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Contamination by the pollutants SO<sub>2</sub> and SO<sup>=</sup><sub>4</sub> was analyzed for the 1989–1992 period at four regional stations in Spain under the auspices of the EMEP-BAPMON program. The evolution of the time series of the daily pollution has also been assessed, and high mean concentrations at La Cartuja and Logroño observed, with values of 3.8 and 4.5 μg m<sup>–3</sup> for SO<sub>2</sub>, respectively. Maximum annual concentrations were recorded in 1989, when SO<sub>2</sub> reached values of 6.24, 5.39, 5.71, and 9.30 μg m<sup>–3</sup> for the stations of La Cartuja, San Pablo de los Montes, Roquetas, and Logroño, respectively. This work attempts to establish a relationship between the concentrations of the pollutants – both SO<sub>2</sub> gas and SO<sup>=</sup><sub>4</sub> aerosol – and the zones of emission or persistence of these long-range transported pollutants. In this way, those regions showing a greater impact on the air quality in each season have been determined. To achieve this, the trajectories of the air masses carrying away the pollution to each of the receiving stations were considered and followed by a sectorial analysis. Nonparametric statistical methods were implemented to contrast the chemical homogeneity among the different sectors. The criterion that several homogeneous sectors form a chemically homogeneous region was used. To improve this sectorial analysis, we have proposed a new technique based on the Potential-Source-Contribution Function (PSCF). Starting out from a set of specified regions, considered to be chemically homogeneous domains, it is possible to determine the likelihood that an air mass with particular characteristics (e.g., that a value of the daily concentration higher than the mean recorded at the station has been obtained) will arrive at a given station after having crossed one of the previously defined regions. Using this technique, it is possible to determine the source regions through which the air masses circulate and bring high pollution concentrations to the studied stations. Thanks to the PSCF, these statistical methods offer, through a sectorial analysis, the possibility to pass from a qualitative to a more quantitative view.
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Some signed measures in turbulence are found to be sign-singular, that is their sign reverses continuously on arbitrary finer scales with a reduction of the cancellation between positive and negative contributions. The strength of the singularity is characterized by a scaling exponent κ, the cancellation exponent. In the present study by using some turbulent samples of the velocity field obtained from spacecraft measurements in the interplanetary medium, we show that sign-singularity is present everywhere in low-frequency turbulent samples. The cancellation exponent can be related to the characteristic scaling laws of turbulence. Differences in the values of κ, calculated in both high- and low-speed streams, allow us to outline some physical differences in the samples with different velocities.
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The effects of cloud shadowing, channelling, cloud side illumination and droplet concentration are investigated with regard to the reflection of shortwave solar radiation. Using simple geometric clouds, coupled with a Monte Carlo model the transmission properties of idealized cloud layers are found. The clouds are illuminated with direct solar radiation from above. The main conclusion reached is that the distribution of the cloud has a very large influence on the reflectivity of a cloud layer. In particular, if the cloud contains vertical gaps through the cloud layer in which the liquid water content is zero, then, smaller more numerous gaps are more influential on the radiation than fewer, larger gaps with equal cloud fraction. At very low solar zenith angles channelling of the radiation reduces the reflection expected on the basis of the percentage cloud cover. At high solar zenith angles the illumination of the cloud edges significantly increases the reflection despite the shadowing of one cloud by another when the width of the gaps is small. The impact of droplet concentration upon the reflection of cloud layers is also investigated. It is found that at low solar zenith angles where channelling is important, the lower concentrations increase the transmission. Conversely, when cloud edge illumination is dominant the cloud distribution is found to be more important for the higher concentrations.
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Radiative properties of cirrus clouds are one of the major unsolved problems in climate studies and global radiation budget. These clouds are generally composed of various ice-crystal shapes, so we tried to evaluate effects of the ice-crystal shape on radiative fluxes. We calculated radiative fluxes of cirrus clouds with a constant geometrical depth, composed of ice crystals with different shapes (hexagonal columns, bullets, bullet-rosettes), sizes and various concentrations. We considered ice particles randomly oriented in space (3D case) and their scattering phase functions were calculated by a ray-tracing method. We calculated radiative fluxes for cirrus layers for different microphysical characteristics by using a discrete-ordinate radiative code. Results showed that the foremost effect of the ice-crystal shape on radiative properties of cirrus clouds was that on the optical thickness, while the variation of the scattering phase function with the ice shape remained less than 3% for our computations. The ice-water content may be a better choice to parameterize the optical properties of cirrus, but the shape effect must be included.
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A numerical model has been used to calculate the atmospheric response to forcing at periods in the region of 12-13.5 h. The results show that the response is enhanced in the neighbourhood of 13 h. These results have been compared with lunar tidal analyses of mesospheric wind data and geomagnetic variations at a number of stations. It is found that the N<sub>2</sub> lunar tidal component (period 12.66 h) is significantly enhanced relative to the main lunar tidal component M<sub>2</sub> (period 12.42 h) in both types of data, compared with what would be expected from the gravitational tidal potential. This supports the predictions of the numerical model. An appreciable phase shift is also found in the experimental data between the N<sub>2</sub> and M<sub>2</sub> tides, agreeing in sense with what would be expected for a resonance at a period around 13 h.
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Computer modelling is used to investigate the possibility of determining ionospheric parameters from slightly oblique ionospheric soundings, using absorption data for decametric radio waves of different polarization. It is shown that with mean square measurement errors of 0.5 dB, and using regularization algorithms to solve the inverse problems, electron collision frequency profiles can be obtained for the night F-region with errors of less than 30%. Both temperatures of electrons and neutrals are also determined to within 10%.
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A statistical investigation of the relationship between VHF radar auroral backscatter intensity and Doppler velocity has been undertaken with data collected from 8 years operation of the Wick site of the Sweden And Britain Radar-auroral Experiment (SABRE). The results indicate three different regimes within the statistical data set; firstly, for Doppler velocities <200 m s<sup>–1</sup>, the backscatter intensity (measured in decibels) remains relatively constant. Secondly, a linear relationship is observed between the backscatter intensity (in decibels) and Doppler velocity for velocities between 200 m s<sup>–1</sup> and 700 m s<sup>–1</sup>. At velocities greater than 700 m s<sup>–1</sup> the backscatter intensity saturates at a maximum value as the Doppler velocity increases. There are three possible geophysical mechanisms for the saturation in the backscatter intensity at high phase speeds: a saturation in the irregularity turbulence level, a maximisation of the scattering volume, and a modification of the local ambient electron density. There is also a difference in the dependence of the backscatter intensity on Doppler velocity for the flow towards and away from the radar. The results for flow towards the radar exhibit a consistent relationship between backscatter intensity and measured velocities throughout the solar cycle. For flow away from the radar, however, the relationship between backscatter intensity and Doppler velocity varies during the solar cycle. The geometry of the SABRE system ensures that flow towards the radar is predominantly associated with the eastward electrojet, and flow away is associated with the westward electrojet. The difference in the backscatter intensity variation as a function of Doppler velocity is attributed to asymmetries between the eastward and westward electrojets and the geophysical parameters controlling the backscatter amplitude.
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The effect of contrails and aircraft water vapour emissions on global climate is studied by means of a general circulation model (GCM). In a first approach water vapour emissions and mean contrail coverage within the main flight routes are prescribed according to current observations in a simplified manner. A hierarchic experiment strategy with gradual increase of the forcing is applied to identify the resulting climate signals. The water vapour increase to be expected from air traffic is too small to force a detectable radiative or climatic response. The sensitivity of the model climate to the occurrence of contrails appears to be higher. For mid-latitude summer conditions, the high cloud increase experiments show a consistent temperature response pattern. However, its magnitude is statistically significant only for a mean contrail coverage exceeding present-day amounts. Moreover, the magnitude of the contrail climate signal is highly sensitive to the details of the experimental setup due to several non-linearities of the cloud-radiative interaction. Hence, the prescription of contrails in the GCM has to be as careful as possible for an optimal treatment of the problem. Respective recommendations are given.
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Some theoretical concepts about the frictionless dynamics of propagation of the barotropic tide over two-dimensional continental slopes of arbitrary shape are developed. A numerical procedure which generalizes the exact solution obtained over a rectilinear sloping topography is given. This technique can be applied to compute the harmonic components of the barotropic tide everywhere over sloping bottom contours of any shape. It permits in particular the avoidance of discontinuities at the boundaries of rectilinear-continental-slope profiles. The barotropic tidal results are used afterwards to calculate the barotropic forcing for the generation of internal tides. Numerical experiments are performed to study the interaction between the tide and some typical sloping topographies. A three-layered model is used for this purpose. Results are compared with those previously obtained over a rectilinear continental slope.
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The SEMAPHORE (Structure des Echanges Mer-Atmosphère, Propriétés des Hétérogénéités Océaniques: Recherche Expérimentale) experiment has been conducted from June to November 1993 in the Northeast Atlantic between the Azores and Madeira. It was centered on the study of the mesoscale ocean circulation and air-sea interactions. The experimental investigation was achieved at the mesoscale using moorings, floats, and ship hydrological survey, and at a smaller scale by one dedicated ship, two instrumented aircraft, and surface drifting buoys, for one and a half month in October-November (IOP: intense observing period). Observations from meteorological operational satellites as well as spaceborne microwave sensors were used in complement. The main studies undertaken concern the mesoscale ocean, the upper ocean, the atmospheric boundary layer, and the sea surface, and first results are presented for the various topics. From data analysis and model simulations, the main characteristics of the ocean circulation were deduced, showing the close relationship between the Azores front meander and the occurrence of Mediterranean water lenses (meddies), and the shift between the Azores current frontal signature at the surface and within the thermocline. Using drifting buoys and ship data in the upper ocean, the gap between the scales of the atmospheric forcing and the oceanic variability was made evident. A 2 °C decrease and a 40-m deepening of the mixed layer were measured within the IOP, associated with a heating loss of about 100 W m<sup>-2</sup>. This evolution was shown to be strongly connected to the occurrence of storms at the beginning and the end of October. Above the surface, turbulent measurements from ship and aircraft were analyzed across the surface thermal front, showing a 30% difference in heat fluxes between both sides during a 4-day period, and the respective contributions of the wind and the surface temperature were evaluated. The classical momentum flux bulk parameterization was found to fail in low wind and unstable conditions. Finally, the sea surface was investigated using airborne and satellite radars and wave buoys. A wave model, operationally used, was found to get better results compared with radar and wave-buoy measurements, when initialized using an improved wind field, obtained by assimilating satellite and buoy wind data in a meteorological model. A detailed analysis of a 2-day period showed that the swell component, propagating from a far source area, is underestimated in the wave model. A data base has been created, containing all experimental measurements. It will allow us to pursue the interpretation of observations and to test model simulations in the ocean, at the surface and in the atmospheric boundary layer, and to investigate the ocean-atmosphere coupling at the local and mesoscales.
