G. Di Donfrancesco

Spectroscopic evidence for NAT, STS, and ice in MIPAS infrared limb emission measurements of polar stratospheric clouds

Spectroscopic evidence for β-NAT, STS, and ice in MIPAS infrared limb emission measurements of polar stratospheric clouds M. Höpfner, B. P. Luo, P. Massoli, F. Cairo, R. Spang, M. Snels, G. Di Donfrancesco, G. Stiller, T. von Clarmann, H. Fischer, and U. Biermann Forschungszentrum Karlsruhe, Institut für Meteorologie und Klimaforschung, Karlsruhe, Germany Institut für Atmosphäre und Klima, ETH-Hönggerberg, Zürich, Switzerland Consiglio Nazionale delle Ricerche, Istituto di Scienze dell’Atmosfera e del Clima, Rome, Italy Forschungszentrum Jülich, Institut für Chemie und Dynamik der Geosphäre, Jülich, Germany Ente per le Nuove tecnologie, l’Energie e l’Ambiente, Rome, Italy Max-Planck-Institut für Chemie, Abteilung Atmosphärenchemie, Mainz, Germany now at: Referat für Umweltund Energiepolitik des SPD-Parteivorstandes, Berlin, Germany

Volcanic aerosol and ozone depletion within the Antarctic polar vortex during the austral spring of 1991

In the spring of 1991 the Antarctic lower stratosphere was characterized by a layer of volcanic aerosol from the Cerro Hudson eruption. This aerosol layer was observed from McMurdo Station, Antarctica, with both lidar and balloonborne particle counters beginning around 10 September. After 20 September the aerosol was observed daily between 9 and 13 km. In this layer homogeneous nucleation of new aerosol was observed with concentrations greater than 6000/cu cm. Comparisons of scattering ratio calculated from measured particle size distributions agree best with the lidar measurements when a real index of refraction near 1.5 is used. In the past 5 years of measurements, ozone below 13 km has been relatively unchanged during the annual Antartic ozone depletion; however, in 1991 ozone below 13 km decreased at a rate of 4 - 8 ppb/day over 30 days. This change began shortly after the appearance of the volcanic aerosol, providing direct measurements correlating volcanic aerosol and ozone depletion. 16 refs.

Polar stratospheric clouds and volcanic aerosol during spring 1992 over McMurdo Station, Antarctica : lidar and particle counter comparisons

Coordinated observations with lidar and balloon-borne particle counters were used to characterize polar stratospheric clouds and to estimate a particle index of refraction. The index of refraction was estimated from comparisons of calculated and measured scattering ratios at a wavelength of 532 nm. The clouds, measured from McMurdo Station, Antarctica (78°S), were observed above 11 km at temperatures below 198 K and were divided into three classes based on their scattering properties and particle size. Clouds with a low scattering ratio, high depolarization, and significant fraction of particles with radii of >2.0 μm had a mean index of refraction of 1.42±0.04 and a mode of 1.43. Clouds with a moderate scattering ratio, low depolarization, and fewer particles of >2.0 μm, had a mean index of refraction of 1.39±0.03 and a mode of 1.37. Ice clouds, apparent from measurements of high scattering ratio, high depolarization, and high concentrations of particles of >1.0 μm, had a mean index of refraction of 1.32±0.02 and a mode of 1.31. Measurements in volcanic aerosol indicated a mean index of 1.43±0.04.

Chemical, microphysical, and optical properties of polar stratospheric clouds

A balloonborne gondola for a comprehensive study of polar stratospheric clouds (PSCs) was launched on 25 January 2000 near Kiruna/Sweden. Besides an aerosol composition mass spectrometer, the gondola carried optical particle counters, two backscatter sondes, a hygrometer, and several temperature and pressure sensors. A mountain wave induced PSC was sampled between 20 and 23 km altitude. Strongly correlated PSC particle properties were detected with the different instruments. A large variability of particle types was measured in numerous PSC layers, and PSC development was followed for about two hours. Liquid ternary PSC layers were found at temperatures near the ice frost point. A large fraction of the sampled cloud layers consisted of nitric acid trihydrate (NAT) particles with a molar ratio H 2 O:HNO 3 close to 3 at temperatures near and below the equilibrium temperature T NAT . The median radius of the NAT particle size distribution was between 0.5 and 0.75 μm at concentrations around 0.5 cm -3 . Below the NAT layers and above T NAT , thin cloud layers containing a few large particles with radii up to 3.5 μm coexisted with smaller solid or liquid particles. The molar ratio in this region was found to be close to two.

Polar stratospheric clouds over McMurdo, Antarctica, during the 1991 spring: Lidar and particle counter measurements

Lidar and balloonborne particle counter measurements were performed simultaneously on two days when polar stratospheric clouds were observed in late August 1991 at McMurdo, Antarctica. Both nitric acid trihydrate and ice clouds were observed in the lower stratosphere between 10 and 23 km in different formation stages and with different cooling rate; however in all cases the size distributions were bimodal. Comparison of scattering ratios measured by lidar and calculated from particle size distributions are in good agreement; however, discrepancies were observed when the lower stratosphere was highly perturbed by wave activity. Lee waves generated by air flowing over the Trans Antarctic Mountains induced ice cloud formation at altitudes as high as 20 km. No PSCs were observed after the end of August in 1991.

Estimate of the Arctic Convective Boundary Layer Height from Lidar Observations: A Case Study

A new automated small size lidar system (microlidar or MULID) has been developed and employed to perform aerosol measurements since March 2010 at Ny Alesund (, ), Svalbard. The lidar observations have been used to estimate the PBL height by using the gradient method based on abrupt changes in the vertical aerosol profile and monitor its temporal evolution. The scope of the present study is to compare several approaches to estimate the PBL height, by using lidar observations, meteorological measurements by radio soundings, and a zero-order one-dimensional model based on a parameterization of the turbulent kinetic energy budget within the mixing layer, under the assumptions of horizontal homogeneity, and neglecting radiation and latent heat effects. A case study is presented here for a convective PBL, observed in June 2010 in order to verify whether the Gradient Method can be applied to lidar measurements in the Arctic region to obtain the PBL height. The results obtained are in good agreement with the PBL height estimated by the analysis of thermodynamic measurements obtained from radio sounding and with the model.

A survey of the signal‐induced noise in photomultiplier detection of wide dynamics luminous signals

In the LIDAR sounding of wide atmospheric ranges, as in middle atmospheric probing, signal‐induced noise (SIN) from the photomultiplier (PMT) has been known since long time as a challenging problem for scientists. In this article the results of investigations on SIN are reported and strategies to minimize its amount proposed. Three different types of PMTs have been tested and their response to pulsed light signals of various duration and amplitude, comparable with close range LIDAR return, have been studied. An analysis of the time properties and spectral characteristics of SIN has been performed, and a method to reduce SIN effects in photon counting acquisition has been proposed. The method is based on a careful choice of discriminator threshold level settings, and exploits differences observed between SIN and signal cumulative pulse height distributions. The method shows dependency on single PMT behavior.

Lidar observations of stratospheric temperature above McMurdo Station, Antarctica

Abstract Stratospheric temperatures were measured by lidar at McMurdo station, Antarctica (78°S, 167°E) during two late spring months (September-October) in 1991 and 1992, and during the period March-October in 1993 and 1994. The stratosphere was found to be quite active, with one major and several minor warmings occurring in 1993 and 1994, and showing the expected behaviour of a distinct region of high temperatures, formed in the polar mesosphere, descending with time and warming the stratopause region. A relative maximum of the stratopause temperature was observed in July 1994, and differences between two years in terms of the time development of average temperature in the different stratospheric layers and in terms of the average temperature variability over single months are pointed out. Monthly mean temperature profiles determined from lidar observations are compared with a reference atmosphere (CIRA86). Fair agreement, with discrepancies less than ±4 K, in June, July and August in the middle stratosphere and just above the stratopause was found.

Calibration method for depolarization lidar measurements

A new method has been developed for calibration of the depolarization measurement of an atmospheric lidar. The technique requires a simple polarizer, and can be performed without interfering with the measurement set-up. The theoretical background of the procedure will be given and results of this calibration procedure on the tropospheric stratospheric aerosol lidar in McMurdo will be presented.

Ultrathin subvisible cirrus clouds at the tropical tropopause

Subvisible cirrus clouds with a vertical thickness of only 100–300 m but horizontal extent of thousands of square kilometers have been detected at the tropical tropopause around 17 km altitude during the European-Union-funded APE-THESEO campaign. The cloud layers have been characterized by measurements on board of two aircraft: the Russian high-flying research aircraft Geophysica, which performed in situ measurements of the cloud layers; and a German Falcon research aircraft flying up to 13 km altitude and directing the Geophysica into these clouds, which remained invisible for the Geophysical pilot even during level flight within the layer. Both in situ and remote measurements suggest that the condensed phase volume ranges between 1 and 5 μm3 cm−3. If the particles consisted of water ice, this would correspond to 10–40 ppbv condensed water. Concerning the condensed mixing ratio this would correspond to the thinnest ice cloud ever observed. As a matter of fact, to condense only 10–40 ppbv of H2O in equili...