Evaporation estimates from Nasser Lake, Egypt, based on three floating station data and Bowen ratio energy budget

Nasser Lake is located in a hyper-arid region in the south of Egypt. Evaporation is by far the most important factor in explaining the water losses from the lake. To obtain better management scenarios for Nasser Lake, an accurate estimation of the lake evaporation losses thus is essential. This paper presents an update of previous evaporation estimates, making use of local meteorological and hydrological data collected from instrumented platforms (floating weather stations) at three locations on the lake: at Raft, Allaqi, and Abusembel (respectively at 2, 75, and 280 km upstream of the Aswan High Dam). Results from six conventional evaporation quantification methods were compared with the values obtained by the Bowen ratio energy budget method (BREB). The results of the BREB method showed that there is no significant difference between the evaporation rates at Allaqi and Abusembel. At Raft, higher evaporation rates were obtained, which were assumed to be overestimated due to the high uncertainty of the Bowen ratio (BR) parameter. The average BR value at Allaqi and Abusembel was used to eliminate this overestimates evaporation. Variance-based sensitivity and uncertainty analyses on the BREB results were conducted based on quasi-Monte Carlo sequences (Latin Hypercube sampling). The standard deviation of the total uncertainty on the BREB evaporation rate was found to be 0.62 mm day^?1. The parameter controlling the change in stored energy, followed by the BR parameter, was found to be the most sensitive parameters. Several of the six conventional methods showed substantial bias when compared with the BREB method. These were modified to eliminate the bias. When compared to the BREB-based values, the Penman method showed most favorably for the daily time scale, while for the monthly scale, the Priestley–Taylor and the deBruin–Keijman methods showed best agreement. Differences in mean evaporation estimates of these methods (against the BREB method) were found to be in the range 0.14 and 0.36 mm day^?1. All estimates were based calculations at the daily time scale covering a 10-year period (1995–2004).     

A comparison of different methods of estimating energy‐limited evapotranspiration in the peace river region of British Columbia

Abstract

The performances of eight methods for estimating daily energy‐limited evapotranspiration (E_e) were compared with reliable values for the Peace River region of British Columbia. They included the methods of Priestley and Taylor (PT), Jensen and Haise (JH), Hargreaves (H) and Makkink (M), the first and second equations of Baier and Robertson (BR1 and BR2), and the modified methods of Blaney and Criddle (BC) and Thornthwaite (T). The reference data were obtained from previous workers, who in 1977 and 1979 made micrometeorological measurements of E_e using the Bowen ratio method.

The relationships between estimated and measured E_e values excluding the T method in 1977 had correlation coefficients (R) ranging from 0.65 to 0.84. These were not significantly different at the 5% level. The Standard Errors of the Estimate (SEE) ranged from 0.43 to 1.69 mm d^?1. The PT, JH, BR1 and T methods had low SEE values, whereas the BR2, H, BC and M methods had high SEE values. Six of the eight methods (PT, JH, H, M, BR2 and BC) were calibrated for local conditions using 1979 data. After calibration, the methods were tested with the 1977 data. The results indicated significant improvement in the fit of four of the methods (H, M, BR2 and BC). Overall, it was concluded that after calibration, six of the eight methods had predictive power and fit that were not significantly different at the 5% level.     

A comparison of two methods for the assimilation of altimeter data into a shallow-water model

Two simple methods for assimilating ocean surface pressure data into a three-layer adiabatic primitive equation model are tested and compared using a twin-experiment approach. One of the methods is based on a recently presented direct insertion scheme using a quasi-geostrophic model. The method explicitly avoids making changes to the potential vorticity fields in the lower layers and the vertical current structure after assimilation is determined by this criterion. The modification required for a primitive equation framework are discussed and a comparison is made with the more familiar ‘nudging’ method of assimilation. We use root mean square errors to quantify the response of the model to the assimilation of a single complete surface pressure field and also to repeated or intermittent data available every 20 days for a period of 1 year. The two methods are almost equally effective in the short term, although the direct insertion scheme appears to be more effective in the longer term. It is suggested that this is because insertion maintains a geostrophic balance at all levels and thus avoids the generation of internal gravity waves which are needed when nudging is used to restore geostrophic currents in the deeper layers.     

南极探空与两套再分析资料的对比分析

基于南极18个站点探空气象观测数据对欧洲中期天气预报中心的再分析数据(ERA-Interim)和美国国家环境预报中心的再分析数据(NECP)在南极地区高层大气的适用性进行验证。结果表明:在南极上空,随着高度抬升,探空气象观测数据与两套再分析数据中四个气象要素的差值均逐渐变大,再分析数据数值愈加偏离实际观测数值。两套再分析数据的位势高度和温度与探空观测数据偏差较小;风向则和探空观测数据相差甚远;两套再分析数据的风速与探空观测数据在300 hPa偏差较大。在季节变化中,南极的春季,再分析数据中的位势高度和温度与探空观测数据相差较大,在其他季节相差相对较小。再分析数据中的风速与探空观测数据在南极的夏季相差较小。再分析数据中的风向与探空观测数据存在较大偏差,且差值没有明显的季节变化。尽管两套再分析数据都存在很大偏差,但ERA-Interim数据整体上优于NCEP数据。对比分析也表明,采用这些再分析资料作为初始条件和边界条件驱动南极区域大气模式将带来较大的误差。未来需要加强南极探空观测,改进再分析资料同化和数值模拟系统。     

Detection of climate signal in dendrochronological data analysis: a comparison of tree-ring standardization methods

Summary Tree-ring standardization methods were compared. Traditional methods along with the recently introduced approaches of regional curve standardization (RCS) and power-transformation (PT) were included. The difficulty in removing non-climatic variation (noise) while simultaneously preserving the low-frequency variability in the tree-ring series was emphasized. The potential risk of obtaining inflated index values was analysed by comparing methods to extract tree-ring indices from the standardization curve. The material for the tree-ring series, previously used in several palaeoclimate predictions, came from living and dead wood of high-latitude Scots pine in northernmost Europe. This material provided a useful example of a long composite tree-ring chronology with the typical strengths and weaknesses of such data, particularly in the context of standardization. PT stabilized the heteroscedastic variation in the original tree-ring series more efficiently than any other standardization practice expected to preserve the low-frequency variability. RCS showed great potential in preserving variability in tree-ring series at centennial time scales; however, this method requires a homogeneous sample for reliable signal estimation. It is not recommended to derive indices by subtraction without first stabilizing the variance in the case of series of forest-limit tree-ring data. Index calculation by division did not seem to produce inflated chronology values for the past one and a half centuries of the chronology (where mean sample cambial age is high). On the other hand, potential bias of high RCS chronology values was observed during the period of anomalously low mean sample cambial age. An alternative technique for chronology construction was proposed based on series age decomposition, where indices in the young vigorously behaving part of each series are extracted from the curve by division and in the mature part by subtraction. Because of their specific nature, the dendrochronological data here should not be generalized to all tree-ring records. The examples presented should be used as guidelines for detecting potential sources of bias and as illustrations of the usefulness of tree-ring records as palaeoclimate indicators.     

Measurements of tropospheric OH concentrations: A comparison of field data with model predictions

Using long path UV absorption spectroscopy we have measured OH concentrations close to the earth's surface. The OH values observed at two locations in Germany during 1980 through 1983 range from 0.7×10⁶ to 3.2×10⁶ cm^-3. Simultaneously we measured the concentrations of O₃, H₂O, NO, NO₂, CH₄, CO, and the light non methane hydrocarbons. We also determined the photolysis rates of O₃ and NO₂. This allows calculations of OH using a zero dimensional time depdendent model. The modelled OH concentrations significantly exceed the measured values for low NO _x concentrations. It is argued that additional, so far unidentified. HO _x loss reactions must be responsible for that discrepancy.     

Evaporation from the desert: Some preliminary results of HEIFE

As part of a feasibility study for HEIFE, a Sino-Japanese Cooperative Research Program on Atmosphere-Land Surface Processes in Heihe River Basin, fluxes of water vapour are estimated above and below the sand surface. It is found that during the daytime under clear skies, the water vapour flux is directed towards the surface from both the atmosphere above and the sand below.     

A comparison of two methods of estimating transpiration rates from a Sitka spruce plantation

Estimates of hourly transpiration from a 16–17 yr old Sitka spruce forest were calculated from the Penman-Monteith combination equation and compared with estimates from an eddy correlation/energy balance method.Canopy conductances were estimated from stomatal conductances measured using null balance diffusion porometers and took account of canopy variations of stomatal conductance and needle area index.Vertical heat fluxes were measured by the eddy correlation method; transpiration fluxes were then estimated from an energy balance of the forest.There was not a 1:1 relationship between the estimates of transpiration from the two methods. The major sources of error were concluded to be (i) difficulties of estimating the variation in stomatal conductance and leaf area through the canopy, (ii) errors in the value of total leaf area index, and (iii) errors in stomatal conductance measurements.The eddy correlation method was suggested as the more useful for future studies of the variation of forest transpiration in time or space, because the Penman-Monteith equation requires extensive biological measurements.     

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.     

Assessing interactions of multiple stressors when data are limited: A Bayesian belief network applied to coral reefs

Bayesian belief networks are finding increasing application in adaptive ecosystem management where data are limited and uncertainty is high. The combined effect of multiple stressors is one area where considerable uncertainty exists. Our study area, the Great Barrier Reef is simultaneously data-rich – concerning the physical and biological environment – and data-poor – concerning the effects of interacting stressors. We used a formal expert-elicitation process to obtain estimates of outcomes associated with a variety of scenarios that combined stressors both within and outside the control of local managers. There was much stronger consensus about certain stressor effects – such as between temperature anomalies and bleaching – than others, such as the relationship between water quality and coral cover. In general, the expert outlook for the Great Barrier Reef is pessimistic, with the potential for climate change effects potentially to overshadow the effects of local management actions.     

Plastic Drawdown: A rapid assessment tool for developing national responses to plastic pollution when data availability is limited,as demonstrated in the Maldives

Governments are increasingly supporting initiatives to address plastic pollution, but efforts are largely opportunistic or driven by national socio-political priorities. There is an urgent need to move away from piecemeal single product instruments (e.g. single use plastic bag taxes or plastic straw bans) to deliver system-wide strategies that minimise the most pervasive sources of plastic pollution. Developing a common understanding of a jurisdiction’s plastic waste stream and the solutions available to decision-makers is vital to build consensus across stakeholders and to align on an evidence-based portfolio of priority instruments.This paper presents the Plastic Drawdown framework as a boundary-spanning tool to quickly create a coherent, relevant, and credible analysis and visualisation for stakeholders of plastic waste, leakage hotspots and minimisation opportunities. Using a new plastic waste modelling framework with a consultative structure, Plastic Drawdown explores plastic waste and leakage over a ten-year period and assesses impacts of policy instruments on this projection. Plastic Drawdown is adaptable to the data poor environment typical of many countries and designed as a rapid assessment tool to support the decision making of governments operating in a highly resource-constrained context.The Maldives is used as a case study to show the utility of the tool, where it highlighted strategies with the potential to reduce leakage of plastic waste into the marine environment by up to 85% by 2030. Plastic Drawdown built the case for phasing out single-use plastic waste across the Maldives and supported the Government’s decision to set ambitious targets, as announced at the United Nations General Assembly in 2019.     

A comparison of the static and flow methods for the detection of ice nuclei

The use of the membrane-filter processing chamber to study ice nuclei concentrations has become wide-spread since its introduction by Bigg et al. in 1961. The technique is convenient because of the simplicity of its operation and because it could be run remote from the place of field study. It has however been found to suffer from a number of drawbacks, namely, the volume effect, the chamber height effect, the vapour depletion effect, etc.Comparison of the results obtained by running a traditional filter processor and a continuous flow chamber under identical temperature and humidity conditions for polluted Manchester air has shown that the latter technique detects more ice nuclei than the former one by a factor of about 14±4. These results confirm that the filter technique suffers from the vapour depletion effect. The present results are in agreement with Bigg et al., Mossop and Thorndike, and King.In the light of our findings the filter technique does not appear to be a standard method. Therefore the ice nuclei data obtained with the filter method should not be extended to clouds in order to study their microphysical properties.     

A comparison of reanalyses in the tropical stratosphere. Part 3: inclusion of the pre-satellite data era

 In this study, previous evaluations of the monthly mean structure of the tropical lower stratosphere in reanalyzed datasets are extended to include the period 1958–1978, when no satellite-based observations were available. It is shown that a large discontinuity, in temperatures near the tropical tropopause, in the NCEP data occurred when the Tiros Operational Vertical Sounder (TOVS) became operational. When only rawinsonde data were available, the tropopause temperatures in the NCEP dataset are in better agreement with ERA data for TOVS period. Both NCEP and NASA reanalyses show similar deviations from the ERA data in the TOVS renalyses show similar deviations from the ERA data in the TOVS period. There is also a stepwise change in the lower stratospheric meridional velocity when the TOVS data were introduced into the NCEP reanalyses. This discontinuity is such that in the 1958–1978 period, the annual cycle in zonal mean meridional velocity in the NCEP data resembles that of the ERA data in the 1979–1993 period. The differences are shown to result from large changes in the local meridional flow in the Indonesian region. The temporal consistency of the QBO is examined; it is shown that the NCEP assimilation system is sensitive to the data available. There is a change in the zonally asymmetric structure of the zonal wind over time, presumably related to the changes in input data and the inability of the model to represent the three-dimensional structure of the tropical lower stratosphere. These results provide further evidence of the value of rawinsonde data in data assimilation systems as well as the need to use satellite radiance data in an appropriate manner. Received: 7 April 1997 / Accepted: 4 September 1998     

A comparison of statistical downscaling and climate change factor methods: impacts on low flows in the River Thames, United Kingdom

Strategic-scale assessments of climate change impacts are often undertaken using the change factor (CF) methodology whereby future changes in climate projected by General Circulation Models (GCMs) are applied to a baseline climatology. Alternatively, statistical downscaling (SD) methods apply climate variables from GCMs to statistical transfer functions to estimate point-scale meteorological series. This paper explores the relative merits of the CF and SD methods using a case study of low flows in the River Thames under baseline (1961–1990) and climate change conditions (centred on the 2020s, 2050s and 2080s). Archived model outputs for the UK Climate Impacts Programme (UKCIP02) scenarios are used to generate daily precipitation and potential evaporation (PE) for two climate change scenarios via the CF and SD methods. Both signal substantial reductions in summer precipitation accompanied by increased PE throughout the year, leading to reduced flows in the Thames in late summer and autumn. However, changes in flow associated with the SD scenarios are generally more conservative and complex than that arising from CFs. These departures are explained in terms of the different treatment of multidecadal natural variability, temporal structuring of daily climate variables and large-scale forcing of local precipitation and PE by the two downscaling methods.     

湍流通量计算方法和误差的比较研究

近地层的风、温、湿梯度以及辐射和土壤热通量资料在过去的几十年间被广泛地用于计算陆-气感热、潜热通量.然而,不同的计算方法之间存在一些差异,随意的选用可能会造成计算结果存在较大的误差.文中利用绿洲系统能量与水分循环过程观测试验的2005年6月甘肃金塔绿洲中部观测资料,在分析了观测期间近地层微气象特征的基础上,对鲍恩比法、空气动力学法(包含两种不同计算方案)和变分法计算的绿洲农田下垫面的湍流通量进行了比较,并通过敏感性实验和理论分析探讨了不同方法间计算结果的差异和误差的来源.结果表明:不同方法计算的湍流通量特征存在明显差异.鲍恩比法从理论上满足能量平衡关系,但在昼夜交替和夜晚时出现了计算不稳定现象,因此在这些时段不适用.变分法避免了鲍恩比法计算夜间湍流通量的不稳定性,而且二者计算的白天湍流通量有较好的相关性;同时变分法由于加入了辐射和土壤热通量信息,明显改善了空气动力学法白天的能量平衡状况;变分法计算得到的潜热通量对辐射和微气象条件有较为合理的响应;对数据误差的敏感性试验表明,鲍恩比法和空气动力学法应用于绿洲下垫面上对观测误差较为敏感,而变分法相对比较稳定.进一步的分析表明,鲍恩比法在-2<β<0的范围内由于方法本身局限而适用性较差;而空气动力学方法计算结果的差异和误差来源于所应用的相似性函数.因而与鲍恩比法和空气动力学法计算的湍流通量值相比较,变分法的计算结果更加合理和稳定,在绿洲下垫面有较好的适用性.该文研究方法和结果可为这些方法的使用提供参考.     

A comparison of microwave radiometric data and modeled snowpack conditions for Dye 2, Greenland

Summary Meteorological observations were recorded at Dye 2, Greenland during the summer of 1993 as part of a research program to identify interannual variations in melt occurrence on the Greenland ice sheet from satellite microwave data. The meteorological observations were used to drive and energy-balance model of the snowpack during 21 June to 13 July 1993. Time series of the meteorological observations and various model outputs were compared to a concurrent time series of Special Sensor Microwave/Imager (SSM/I) data for scan cells centered within 25 km of Dye 2. The satellite microwave observations clearly show an increase in snowpack emissivity at the same time that the model indicates liquid water forming in the snow. Diurnal melt-freeze cycles that occurred during mid June to early July resulted in an increase in the 37 GHz brightness temperature as great as 60K from the dry, refrozen snow in the morning to the wet snow of some afternoons. The effects of fresh snowfall, which tend to increase the brightness temperature, and of snow growth from melt-freeze metamorphism, which tend to decrease the brightness temperature, are also apparent in the microwave observations. The results of this work demonstrate the influence of daily weather variations on the microwave emissivity in the ice sheet's percolation zone and the usefulness of swath data to diagnose the diurnal cycle of melt.With 4 Figures     

A comparison of some aerodynamic resistance methods using measurements over cotton and grass from the 1991 California ozone deposition experiment

Measurements of dry, deposition velocities (V _d ) of O₃ (using the eddy correlation technique) made over a cotton field and senescent grass near Fresno California during July and August 1991 were used to test some dry deposition velocity models. Over the cotton field, the observed maximum daytimeV _d was about 0.8 cm s^–1 and the average nighttime value was about 0.2 cm s^–1. Over the grass, daytime values averaged about 0.2 cm s^–1 and nighttime values about 0.05 cm s^–1. Application of a site-specific model known as ADOM (Acid Deposition and Oxidant Model) over the cotton field generally overestimated the observations except for a few hours in the afternoon when the observations were underestimated The overestimation was attributed to inadequacies in the surface resistance formulation and the underestimation to uncertainties in the aerodynamic formulation. Unlike previous studies which focused on the role of surface resistance, we perform additional tests using a large variety of aerodynamic resistance formulae, in addition to those in ADOM, to determine their influence on the modelledV _d of O₃ over, cotton. Over grass, ADOM considerably overestimated the observations but showed improvement when other surface resistance formulations were applied.