Regular and seasonal behaviour of atmospheric radio noise field strength (ARNFS) over low latitude station calcutta

Summary The electromagnetic radiation of cloud discharge known as atmospheric radio noise field strength (ARNFS) shows a gradual fall from a frequency of 9 kHz to 81 kHz as studied over a period of two years at Calcutta, very close to Bay of Bengal. The main characteristic features of ARNFS at Calcutta are that-(i) ARNFS shows that midday median value is smaller than midnight median value in all months, (ii) level of daily minimum is higher in February and monsoon compared to other seasons, (iii) sunrise effect and sunset effect are well correlated with local sunrise and sunset times, (iv) the magnitude of sunrise fade and sunrise fade rate are maximum in April and lowest during winter period, (v) the magnitude of sunset fade is higher in premonsoon and postmonsoon while it is lowest in monsoon, (vi) number of occurrence of both sunrise effect and sunset effect is remark-ably smaller in monsoon. The positions of the sun and of atmospheric sources are jointly the causes of seasonal and diurnal variations. The missing of sunrise effect and sunset effect are due to local cloud activity and variation of electron density during geomagnetic storms.With 7 Figures     

PHYSICAL IMPACTS OF CLIMATIC CHANGE ON DELTAIC COASTAL SYSTEMS (II): DRIVING TERMS

The evaluation of the impact of climatic change on deltaic coastal systems requires establishing a reference situation. As deltaic coastal zones are highly dynamic areas, it is necessary to define the reference situation as a function of their present evolutive stage. This implies characterizing coastal processes as a complex system defined by the links between driving or forcing terms and the coastal response at different temporal and spatial scales (long/large, medium and episodic scales). Starting from classical previous works on deltaic systems and including all available field and conceptual knowledge to properly consider present conditions (largely regulated rivers, subsidence, etc.) a detailed methodology to analyse forcing or driving agents at the three mentioned scales is presented. This methodology which developed the general approach presented in a companion paper, will be followed by a future paper focusing on the analysis of the coastal response.     

On Trajectory Curvature as a Selection Criterion for Valid Lagrangian Stochastic Dispersion Models

Recently Wilson and Flesch (Boundary-Layer Meteorology, 84, 411-426, 1997) suggested that the average increment d z to the orientation = arctan(w/u) of the Lagrangian velocity-fluctuation vector can be used to distinguish the better Lagrangian stochastic models within the well-mixed class. Here it is demonstrated that the specification of d z constitutes neither a sufficient or universally applicable criterion to distinguish the better Lagrangian stochastic models within the well-mixed class. The hypothesis made by Wilson and Flesch that Lagrangian stochastic models with /P_E irrotational are zero-spin models, having d z=0, is proven     

Stable boundary layers: Observations,models and variability part I: Modelling and measurements

Selected field measurements of evening stable boundary layers are presented in detail comparable with published Large Eddy Simulation results. Such models appear to match idealized theories more closely than do some boundary-layer observations. Any attempt to compare detailed observations with idealized models therefore highlights the variability of the real boundary layer.Here direct turbulence measurements across the stable boundary layer from a heterogeneous and an ideal site are contrasted. Recommendations are made for the information needed to distinguish heterogeneous and ideal cases.The companion paper (Part II) discusses further the issues of data, analysis in the presence of variability, and the effects of averaging over heterogeneous terrain.Part of UK Meteorological Office Atmospheric Process Research Division.     

A radiation and energy balance study of mature forest and clear-cut sites

Components of the radiation and energy balances were measured over a clear-cut area and a mature, mixed forest during the summer of 1981 at the Petawawa National Forestry Institute, Chalk River, Ontario. The work concentrated on the clear-cut site which supported a canopy layer composed primarily of bracken fern and logging remnants.Forty days of radiation data were collected at the clear-cut site. After the first four weeks of measurements (the green season), most of the ferns quickly died, and their foliage changed appearance from a green to brownish colour (the brown season). The daily mean reflection coefficient of solar radiation determined over the green season was 0.20 and decreased to 0.13 for the brown season. The corresponding value for the forest was 0.13, based on a limited amount of data. The clear-cut site received 11% and 21% less net radiation than the forest on a 24-hr and daylight-hours basis, respectively, as a consequence of the higher reflection coefficient and larger daytime longwave radiation emission.A reversing temperature difference measurement system (RTDMS), incorporating ten-junction thermopiles was employed at each site in order to determine Bowen ratios () via differential psychrometry. Both systems performed well, especially the RTDMS over the forest which was capable of resolving very small differences of temperature, typically less than 0.2 °C over a height of 3 m. The mean hourly Bowen ratio, calculated from values from 0800 to 1600 hr, varied from 0.2 to 1.0 for the forest and from 0.4 to 0.8 for the clear-cut site in the green season.A significant canopy heat storage component of the energy balance, Q _S , was found at the clear-cut site. In the early morning, a portion of the available energy was used to heat the biomass materials and air within the canopy layer. The stored heat within the canopy was released later in the day, increasing the available energy total.The daily mean value of the Priestley-Taylor coefficient (Priestley and Taylor, 1972) for the green season at the clear-cut site was 1.14, and individual values tended to increase during wet surface conditions and decrease when the surface dried. The daylight mean value during dry canopy conditions at the forest was 1.05, and much higher values occurred when the canopy was wet. The enhancement of for the wet forest was a result of the evaporation of intercepted rain (which is not limited by stomatal resistance) and the concomitant transfer of sensible heat to the forest.     

AN E – ε – ℓ TURBULENCE CLOSURE SCHEME FOR PLANETARY BOUNDARY-LAYER MODELS: THE NEUTRALLY STRATIFIED CASE

The standard E – model generates aplanetary boundary layerthat appears to be much too deep. The cause of theproblem is traced to the equation for the dissipationrate () of turbulent kinetic energy (E), specifically theparameterization of dissipation production anddestruction. In the context of atmosphericboundary-layer modelling, we argue that a part of thedissipation production should be modelled as the inputto the spectral cascade from the energy-containingpart of the spectrum, with a characteristic length , while the equilibrium imbalancebetween local production and destruction ofdissipation is modelled as proportional toE2/E, as in the standard model. Wepropose an E – – turbulence closurescheme, in which both the mixing length, m, and are prescribed. The importance ofthe equation is diminished, though itstill determines the dissipation rate in the Eequation.     

Occurrence of roll circulations in a shallow boundary layer

Meteorological measurements taken at the Näsudden wind turbine site during slightly unstable conditions have been analyzed. The height of the convective boundary layer (CBL) was rather low, varying between 60 and 300 m. Turbulence statistics near the ground followed Monin-Obukhov similarity, whereas the remaining part of the boundary layer can be regarded as a near neutral upper layer. In 55% of the runs, horizontal roll vortices were found. Those were the most unstable runs, with -z _i/L > 5. Spectra and co-spectra are used to identify the structures. Three roll indicators were identified: (i) a low frequency peak in the spectrum of the lateral component at low level; (ii) a corresponding increase in the vertical component at mid-CBL; (iii) a positive covariance {ovvw} together with positive wind shear in the lateral direction (V/z) in the CBL. By applying these indicators, it is possible to show that horizontal roll circulations are likely to be a common phenomenon over the Baltic during late summer and early winter.     

Scavenging Efficiency of ‘Aerosol Carbon’ and Sulfate in Supercooled Clouds at Mt. Sonnblick (3106 m a.s.l., Austria)

Cloud water and interstitial aerosol samples collected at Mt. Sonnblick (SBO) were analyzed for sulfate and aerosol carbon to calculate in-cloud scavenging efficiencies. Scavenging efficiencies for sulfate (_SO) ranged from 0.52 to 0.99 with an average of 0.80. Aerosol carbon was scavenged less efficiently with an average value (_AC) of 0.45 and minimum and maximum values of 0.14 and 0.81, respectively. Both _SO and _AC showed a marked, but slightly different, dependence on the liquid water content (LWC) of the cloud. At low LWC, _SO increased with rising LWC until it reached a relatively constant value of 0.83 above an LWC of 0.3 g/m³. In the case of aerosol carbon, we obtained a more gradual increase of _AC up to an LWC of 0.5 g/m³. At higher LWCs, _{_} remained relatively constant at 0.60. As the differences between _SO and _A varied across the LWC range observed at SBO, we assume that part of the aerosol carbon was incorporated into the cloud droplets independently from sulfate. This hypothesis is supported by size classified aerosol measurements. The differences in the size distributions of sulfate and total carbon point to a partially external mixture. Thus, the different chemical nature and the differences in the size and mixing state of the aerosol particles are the most likely candidates for the differences in the scavenging behavior.     

The Askervein Hill Project: Mean wind variations at fixed heights above ground

This is one of a series of papers on the Askervein Hill Project. It presents results on the variations in mean wind speed at fixed heights (z) above the ground from linear arrays of anemometer posts and towers. Most of the data are for z = 10 m but some are for z = 3 m. Selected and directionally grouped data from the 55 Mean Flow runs are presented together with mean flow data from Askervein '83 Turbulence runs. Comparisons are made between the data and guideline estimates of fractional speed-up ratio at hilltop locations and between the data and MS3DJH/3 model predictions along the tower lines. There is good agreement in most cases.     

Properties of spatial temperature derivatives in the atmospheric surface layer

A pair of parallel cold wires separated in either the vertical or lateral direction was used to obtain the three components _x, _y, _z of the temperature derivative in the streamwise, lateral and vertical directions, respectively. The average absolute skewness values of _x and _z are nonzero and approximately equal, while the skewness of _y is approximately zero. These results appear to be consistent with the presence of a large, three-dimensional organised structure in the surface layer. There is an apparent low-frequency contamination in the spectral density of _y and _z due mainly to small errors in estimating the sensitivity of the cold wires. The temperature derivatives were high-pass filtered, the filter being set to remove possible contributions from the large structure and to minimise low-frequency sensitivity contamination. The filtered rms ratios \~_x/\~_y and \~_x/\~_z were in the range 0.7 to 0.9, a result in qualitative agreement with that obtained in the laboratory boundary layer by Sreenivasan et al. (1977). The skewness of filtered _x or _z is negligible, consistent with local isotropy of small-scale temperature fluctuations and in support of the high wavenumber spectral isotropy discussed in Antonia and Chambers (1978).     

Observed variations of σθ and σΦ in the nocturnal drainage flow in a deep valley

The variations of and in the drainage flow in the Brush Creek valley of western Colorado are investigated using data from Doppler acoustic sodars and instrumented towers. The data were obtained on two experimental nights during the 1984 ASCOT field study. There is good agreement between the variations derived from low-level observations of the sodars and those derived from the towers located throughout the valley. The observed hourly average and in the nocturnal drainage flow are about 20 ° to 25 ° and 5 °, respectively; these values are much larger than those generally observed over flat terrain during nighttime stable conditions. After sunrise (about 0600 MST), as the valley warms and the flow direction changes to up-valley, these parameters increase sharply to their peak values at about 0800 MST and then decrease to their normal daytime values after about two hours.In the drainage flow, the hourly average varies inversely with wind speed according to the relation u 0.7ms^-1. The vertical standard deviation is much less enhanced by complex terrain than the horizontal standard deviation. The observed values are predicted fairly well by the local similarity theory.Oak Ridge Associated Universities (ORAU) Summer Research Participant at ATDD in 1987 andOak Ridge Associated Universities (ORAU) Summer Research Participant at ATDD in 1987 and     

Monitoring of Short-Term Heavy Metal Deposition by Accumulation in Epiphytic Lichens (Hypogymnia Physodes (L.) Nyl.)

Many lichens are very sensitive to air pollution due to their symbiotic nature. However, they are generally less sensitive to toxic effects of trace elements; therefore they can be used as accumulator organisms for estimating concentrations of these elements in the environment. Heavy metal accumulation in lichens is a commonly used bioindication method for assessing heavy metal ambient levels. An active biomonitoring method was used for the determination of short-term accumulation of zinc, lead, arsenic and cadmium by epiphytic lichens transplanted at different localities in Slovenia polluted by heavy metals. The atomic absorption spectrophotometry method was used for the determination of heavy metal content in lichen thalli. The content of heavy metals in lichens collected in the background area with clean air (Rogla, Pohorje Mountains) was used as the reference value. Lichens were transplanted from Rogla using the branch transplantation technique for a period of 6 months in the surroundings of Slovenian thermal power plants (Velenje, otanj, Zavodnje, Veliki Vrh, Vnajnarje, and Dobovec) and close to the lead and zinc ore smelter at erjav. The monthly accumulation of heavy metals was comparable within years at selected locations. Heavy metal pollution was the highest at erjav despite of remediation of lead and zinc ore smelter. The monthly accumulation of all four heavy metals was statistically significantly higher in lichens exposed at erjav than at other localities.     

Small-Scale Variability in the Coastal Atmospheric Boundary Layer

The influence of the main large-scale wind directions on thermally driven mesoscale circulations at the Baltic southwest coast, southeast of Sweden, is examined. The aim of the study is to highlight small-scale alterations in the coastal atmospheric boundary layer. A numerical three-dimensional mesoscale model is used in this study, which is focused on an overall behaviour of the coastal jets, drainage flows, sea breezes, and a low-level eddy-type flow in particular. It is shown that synoptic conditions, together with the moderate terrain of the southeast of Sweden (max. height h₀ 206 m), governs the coastal mesoscale dynamics triggered by the land-sea temperature difference T. The subtle nature of coastal low-level jets and sea breezes is revealed; their patterns are dictated by the interplay between synoptic airflow, coastline orientation, and T.The simulations show that coastal jets typically occur during nighttime and vary in height, intensity and position with respect to the coast; they interact with downslope flows and the background wind. For the assigned land surface temperature (varying ±8 K from the sea temperature) and the opposing constant geostrophic wind 8 m s^-1, the drainage flow is more robust to the opposing ambient flow than the sea breeze later on. Depending on the part of the coast under consideration, and the prevailing ambient wind, the sea breeze can be suppressed or enhanced, stationary at the coast or rapidly penetrating inland, locked up in phase with another dynamic system or almost independently self-evolving. A low-level eddy structure is analyzed. It is governed by tilting, divergence and horizontal advection terms. The horizontal extent of the coastal effects agrees roughly with the Rossby radius of deformation.     

A SIMPLE FORMULA FOR ATTENUATION OF EDDY FLUXES MEASURED WITH FIRST-ORDER-RESPONSE SCALAR SENSORS

A simple formula, (1 + (2fmc))-1,is proposed to estimate the attenuation of a scalar flux measurement made by eddy-correlation using a fast-response anemometer and a linear, first-order-response scalar sensor with a characteristic time constant c.In this formula, =7/8 for neutral and unstable stratification within the surface-flux layer and =1 both within the convective boundary layer (CBL) and for stable stratification in the surface layer.fm is the frequency of the peak of the logarithmic cospectrum and can be estimated from fm = nm /z, where z is the measurement height and is thewind speed at that height. The dimensionless frequency at the cospectral maximum nm is estimated here from observations of its behavioras a function of atmospheric stability, z/L within the surface layeror z/zi within the CBL, where L is the Obukhov stability length and zi is the depth of the CBL. The predicted dependence of flux attenuation on measurement height is discussed.     

Atmospheric correction of infrared measurements of sea surface temperature using channels at 3.7, 11 and 12 Μm

Atmospheric effects upon the radiometric determination of surface temperature were studied for channels centered at 3.7, 11 and 12 m. The error due to the atmosphere is least for the channel centered at 3.7 m, which is a real advantage. The use of a linear combination of two or all three of these channels allows one to eliminate most of the atmospheric effect. If instrumental noise of from 0.1 to 0.2 K is accounted for in each channel, the best results are obtained by a combination of the two channels at 3.7 and 12 m.     

Numerical modelling of the turbulent flow developing within and over a 3-d building array,part ii: a mathematical foundation for a distributed drag force approach

In this paper, we lay the foundations of a systematic mathematical formulation for the governing equations for flow through an urban canopy (e.g., coarse-scaled building array) where the effects of the unresolved obstacles on the flow are represented through a distributed mean-momentum sink. This, in turn, implies additional corresponding terms in the transport equations for the turbulence quantities. More specifically, a modified k-- model is derived for the simulation of the mean wind speed and turbulence for a neutrally stratified flow through and over a building array, where groups of buildings in the array are aggregated and treated as a porous medium. This model is based on time averaging the spatially averaged Navier--Stokes equations, in which the effects of the obstacle--atmosphere interaction are included through the introduction of a volumetric momentum sink (representing drag on the unresolved buildings in the array).The k-- turbulence closure model requires two additional prognostic equations, namely one for the time-averaged resolved-scale kinetic energy of turbulence,, and another for the dissipation rate, , of . The transport equation for is derived directly from the transport equation for the spatially averaged velocity, and explicitly includes additional sources and sinks that arise from time averaging the product of the spatially averaged velocity fluctuations and the distributed drag force fluctuations. We show how these additional source/sink terms in the transport equation for can be obtained in a self-consistent manner from a parameterization of the sink term in the spatially averaged momentum equation. Towards this objective, the time-averaged product of the spatially averaged velocity fluctuations and the distributed drag force fluctuations can be approximated systematically using a Taylor series expansion. A high-order approximation is derived to represent this source/sink term in the transport equation for . The dissipation rate () equation is simply obtained as a dimensionally consistent analogue of the equation. The relationship between the proposed mathematical formulation of the equations for turbulent flow within an urban canopy (where the latter is treated as a porous medium) and an earlier heuristic two-band spectral decomposition for parameterizing turbulence in a plant canopy is explored in detail.     

Effect of land management on ecosystem carbon fluxes at a subalpine grassland site in the Swiss Alps

Summary The influence of agricultural management on the CO₂ budget of a typical subalpine grassland was investigated at the Swiss CARBOMONT site at Rigi-Seebodenalp (1025m a.s.l.) in Central Switzerland. Eddy covariance flux measurements obtained during the first growing season from the mid of spring until the first snow fall (17 Mai to 25 September 2002) are reported. With respect to the 10-year average 1992–2001, we found that this growing season had started 10 days earlier than normal, but was close to average temperature with above-normal precipitation (100–255% depending on month). Using a footprint model we found that a simple approach using wind direction sectors was adequate to classify our CO₂ fluxes as being controlled by either meadow or pasture. Two significantly different light response curves could be determined: one for periods with external interventions (grass cutting, cattle grazing) and the other for periods without external interventions. Other than this, meadow and pasture were similar, with a net carbon gain of –128±17g Cm^–2 on the undisturbed meadow, and a net carbon loss of 79±17g Cm^–2 on the managed meadow, and 270±24g Cm^–2 on the pasture during 131 days of the growing season, respectively. The grass cut in June reduced the gross CO₂ uptake of the meadow by 50±2% until regrowth of the vegetation. Cattle grazing reduced gross uptake over the whole vegetation period (37±2%), but left respiration at a similar level as observed in the meadow.     

Analytical investigations of horizontal meso-scale momentum equations with Newtonian nudging

Summary This study presents an analytical investigation of the local behaviour of the solution to a mesoscale model with Newtonian nudging when observed winds are time varying. The analysis examines each Fourier component of the time series of observed winds. Unlike the case with a constant observed wind, the nudged wind vector does not asymptotically approach the observed wind. In response to sinusoidal oscillation of the observed wind, the nudged wind vector is always on a half circle connecting the vector ends of the observed and un-nudged modelled winds. When nudging parameter 0, the nudged wind vector approaches the un-nudged wind; when , the nudged wind vector approaches the observed wind. For commonly used values of nudging parameter , the modelled wind field always carries errors.A target nudging scheme is devised in this study in order to ensure the model result is identical to observed winds with sinusoidal oscillation. Investigation shows that such a target wind exists for a finite value of , and the magnitude of the target-nudging term is about the same as that of a normal nudging term if f, wheref is the Coriolis parameter and is the frequency of the wind oscillation.With 7 Figures     

La conversion du rayonnement sphérique en rayonnement global

Résumé Une étude théorique permet d'estimer les sommes journalières soit du rayonnement global tombant sur un récepteur plan et horizontal, soit du rayonnement sphérique ou circumglobal tombant sur une sphère. Ces deux sommes dépendent fortement des conditions atmosphèriques: atmosphère pure, brume, humidité et nébulosité, ainsi que de la latitude géographique, de la déclinaison du soleil et de l'albédo du sol. On montre qu'il est quand-même possible de trouver une relation du deuxième ou troisième degré entre les deux types de mesures où les coefficients ne dépendent que de la différence – entre latitude et déclinaison et de l'albédo du sol. Cette relation est valable dans les basses latitudes <30° avec une faible dispersion des valeurs journalières; dans les latitudes plus élevées sa validité semble limitée au semestre estival.

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Summary The daily totals of radiation falling on a horizontal receiver as well as on a spherical one are calculated by theoretical approach. Both types of values change very much with atmospherical conditions, such as pure atmosphere, haze and humidity of the atmosphere and cloudiness and furthermore with the geographical latitude, the declination of sun and finally with the albedo of the ground. It is demonstrated that it is nevertheless possible to establish a relation of second or third order between both these types of values. The coefficients are function only of –, the difference between latitude and declination and of the albedo . This relation is valid in low latitudes <30° for daily totals; in higher latitudes its validity is limited to days of the summer period.

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Zusammenfassung Auf theoretischem Wege werden die Tagessummen sowohl der auf einen horizontalen Empfänger fallenden Globalstrahlung von Sonne und Himmel als auch der auf eine Kugel fallenden Zirkumglobalstrahlung ermittelt. Beide Werte hängen sehr stark von den atmosphärischen Verhältnissen, wie Reinheit der Atmosphäre, Dunst, Feuchtigkeit und Bewölkung, sowie von der geographischen Breite, der Deklination der Sonne und der Albedo der Bodenoberfläche ab. Es wird nun gezeigt, daß es trotzdem möglich ist, eine Beziehung zweiten oder dritten Grades zwischen den beiden Arten von Tagessummen herzuleiten, in der die Koeffizienten nur von der Differenz – zwischen geographischer Breite und Deklination der Sonne einerseits und der Albedo andererseits abhängen. Diese Beziehung gilt in niederen Breiten <30° für alle Einzelwerte, in höheren Breiten nur im Sommerhalbjahr.

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Avec 8 Figures     

An invariant theory approach to the problem of closure

A solution to the kinematical aspects of the problem of closure, from the standpoint of the theory of tensor invariants, is presented. The solution is effected for isotropic turbulence by employing a modified tensor invariant theory and it is shown how a similar approach will lead to the solution of the corresponding closure problem for a simple type of nonhomogeneous turbulence. A geometric significance is attached to the results of this approach by introducing the concept of a multiplicative product of two vectors. The theoretical results are applied to the case of isotropic atmospheric turbulence. The kinetical implication of the proposed solution to the problem is discussed.Associate Professor of Physics and Mathematics, Del Mar College, Corpus Christi, Texas (on leave of absence).