Assimilating QuikSCAT Ocean Surface Winds with the Weather Research and Forecasting Model for Surface Wind-Field Simulation over the Chukchi/Beaufort Seas

To achieve a high-quality simulation of the surface wind field in the Chukchi/Beaufort Sea region, quick scatterometer (QuikSCAT) ocean surface winds were assimilated into the mesoscale Weather Research and Forecasting model by using its three-dimensional variational data assimilation system. The SeaWinds instrument on board the polar-orbiting QuikSCAT satellite is a specialized radar that measures ice-free ocean surface wind speed and direction at a horizontal resolution of 12.5 km. A total of eight assimilation case studies over two five-day periods, 1–5 October 2002 and 20–24 September 2004, were performed. The simulation results with and without the assimilation of QuikSCAT winds were then compared with QuikSCAT data available during the subsequent free-forecast period, coastal station observations, and North American Regional Reanalysis data. It was found that QuikSCAT winds are a potentially valuable resource for improving the simulation of ocean near-surface winds in the Chukchi/Beaufort Seas region. Specifically, the assimilation of QuikSCAT winds improved, (1) offshore surface winds as compared to unassimilated QuikSCAT winds, (2) sea-level pressure, planetary boundary-layer height, as well as surface heat fluxes, and (3) low-level wind fields and geopotential height. Verification against QuikSCAT data also demonstrated the temporal consistency and good quality of QuikSCAT observations.     

Verification of a Synthesized Method for the Calculation of Low-Level Climatological Wind Fields Using a Mesoscale Boundary-Layer Model

Low-level climatological wind fields over the La Plata River region of South America are synthesized with a dry, hydrostatic mesoscale boundary-layer numerical model. The model is forced at the upper boundary with the 1200 UTC local radiosonde observations and at the lower boundary with a land-river differential heating function defined from the daily meteorological observations of the region. The climatological wind field is defined as the mean value of a series of individual daily forecasts, employing two methods. The simplified method considers a 192-member ensemble (16 wind directions and 12 wind-speed classes at the upper boundary). Each member has a probability of occurrence that is determined from the 1959–1984 observations; the daily method uses a total of 3,248 days with available data during the same period. In both methods each realization is a daily forecast from which the mean wind distributions at 0300, 0900, 1500 and 2100 local standard time are calculated and compared to the observations of five meteorological stations in the region. The validation of the climatological wind fields for both methods is evaluated by means of the root-mean-square error of the wind-direction frequency distribution and mean wind speed by wind sector. The results obtained with the two methods are similar, and the errors in wind speed are always smaller than those in wind direction. The combined errors of wind direction and wind speed show that the ensemble method is outperformed by the daily method, on average by meteorological station in only one out of five of them, and on average by the time of the day in only one out of 4 h. The conclusion of the study is that the ensemble method is an appropriate methodology for determining high resolution, low-level climatological wind fields, with the boundary-layer model applied to a region with a strong diurnal cycle of surface thermal contrast. The proposed methodology is of particular utility for synthesizing wind fields over regions with limited meteorological observations, since the 192-member matrix can be easily defined with few observing points, as well as in the case of relatively incomplete records.     

Measurements and Modelling of the Wind Speed Profile in the Marine Atmospheric Boundary Layer

We present measurements from 2006 of the marine wind speed profile at a site located 18 km from the west coast of Denmark in the North Sea. Measurements from mast-mounted cup anemometers up to a height of 45 m are extended to 161 m using LiDAR observations. Atmospheric turbulent flux measurements performed in 2004 with a sonic anemometer are compared to a bulk Richardson number formulation of the atmospheric stability. This is used to classify the LiDAR/cup wind speed profiles into atmospheric stability classes. The observations are compared to a simplified model for the wind speed profile that accounts for the effect of the boundary-layer height. For unstable and neutral atmospheric conditions the boundary-layer height could be neglected, whereas for stable conditions it is comparable to the measuring heights and therefore essential to include. It is interesting to note that, although it is derived from a different physical approach, the simplified wind speed profile conforms to the traditional expressions of the surface layer when the effect of the boundary-layer height is neglected.     

The Kwinana Coastal Fumigation Study: III – Meteorological and Turbulence Modelling on Selected Days

The Kwinana Coastal Fumigation Study took place inearly 1995 at Kwinana near Perth in Western Australia.The study involved surface and elevated meteorologicaland plume fumigation measurements in sea-breeze flowsnear the coast, and has yielded a comprehensive dataset that is suitable for assessing meteorological andplume dispersion models. In this paper, wesimulate the meteorology and turbulence on four casestudy days, and compare model results with thedetailed surface and aircraft measurements takenduring the study. These days had surface synopticwinds ranging from southerly to northeasterly, witheither stable or near-neutral temperature profilesover the sea.The model used was based on that developed by Hurley(Boundary-Layer Meteorol. 83, 43–73, 1997), but extended here to allow domain nesting,optional non-hydrostatic simulations, and a vegetativecanopy at the surface. The model was forced bystandard weather service synoptic data, and thesimulations have captured the essential features ofthe strong sea-breeze circulation observed on thesedays. The boundary-layer structure over the sea waspredicted to be near-neutral or stable in agreementwith the observations on the particular day. The windspeed and direction in the sea-breeze flow weregenerally predicted well, although the predictedmaximum inflow speed over the land was a little toohigh. The potential temperature was generallyover-predicted, but temperature gradients agreed well.Predicted turbulence levels in the bottom-half of thethermal internal boundary layer compared well to theobservations, but under-estimated the observations inthe in the upper half of this layer. Near-surfacemeasurements of meteorological variables werepredicted well over the entire diurnal cycle, althoughthe predicted sea-breeze onset was generally tooearly. A quantitative model evaluation for thenear-surface sites showed the model performance to bebetter than that from other studies, with Index ofAgreement (IOA) values of 0.8 (wind speed) and 0.96(temperature), compared with values of 0.5–0.6 (windspeed) and 0.33 (temperature) obtained from otherstudies.The availability of new higher resolution synopticanalyses should obviate the lack of spatial andtemporal resolution in synoptic inputs. Theincorporation of these higher resolution synopticinputs and new parameterisation schemes should improvefuture model performance.     

The KABEG’97 field experiment: An aircraft-based study of katabatic wind dynamics over the Greenland ice sheet

The aircraft-based experiment KABEG97 (Katabatic wind and boundary-layer front experiment around Greenland) was performed in April/May 1997. During the experiment, surface stations were installed at five positions on the ice sheet and in the tundra near Kangerlussuaq, West Greenland. A total of nine katabatic wind flights were performed during quite different synoptic situations and surface conditions, and low-level jets with wind speeds up to 25m s^-1 were measured under strong synoptic forcing of the katabatic wind system. The KABEG data represent a unique data set for the investigation of katabatic winds. For the first time, high-resolution and accurate aircraft measurements can be used to investigate the three-dimensional structure of the katabatic wind system for a variety of synoptic situations.Surface station data show that a pronounced daily cycle of the near-surface wind is present for almost all days due to the nighttime development of the katabatic wind. In a detailed case study the stably-stratified boundary layer over the ice and the complex boundary-layer structure in the transition zone ice/tundra are investigated. The katabatic wind system is found to extend about 10 km over the tundra area and is associated with strong wind convergence and gravity waves. The investigation of the boundary-layer dynamics using the concept of a two-layer katabatic wind model yields the results that the katabatic flow is always a shooting flow and that the pure katabatic force is the main driving mechanism for the flow regime, although a considerable influence of the large-scale synoptic forcing is found as well.     

Near-surface wind estimates using statistics from a planetary boundary-layer model

This paper shows the possibilities of a procedure for estimating near-surface wind statistics, by means of the numerical integration of a simple boundary-layer model with a second-order turbulent closure. Standard and easily available synoptic data are used as initial and boundary conditions. The development of this methodology is impelled by increasing requirements of a quick and precise knowledge of the wind characteristics in many regions of South America, which confronts the serious limitation of a reduced number of extended observational series, scattered over a vast continent. In order to evaluate the methodology, near-surface wind statistics from observed data at two locations are compared with model output statistics. Relative errors are about 0.2 for daily mean velocities and about 0.1 for weekly mean velocities, observed and computed time series being highly correlated in both cases. Calculated frequency distribution of wind directions is in good agreement with the observed one, and the absolute mean error in the daily mean wind direction is about 20 deg. Even though a wide variety of large-scale synoptic situations has been indirectly considered through boundary conditions, basic model output statistics resemble fairly well those observed at different levels between the surface and 100 m.     

夏秋季节天气系统对边界层内大气中PM10浓度分布和演变过程的影响

边界层内的气压场直接影响区域性的大气环境质量,天气系统的变化与边界层气压场形势直接相关。根据2000～2006年大气环境监测资料的日均值和日增(减)量分布图,同时对夏秋季节主要的天气系统,副热带高压和台风进行耦合诊断分析,得出西太平洋高压和台风的时空演变对我国环境质量有十分重要的影响,这种影响主要是形成的高压均压场对污染物有累积效应,也出现污染物的汇聚,而其周边流场对区域污染物有输送作用。此外,天气系统的降水分布又对大气污染物有清除的作用。而且天气形势演变的空间和周期性形成了大气环境的区域性和过程性等复杂的特征。本文选择典型个例,进行剖析研究。在地面高压或500 hPa高度上5880 gpm等高线控制区内,造成大范围的静稳型区域性污染物的增量过程。在副高周边地区的雨区内经常是PM10的谷值期。夏秋季节台风近周边和远周边的影响区,经常是PM10峰值或较重污染物浓度出现区域。     

A Statistical Model for the Prediction of Wind-Speed Probabilities in the Atmospheric Surface Layer

Wind fields in the atmospheric surface layer (ASL) are highly three-dimensional and characterized by strong spatial and temporal variability. For various applications such as wind-comfort assessments and structural design, an understanding of potentially hazardous wind extremes is important. Statistical models are designed to facilitate conclusions about the occurrence probability of wind speeds based on the knowledge of low-order flow statistics. Being particularly interested in the upper tail regions we show that the statistical behaviour of near-surface wind speeds is adequately represented by the Beta distribution. By using the properties of the Beta probability density function in combination with a model for estimating extreme values based on readily available turbulence statistics, it is demonstrated that this novel modelling approach reliably predicts the upper margins of encountered wind speeds. The model’s basic parameter is derived from three substantially different calibrating datasets of flow in the ASL originating from boundary-layer wind-tunnel measurements and direct numerical simulation. Evaluating the model based on independent field observations of near-surface wind speeds shows a high level of agreement between the statistically modelled horizontal wind speeds and measurements. The results show that, based on knowledge of only a few simple flow statistics (mean wind speed, wind-speed fluctuations and integral time scales), the occurrence probability of velocity magnitudes at arbitrary flow locations in the ASL can be estimated with a high degree of confidence.     

2011年12月3-7日山东省连续大雾分析

利用2011年12月1-7日的常规探测资料和NCEP再分析资料,采取诊断分析的方法,分析了山东省大范围连续性大雾产生的天气背景、温湿条件和大气层结等特征。结果表明：此次连续大雾发生在稳定的大气层结下,高空中纬度环流平直,地面气压场上表现为从东西伯利亚到华南一个庞大的变性高压区（山东处于高压的中部）,天气形势稳定;在高分辨率可见光云图上,大雾图像顶部较光滑、边缘较清晰,在红外云图上特征不明显;近地面层是弱的东到东北风（风力在2 m·s^-1以下）,既有利于海洋上暖湿气流的平流输送,也为较冷的下垫面的建立创造了条件;大气边界层湍流活跃,湍流动力垂直输送明显;大雾初期低空有明显的逆温层,天气状况较好,辐射降温明显,大雾性质以辐射雾为主,中后期是暖湿空气平流到温度较低的下垫面上时冷却而形成,以平流雾为主。     

Boundary-layer dynamics of the extremely arid northern part of Chile

A boundary-layer field experiment in the extremely arid northern part of Chile was carried out in July, 1970. Captive radiosonde data, pibal soundings and surface observations in a three-point zonal cross-section at 23 deg lat. S are analyzed. Evidence of a thermal response in the boundary-layer circulation and daytime subsidence enhancement is presented. Surface windspeeds seem to increase in the general upslope direction while wind components along the contour lines are apparent in the central strip, in accordance with earlier results for Pampa de la Joya, Peru.     

Mesoscale modeling of katabatic winds over Greenland and comparisons with AWS and aircraft data

Summary Simulations of the katabatic wind system over the Greenland ice sheet for the two months April and May 1997 were performed using the Norwegian Limited Area Model (NORLAM) with a horizontal resolution of 25 km. The model results are intercompared and validated against observational data from automatic weather stations (AWS), global atmospheric analyses and instrumented aircraft observations of individual cases during that period. The NORLAM is able to simulate the synoptic developments and daily cycle of the katabatic wind system realistically. For most of the cases covered by aircraft observations, the model results agree very well with the measured developments and structures of the katabatic wind system in the lowest 400 m. Despite NORLAM’s general ability of reproducing the four-dimensional structure of the katabatic wind, problems occur in cases, when the synoptic background is not well captured by the analyses used as initial and boundary conditions for the model runs or where NORLAM fails to correctly predict the synoptic development. The katabatic wind intensity in the stable boundary layer is underestimated by the model in cases when the simulated synoptic forcing is too weak. An additional problem becomes obvious in cases when the model simulates clouds in contrast to the observations or when the simulated clouds are too thick compared to the observed cloud cover. In these cases, the excessive cloud amount prevents development of the katabatic wind in the model. Received September 22, 2000/Revised March 16, 2001     

The use of successive dynamic initialization by nudging to simulate cyclogenesis during GALE IOP 1

Summary Synoptic/diagnostic case studies have increasingly come to rely on numerical simulations started from some initial state after which the model generated fields receive no further information from observed data. The purpose of this paper is to demonstrate the use of a dynamic data assimilation technique based on nudging to create a dynamically consistent high-resolution four-dimensional data set that can be used for synoptic diagnostic studies. The nudging technique is applied in the Goddard Mesoscale Atmospheric Simulation System (GMASS) using the 3-h radiosonde data collected during GALE IOP 1. A unique aspect of this application is nudging toward data analyses for which the areal coverage shifts with time. One of the two nudging simulations assimilates surface pressure in addition to the temperature, mixing ratio, and wind components. The nudging values are determined by linear interpolation between 3-h observation times. Assuming a linear variation of the assimilated value in time leads to estimates of the nudging coefficients which take into account the accuracy of the observations.Both nudging simulations are more accurate in terms of S hand root-mean-square error (RMSE) scores than a control sunulation without successive initialization. The nudging simulation with surface pressure is more accurate than the nudging simulation without surface pressure assimilation for this case. The simulation with surface pressure nudging captures the surface cyclogenesis and the associated strong rise-fall couplet in the 500 hPa height field. It also exhibits the strongest ageostrophic flow and exit region vertical circulation associated with a jet streak on the western side of the intensifying upper-level trough.The data sets made possible by the dynamic assimilation/ simulation cycles are dynamically consistent, have high spatial and temporal resolution and are ideally suited for diagnostic studies. Examples presented include the evolution of the ageostrophic flow associated with the exit region of an upper-level jet propagating toward the base of an intensifying trough with increasing cyclonic curvature of the flow. The nudging simulation with surface pressure provides the resolution and accuracy required to depict the rapid transformation (within a 12-h period) of the exit region ageostrophic flow from predominantly cross contour to along contour as the jet streak approaches the base of the trough.With 19 Figures     

用EVAP方法反演冷流暴雪的风场及检验

EVAP(Extended VAP)雷达反演方法是在忽略空气垂直运动速度而考虑云雨粒子下落速度影响的条件下,导出从雷达多仰角多普勒速度资料产生不同高度上的水平径向速度计算公式,然后利用VAP方法来反演出等高面上的二维水平风场.利用烟台多普勒天气雷达资料,采用EVAP方法反演了2005年12月山东半岛两个风场背景条件和暴雪落区均有明显差异的冷流暴雪个例水平风场.将雷达实测的径向速度、反射率因子插值到各等高面上,并与反演的同等高面风场叠加显示,以此分析反演的风场是否与原始雷达数据相符.此外,还与地面自动站风场、高空风场等实测资料进行对比,综合检验反演结果的合理性和可靠性,并进一步分析了冷流暴雪的中尺度风场结构.结果表明,反演风场与实测资料相符;EVAP方法对强风和弱风的冷流暴雪个例都可反演出不同高度的中尺度风场结构,反演的西南风与东北风、西北风与东北风之间的切变线和强回波带相对应,揭示了暴雪产生的动力机制,说明EVAP方法对不同风场条件的暴雪中尺度系统都有一定的反演能力.高时空分辨率的EVAP反演风场弥补了天气图上仅能分析出山东半岛西北风的缺憾,且反演过程中一个体扫的计算时间不足1 min,因此EVAP反演的中尺度风场信息适合在冷流降雪的短时临近预报业务中应用.     

Geostrophic Wind, Gradient Wind, Thermal Wind And The Vertical Wind Profile – A Sample Analysis Within A Planetary Boundary Layer Over Arctic Sea-Ice

For the interpretation of many boundary-layer field experiments the geostrophic wind is needed as an external parameter. However, quite often and especially in remote areas this wind is not known at all or difficult to determine because there are not enough measurements of the surface pressure.Here it is shown how measurements carried out with the HELIPOD system, a helicopter-borne meteorological turbulence measuring system, may be used to evaluate the geostrophic wind. This is done by the analysis of the pressure field at different heights. An additional analysis of the temperature field in the same heights allows for the discussion of the quality of the derived geostrophic wind. An intercomparison with the vertical wind profile enables us to discuss the influence of the curvature of the isobars. From this, finally, also the gradient wind can be estimated.     

Simulation of three-dimensional wind flow over complex terrain in the atmospheric boundary layer

A three-dimensional model for wind prediction over rough terrain has been developed for practical use. It is a compromise between hydrodynamic and objective wind models. The proposed model includes: (1) a statistical model to predict the wind velocity and potential temperature at anemometer height at observing stations, (2) the drainage wind model expressed by Prandtl's analytic solution for the slope wind, (3) the Businger-Dyer surface-layer formulation which considers the surface energy budget and (4) the model for three-dimensional boundary-layer solutions to the stationary flow. In this model, mass consistency is guaranteed by using flow fields that satisfy the continuity equation. Model predictions show good agreement with the observations.     

The impact of the assimilation of scatterometer winds on surface wind and wave forecasts

Recent work has demonstrated that surface marine winds from the Bureau of Meteorology's operational Numerical Weather Prediction (NWP) systems are typically underestimated by 5 to 10%. This is likely to cause significant bias in modelled wave fields that are forced by these winds. A simple statistical adjustment of the wind components is shown to reduce the observed bias in Significant Wave Height considerably. The impact of increasing the vertical resolution of the NWP model and assimilating scatterometer data into the model is assessed by comparing the resulting forecast wind and waves to observations. It is found that, in general, the inclusion of scatterometer observations improves the accuracy of the surface wind forecasts. However, most of the improvement is shown to arise from the increased number of vertical levels in the atmospheric model, rather than directly from the use of the observations. When the wave model is forced with surface winds from the NWP model that includes scatterometer data, it is found that the scatterometer assimilation does not reduce the systematic bias in surface wave forecasts, but that the random errors are reduced.     

Land-Surface Heterogeneity Effects in the Planetary Boundary Layer

We investigate the cumulative added value of assimilating temperature, moisture, and wind observations in the three-dimensional non-hydrostatic Fifth-Generation Pennsylvania State University/National Center for Atmospheric Research Mesoscale Model MM5 and use these forecasts to analyze the relationship between surface forcing and planetary boundary-layer (PBL) depth. A data assimilation methodology focused on the surface and the PBL, previously tested in a one-dimensional version of MM5, is applied to 29 May, 6 June, and 7 June 2002 during the International $\hbox {H}_{2}\hbox {O}$ Project over the Southern Great Plains. Model-predicted PBL depth is evaluated against PBL depth diagnosed from data across 4,800 km of airborne lidar data (flight tracks 100–300 km long). The forecast with data assimilation verifies better against observations and is thus used to investigate the environmental conditions that govern PBL depth. The spatial structure in PBL depth is found to be most affected by spatial variations in surface buoyancy flux and capping inversion strength. The spatial scales of surface flux forcing reflected in the PBL depth are found through Fourier analysis and multiresolution decomposition. Correlations are ${<}0.50$ at scales of 64 km or less and increase at larger scales for 29 May and 6 June, but on 7 June low correlations are found at all scales, possibly due to greater within-PBL wind speeds, a stronger capping inversion on this day, and clouds. The results suggest a minimum scale, a function of wind speed, below which heterogeneity in surface buoyancy fluxes is not reflected directly in PBL depth.