A Theoretical Model for the Study of Convective Turbulence Decay and Comparison with Large-Eddy Simulation Data

The development of a theoretical model fora decaying convective boundary layeris considered. The model relies on thedynamical energy spectrumequation in which the buoyancy andinertial transfer terms are retained,and a closure assumptionmade for both. The parameterization for thebuoyancy term is given providing a factorizationbetween the energy source termand its temporal decay. Regarding the inertialtransfer term a hypothesis ofsuperposition is used to describe theconvective energy source and time variationof velocity correlation separately.The solution of the budget equation for theturbulent kinetic energy spectrum is possible,given the three-dimensional initial energyspectrum. This is doneutilizing a version of the Kristensen et al.(see Boundary-Layer Meteorol. 47, 149–193)model valid for non-isotropic turbulence. During thedecay the locus of the spectralpeak remains at about the sameposition as the heat flux decreases.Comparison of the theoretical modelis performed against large-eddy simulationdata for a decaying convectiveboundary layer.     

Evidence for zonally-trapped propagating waves in the eastern atlantic from satellite sea surface temperature observations

It has been hypothesized (Moore et al., 1978; O'Brien et al., 1978), that equatorial upwelling and subsequent coastal upwelling on the eastern boundary of the Atlantic Ocean are the result of eastward propagating equatorially trapped Kelvin waves in the Atlantic. Concurrent satellite and ship sea surface temperature observations taken during the GATE experiment permit validation of the satellite data as well as relating sea surface temperature (SST) variability to the local current dynamics. A method based on cross-correlations and cross-spectra of the SST field at various locations is utilized to test the Kelvin wave hypothesis. Significant periodic variation of time lags in the SST variability in the eastern Atlantic is observed by the spectral techniques. Satellite data for the 1974 summer show periodic variability which fits either eastward or westward propagating waves with 1 m s^-1 phase speed, i.e., SST supports the quasi-continuous presence of Kelvin or Yanai waves. We find no evidence for a seasonally solitary eastward propagating signal in the eastern Atlantic from SST.     

Do large-area-average temperature series have an urban warming bias?

Conclusions The arguments presented by Wood and his criticisms of the methods used by Jones et al. are largely fallacious and are generally based on misconceptions and unwarranted assumptions. This does not, of course, mean that the Jones et al. data are perfect. Jones et al. (1986a, b) have clearly stated that, at the regional level, there may be residual uncertainties in their gridded data set. Furthermore, they have noted that there is a residual uncertainty in the global-mean change since late last century of ±0.2 °C (Wigley et al., 1986), although the main reasons for this uncertainty do not relate to urban warming. Further studies of possible urban warming biases are certainly warranted, particularly at the regional scale. However, for averages over continental-scale areas upwards, it is unlikely that any significant urban warming bias remains.     

Stability dependence of the temperature structure parameter

Experimental data of C _T ², determined during various experiments in the surface layer, are compared with several functions giving the stability dependence of the temperature structure parameter. The universal function of the dimensionless temperature gradient by Skeib (1980) follows very well the experimental data and the empirical function by Wyngaard et al. (1971). This function can be used in an inertial-dissipation method.     

An improved method for integrating the Mixed Spectral Finite Difference (MSFD) model equations

In their Mixed Spectral Finite Difference (MSFD) model for flow over complex terrain, Beljaars et al. (1987) solve a set of coupled, second-order ordinary differential equations (ODEs) for the first-order perturbations to the logarithmic velocity profile caused by nonuniform surface roughness and topography. To solve this set of ODEs, they employ a Forward Euler Shooting Method. It is demonstrated here that the shooting method is computationally unstable for this problem. An absolutely stable finite-difference method based on a block tridiagonal LU factorization of the finite-difference matrix is presented. The advantages of the present algorithm over the method used by Beljaars et al. are demonstrated both by theoretical argument and numerical experiment.     

Formulation of the thermal internal boundary layer in a mesoscale model. II. Simulations with a level-2.5 turbulence closure

Physick et al. (1989) have discussed some difficulties associated with simulation of the sea-breeze thermal internal boundary layer (TIBL), using a numerical model containing a profile exchange coefficient formulation developed by O'Brien (1970). They suggested that a closure scheme based on a prediction of turbulent kinetic energy (TKE) would be able to resolve the TIBL better than a profile scheme. This Research Note reports simulations of the case discussed in I, using a numerical model with a TKE-based turbulence closure.     

Incorporation of Non-Linear Effective Cross-Section Parameterizations into a Fast Photolysis Computation Code (Fast-J)

We have investigated several methods of incorporating laboratorymeasurements of effective cross-sections (quantum yield timesabsorption cross-section) that are non-linear functions oftemperature, pressure, and wavelength, into the Fast-J photolysis codeof Wild et al. (2000). The best method was found to beparameterization of the solar weighted effective cross-section usingthe form of the effective cross-section, with numerical fitting of theparameterization constants. Using this technique on the acetone dataof Gierczak et al. (1998) produces aparameterization which fits all points to within 1%. This methodshould also benefit other photolysis codes with large wavelength bins.     

Product study of the reaction of OH radicals with isoprene in the atmosphere simulation chamber SAPHIR

Atmospheric oxidation of isoprene and its oxidation products methacrolein (MACR) and methyl vinyl ketone (MVK) have an important impact on the photochemical activity in the boundary layer, in particular in forested areas. The oxidation of isoprene by OH radicals was investigated in chamber experiments conducted under tropospheric conditions in the atmosphere simulation chamber SAPHIR at the Research Center Jülich. The aim was to determine the product yield of MVK and MACR in the OH-induced isoprene oxidation and the rate constant of their reaction with OH under real atmospheric conditions. The recently published updated degradation scheme for isoprene from Geiger et al. (2003) was used to determine rate constants and product yields. The fractional yields in the isoprene peroxy radical reaction with NO were found to be 0.41±0.03 for MVK and 0.27±0.03 for MACR. The rate coefficient for MACR with OH was found to be in very good agreement with the recommended value of IUPAC Atkinson (Atkinson et al., 2005). while the rate coefficient for MVK with OH was 27% lower.     

Reply to comment by Ben-Zvi,A., D. Rosenfeld and A. Givati on the paper: Levin,Z., N. Halfon and P. Alpert, “Reassessment of rain experiments and operations in Israel including synoptic considerations,” Atmos. Res. 97, 513–525. DOI: 10.1016/j.atmosres.2010.06.011

Levin et al. (2010; hereafter LHA) (Levin, Z., Halfon, N., Alpert, P., 2010. Reassessment of rain experiments and operations in Israel including synoptic considerations. Atmos. Res. 97, 513–525. DOI:10.1016/j.atmosres.2010.06.011.), reanalyzed the results of the operational seeding in northern Israel between 1975 and 2007 and the preceding Israel 2 cloud seeding experiment (1969–1975) and concluded that there is no net increase in precipitation over the target areas. Our analysis revealed that a synoptic bias during Israel 2 is one of the reasons for the apparent positive effect of seeding in the northern target area and the negative effect in the southern area both of which disappeared in the following experiment in the south (Israel 3; 1975–1995) and the operational seeding in the north.Ben-Zvi et al. (2010;hereafter BRG) criticized our paper primarily on the ground that we did not consider the positive results of Israel 1 experiment (1960–1967). It should be noted that in Israel 1 different seeding lines were used from those in both Israel 2 and the operational period. In addition, its raw data is not accessible anymore for reanalysis. Furthermore, Israel 2 had been designed as a confirmatory cross-over experiment to Israel 1 and failed to reproduce its promising results with double ratio (DR) of ~ 1.00, namely, zero rainfall enhancements. The same DR values were also found in Israel 3 and in the operational seeding. Therefore, because of the differences in the two experiments, the lack of access to the raw data and the disappointing results of the confirmatory experiment, we decided to concentrate our analysis on the more recent seeding activities.The attempt by BRG to explain the reduction of the DR to ~ 1.00 in the operational seeding period by the suppression due to pollution have been disproved by Alpert et al., 2008, Alpert et al., 2009 and also fail to explain the sharp decline of the target/control ratio right at the beginning of the operational seeding period when the lucky draw in this area came to its end (see LHA).     

二维均匀剪切湍流的湍涡闭合方案

通过仔细分析Thorpe与Woods等对湍流的典型实验,得到如下结论:(1)提出了湍流的太极模型;(2)指出了湍流串级的内容包括2个内容,一是湍流外圈层(大涡)在内旋到内部时产生新的圈层(小涡),二是在阴阳旋臂相切的锋面由于依然存在较大切变而产生新的次尺度湍涡;(3)指出湍流的耗散机制是通过湍涡,把具有切变的2流场的交界面通过旋转而无限拉长以增大2者的接触面积,加强了交界面之间的分子交换,从而实现了湍流层层混合的高效率;(4)基于湍涡结构,提出了湍涡闭合方案,给出了Reynolds应力的表达式,闭合了雷诺方程;(5)混合长反映了湍涡的大小,Karman常数则反映了湍涡内外半径比的关系;(6)指出Reynolds方程算子是湍流方程推导的关键,提出了湍流算子的概念;(7)推导出了新型的湍流运动方程;(8)提出了关系不变性原理。     

Wind and temperature profile prediction for diabatic surface layers including strong inversion cases

The micrometeorological research program in Antarctica has provided extensive data on wind and temperature profile structure under strong to extreme inversion conditions (Dalrymple et al., 1966; Lettau et al., 1977). The basic similarity hypotheses and limiting conditions for prediction of diabatic surface layer profiles are summarized. The model by Businger et al. (1971) for dimensionless shear and temperature gradients is revised to conform with the new results for strong stability. A novel similarity hypothesis is introduced to complete the step from shear and gradient prediction to prediction of absolute wind speed, wind energy, and temperature on the basis of prescribed external factors of surface layer structure. The physics of interactions between predicted profile tilting and curving are discussed and used to explain several micrometeorological paradoxes, including that of the elevated minimum of air temperature observed occasionally near the active surface when the energy budget is of the nocturnal type.     

Cloud changes in a warmer Europe

Cloud cover records for western Europe have been analysed in the context of the warming world analogue model described by Lough et al. (1983). It is found that cloud cover has generally increased in moving from a cold period (1901–1920) to a warm period (1934–1953). The exception to this general trend is over the central part of the area considered (Germany, France and some parts of Spain) where there is a tendency towards decreasing cloud as warming occurs. While the results presented here are not closely correlated with the temperature and precipitation results of Lough et al. (1983), there is support for their hypothesis that cloudiness increased in autumn over northern Europe. The suggestion that successful performance of numerical climate models in seasonal simulations might demonstrate adequacy in other climatic simulation modes is also examined It is shown that whilst there is good agreement with observations in one such numerical model in the seasonal simulation, there is no agreement in the case of a warming world in either the direction or the amount of cloudiness change.     

Oxidation of Elemental Mercury in the Atmosphere; Constraints Imposed by Global Scale Modelling

We use the global mercury model published by Bergan et al. (1999) to evaluate the potential role of ozone and the hydroxyl radical as gas phase oxidants for the oxidation of elemental mercury in the atmosphere. The magnitude of natural and man-made mercury emissions are taken from recent literature estimates. We consider only two mercury reservoirs, elemental mercury, Hg⁰, and the more soluble divalent form, HgII. Wet and dry deposition of HgII is explicitly treated.Applying monthly mean fields of ozone for the oxidation of gas phase Hg⁰ and using the reaction rate by Hall (1995) yields a global transformation of Hg⁰ to HgII which is too slow to keep the simulated concentration of Hg⁰ near observed values. This shows that there must be additional important removal processes for Hg⁰ or that the reaction rate proposed by Hall (1995) is too slow. A simulation in which the oxidation rate was artificially increased, so that the global turn-over time of Hg⁰ is one year and the simulated average concentration of Hg⁰ realistic, produces latitudinal and seasonal variations in Hg⁰ that do not support the hypothesis that gas phase reaction with O₃ is the major oxidation process for Hg⁰.Recent studies indicate that OH may be an important gas phase oxidant for Hg⁰ (Sommar et al., 2001). Using OH as the sole oxidantand applying the oxidation rate by Sommar et al., we calculate aconcentration of Hg⁰ well below (about a factor of three) the observations. By prescribing a slower rate, corresponding to a turn-over time of Hg⁰ of one year, we calculate concentrations of both Hg⁰ in surface air and HgII in precipitation which correspond reasonably well, both in magnitude and temporal variation, with seasonal observations in Europe and North America. This result supports the suggestion that the oxidation by OH is an important pathway for the removal of Hg⁰. In view of the uncertainties associated with our calculations, this conclusion should still be regarded as tentative.     

Fluctuations of the horizontal wind under unstable conditions

The similarity relations for _u/u_* proposed by Panofskyet al. (1977) and Højstrup (1982) have been verified using eddy-correlation data collected during the EFEDA-experiment, conducted over the extensive plain of La Mancha (Spain), where vine plants form a primary crop. Also, the standard Monin-Obukhov relation is considered. It is found that the expressions by Panofskyet al. and Højstrup both yield almost identical results, and are better than the Monin-Obukhov expression. Also, _u measured with a cup anemometer obeys the similarity expressions well, provided that the effect of the variation of wind direction on propellor wind speed is accounted for. The relationship of Panofskyet al. works rather well even when the boundary-layer height scaleh is replaced by a fixed height,h _c. Best results were obtained forh _c=1800 m. This height scale is possibly associated with the horizontal variability in the surface sensible heat flux pattern.     

Estimation of net radiation and surface heat fluxes using NOAA-7 satellite infrared data during fair-weather cloudy situations of Mesogers-84 experiment

Estimation of radiation during fair weather cloudy situations of the MESOGERS-84 experiment has been examined using micrometeorological observations and satellite data. Diurnal variation of cloudiness is empirically determined using satellite information as a function of global radiation, and relationships between net incoming radiation and global radiation are analyzed. Particularly, it has been found that a very simple relationship between global radiation, cloudiness and net radiative heat flux proposed by Nielsenet al. (1981) can be used with satellite data and applied to the Mesogers region in Southwest France. The different relationships between cloudiness and radiation are utilized to modify and to validate Taconet'set al. model (1986) to get fluxes related to a cloudy situation without advection.     

Possible climatic impacts of land cover transformations,with particular emphasis on tropical deforestation

The climatic impact of albedo changes associated with land-surface alterations has been examined. The total surface global albedo change resulting from major land-cover transformations (i.e. deforestation, desertification, irrigation, dam-building, urbanization) has been recalculated, modifying the estimates of Sagan et al., (1979). Tropical deforestation (11.1 million ha yr^-1, or 0.6% yr^-1, Lanly, 1982) ranks as a major cause of albedo change, although uncertainties in the areal extent of desertification could conceivably render this latter process of similar significance. The maximum total global albedo change over the last 30 yr for the various processes lies between 0.000 33 and 0.000 64, corresponding to a global temperature decrease of between 0.06 K and 0.09 K (scaled from the 1-D radiative convective model of Hansen et al., 1981), which falls well below the interannual and longer period variability.An upper bound to the impact of tropical deforestation was obtained by concentrating all vegetation change into a single region. The magnitude of this modification is equivalent to 35–50 yr of global deforestation at the current rate, but centered on the Brazilian Amazon. The climatic consequences of such tropical deforestation were simulated, using the GISS GCM (Hansen et al., 1983). In the simulation, a total area of 4.94 × 10⁶ km² of tropical moist forest was removed and replaced by a grass/crop cover. Although surface albedo increased from 0.11 to 0.19, the effect upon surface temperature was negligible. However, other climate parameters were altered. Rainfall decreased by 0.5–0.7 mm day^-1 and both evapotranspiration and total cloud cover were reduced. The absence of a temperature decrease in spite of the increased surface albedo arises because the reduction in evapotranspiration has offset the effects of radiative cooling. The decrease in cloud cover also counteracts the increase in surface albedo. These locally significant changes had no major impact on regional (Hadley or Walker cells) or the global circulation patterns.We conclude that the albedo changes induced by current levels of tropical deforestation appear to have a negligibly small effect on the global climate.     

Measurements and Langevin simulations of mean tracer concentration fields downwind from a circular line source inside an alfalfa canopy

Two Langevin simulations of trajectories of marked fluid elements in inhomogenous turbulence, where the Lagrangian length and vertical velocity scales are height dependent, were compared with field data. A CO₂ tracer was released from a circular line source and the concentration profiles were measured for diffusion distances of 50 and 100 cm inside and above an alfalfa canopy.One of the simulations, suggested by Wilson et al. (1983), biases the vertical velocities by adding a mean upward drift. The second simulation proposed here by-passes this difficulty by reflecting marked particles according to a probability calculated from the gradient in vertical velocity variance between the beginning and the end of each step. This simulation also makes use of a constant time-scale within the canopy, following preliminary results from a turbulence experiment within a forest (Leclerc, 1987).Comparing the results of these simulations with the field data shows that the simulation proposed by Wilson et al. (1983) does not correctly reproduce the difusion for the larger fetch in systems exhibiting strong gradients in vertical velocity variance. Instead, the modelled plumes exhibit a bulge at the source height whereas the field data show smooth profiles. In addition, the modelled plumes overestimate the vertical spread of the plumes, which is possibly due to the inadequacy of the approach in severely inhomogeneous systems. In contrast, the results from the tracer experiments indicate that the diffusion can be better reproduced with the use of a reflection probability calculated at each step. The discrepancies between the experimental results and the simulation using a reflection probability are attributed to stability effects.     

Ozone UV spectroscopy I: Absorption cross-sections at room temperature

Because of the importance of the absorption cross-sections of ozone in the atmosphere, we give a new set of data at ambient temperature for the whole spectral region within the 195–345 nm range. Two new methods have been investigated for the experiments to measure the ozone pressures. Our results are compared with all of the previous data. The agreement between the more recent works is relatively good and can be improved by making some corrections in the WMO set. The new reference value at 254 nm proposed by Mauersberger et al. is confirmed and we suggest taking it as a new standard.     

An evaluation of autocatalytic ozone production from vibrationally excited oxygen in the middle atmosphere

The possibility of significant autocatalytic ozone production in the middle atmosphere has been demonstrated by Toumi et al. A detailed framework including five processes previously not considered is presented here. When these processes are included, particularly wavelength-dependent vibrational distributions of O₂, calculated ozone enhancements are less than 10% in the upper stratosphere and lower mesosphere. The vibrational distributions and the rate of vibrational quenching are identified as key issues which need to be addressed by experiments.