The Value Of The β Coefficient In The Relaxed Eddy Accumulation Method In Terms Of Fourth-Order Moments

The commonly measured value of in the relaxed eddy accumulationmethod of about 0.56is shown to arise from the non-Gaussiannature of turbulence. Fourth-orderGram–Charlier functions forthe two-dimensional probability distributionsof variation in the horizontal component of wind velocityand concentrations of water vapour, carbondioxide and methane with respect to thevertical component of wind velocity are used to examinethe value of .An analytical solution for ispresented in terms of fourth-order moments.Under mean conditions, this solution givesa value for of0.557. Variation of is shown to be controlledprimarily by the ratio of the mean ofc'w3 (where c'is relevant to the entity of interest andw' is vertical component of windvelocity) to the correlationcoefficient between the entity concentrationand vertical component of wind velocity.     

Equilibrium evaporation beneath a growing convective boundary layer

Expressions for the equilibrium surface Bowen ratio ( _s ) and equilibrium evaporation are derived for a growing convective boundary layer (CBL) in terms of the Bowen ratio at the top of the mixed layer _i and the entrainment parameter A _R . If A_R is put equal to zero, the solution for _s becomes-that previously obtained for the zero entrainment or closed box model. The Priestley-Taylor parameter is also calculated and plotted in terms ofA _R and _i . Realistic combinations of the atmospheric parameters give values of in the range 1.1 to 1.4.     

ALPEX-Simulation

Summary In a project ALPEX-Simulation, sponsored by the Österreichischer Fond zur Förderung der wissenschaftlichen Forschung (FWF), all eight cases of ALPEX-SOP cyclones were numerically simulated with a fine mesh isentropic model of the atmosphere. These numerical simulations in six-hourly intervals allow a deeper insight into the synoptics and dynamics of the cyclogeneses in the Western Mediterranean, especially into the genesis of the two basic types of cyclones: the so-called Überströmungs-type and Vorderseiten-type. In the first phase of cyclogenesis of the Überströmungs-type, the blocking and flow splitting of the cold air due to the Alps and the canalization between the Alps and the Massif Central are important. Cold air flows cyclonically around the western part of the Alps, creating a vorticity maximum at the south western edge of the Alpine, bow and leads also to an enhanced PV. In connection with warm air in the Mediterranean, a strong baroclinic zone is generated. The interaction between the arriving PV maximum in the upper troposphere and the enhanced PV at the bottom leads to cyclogenesis in the Western Mediterranean. In the case of the Vorderseiten-type warm air advection dominates with the exception of a shallow layer of cold air in the inner Po-Valley, which is shielded by the Alpine ridge. A well-pronounced PV maximum builds up and couples with the PV maximum arriving at upper levels, even before the cold air, coming from the north-west, has surrounded the Alps. The cold air only intensifies the development by raising the baroclinity. Therefore, the Vorderseiten-cyclogenesis is an orographically modified cyclogenesis, in the course of which the cyclonic development is triggered by the Alps, whereas the Überströmungs-cyclogenesis is an orographically induced cyclogenesis i.e. a true lee cyclogenesis.With 14 FiguresDied in a tragic traffic accident on June 6, 1993.     

An explicit three-dimensional nonhydrostatic numerical simulation of a tropical cyclone

Summary A nonhydrostatic numerical simulation of a tropical cyclone is performed with explicit representation of cumulus on a meso- scale grid and for a brief period on a meso- scale grid. Individual cumulus plumes are represented by a combination of explicit resolution and a 1.5 level closure predicting turbulent kinetic energy (TKE).The results demonstrate a number of expected and unexpected important scale interaction processes. Within the central core of the developing cyclone, meso- convective regions grow and breakdown into propagating inertiagravity waves throughout the lifecycle of the cyclone. In the early stages, the amplitude of pressure fluctuations associated with the meso- scale convection exceed the central pressure of the cyclone and strongly modulate its intensity. With each meso- scale pulsation, the cyclone core increases in strength, measured by the central pressure deficit. The increasingly strong inertial frequency of the storm core acts to increasingly trap the convection induced heating within the core by balancing the tangential wind against the low central pressure, before the meso- scale convection breaks down and sends the warmth away as a propagating wave. Eventually, the slow manifold's amplitude exceeds the amplitude of the meso- scale oscillations and a stable eye region is formed. As inertial instability increases, increasingly high thermal warmth can be protected in the core, allowing persistent subsidence to form and to clear out the cyclone eye.On the outside of the eye wall, strong inertial stability gradients in the troposphere cause convective warming to split the inflow to the eye wal! and spawn outwardly propagating inertia gravity waves. These waves carry away all of the heating forced by convection that is not inertially trapped by the eye wall and act as a moderating influence on storm intensity.Inertia gravity waves are also spawned in the stratosphere at the top of the eye wall by the revolution of asymmetric cumulus structures. In all instances, the tropospheric waves are coupled to the propagating stratospheric waves which both move at 35 ms^–1, although there are many instances where the stratospheric waves seem to have no tropospheric counterpart. Hence the anvil top forcing and low level breakdown are linked.The outwardly propagating inertia gravity waves act to initiate outer bands of convection. This initiation is with the assistance of low level boundary layer variations of density related to previous convection and to virga falling from the anvil which moistens and destabilizes the mid levels of _e minimum. The convection initiated by these waves does not move substantially outward with the wave, although may appear to develop outward discontinuously.With 12 Figures     

Convective Profile Constants Revisited

This paper examines the interpolation betweenBusinger–Dyer (Kansas-type) formulae,_u = (1 -1 6 )^-1/4 and_t = (1 - 16 )^-1/2, and free convection forms. Based on matching constraints, the constants, a_u and a_t, in the convective flux-gradient relations, _u = (1 - a_u )^-1/3 and _t = (1 - a_t )^-1/3, are determined. It isshown that a_u and a_t cannot be completely independent if convective forms are blended with theKansas formulae. In other words, these relationships already carryinformation about a_u and a_t. This follows because the Kansas relations cover a wide stability range (up to = - 2), which includes a lower part of the convective sublayer (about 0.1 < - < 2). Thus, there is a subrange where both Kansas and convective formulae are valid. Matching Kansas formulae and free convection relations within thesubrange 0.1 < - < 2 and independently smoothing ofthe blending function are used to determine a_u and a_t. The values a_u = 10 for velocity and a_t = 34for scalars (temperature and humidity) give a good fit. This new approacheliminates the need for additional independent model constants and yields a`smooth' blending between Kansas and free-convection profileforms in the COARE bulk algorithm.     

A study of a fair weather boundary layer in TOGA-COARE: Parameterization of surface fluxes in large scale and regional models for light wind conditions

The study focuses on a way to parameterize the effect of subgrid scale convective motions on surface fluxes in large scale and regional models for the case of light surface winds. As previously proposed, these subgrid effects are assumed to scale with the convection intensity through the relationship: where is the mean velocity of the wind, U₀ the velocity of the mean wind, w_* the free convection velocity, and an empirical coefficient to be determined. Both observations and numerical simulation are presently used to determine the free convection coefficient .Large eddy simulation of a fair weather convective boundary layer case observed during TOGA-COARE is performed. Comparisons between observations and the simulation of surface properties and vertical profiles in the planetary boundary layer are presented. The simulated vertical turbulent fluxes of heat, moisture and buoyancy range well within estimates from aircraft measurements.The most important result is that the true free convection coefficient , directly estimated from simulation, leads to a value of 0.65, smaller than the ones estimated from temporal and spatial variances. Using observations and simulation, estimates of from temporal and spatial variances are obtained with similar values 0.8. From both theoretical derivations and numerical computations, it is shown that estimates of the true from variances are possible but only after applying a correction factor equal to 0.8. If this correction is not used, is overestimated by about 25%. The time and space sampling problem is also addressed in using numerical simulations.     

Numerical Simulation of Strongly Stratified Flow Over a Three-Dimensional Hill

A numerical study of stably stratified flow over a three-dimensional hill is presented. Large-eddy simulation is used here to examine in detail the laboratory experimental flows described in the landmark work of Hunt and Snyder about stratified flow over a hill. The flow is linearly stratified and U/Nh is varied from 0.2 to 1.0. Here N and U are the buoyancy frequency and freestream velocity respectively, and h is the height of the hill. The Reynolds number based on the hill height is varied from 365 to 2968. The characteristic flow patterns at various values of U/Nh have been obtained and they are in good agreement with earlier theoretical and experimental results. It is shown that the flow field cannot be predicted by Drazin's theory when recirculation exists at the leeside of the hill even at UNh 1. The wake structure agrees well with a two-dimensional wake assumption when U/Nh 1 but lee waves start to influence the wake structure as U/Nh increases. The dividing-streamline heights obtained in the simulation are in accordance with experimental results and Sheppard's formula. The energy loss along the dividing streamline due to friction/turbulence approximately offsets the energy gained from pressure field. When lee waves are present, linear theory always underestimates the amplitude and overestimates the wavelength of three-dimensional lee waves. The simulated variations of drag coefficients with the parameterK (=ND/ U) are qualitatively consistent with experimental data and linear theory. Here D is the depth of the tank.     

A study of the sensitivity of bare soil evaporation schemes to soil surface wetness,using the coupled soil moisture and surface temperature prediction model,BARESOIL

Summary The performance of evaporation schemes with and approach and their combination within resistance representation of evaporation from bare soil surface is discussed. For this purpose nine schemes, based on different functions of or , on the ratio of the volumetric soil moisture content and its saturated value are used.The quality of the chosen schemes has been evaluated using the results of time integration by the coupled soil moisture and surface temperature prediction model, BARESOIL, using in situ data. A sensitivity analysis was made using two sets of data derived from the volumetric soil moisture content of the top soil layer. One with values below the wilting point (0.17 m³m^–3) and the second with values above 0.20m³m^–3. Data sets were obtained at the experimental site Rimski anevi, Yugoslavia, from the bare surface of a chernozem soil.With 4 Figures     

Gas and Particle Products Distribution from the Reaction of β-Caryophyllene with Ozone

Gas and particulate reaction products from the ozonolysis of -caryophyllene (I) in the presence of atmospheric air were investigated using a combination of gas chromatography-mass spectrometry (GC-MS) and high performance liquid chromatography (HPLC). A Scanning Mobility Particle Sizer system (3936, TSI) and a Condensation Particle Counter (3025A, TSI) were used to study secondary organic aerosol formation. The nighttime oxidation was carried out in a large outdoor smog chamber (190 m³). A wide range of ring retaining and ring opening products in the gas and particle phase are reported over the course of the reaction. On average, measured gas and particle phase products accounted for 64% of the reacted -caryophyllene (I) carbon. Measurements show that a number of reaction products with low vapor pressure (e.g., -caryophyllone aldehyde (IV), -norcaryophyllone aldehyde (V), -caryophyllonic acid (VIII), -14-hydroxycaryophyllonic acid (XIV)) were found in the sample taken during the first 20 min of the reaction and may play an important role in the early formation of secondary organic aerosol. A detailed mechanism is proposed to account for most products observed in this investigation.     

An improved calibration for tethered balloon wind measurements

A previously published technique for using tethered spherical balloons as anemometers for measuring light low-level winds has been further developed. Earlier data on the relationship between the aerodynamic drag coefficient and the Reynolds number of spherical rubber balloons were combined with a large number of new data and re-analysed; and the errors in the relationship were estimated. The results allowed a more accurate calculation of wind speed from the deflection of a tethered balloon from the vertical. When combined with a new technique for calculating the effects of the tether, this enabled light to moderate low-level winds at fixed heights up to 600 m or more to be measured with simple, cheap, and readily mobile equipment; and a slight modification of the technique allowed measurement of winds in and above fog. Wind speeds measured by the ballon technique showed reasonably good agreement with measurements by an anemometer carried beneath the balloon.Glossary of Symbols a, b, c Coefficients in the relationship between lnC _d and lnR - A Quantity under square root in solution for lnV whena0 - C _d Wind drag coefficient for balloon - C _dc Value ofC _d given by calibration curve of Table I - D Dynamic wind pressure force on balloon - F Buoyant free lift of balloon with load - Re Reynold's number of balloon (sphere) - R = Re/10⁵ - r Radius of sphere - T Tension in tether - V Wind speed - ₈₃() =(lnC _dc -lnC _d ) when 83° , or 0 for other - Error in lnC _d - Elevation of tether where attached to balloon - Elevation of balloon from ground tether point - Molecular viscosity of air - Ratio of circumference to diameter of circle - Density of air     

Relation of Richardson number to the curvature of the wind profile

Values of Richardson number (Ri) and the wind profile curvature () in the atmospheric surface layer are evaluated from experimental data. The relationship between Ri and shows some scatter, but gives agreement with the theoretical relation derived by Businger et al. (1971).     

Formation of Organic Aerosols from the Oxidation of Biogenic Hydrocarbons

Measurements of aerosol formation during thephotooxidation of -pinene, -pinene,d-3-carene, d-limonene, ocimene, linalool, terpinene-4-ol, andtrans-caryophyllene were conducted in anoutdoor smog chamber. Daylight experiments in thepresence of and dark experiments withelevated ozone concentrations were performed. Theevolution of the aerosol was simulated by theapplication of a gas/particle absorption model inconnection with a chemical reaction mechanism. Thefractional aerosol yield is shown to be a function ofthe organic aerosol mass concentration andtemperature. Ozone and, for selected hydrocarbons, theNO₃ reaction of the compounds were found torepresent efficient routes to the formation ofcondensable products. For initial hydrocarbon mixingratios of about 100 ppb, the fractional aerosol yieldsfrom daylight runs have been estimated to be 5%for open-chain hydrocarbons, such as ocimene andlinalool, 5–25% for monounsaturated cyclicmonoterpenes, such as -pinene, d-3-carene, orterpinene-4-ol, and 40% for a cyclic monoterpenewith two double bonds like d-limonene. For the onlysesquiterpene investigated, trans-caryophyllene, afractional aerosol yield of close to 100% wasobserved. The majority of the compounds studied showedan even higher aerosol yield during dark experimentsin the presence of ozone.     

Gas-Phase Ozone Oxidation of Monoterpenes: Gaseous and Particulate Products

Atmospheric oxidation of monoterpenes contributes to formation of tropospheric ozone and secondary organic aerosol, but their products are poorly characterized. In this work, we report a series of outdoor smog chamber experiments to investigate both gaseous and particulate products in the ozone oxidation of four monoterpenes: -pinene, -pinene, ³-carene, and sabinene. More than ten oxygenated products are detected and identified in each monoterpene/O₃ reaction by coupling derivatization techniques and GC/MS detection. A denuder/filter pack sampling system is used to separate and simultaneously collect gas and aerosol samples. The identified products, consisting of compounds containing carbonyl, hydroxyl, and carboxyl functional groups, are estimated to account for about 34–50%, 57%, 29–67%, and 24% of the reacted carbon mass for -pinene, sabinene, -pinene, and ³-carene, respectively. The identified individual products account for >83%, 100%, >90%, and 61% of the aerosol mass produced in the ozone reaction of -pinene, sabinene, -pinene, and ³-carene. The uncertainty in the yield data is estimated to be ±50%. Many of the products partition between gas and aerosol phases, and their gas-aerosol partitioning coefficients are determined and reported here. Reaction schemes are suggested to account for the products observed.     

Transport-Dissipation Analytical Solutions to the E-∈Turbulence Model and their Role in Predictions of the Neutral ABL

E- turbulence model predictions of the neutralatmospheric boundary layer (NABL) are reinvestigated to determine thecause for turbulence overpredictions found in previous applications. Analytical solutions to the coupled E and equations for the case of steady balance between transport and dissipation terms, the dominant balance just below the NABL top, are derived. It is found that analytical turbulence profiles laminarizeat a finite height only for values of closure parameter ratio c ₂ /_e equal toor slightly greater than one, with laminarization as z for greater . The point = 2 is additionally foundthat where analytical turbulent length scale (l) profilesmade a transition from ones ofdecreasing ( < 2) to increasing ( > 2)values with height. Numerically predicted profiles near the NABL topare consistent with analytical findings. The height-increasingvalues of l predicted throughout the NABL with standard values ofclosure parameters thus appear a consequence of 2.5(> 2), implied by these values (c ₂ = 1.92, _{= 1.3}, _{e = 1}). Comparison of numericalpredictions with DNS data shows that turbulence overpredictions obtained with standard-valued parameters are rectifiedby resetting and _e to 1.1 and 1.6, respectively, giving, with c ₂ = 1.92, 1.3, and laminarization of the NABL's cappingtransport-dissipation region at a finite height.     

The validation of various schemes for parameterizing evaporation from bare soil for use in meteorological models: A numerical study using in situ data

Parameterization of evaporation from a non-plant-covered surface is very important in the hierarchy strategy of modelling land surface processes. One of the representations frequently used in its computation is the resistance formulation. The performance of the evaporation schemes using the , , and their combination resistance approaches to parameterize evaporation from bare soil surfaces is discussed. For that purpose, the nine schemes, based on a different dependence of and on volumetric soil moisture content and its saturated value, are used.The tests of performances of the considered schemes are based on time integrations by the land surface module (BARESOIL) using observed data. The 23 data sets at a bare surface experimental site in Rimski anevi, Yugoslavia on chernozem soil, were used for the resistance algorithm evaluation. The quality of the schemes was compared with the observed values of the latent heat flux using several statistical parameters.     

Latitudinal variation of tropospheric temperature lapse rate

Summary The latitudinal variation of tropospheric temperature lapse rate is examined on the basis of recently published data for the northern hemisphere. In January, the lapse rate systematically decreases from 6.4°C/km. for the equator to 3.6°C/km. for 70°N; in July, the lapse rate is nearly constant in respect of latitude between the equator and 50°N, but decreases from the latter poleward. The average January–July lapse rate decreases systematically from 6.3°C/km. for the equator to 4.7°C/km. for 70°N. This variation is closely described by the equation =6.25–2 sin⁴, where is the lapse rate, in °C/km, for latitude . For the north pole, the equation gives a lapse rate of 4.25°C/km.

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Zusammenfassung Die Änderung des vertikalen Gradienten der Troposphärentemperatur mit der geographischen Breite wird auf Grund neuen Zahlenmaterials für die Nordhemisphäre untersucht. Im Januar nimmt der Temperaturgradient allmählich von 6.4°C/km am Äquator bis zu 3.6°C/km in 70°N Breite ab; im Juli ist der Temperaturgradient in den Breiten zwischen dem Äquator und 50°N Breite fast gleich und nimmt von hier aus polwärts ab. Der mittlere Temperaturgradient für Januar–Juli nimmt allmählich von 6.3°C/km am Äquator bis auf 4.7°C/km in 70°N Breite ab. Die Breitenabhängigkeit läßt sich gut durch die Gleichung =6.25–2 sin⁴ darstellen, in der der Temperaturgradient in °C/km in der Breite ist. Für den Nordpol ergibt die Gleichung einen Temperaturgradienten von 4.25°C/km.

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Résumé La variation latitudinale du gradient thermique vertical de la température troposphérique est examinée en se basant sur des données publiées récemment d'une valeur de 6.4°C/km pour l'équateur à 3.6°C/km pour 70°N; en juillet le gradient est presque constant pour les latitudes situées entre l'équateur et 50°N, mais il diminue d'ici vers le pôle. Le gradient moyen janvier/juillet diminue systématiquement de 6.3°C/km pour l'équateur à 4.7°C/km pour 70°N. Cette variation peut être représentée par l'équation =6.25–2 sin⁴ où est le gradient en °C/km pour la latitude . Pour le pôle nord, l'équation donne un gradient de 4.25°C/km.

     

Bistability of the thermohaline circulation identified through comprehensive 2-parameter sweeps of an efficient climate model

The effect of changes in zonal and meridional atmospheric moisture transports on Atlantic overturning is investigated. Zonal transports are considered in terms of net moisture export from the Atlantic sector. Meridional transports are related to the vigour of the global hydrological cycle. The equilibrium thermohaline circulation (THC) simulated with an efficient climate model is strongly dependent on two key parameters that control these transports: an anomaly in the specified Atlantic–Pacific moisture flux (F_a) and atmospheric moisture diffusivity (K_q). In a large ensemble of spinup experiments, the values of F_a and K_q are varied by small increments across wide ranges, to identify sharp transitions of equilibrium THC strength in a 2-parameter space (between Conveyor On and Off states). Final states from this ensemble of simulations are then used as the initial states for further such ensembles. Large differences in THC strength between ensembles, for identical combinations of F_a and K_q, reveal the co-existence of two stable THC states (Conveyor On and Off)—i.e. a bistable regime. In further sensitivity experiments, the model is forced with small, temporary freshwater perturbations to the mid-latitude North Atlantic, to establish the minimum perturbation necessary for irreversible THC collapse in this bistable regime. A threshold is identified in terms of the forcing duration required. The model THC, in a Conveyor On state, irreversibly collapses to a Conveyor Off state under additional freshwater forcing of just 0.1 Sv applied for around 100 years. The irreversible collapse is primarily due to a positive feedback associated with suppressed convection and reduced surface heat loss in the sinking region. Increased atmosphere-to-ocean freshwater flux, under a collapsed Conveyor, plays a secondary role.     

Effects of water vapour on the structure parameter of the refractive index for near-infrared radiation

The refractive index of air (n) mainly depends on temperature andwater vapour content. For near-infrared radiation, temperature isthe main determining factor. To determinethe structure parameter of temperature ( ) fromthe structure parameter of the refractive index( ), the influence ofwater vapour content on n needs to be taken into account as acorrection. Three levels of approximation are presented. The firstinvolves the standard deviations of T and q (_T and_q) as well as the correlation coefficient betweenT and q (R_Tq). The second approximation involvesR_Tq and the Bowen ratio (), and the last uses only the Bowen ratio.The latter is the classical Bowen ratio correction.Evaluation of the validity of the assumptions used in the derivationreveals that a large error may be introduced (for situations withR_Tq < 0.8) by replacing _T/_qwith the Bowen ratio.Consequently, estimating from using the Bowen ratiocan contain errors of 5–40% for || < 1. Adding R_Tqas extra information reduces the errors only marginally. For || > 1,the correction is small, and all three approximations give errors of lessthan 1% in . When is used to compute the sensible heat flux,the influence of the quality of the correction forwater vapour fluctuations on the measured energy balance is small: forsmall ||, the correction is large, but the absolute value ofthe heat flux is small, whereas for large ||, the correctionis insignificant.     

Turbulence mechanisms at an agricultural site

An extensive set of turbulence data from the 3- and 12-m heights taken over an agricultural site (Marsta, Sweden) are analyzed and compared with data from ideal sites.In unstable air, Monin-Obukhov similarity is found to be valid for the non-dimensional gradients of wind, _m , temperature, _h , and humidity, _e , for (only a few data), for _T /|T _*|,/ _E /|E _*| and for the non-dimensionalized inertial subrange spectra of temperature and humidity. Where comparison is possible, the unstable data also agree with those found in the Kansas study, with one remarkable exception, the inertial subrange constant of the temperature spectrum, ₁, being only 0.39, compared to the value 0.80 found at the Kansas site.On the stable side, most similarity predictions break down, with most of the data differing systematically from the corresponding Kansas results, the only exception being . The inertial subrange constants for temperature, ₁, and for humidity, ₁ are found to have the same values, 0.39 and 0.30, respectively, as they do on the unstable side. Remarkable similarity is found for the shape of the stable u- and - and e-spectra. In addition, this shape is found to be identical with that found in Kansas. The peak wavelength of the stable u-, and -spectra is found to be about four times larger than it is for the corresponding Kansas spectra. This is interpreted to be a result of the increased macro-roughness at the Marsta site as compared with that at the Kansas site. A possible explanation for the low ₁-value is discussed, suggesting that ₁ is not a universal constant, but instead dependent on the turbulent structure.