A climatological study of polar stratospheric clouds (1989-1997) from LIDAR measurements over Dumont d’Urville (Antarctica).

Backscatter lidar data from the French Antarctic base in Dumont d'Urville (66.40°S, 140.01°E), including aerosol background and observations of polar stratospheric clouds (PSCs), have been collected since 1989. In the present work we present a climatological study of PSCs, using a data base consisting of almost 90 observations. The seasonal evolution of PSCs, their optical classification, and their relationship with the observation temperature were studied. The first PSC was observed on day number 175 (15 June) and the last on day number 260 (7 September). The characteristic mid‐cloud altitude decreases through the season at a rate of 2.5 km/month. Type Ia, Ib, and II PSCs — identified by their optical properties — have been observed. External mixtures of these types have also been observed. These observations have been related to the local temperature measured by radiosondes. The relationship between PSC type and the period of the winter season was also investigated. Mixed (solid and liquid) type I clouds are mostly observed at the beginning of the winter. Type II clouds are observed only during the coldest period around midwinter, although temperatures below the frost point begin earlier and persist longer than this. Type Ia, solid‐particle, clouds are observed mostly at the end of the winter

Unusual PSCs observed by LIDAR in Antarctica

International audiencePolar Stratospheric Cloud (PSC) measurements by ground‐based LIDAR were carried out at Dumont d'Urville, Antarctica, during the years 1989 to 1993. From such measurements it can be seen that there are cases of PSCs that are not consistent with the simplest nitric acid trihydrate (NAT) theories. Several cases of long‐lasting, non‐depolarizing PSCs were detected at temperatures below or close to the NAT freezing threshold, at about 195°K, suggesting the presence of durable supercooled droplets. PSC cases showing depolarizing (frozen) particles well above the NAT expected threshold are also shown. These results seems to be more consistent with recent laboratory and in situ findings, suggesting a close link between sulfate and PSC particles through the HNO3‐H2O‐H2SO4 ternary system. In this framework, non‐depolarizing clouds observed below and close to 195°K would arise from the uptake of HNO3 and water by the background particles before the freezing of the ternary system and the successive growth in nitric‐acid hydrates. Depolarizing “warm” PSCs are also consistent with the laboratory‐observed high melting point of the frozen sulfate core, that remains after NAT evaporation. A problematic PSC case, that cannot be easily explained by this theory is also shown

ENVISAT tropical validation of cloud and ozone parameters by high-altitude aircraft.

The validation of cloud top and ozone vertical column, measured by SCIAMACHY, were carried out respectively by lidars and in-situ and remote-sensing ozone instruments on-board the high altitude Geophysica aircraft. Cloud top and ozone measurements were conducted during the transfer flights of the Geophysica from Europe to Brazil and in the Tropics, from Araçatuba, from January to the end of February 2005. The Validation campaign, financed by ESA, was embedded within a scientific campaign in the frame of two EC projects: APE-INFRA and Troccinox. Validation of MIPAS-ENVISAT products were planned by means of the corresponding instrument MIPAS-STR which was also on-board the Geophysica, and by means of other in-situ instruments. Some results of MIPAS-STR are reported here; however, the MIPAS data from the ENVISAT are not available. In general the validations show some discrepancies between the data collected by the Geophysica and the instruments on board of ENVISAT, which cannot easily be explained by the displacement of the satellite and aircraft measurements

Polar stratospheric cloud observations over the Antarctic continent at Dumont d'Urville

International audienceThe Istituto di Ricerca sulle Onde Electromagnetiche (IROE) two-channel elastic backscattering lidar [Sacco et al., 1989], suitable for depolarization measurements, has been operated since January 8, 1989, at the French Antarctic base of Dumont d'Urville (66°40'S, 140°01'E). A continuous monitoring of the stratosphere was performed, which permitted measurement of the evolution of the background stratospheric aerosols and of polar stratospheric clouds (PSC) throughout the year. The data reported in this article correspond to the first year of measurements. Depolarization of the lidar signals was measured in order to obtain information on the type of clouds observed and on their particle size distribution. Both low (10%) depolarization ratios were detected, permitting discrimination between types Ia, Ib, and II PSC according to the classification given by Toon et al. (1990). Temporal continuity and high time resolution of the lidar measurements are evidence for altitude decreases in the PSC layers over periods of a few hours. These motions, if linked to sedimentation processes, led to values of velocity (≃10 cm s-1) compatible with large particles

Clouds at the tropical tropopause: a case study during the APE-THESEO campaign over the western Indian Ocean.

In this paper, we report a detailed description of a thin cirrus at the tropopause above a cumulonimbus (Cb) convective cluster observed during the Airborne Platform for Earth Observation–Third European Stratospheric Experiment for Ozone (APE-THESEO) campaign in February–March 1999 in the western Indian Ocean. The thin cirrus (Ci) has an optical depth at 532 nm below 0.1, with extended subvisible stretches, and is located directly below the tropopause, which was supersaturated with respect to ice. A direct comparison between the optical depth retrieved by Meteosat and that obtained by means of the hygrometers installed on the M55-Geophysica aircraft is discussed showing discrepancies ranging from 10 to 20%. Combining satellite and aircraft data, we show that the observed Ci is not due to cirrus outflow from Cb anvils. In the absence of any deeply convective clouds reaching altitudes above 15 km, we propose a possible mechanism of Ci formation based on a net mesoscale transport of water vapor from altitudes above 16 km to the tropopause region around 18 km. This transport could be driven by the critical layer and turbulence induced by gravity waves that could have been generated by lower level Cb cluster activity. The proposed mechanism for high-altitude Ci formation corroborates the new paradigm of a tropical tropopause layer (TTL) or “substratosphere,” several kilometers thick, which is decoupled from the convection-dominated lower troposphere