic acceleration. The frontal shape is determined by the near-surface thermal stability and ambient wind. The flow initiation is distinctly different when it is linked to the local surface cooling, in which case it develops gradually and produces a slight local warming.The erosion of the katabatic layer at mid-slope precedes that at the foot and is closely linked to dilution of the local surface inversion. The flow erosion at the foot is often delayed, as the warming of air proceeds uniformly at all heights near the ground, so maintaining the inversion due to warming produced by mixing and advective processes linked to the upslope flow development. The latter initiates first at mid-slope and then at the foot, where for a non-negligible time period it flows over the persistent katabatic flow. The prerequisite for the development of this structure is the maintenance of a shallow inversion in the first 2–3 m above the ground surface.The morning dilution of the katabatic flow is apparently different from common experience over simple slopes and may be attributed to the steep upper portion of the slope in association with its easterly orientation, which results in strong non-uniformity of the solar heating along the slope.     

重庆雾的二维非定常数值模拟

重庆是著名的“雾都”.本文建立了一个复杂地形下详细考虑长波辐射冷却、地表热量平衡、雾水沉降、水汽凝结和蒸发、湍流交换系数等的二维非定常雾模式,以实测资料为初始场,研究了一次重庆雾的形成和发展过程.对江河、热岛、辐射、地形和山城的影响,模式分别进行了数值试验.     

基于WRF模式的三峡地区局地下垫面效应的数值试验

基于WRF模式,在模式下垫面中嵌入长江水体以及改变地形高度,研究了三峡地区长江水体和局地地形在有降水和无降水事件中的局地效应.结果表明,在有降水事件中,水体下垫面为降水提供了局地水汽,同时在局地喇叭口和峡谷地形动力强迫作用下,近地面河谷风增加,偏南气流携带水汽北上,三峡地区下游段在峡谷地形的"狭管"效应作用下,河谷风偏...     

Impact of Surface Exchange Coefficients and Sea Surface Cooling on Tropical Cyclone Simulation

The surface flux exchange associated with the exchange coefficients and upper ocean conditions is essential to the development of tropical cyclones (TCs). Using the Weather Research and Forecasting (WRF) model, the present study has investigated the impact of exchange coefficients and ocean coupling during Super Typhoon Saomai (2006). Firstly, two experiments with different formula of roughness are conducted. The experiment with the Donelan formula for drag coefficient (Cd) and ramped formula for enthalpy coefficient (Ck) can simulate stronger intensity compared to other experiments due to the increased surface wind and enthalpy fluxes. That is because the new formulas allows for a smaller Cd and larger Ck in the high wind regime than the former formulas did. Moreover, two coupled simulations between WRF and a one-dimensional ocean model are conducted to examine the feedback of sea surface cooling to the TC. In the experiments with a horizontal uniform mixed layer depth of 70 m, the sea surface cooling is too weak to change the evolution of TC. While in the experiment with an input mixed layer calculated using the Hybrid Coordinate Ocean Model (HYCOM) data, the significant sea surface cooling induces obvious impact on TC intensity and structure. Under the negative feedback of sea surface cooling, the sensible and latent heat fluxes decreases, especially in the right part of Saomai (2006). The negative feedback with coupled ocean model plays a vital role in simulating the intensity and structure of TC.     

二维界面流的稳定性及相路方程的椭圆型条件

针对天气实践的一些问题,本文改变了文献[1]的某些作法,分别就上、下行波的稳定性情况,切变邻近区域相路方程的类型条件作了较细致的讨论。结果表明:不稳定极易发生在背急流方向而立的左侧,并在不连续界面的两侧其稳定性是不同的,其相路方程也是有条件的——急流轴及其左侧邻近地区,一般满足椭圆型条件,而其右侧则有可能出现双曲型,从而揭示了扰动沿东西向传播时不稳定易于维持的原因,并与天气实践吻合。     

中国区域多种数值模式资料的地面气象要素评估

利用中国气象局国家级自动站(2421个站)的观测资料,分别对2010年7月1日至2013年6月30日的欧洲中期数值预报中心(ECMWF)、日本气象厅(JMA)和美国国家海洋大气局的全球预测系统(GFS)数值模式资料的地面气温、相对湿度和风速在中国区域的适用性进行了比较研究。结果表明,3种数值模式资料都能在一定程度上反映观测资料所具有的时间和空间分布特征,东部地区的适用性要高于西部地区。不同的地面气象要素,差异较大:对于地面气温和地面相对湿度,ECMWF比JMA和GFS更接近实际观测,ECMWF和JMA的分析场数据质量要好于预报场;ECMWF和JMA地面气温在冬季与观测差异最大,GFS地面气温在夏季与观测差异最大;3种数值模式资料的地面相对湿度在秋季和冬季与观测差异最大;对于地面风速,在江淮流域和华南等东部区域JMA与实际观测差异最小,在北部和西部区域ECMWF最好,JMA和GFS地面风速在冬季与实际观测差异最大。     

Large-Eddy Simulation of Flow and Pollutant Transports in and Above Two-Dimensional Idealized Street Canyons

A large-eddy simulation (LES) model, using the one-equation subgrid-scale (SGS) parametrization, was developed to study the flow and pollutant transport in and above urban street canyons. Three identical two-dimensional (2D) street canyons of unity aspect ratio, each consisting of a ground-level area source of constant pollutant concentration, are evenly aligned in a cross-flow in the streamwise direction x. The flow falls into the skimming flow regime. A larger computational domain is adopted to accurately resolve the turbulence above roof level and its influence on the flow characteristics in the street canyons. The LES calculated statistics of wind and pollutant transports agree well with other field, laboratory and modelling results available in the literature. The maximum wind velocity standard deviations σ _i in the streamwise (σ _u ), spanwise (σ _v ) and vertical (σ _w ) directions are located near the roof-level windward corners. Moreover, a second σ _w peak is found at z ≈ 1.5h (h is the building height) over the street canyons. Normalizing σ _i by the local friction velocity u _*, it is found that σ _u /u _* ≈ 1.8, σ _v /u _* ≈ 1.3 and σ _w /u _* ≈ 1.25 exhibiting rather uniform values in the urban roughness sublayer. Quadrant analysis of the vertical momentum flux u′′w′′ shows that, while the inward and outward interactions are small, the sweeps and ejections dominate the momentum transport over the street canyons. In the x direction, the two-point correlations of velocity R _v,x and R _w,x drop to zero at a separation larger than h but R _u,x (= 0.2) persists even at a separation of half the domain size. Partitioning the convective transfer coefficient Ω _T of pollutant into its removal and re-entry components, an increasing pollutant re-entrainment from 26.3 to 43.3% in the x direction is revealed, suggesting the impact of background pollutant on the air quality in street canyons.     

Oscillation of Surface PM_(2.5) Concentration Resulting from an Alternation of Easterly and Southerly Winds in Beijing: Mechanisms and Implications

We used simultaneous measurements of surface PM_(2.5) concentration and vertical profiles of aerosol concentration,temperature, and humidity, together with regional air quality model simulations, to study an episode of aerosol pollution in Beijing from 15 to 19 November 2016. The potential effects of easterly and southerly winds on the surface concentrations and vertical profiles of the PM_(2.5) pollution were investigated. Favorable easterly winds produced strong upward motion and were able to transport the PM_(2.5) pollution at the surface to the upper levels of the atmosphere. The amount of surface PM_(2.5) pollution transported by the easterly winds was determined by the strength and height of the upward motion produced by the easterly winds and the initial height of the upward wind. A greater amount of PM_(2.5) pollution was transported to upper levels of the atmosphere by upward winds with a lower initial height. The pollutants were diluted by easterly winds from clean ocean air masses. The inversion layer was destroyed by the easterly winds and the surface pollutants and warm air masses were then lifted to the upper levels of the atmosphere, where they re-established a multi-layer inversion. This region of inversion was strengthened by the southerly winds, increasing the severity of pollution. A vortex was produced by southerly winds that led to the convergence of air along the Taihang Mountains. Pollutants were transported from southern–central Hebei Province to Beijing in the boundary layer. Warm advection associated with the southerly winds intensified the inversion produced by the easterly winds and a more stable boundary layer was formed. The layer with high PM_(2.5) concentration became dee-per with persistent southerly winds of a certain depth. The polluted air masses then rose over the northern Taihang Mountains to the northern mountainous regions of Hebei Province.     

一次局地暴雪过程低层降温机制分析

2015年12月5日在强西南暖湿气流和弱冷空气配合之下,浙江杭州到安徽黄山一带出现了局地大到暴雪,此次过程预报偏差较大。通过对水汽、动力和温度条件分析表明:强西南暖湿气流和冷暖空气的辐合为大量地面降水的产生提供了水汽和动力条件;深厚的湿层和合适的中层温、湿条件有利于生成大量可供降落的冰雪晶;转雪前杭州站低层异常降温是形成降雪的关键原因。通过对降温机制的分析表明:引起杭州站低层降温的原因主要是水物质相变相关的非绝热加热,冷平流的作用很弱。5日08时之前低层降温主要是降水粒子蒸发吸热引起的;5日白天低层降温主要是由大量冰(雪)晶融化吸收环境潜热导致,冷平流和垂直输送项也有部分贡献。预报中对降温机制分析不足,可能是导致预报偏差的主要原因。该个例中平流等降温机制较弱,冰晶融化吸热作用导致中低层形成0℃的均温层,从而使冰晶顺利到达地面。中低层温度与常用的预报相态的温度阈值相差较大,说明预报中不能机械依赖温度指标,而应全面分析降雪形成的物理机制。当降水量大时,融化吸热可以成为降温的主要机制,预报中应予以充分考虑。     

一次局地大暴雨过程的数值模拟和诊断分析

采用中尺度数值模式MM5(V3)对2002年8月15-16日发生在江苏启东附近的一次局地大暴雨过程进行了数值模拟,较好地模拟出了本次过程的暴雨中心落区和降水量,以及相关的中尺度系统的发生、发展过程.探讨了中尺度涡旋、局地环流对这次暴雨过程的影响,结果表明,中尺度涡旋对暴雨的发生起了至关重要的作用,局地环流的存在,也有利于降水的发生.