Marco Cacciani

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.

Volcanic aerosol layers observed by lidar at South Pole, September 1991–June 1992

During 1991 the terrestrial stratosphere went through a substantially increased aerosol load, due mainly to the eruptions of Mt. Pinatubo in the Philippines, and, to a lesser extent, to those of Mt. Hudson in Chile. This paper reports lidar observations of the stratospheric aerosols at South Pole. Two layers were present at different altitudes during the austral summer, but only the higher one persisted in the stratosphere until the onset of the polar stratospheric cloud (PSC) phenomenon. Data have been analyzed in terms of the integrated backscattering coefficient and the aerosol mass content has been estimated.

Lidar observations of the Pinatubo aerosol layer at Thule, Greenland

Lidar measurements of the stratospheric aerosol content have been carried out in Thule (Greenland), in connection with the European Arctic Stratospheric Ozone Experiment (EASOE). Aerosols attributable to the Pinatubo eruption were detected in the stratosphere late in September; during autumn and early winter the stratospheric aerosol load slowly increased. In the region above approximately 18 km the aerosol load depended on the location of the station relative to the polar vortex. A temporary increase of the aerosol concentration was detected up to about 26 km at the beginning of February, when Thule was located for a few days outside the vortex. A final increase of the aerosol load was related with the final breakup of the vortex. The aerosol integrated backscatter values ranged between 0.001 and 0.005 sr−1 during most of the winter, and increased up to 0.007 sr−1 in early spring; in this period the aerosol columnar mass was estimated to be as high as 0.1 g m−2.

Stratospheric clouds at south pole during 1988 1. Results of lidar observations and their relationship to temperature

An optical radar—lidar—has been operational at the Amundsen-Scott South Pole Station since summer 1987–1988. The observations were specially directed to the detection of aerosol layers and polar stratospheric clouds (PSCs). The lidar utilized a Nd-YAG laser followed by a second harmonic generator, and a 0.5-m diameter Cassegrain receiving telescope. Results obtained during the period May–October 1988 are summarized. Some 10,000 profiles of the lidar echoes, each the result of 1-min averaging, were obtained. Data sets consisting of profiles of the scattering ratio and of the backscattering cross section Ba, based on half-hour averaging, are presented. The data can be related to profiles of the atmospheric temperature T, usually obtained on a daily basis at South Pole. Stratifications appear to have two distinct types of structures: one structure shows only a modest variation with height; the other is characterized by sharp features, with large changes of the cross section with height. The basic results, the relationship between Ba and T, and their statistical relevance are considered in this paper. The microphysical interpretation, the attribution of these structures to PSC Type I and Type II, respectively involving the condensation of nitric acid trihydrate and of water ice, and the seasonal evolution of the phenomena are treated in a companion paper.

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.

Lidar observations of polar stratospheric clouds at the South Pole. 1. Stratospheric unperturbed conditions, 1990

Observations of polar stratospheric clouds (PSCs) carried out at the Amundsen Scott South Pole Station by lidar in the winter of 1990 are reported. Echoes attributable to PSCs began to appear in late May, were a persistent feature till early September, and from that time until the end of October were sporadically observed. Values of the backscatter ratio as high as 16 were recorded in late August at 13 km. Analyses based on the sensitivity of the backscattering to temperature confirm that the attribution of the echoes to PSC type I or II is possible in some cases. Those that could be ascribed to type II were observed mostly during July and August, while those attributable to type I appeared only at the beginning of the winter. The comparison of water vapor mixing ratios derived from PSC type II occurrence temperatures and frost point hygrometer measurements indicates a fast dehydration in the beginning of the winter.

Observations of correlated behavior of stratospheric ozone and aerosol at Thule during winter 1991-1992

Using a recently installed lidar, a series of measurements of aerosol concentrations have been carried out between 12-1991 and 3-1992. This work was in conjunction with the European Arctic Stratospheric Ozone Experiment. Scattering ratios were very high because of the presence of aerosols from the Mt Pinatubo volcanic eruption. Ozone observations were made in this period by sondes. This paper reports on correlations between these observations. The correlations observed were not always positive. Both observations saw distinct layered structures representing density variations with height.

ABLE: Development of an Airborne Lidar

Abstract The acronym ABLE (Airborne Lidar Experiment) identifies a project to develop and fly an optical radar on a stratospheric platform for studies related to atmospheric radiation and composition. The prototype, ABLE 1, has been successfully flown on board the M55 Geophysica aircraft in the Arctic campaign of December 1996–January 1997 to observe stratospheric clouds and aerosol. The lidar, which runs automatically, has been installed in the unpressurized bay of the aircraft where the temperature approaches the low values of external air. The lidar transmitter is based on a Nd:YAG laser, with second and third harmonic outputs. The receiver consists of a 0.3-m Cassegrain telescope and several detection channels to look at different wavelengths and polarizations. A fluid circulation unit connected to the aircraft provides heating control. The instrument can point to the zenith or to the nadir. In the past campaign only λ = 532 nm was utilized: observations were carried out at two polarizations, pointing...

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.

Stratospheric clouds at South Pole during 1988 2. Their evolution in relation to atmospheric structure and composition

A lidar was installed at the Amundsen-Scott South Pole Station in the austral summer 1987–1988: aerosol layers, generically identified as polar stratospheric clouds, were frequently observed in the period May–October 1988 through the altitude range 8–20 km. On the basis of previous work (Fiocco et al., 1991) the behavior of the aerosol backscattering cross section Ba has been related to the temperature T through linear fits between the two variables (Fiocco et al., this issue). The resulting coefficients, namely the slope b = dBa/dT and Tf, the temperature at which the onset of condensation occurs, help to classify these clouds as Type I or II, in view of the thermodynamic properties of the condensing species. A strong dependence of Ba on T, represented by a large value of b, is interpreted as evidence of ice condensation and leads us to identify the cloud as Type II, while a lower value of b, characteristic of diffuse structures, is taken as evidence for Type I clouds, composed of nitric acid trihydrate. The evolution of these features is reported. Type II clouds, appearing as sharp layers of moderate vertical extent, are almost always present in the interval from June to early September. The temperature at the base of the lowest layer spans an initial value around 198 K to a final value in September around 196 K, with a corresponding height value from around 13.5 km to 10.5 km. Layers that we identify as Type I clouds appeared at higher temperatures with an onset at the beginning of the season around 200 K to a final value at mid-October around 203 K.