A. di Sarra

Saharan dust profiles measured by lidar at Lampedusa

Lidar observations of the tropospheric aerosols were carried out at the island of Lampedusa (35.5°N, 12.6°E) in the Mediterranean during May-June 1999, within the Photochemical Activity and Ultraviolet Radiation II experiment. The measurements indicate that the troposphere is often loaded with large amounts of aerosol particles, producing relatively large values of the depolarization ratio. The aerosol content below and above 2 km shows a somewhat different behavior. In the upper region, large aerosol concentrations last for a few days; during these events aerosol is often detected up to 7 or 8 km. Large amounts were detected in mid-May and were very often observed in June. By using meteorological analyses and isentropic backward trajectories, the aerosol behavior above Lampedusa has been related to the large-scale transport patterns and to the source regions. Large aerosol loads are clearly due to dust transport from Africa, occurring through two main paths: from central Sahara, when a high-pressure system was centered over northern Libya, and following the northwestern African coast, often along the Atlas Mountains, when the anticyclone is over Algeria or Libya, at latitudes lower than 30°N. Large aerosol loads are observed even when the air mass trajectories marginally overpass Africa, often up to 5–6 km. According to the isentropic trajectories, large vertical motions occur when the air masses travel over Africa. Significant differences in the aerosol profiles are found, depending on the origin of the air masses, and on the strength of the vertical motion. All the air masses that have traveled over Africa show an enhanced aerosol concentration, indicating that in this period the conditions were always favorable to dust mobilization and vertical propagation. The identified transport patterns appear to mainly affect the southern Mediterranean; in rare cases, forward trajectories reached southern Italy, Greece, Turkey, and the eastern Mediterranean.

Latitudinal distribution of stratospheric aerosols during the EASOE winter 1991/92

This paper summarizes lidar measurements of stratospheric aerosols spanning the latitude range from 44[degrees]N to 79[degrees]N, during the period of the EASOE campaign. The Arctic region measurements show no aerosol content above roughly 16 km, but the density is fairly constant at lower altitudes independent of latitude. The authors argue this indicates latitudinal transport of aerosol throughout the winter.

Convective characteristics of the nocturnal urban boundary layer as observed with Doppler sodar and Raman lidar

Convective plume patterns, characteristic of clear sky and light wind daytime boundary layers over land, were observed for two nights with a tri-axial Doppler sodar operated in the central area of Rome during the summer of 1994. An urban heat island effect, combined with a continuation of a breeze from the sea late into night during both days, is believed to be responsible for the observed nocturnal convection. Estimates of the surface heat flux and the vertical velocity scaling parameter are obtained from profiles of vertical velocity variance, and the Raman lidar water vapor measurements are used to obtain the humidity scaling parameter. Convective scaling results for vertical wind and humidity fairly agree with the results of other experiments and models. On the basis of available measurements, it appears that mixed-layer similarity formulations used to characterize the daytime convective boundary layer over horizontally homogeneous surfaces can also be applied to the nocturnal urban boundary layer during periods of reasonable convective activity.

Backscatter measurements of stratospheric aerosols at Thule during January‐February 1992

Within the European Arctic Stratospheric Ozone Experiment (EASOE) aerosols from the volcanic eruption of Mt. Pinatubo have been observed during 6 balloonborne backscatter soundings in January and February 1992 under different polar vortex conditions from Thule, Greenland. The vortex boundary seemed to retard stratospheric aerosol mixing into the inner parts of the vortex from lower latitudes; however, when Thule was outside the vortex large aerosol loadings were measured. The aerosols were simultaneously observed by groundbased lidar, whereby aerosol backscatter measurements in three different wavelengths made it possible to obtain information about the particle sizes. Parameters of lognormal distributions have been derived, using a least square approach between observed and calculated backscatter coefficients from Mie theory. The aerosol surface area density was increased by factors 10–50, compared to the background levels at pre-volcanic conditions.

Interannual variability of atmospheric CO2 in the Mediterranean: measurements at the island of Lampedusa

The atmospheric concentration of carbon dioxide and other greenhouse gases has been measured weekly since 1992 at the island of Lampedusa, in the Mediterranean sea. Lampedusa is a small island located approximately 100 km east of Tunisia, and 250 km south of Sicily. The 10-yr CO2 data set has been analysed to quantify trends, and characterize semi-annual, annual and inter-annual variability. The data show an average trend of ×1.7 ppmv yr−1; the average annual cycle has an amplitude of about 9 ppmv. In the period of investigation the annual growth rate varies between 0.5 and 4.5 ppmv yr−1, and the amplitude of the annual cycle between 7 and 11 ppmv yr−1. By comparing the observed growth rate with recent estimates of carbon dioxide emissions, it is calculated that 58–61% of the emitted CO2 remains in the atmosphere. The CO2 growth rate appears to be related to large-scale dynamic phenomena, primarily El Niño/Southern Oscillation (ENSO). An evident signature of the 1997–98 El Niño is apparent in the CO2 record, and corresponds to a weakening of the exchange with the biosphere. A high correlation between the global average temperature and the 12-month average carbon dioxide growth rate is also found. Wind direction displays a significant inter-annual variability throughout the measurement period, possibly influencing the observed evolution of the CO2 concentration.

An improved algorithm for the determination of aerosol optical depth in the ultraviolet spectral range from Brewer spectrophotometer observations

Methods to derive aerosol optical depth in the UV spectral range from ground-based remote-sensing stations equipped with Brewer spectrophotometers have been recently developed. In this study a modified Langley plot method has been implemented to retrieve aerosol optical depth from direct sun Brewe rm easurements. The method uses measurements over an extended range of atmospheric airmasses obtained with two different neutral density filters, and accounts for short-term variations of total ozone, derived from the same direct sun observations. The improved algorithm has been applied to data collected with a Brewer mark IV, operational in Rome, Italy, and with a Brewer mark III, operational in Lampedusa, Italy, in the Mediterranean. The efficiency of the improved algorithm has been tested comparing the number of determinations of the extraterrestrial constant against those obtained with a standard Langley plot procedure. The improved method produces a larger number of reliable Langley plots, allowing for a better statistical characterization of the extraterrestrial constant and a better study of its temporal variability. The values of aerosol optical depth calculated in Rome and Lampedusa compare well with simultaneous determinations in the 416–440 nm interval derived from MFRSR and CIMEL measurements.

Altitude‐resolved shortwave and longwave radiative effects of desert dust in the Mediterranean during the GAMARF campaign: Indications of a net daily cooling in the dust layer

Desert dust interacts with shortwave (SW) and longwave (LW) radiation, influencing the Earth radiation budget and the atmospheric vertical structure. Uncertainties on the dust role are large in the LW spectral range, where few measurements are available and the dust optical properties are not well constrained. The first airborne measurements of LW irradiance vertical profiles over the Mediterranean were carried out during the Ground-based and Airborne Measurements of Aerosol Radiative Forcing (GAMARF) campaign, which took place in spring 2008 at the island of Lampedusa. The experiment was aimed at estimating the vertical profiles of the SW and LW aerosol direct radiative forcing (ADRF) and heating rates (AHR), taking advantage of vertically resolved measurements of irradiances, meteorological parameters, and aerosol microphysical and optical properties. Two cases, characterized respectively by the presence of a homogeneous dust layer (3 May, with aerosol optical depth, AOD, at 500 nm of 0.59) and by a low aerosol burden (5 May, with AOD of 0.14), are discussed. A radiative transfer model was initialized with the measured vertical profiles and with different aerosol properties, derived from measurements or from the literature. The simulation of the irradiance vertical profiles, in particular, provides the opportunity to constrain model-derived estimates of the AHR. The measured SW and LW irradiances were reproduced when the model was initialized with the measured aerosol size distributions and refractive indices. For the dust case, the instantaneous (solar zenith angle, SZA, of 55.1°) LW-to-SW ADRF ratio was 23% at the surface and 11% at the top of the atmosphere (TOA), with a more significant LW contribution on a daily basis (52% at the surface and 26% at TOA), indicating a relevant reduction of the SW radiative effects. The AHR profiles followed the aerosol extinction profile, with comparable peaks in the SW (0.72 ± 0.11 K d−1) and in the LW (−0.52 ± 0.12 K d−1) for the considered SZA. On a daily basis, the absolute value of the heating rate was larger in the LW than in the SW, producing a net cooling effect at specific levels. These are quite unexpected results, emphasizing the important role of LW radiation.

Observation of lump structures in the nocturnal atmospheric boundary layer with Doppler sonar and Raman lidar

Atmospheric lump structures were observed at Rome during two nights in the summer of 1994 by a tri-axial Doppler sodar operating in monostatic mode. They appeared on sodar facsimile records at height ranges between 300 and 800 m for several hours each night. The vertical velocities within the lump structures ranged around ±2 m/sec. Their occurrence was found to increase as the nights progressed. A Raman lidar, which was also simultaneously operated, provided high-resolution water vapor profiles. The sodar-derived wind profiles confirmed the presence of sea breeze during both nights while the water vapor mixing ratio showed steep gradients at the height of lump structures. The analysis of the two data sets provided evidence that the lumps had originated, in an otherwise neutral atmosphere, from small-scale temperature fluctuations generated in isolated regions due to thermodynamical processes of water vapor by way of condensation and re-evaporation.

Observed influence of liquid cloud microphysical properties on ultraviolet surface radiation

Measurements of different UV quantities (UV index, ozone photolysis rates, global and diffuse irradiances, and actinic flux spectra) and cloud properties were collected during a field campaign carried out in Southern Italy in May–June 2010. Independent measurements of cloud liquid water path and optical depth allowed retrieving the cloud effective radius. The cloud modification factor (CMF) is used to investigate the influence of liquid cloud properties on the UV radiation under overcast conditions. CMF was also simulated using a 1-D radiative transfer model. Experimental and simulated CMF values for UV index (under overcast conditions) show a normalized root-mean-square error around 11%. Clouds with small effective radius determine a higher UV radiation attenuation than clouds formed by large particles. The CMFs for the UV index and the global spectral irradiance show a very weak dependence on the solar zenith angle (SZA), while the CMFs for actinic flux (both integrated and spectral) and diffuse spectral irradiance show a variation with SZA. The irradiance is more effectively attenuated at low SZA, while the actinic flux at high SZA. These effects are due to the different weight given to the direct and the diffuse components.

Stratospheric aerosols observed by lidar over northern Greenland in the aftermath of the Pinatubo eruption

Lidar observations were carried out from Thule (76.5°N, 68.8°W), Greenland, during the period September 1991 to February 1996. The measurements were carried out with relative continuity throughout the solar year, with the exception of the summer 1992. A total of over 300 profiles of the stratospheric aerosol backscattering and depolarization were obtained. The buildup and decay of the volcanic aerosols, originated from the eruption of Mount Pinatubo (June 1991, Philippines), were followed in detail. Maxima of the backscatter ratio, around 6, were recorded in March 1992. The aerosol decay, mainly controlled by gravitational sedimentation and stratosphere-troposphere exchange, shows a strong modulation induced by dynamical phenomena: an annual oscillation of the aerosol integrated backscattering, with a winter maximum at altitudes below 20 km, distinctly appears. Subsidence during winter and, possibly, upward motion during summer lead to a consistently different winter-to-summer aerosol vertical distribution. Discontinuities in the evolution of the aerosol backscattering may be related to the influence of the quasi-biennial oscillation (QBO) on the poleward transport and on the mean diabatic circulation. The QBO appears also to affect the stratosphere-troposphere exchange during winter. By fitting the decreasing phase of the volcanic aerosol integrated backscattering with an analytical expression given by an exponential decrease and an annual oscillation, an e-folding time of approximately 9.4 months is derived. The e-folding time varies however with time and depends on altitude. A distinct transition occurs in early summer 1994, with a fast decrease of the aerosol load below 20 km. The observation of backscattering values larger than in the pre-Pinatubo period still in early 1996 above 20 km seems to be consistent with modifications of the particle size distribution at these altitudes. Upper limit estimates for the diabatic descent in and outside the polar vortex could also be derived from the lidar observations in the winter months.