A. Longhetto

Localization of source and sink regions of carbon dioxide through the method of the synoptic air trajectory statistics

Abstract The main purpose of this paper is to contribute to the improvement of the present knowledge concerning the transient components of the global carbon cycle, superimposed to the periodic seasonal oscillation and to the yearly trend. This purpose has been achieved through the comparison among the calculated concentration fields of atmospheric CO2 and its comparison with the sea-surface temperature patterns, forestation maps, forest fires, and the anthropogenic emissions extracted from Edgar V.2.0 database. In order to identify with high spatial resolution the most relevant areas of CO2 sources and sinks, we have applied a methodology based on a statistical analysis of simulated back-trajectories related to atmospheric concentration values measured at some receptor sites where the back-trajectories originate. In particular, we have used a 2-year time series (1996 and 1997) of CO2 concentration data observed in three receptor sites located in high mountain areas, in order to reduce significantly the effects due to local influences (such as emissions from industries and urban areas or the absorption processes due to the vegetation). The back-trajectories were computed by means of the wind fields provided by the ECMWF analysis (T213/L31 model) on a regular grid. The area investigated was from 11°W to 36°E in longitude and from 30°N to 57°N in latitude. The final concentration field was computed by means of a statistical source–receptor model, based on a methodology developed by Stohl (Atmos. Environ. 30 (1996) 579) and adapted here with some modifications in the pre- and post-processing phases. Before applying the model, a careful evaluation of its sensitivity to the input data has been performed, followed by an analysis to identify the optimal configuration of the model. The results have shown a satisfactory accuracy in the identification of the major sources and sinks considered.

A study of the performance of a land surface process model (LSPM)

In this paper, a reliable Land-Surface Process Model (LSPM), which is a new version of the LPM of Ji and Hu (1989), is described. The LSPM has been validated with experimental data measured at two stations in the Po Valley (Northern Italy).

Acoustic sounding of land and sea breezes

Sodar measurements have been made at La Spezia, Italy during land- and sea-breeze conditions. The backscatter returns are discussed qualitatively, including their relation to the vertical structure of the boundary layer as revealed by vertical soundings of wind and temperature. During inversion conditions, the sodar signals may be difficult to interpret especially when there is a land breeze flowing over irregular terrain.

Experimental study of higher-order moments in shear-driven boundary layers with rotation

The results of laboratory wall turbulence experiments on a shear-driven rotating boundary layer are presented. The experiments were carried out in the Turin University Laboratory rotating water tank. The flow was generated by changing the rotation speed of the platform and measured by means of particle image velocimetry. In order to analyse the influence of the rotation and of surface roughness, different cases were examined. Several rotation periods were considered. The measurements were performed both over a smooth surface and over a rough-to-smooth transition. Mean flows and the higher-order moments of the velocity probability density function are shown and discussed together with a comparison of the different experimental cases, theory and large-eddy simulations.

A study of greenhouse gases and air trajectories at Plateau Rosa

SummaryThe attention paid to changes in atmospheric concentrations of greenhouse gases has recently increased, with particular regard to the relationships between variations of their annual trends and of their shorter-period fluctuations on the one hand, and some geophysical and biological natural processes (like atmospheric transport, volcanic eruptions, ENSO events, seasonal vegetation cycles, and so on) on the other hand. Careful analysis and interpretation of the above relationships, in fact, can often disclose unknown mechanisms acting on modulations of these gases and make it possible for us to better understand the contributions made to them by natural causes, contrasted with anthropogenic ones. This paper presents and discusses the results of a series of CO2 air concentration data, measured at Plateau Rosa (3480 m a.s.l.—Italian Western Alps) over a period of about four years, and correlated with atmospheric transport processes on synoptic scale. These results have shown the leading role played by some kinds of atmospheric circulation patterns in creating situations of not fully mixed air streams (and, as such, not representative of background conditions) and have also suggested a «meteorological» selection scheme for CO2 data to be used in evaluating more reliable annual trends.

Experimental study of atmospheric gravity waves and visibility oscillations in a fog episode

SummaryA fog episode characterized by quasi-periodic oscillations in visibility is described. Gravity waves are also present, and frequencies at which pressure and transparency fluctuate are compared. The existence of intervals during which they are synchronized and the presence of strong winds aloft suggest that shear instabilities in the troposphere can influence the fog behaviour at ground.

A laboratory model of wake-affected stack's emissions

Abstract The high demand for electric energy during winter months, which coincides with the most critical conditions from the point of view of atmospheric pollution, makes it necessary to exploit at their maximum capacity even the oldest and smallest power plants, where dispersion is usually inefficient and greatly influenced by the aerodynamic wake of the buildings. In this paper the results of a series of hydrodynamic simulations, concerning the emissions of two 70 MW units of an electric power plant are presented. It was, in fact, necessary to decide whether a certain height-increase of the stacks, within the limits imposed by the building structures, would be sufficient to reduce the elevated ground-level concentrations found in some critical wind conditions, or if new stacks, external to the power plant buildings, would be needed.

Inspection of high-concentration CO2 events at the Plateau Rosa Alpine station

The Plateau Rosa Alpine station (Italy) has collected atmospheric concentrations of carbon dioxide since 1989. If the complete set of hourly data is observed, two distinct and exceptional very high concentration events are evident for February 2004. Similar and almost contemporary peaks were registered at the European high–altitude stations of Zugspitze–Schneefernerhaus and Sonnblick in the Alps, and at Mt. Cimone in the Northern Apennines. A regional meteorological model (the Weather Research and Forecast) was applied over a medium–high resolution grid to study the evolution of the meteorological fields and to identify the trajectories of the polluted air masses during the CO2 observed peaks. The results show that, during both episodes, atmospheric circulation conveyed highly polluted air from the European plains to the Alpine stations. This conclusion has been also confirmed through concentration measurements of the atmospheric trace gases in the same area.

A laboratory simulation of mesoscale flow interaction with the Alps

Abstract A series of laboratory experiments, aimed at the simulation of some aspects of Alpine lee cyclogenesis has been carried out in the rotating tank of the Coriolis Laboratory of LEGI-IMG in Grenoble. Dynamic and thermodynamic processes, typical of baroclinic development triggered by the orography, were simulated. The background flow simulating the basic state of the atmosphere consisted of a stream of intermediate density fluid introduced at the interface between two fluid layers. The structure of the intermediate current was established by mixing fluid obtained from the upper layer of fresh water with fluid removed from the heavier salty layer below. The dynamical similarity parameters are the Rossby ( Ro ), Burger ( Bu ) and Ekman ( Ek ) numbers, although this last, owing to its small values, need not be matched between model and prototype, since viscous effects are not important for small time scales. The flow in both the prototype and laboratory simulation is characterized by hydrostatics; this requires ( Ro 2 δ 2 / Bu )⪡1 (where δ = H / L is the aspect ratio of the obstacle) which is clearly satisfied, in the atmosphere and oceans, and for the laboratory experiment. A range of experiments for various Rossby and Burger numbers were conducted which delimited the region of parameter space for which background flows akin to that found to the northwest of the Alps prior to baroclinic cyclogenesis events, were observed. One such experiment was carried out by placing a model of the Alps at the appropriate place in the flow field. The subsequent motion in the laboratory was observed and dye tracer motions were used to obtain the approximate particle trajectories. The density field was also analyzed to provide the geopotential field of the simulated atmosphere. Using standard transformations from the similarity analysis, the laboratory observations were related to the prototype atmosphere. The flow and the geopotential fields gave results compatible with the particular atmospheric event presented.

Laboratory simulation of inertial and frictional effects on barotropic rotating flows over and past obstacles: Comparison with simple numerical and analytical models

SummaryThis paper presents the results of laboratory experiments designed to simulate some basic process of large-scale flows interacting with obstacles, also in order to better understand details of subsynoptic disturbances that are created in the lee of large topographic features. For this event, the experimental facilities of the Istituto di Cosmogeofisica of Consiglio Nazionale delle Ricerche (CNR) were exploited, consisting in a hydraulic channel mounted on a rotating platform, along whose longitudinal axis a hemispherical obstacle was towed at various speeds. Because of the conversion of potential vorticity, the experimental results showed, as expected, the existence of a region of anticyclonic circulation, located above the obstacle; however, also an asymmetric pattern of positive vorticity located downwind of the obstacle did appear, which cannot be interpreted in terms of simple quasi-geostrophic inviscid dynamics. This behaviour is not surprising, if one considers that the real flow near the obstacle could hardly ever match the conditions of inviscid quasi-geostrophy (Ro≪1,E∼0), but was similar to that characterising the zone close to the surface of the obstacle, where inertial and viscous effects are not negligible. Finally, in order to investigate the importance of these effects on the interaction processes, simple numerical and analytical models were applied, by which the consistency of some laboratory simulations, chosen among the most significant ones, could be compared.