A Comparison of Two Canopy Radiative Models in Land Surface Processes

This paper compares the predictions by two radiative transfer models-the two-stream approximation model and the generalized layered model （developed by the authors） in land surface processes -for different canopies under direct or diffuse radiation conditions. The comparison indicates that there are significant differences between the two models, especially in the near infrared （NIR） band. Results of canopy reflectance from the two-stream model are larger than those from the generalized model. However, results of canopy absorptance from the two-stream model are larger in some cases and smaller in others compared to those from the generalized model, depending on the cases involved. In the visible （VIS） band, canopy reflectance is smaller and canopy absorptance larger from the two-stream model compared to the generalized model when the Leaf Area Index （LAI） is low and soil reflectance is high. In cases of canopies with vertical leaf angles, the differences of reflectance and absorptance in the VIS and NIR bands between the two models are especially large. Two commonly occurring cases, with which the two-stream model cannot deal accurately, are also investigated. One is for a canopy with different adaxial and abaxial leaf optical properties; and the other is for incident sky diffuse radiation with a non-uniform distribution. Comparison of the generalized model within the same canopy for both uniform and non-uniform incident diffuse radiation inputs shows smaller differences in general. However, there is a measurable difference between these radiation inputs for a canopy with high leaf angle. This indicates that the application of the two-stream model to a canopy with different adaxial and abaxial leaf optical properties will introduce non-negligible errors.     

Southern Hemisphere blocking: A comparison between two indices

Summary Two different blocking indices are computed for the Southern Hemisphere for the period 1979–1985, one is based on sea level pressure and the other on upper tropospheric zonal wind. The Southern Hemisphere blocking climatology based on these two indices separately presents very similar results, mainly in identifying the region of maximum occurrence over eastern Australia and the western Pacific. The sea level presure index shows strong orographic influences, while the 250hPa zonal wind index is not affected by orography but does exaggerate seasonal variations. It is apparent that blocking occurrence in the eastern Australia and neighbouring areas has a link with climatological atmospheric features.With 8 Figures     

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.     

Uncertainty of global summer precipitation in the CMIP5 models: a comparison between high-resolution and low-resolution models

Worldwide, the majority of rapidly growing neighborhoods are found in the Global South. They often exhibit different building construction and development patterns than the Global North, and urban climate research in many such neighborhoods has to date been sparse. This study presents local-scale observations of net radiation (Q _* ) and sensible heat flux (Q _H ) from a lightweight low-rise neighborhood in the desert climate of Andacollo, Chile, and compares observations with results from a process-based urban energy-balance model (TUF3D) and a local-scale empirical model (LUMPS) for a 14-day period in autumn 2009. This is a unique neighborhood-climate combination in the urban energy-balance literature, and results show good agreement between observations and models for Q _* and Q _H . The unmeasured latent heat flux (Q _E ) is modeled with an updated version of TUF3D and two versions of LUMPS (a forward and inverse application). Both LUMPS implementations predict slightly higher Q _E than TUF3D, which may indicate a bias in LUMPS parameters towards mid-latitude, non-desert climates. Overall, the energy balance is dominated by sensible and storage heat fluxes with mean daytime Bowen ratios of 2.57 (observed Q _H /LUMPS Q _E )–3.46 (TUF3D). Storage heat flux (ΔQ _S ) is modeled with TUF3D, the empirical objective hysteresis model (OHM), and the inverse LUMPS implementation. Agreement between models is generally good; the OHM-predicted diurnal cycle deviates somewhat relative to the other two models, likely because OHM coefficients are not specified for the roof and wall construction materials found in this neighborhood. New facet-scale and local-scale OHM coefficients are developed based on modeled ΔQ _S and observed Q _* . Coefficients in the empirical models OHM and LUMPS are derived from observations in primarily non-desert climates in European/North American neighborhoods and must be updated as measurements in lightweight low-rise (and other) neighborhoods in various climates become available.     

Surface-layer flow in complex terrain: Comparison of models and full-scale observations

Four models of surface boundary-layer flow in complex terrain are compared with observations made at Blashaval Hill, North Uist, Scotland. The field experiment is described by Mason and King (1985). Three of the models are derived from the two-dimensional theory of Jackson and Hunt (1975) and are described in Mason and King (1985), Walmsley et al. (1986) and Troen and Petersen (1989). The fourth is a mass-consistent code based on Traci et al. (1979). The model results are in good agreement with each other and are generally within the observed range of variation ( ~ ± 16%) in normalized wind speed. For most wind direcions (7 of 11), model results of normalized wind speed at the summit were within 7% of the observed mean values. For some wind directions, calculations using the Guidelines of Walmsley et al. (1989) suggested that variations in surface roughness were important. This led us to apply one of our models incorporating nonuniform surface roughness. The lack of significant improvement for cases when water lay upstream of Blashaval Hill is attributed to compensating changes at summit and reference sites and to very local effects on the wind data. Sensitivity to topography lying to the west and northwest of Blashaval was also investigated. Results suggested an influence from those distant topographic features for some wind directions. When those features were incorporated, maximum errors in normalized wind speed at the summit were reduced from 18 to 13%.     

Productivity response of climax temperate forests to elevated temperature and carbon dioxide: a north american comparison between two global models

We assess the appropriateness of using regression- and process-based approaches for predicting biogeochemical responses of ecosystems to global change. We applied a regression-based model, the Osnabruck Model (OBM), and a process-based model, the Terrestrial Ecosystem Model (TEM), to the historical range of temperate forests in North America in a factorial experiment with three levels of temperature (+0 °C, +2 °C, and +5 °C) and two levels of CO₂ (350 ppmv and 700 ppmv) at a spatial resolution of 0.5° latitude by 0.5° longitude. For contemporary climate (+0 °C, 350 ppmv), OBM and TEM estimate the total net primary productivity (NPP) for temperate forests in North America to be 2.250 and 2.602 × 10¹⁵ g C ? yr^?1, respectively. Although the continental predictions for contemporary climate are similar, the responses of NPP to altered climates qualitatively differ; at +0 °C and 700 ppmv CO₂, OBM and TEM predict median increases in NPP of 12.5% and 2.5%, respectively. The response of NPP to elevated temperature agrees most between the models in northern areas of moist temperate forest, but disagrees in southern areas and in regions of dry temperate forest. In all regions, the response to CO₂ is qualitatively different between the models. These differences occur, in part, because TEM includes known feedbacks between temperature and ecosystem processes that affect N availability, photosynthesis, respiration, and soil moisture. Also, it may not be appropriate to extrapolate regression-based models for climatic conditions that are not now experienced by ecosystems. The results of this study suggest that the process-based approach is able to progress beyond the limitations of the regression-based approach for predicting biogeochemical responses to global change.     

两种反演地表净太阳辐射的参数化模式在北京地区的比较

地表净太阳辐射(NSSR)在地表辐射平衡、地-气能量交换、天气预报、气候变化和太阳能利用等研究方面具有重要的作用.利用卫星遥感技术能在广大的空间区域内快速地获取地表净太阳辐射的分布情况,特别是在无人观测的极地和海洋区域.本文基于CERES/SSF卫星数据,分别采用两种地表净太阳辐射(NSSR)的卫星反演参数化模式--Li模式与Masuda模式计算了北京市上甸子地区2005年1、4、7和10月的NSSR,并将反演结果与上甸子大气本底站的实测结果进行了对比分析,并对两种模式进行了修正以更好地反演北京地区的地表净太阳辐射.结果显示:Li模式和Masuda模式反演结果与实测结果间的平均绝对误差在晴空条件下分别为62.2和50.8W/m2,在云天条件下分别为82.1和71.8 W/m2;Li模式相对于Masuda模式具有偏大的趋势,在晴天和云天条件下两者的平均绝对偏差分别为11.5和10.2 W/m2;对Li模式和Masuda模式进行线性拟合回归后能有效减小反演过程中的系统性偏差,修改后的Li模式和Masuda模式在所有天空状况下反演的月平均NSSR值与观测值问的平均绝对误差分别为-1.8和-3.4 W/m2,均方根误差分别为19.6和26.0 W/m2.     

Some laboratory models of flow past the Alpine/Pyrenees mountain complex

Summary A series of laboratory experiments have been conducted to simulate the dynamical effects of the flow over and around a model Alpine/Pyrenean topography. The experimental study has employed a linearly salt-stratified rotating water channel through which the model topography is towed. Data have been collected for ranges of the similarity parameters (Rossby, Burger, Ekman and Froude numbers and the topography height to fluid depth ratio) which accord with their atmospheric values; only four of these parameters are independent. The ratio of the fluid depth to topography width need not be simulated because it does not appear as an independent parameter in the leading terms of the governing equations and boundary conditions. The results demonstrate the effects of flow direction on the horizontal motion patterns at various heights and numerous system parameter combinations. Vortex shedding is shown to occur for parameter ranges similar to those appropriate for the atmosphere; an empirical relation between the Strouhal and Rossby numbers is obtained and it is shown that the shedding periods is approximately twice the inertial period. The experiments demonstrate the flow splitting occurs in the vicinity of Geneva for both westerly and northwesterly flow. The importance of the Froude number in controlling the conditions under which fluid flows over or around the topography is illustrated, and the occurrence of lee waves for different values of the Froude and Rossby numbers (and different locations and flow directions) is described. Finally, it is shown that the advection of a parent cyclone from west to east along a path north of the Alps can generate a cyclone in the lower levels of the atmosphere in the vicinity of the Gulf of Genoa. It is suggested that these dynamical effects may play a role in triggering a lee cyclone which then might grow by other physical processes.

---

Einige Labormodelle von Strömungen im Einfluß des Alpen/Pyrenäen-Gebirgskomplexes

Zusammenfassung Es wurden eine Reihe von Laborexperimenten durchgeführt, um die dynamischen Effekte der Strömung über und um ein Modell der Alpen-Pyrenäentopographie zu simulieren. Bei der Experimentaluntersuchung wurde ein mit Salz linear geschichteter rotierender Wasserkanal, durch den das Modellgelände gezogen wird, verwendet. Es wurden Daten für Bereiche der Ähnlichkeitsparameter (Rossby-, Burger-, Ekman- und Froude-Zahl und das Verhältnis Geländehöhe zu Flüssigkeitstiefe) gesammelt entsprechend ihren atmosphärischen Werten. Nur vier von diesen Parametern sind unabhängig. Das Verhältnis der Flüssigkeitstiefe zur Geländebreite braucht nicht simuliert zu werden, weil es in den Haupttermen der bestimmenden Gleichungen und Randbedingungen nicht als unabhängiger Parameter aufscheint. Die Resultate veranschaulichen die Effekte der Strömungsrichtung auf die horizontalen Bewegungsmuster in verschiedenen Höhen und bei zahlreichen Kombinationen von Systemparametern. Es zeigt sich, daß Wirbelablösung in ähnlichen Parameterbereichen vorkommt, wie sie für die Atmosphäre geeignet sind. Man erhält eine empirische Beziehung zwischen der Strouhal- und der Rossby-Zahl und es zeigt sich, daß die Ablösungsperiode annähernd das Doppelte der Trägheitsperiode ausmacht. Die Experimente zeigen, daß Strömungsaufspaltung im Bereich von Genf sowohl für westliche, als auch für nordwestliche Strömungen vorkommt. Weiters wird die Bedeutung der Froude-Zahl für die Kontrolle der Bedingungen, unter welchen die Flüssigkeit über oder um die Topographie fließt, veranschaulicht und das Auftreten von Lee-Wellen für verschiedene Werte der Froude- und der Rossby-Zahl (und verschiedene Stellen und Strömungsrichtungen) beschrieben. Schließlich wird gezeigt, daß die Advektion einer Mutterzyklone von West nach Ost nördlich der Alpen eine Zyklone in den unteren Schichten der Atmosphäre im Bereich des Golfs von Genua erzeugen kann. Es wird darauf hingewiesen, daß diese dynamischen Effekte beim Auslösen einer Lee-Zyklone, welche dann durch andere physikalische Prozesse wachsen könnte, eine Rolle spielen kann.

---

---

With 15 Figures     

A stochastic weather generator applied to hydrological models in climate impact analysis

Summary A pattern recognition methodology for estimating local climate variables such as regional precipitation and air temperature using local observation and scenario information provided by GCMs is presented. We have adopted a three step approach: (a) Feature information extraction of climate variables, where weather patterns are expanded by the Karhunen-Loeve (K-L) orthogonal functional series; (b) Grey associative clustering of the feature vectors; (3) Stochastic weather generation by a Monte Carlo simulation. The methods described in this paper were verified using the temperature and precipitation data set of Wuhan, Yangtze river basin and the Shun Tian catchment, Dongjiang River in China. The proposed method yields good stochastic simulations and also provides useful information on temporal or spatial downscaling and uncertainty.With 4 Figures     

Boundary-layer flow over analytical two-dimensional hills: A systematic comparison of different models with wind tunnel data

Two mass consistent models (MATHEW and MINERVE) and two dynamic linearized models (MS3DJH/3R and FLOWSTAR) are used to simulate the mean flow over two-dimensional hills of analytical shape and of varying slope. The results are compared with detailed wind tunnel data (RUSHIL experiment at US EPA). Different numerical experiments have been performed, varying input data and control parameters, to test the data-processing methodology and to evaluate the minimum input data (for mass consistent models only) necessary to obtain a reliable flow field. The models behave differently according to the physical assumptions made and numerical procedure used: an assessment is then made in order to identify the proper solution for the different conditions of topography and wind data.     

Water droplet growth study in a continuous flow diffusion cloud chamber

A continuous flow diffusion cloud chamber has been used to study the growth of water droplets on NaCl aerosols of known size. Settling distances of droplets in the chamber have been recorded and compared to the theoretical ones generated on a computer. Using the droplet growth theory of Fukuta and Walter and a thermal accommodation coefficient of 1.0, the condensation coefficient was found to be 0.01.     

A comparison of greenness measures in two semi-arid grasslands

A field experiment was conducted during 1988 in a semi-arid grassland along a portion of the U.S./Mexico border in which the utility of NDVI-transformed data for estimating green vegetation amount and cover was explored. Results showed that, although there was significant difference in green and total biomass between the U.S. and Mexico sites, we were unable to find any correlation between either these vegetation parameters and NDVI values calculated from reflectance measurements. Based on these results, it would appear that semi-arid perennial grasslands can pose problems for remote sensing analysis due to their erectophile structure, the likelihood of significant accumulation of senescent biomass, and dominance of soil background in sparsely vegetated areas. Therefore, if remotely-sensed data, and indices derived from them, are to be used in global change models, it is critical to establish if these indices are sufficiently sensitive to distinguish long-term change from the seasonal and spatial variability in vegetation biomass normally found within these perennial grassland communities.     

Instability in Lagrangian stochastic trajectory models,and a method for its cure

A number of authors have reported the problem of unrealistic velocities (“rogue trajectories”) when computing the paths of particles in a turbulent flow using modern Lagrangian stochastic (LS) models, and have resorted to ad hoc interventions. We suggest that this problem stems from two causes: (1) unstable modes that are intrinsic to the dynamical system constituted by the generalized Langevin equations, and whose actual triggering (expression) is conditional on the fields of the mean velocity and Reynolds stress tensor and is liable to occur in complex, disturbed flows (which, if computational, will also be imperfect and discontinuous); and, (2) the “stiffness” of the generalized Langevin equations, which implies that the simple stochastic generalization of the Euler scheme usually used to integrate these equations is not sufficient to keep round-off errors under control. These two causes are connected, with the first cause (dynamical instability) exacerbating the second (numerical instability); removing the first cause does not necessarily correct the second, and vice versa. To overcome this problem, we introduce a fractional-step integration scheme that splits the velocity increment into contributions that are linear (U _i ) and nonlinear (U _i U _j ) in the Lagrangian velocity fluctuation vector U, the nonlinear contribution being further split into its diagonal and off-diagonal parts. The linear contribution and the diagonal part of the nonlinear contribution to the solution are computed exactly (analytically) over a finite timestep Δt, allowing any dynamical instabilities in the system to be diagnosed and removed, and circumventing the numerical instability that can potentially result in integrating stiff equations using the commonly applied explicit Euler scheme. We contrast results using this and the primitive Euler integration scheme for computed trajectories in a drastically inhomogeneous urban canopy flow.     

A comparison of boundary layer diffusion schemes in unstable conditions over land

We compare the results of a local and a nonlocal scheme for vertical diffusion in the atmospheric boundary layer with observations at the 200 m tower at Cabauw. This is done for a 12 h period during daytime on 31 May 1978, which is characterised by strong insolation, clear skies, moderately strong winds and weak advection. The local diffusion scheme uses an eddy diffusivity determined independently at each point along the vertical based on local vertical gradients of wind and virtual potential temperature, similar to the usual approach in atmospheric models. The nonlocal scheme determines an eddy diffusivity profile based on a diagnosed boundary-layer height and a turbulent velocity scale. It also incorporates nonlocal (vertical) transport effects for heat and moisture. The boundary-layer diffusion schemes are forced with the locally observed fluxes for heat and moisture. The outputs of the scheme are compared with the observed mean structure along the Cabauw tower, and the radiosonde profile at a nearby location (De Bilt). Overall, the nonlocal scheme transports moisture away from the surface more rapidly than the local scheme, and deposits the moisture at higher levels. The local scheme tends to saturate the lowest model levels unrealistically in comparison with the observations. We also compare the outputs of the two diffusion schemes with the results of a transilient model simulation. Subsequently, we study the impact on the model behaviour by varying important parameters in both diffusion schemes and we investigate the sensitivity to uncertainty in the environmental conditions. Finally, we study the interaction of the diffusion schemes with a simple surface flux scheme.     

Storm tracks and zonal mean flow variability: a comparison between observed and simulated data

A statistical technique is used to analyze the relation between monthly mean zonal flow and storm tracks activity in the observations and numerical simulations (ECHAM4 model). The singular value decomposition technique (SVD) has been used to correlate storm tracks and monthly mean wintertime anomaly fields. The analysis has been performed on data from January 1980 to December 1989 (NMC analyses) and on an ensemble of AGCM simulations with prescribed SST for the same period, separately in the Euro-Atlantic and Pacific sectors. We found good correlation between storm tracks activity and zonal flow in both regions. In both data and simulations the dominant SVD modes show that the storm tracks spatial displacement is in conjunction with jet shifts in the same direction. Our analysis suggests that the model is highly sensitive to the equatorial ocean forcing. Although the model produces an excessive response to El Niño and La Niña phases, it shows good capability of simulating the dynamical connection between storm tracks and jet.     

Disagreements between gradient-diffusion and Lagrangian stochastic dispersion models,even for sources near the ground

It is well known that if turbulent mass convection is modelled as diffusion, errors result unless trajectories from the source (ath) to the point of observation (z _p ) comprise many statistically-independent segments (Taylor, 1921). We show that this is not guaranteed merely by the Lagrangian timescale () at the source being small (e.g., source at ground), but that a better criterion istmax[(h), (z _p )], wheret is a typical travel time toz _p .     

A performance comparison for two lagrangian drifter designs

Abstract

We compare the water‐parcel‐following and position‐reporting performance of two recent drifter designs. One (Tristar‐II ) uses the ARGOS satellite navigation system, has a very large drogue: float frontal area ratio and a small velocity error resulting from wind drag and surface wave forces. The second design uses an internal Loran receiver and transmits its position every half hour by radio. Its drogue: float frontal area ratio is much smaller than the TRISTAR's; we wished to know if this difference caused a significant change in drifter trajectory. Over a 78‐h joint deployment, the two designs had nearly identical trajectories. Measured drift speeds ranged from 10 to 30 cms^?1. Wind speeds (an important potential biasing factor) ranged up to 15 ms^?l. Net velocity difference over the full deployment was only 0.1 cms^?1. Frontal trapping of both drifters during the latter part of the comparison period may have helped to minimize this difference. But we also observed little or no downwind slippage (the net velocity difference was less than 2 cm s^?1 and was nearly normal to the wind and wave directions) during the first third of the deployment, when winds were strongest and the constancy of the water properties measured at drogue depth indicated no frontal trapping. Because the trajectories of the drifters were so similar, the most important differences in their performance were due to their position‐reporting and deployment characteristics. Initial deployment of the tristar was easier, and its positioning method and lower power requirement allow much longer untended deployments. The Loran design gave more frequent and precise positioning information and, therefore, better resolution of short‐time‐scale velocity fluctuations. It was also easier to find at sea, and to recover and redeploy.