L. Stefanutti

Airborne polar experiment - polar ozone, leewaves, chemistry, and transport (APE-POLECAT): Rationale, road map and summary of measurements

The Airborne Polar Experiment-Polar Ozone, Leewaves, Chemistry and Transport (APE-POLECAT) mission took place between December 19, 1996, and January 16, 1997. APE-POLECAT comprised the inaugural mission of the high-altitude research aircraft, the M-55 Geophysica, flights by the DLR Falcon, measurements from a number of Arctic ground stations, and atmospheric modeling. Both aircraft flew out of Rovaniemi in Finland. The Geophysica was equipped with a payload designed to probe the chemistry and microphysics of polar stratospheric clouds (PSCs) at, or above, the aircraft altitude (up to 20 km geometric altitude). The Deutsches Zentrum fur Luft und Raumfahrt (DLR) Falcon was fitted with an aerosol lidar called OLEX, which looked upward. Ground-based measurements included aerosol lidar, meteorological sondes, and ozone sondes, from both sides of the Scandinavian Mountains, and from the southern Arctic Ocean. The original primary aim of the mission, to study PSC processes in situ, was modified in the light of unfavorable meteorological conditions. Flights concentrated on studies of transport and chemistry around the polar vortex, and on remote sensing of very high, mountain-wave-induced, PSCs. Here we report the objectives and rationale of the mission, provide basic descriptions of the conditions of the stratosphere at the time of each flight, and give a summary of the measurements made.

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.

Tropopause and hygropause variability over the equatorial Indian Ocean during February and March 1999

Measurements of temperature, water vapor, total water, ozone, and cloud properties were made above the western equatorial Indian Ocean in February and March 1999. The cold-point tropopause was at a mean pressure-altitude of 17 km, equivalent to a potential temperature of 380 K, and had a mean temperature of 190 K. Total water mixing ratios at the hygropause varied between 1.4 and 4.1 ppmv. The mean saturation water vapor mixing ratio at the cold point was 3.0 ppmv. This does not accurately represent the mean of the measured total water mixing ratios because the air was unsaturated at the cold point for about 40% of the measurements. As well as unsaturation at the cold point, saturation was observed above the cold point on almost 30% of the profiles. In such profiles the air was saturated with respect to water ice but was free of clouds (i.e., backscatter ratio <2) at potential temperatures more than 5 K above the tropopause and hygropause. Individual profiles show a great deal of variability in the potential temperatures of the cold point and hygropause. We attribute this to short timescale and space-scale perturbations superimposed on the seasonal cycle. There is neither a clear and consistent “setting” of the tropopause and hygropause to the same altitude by dehydration processes nor a clear and consistent separation of tropopause and hygropause by the Brewer-Dobson circulation. Similarly, neither the tropopause nor the hygropause provides a location where conditions consistently approach those implied by a simple “tropopause freeze drying” or “stratospheric fountain” hypothesis.

Stratospheric aerosol measurements in the Arctic winter of 1996/1997 with the M-55 Geophysika high-altitude research aircraft

In-situ aerosol measurements were performed in the northern hemispheric stratosphere up to altitudes of 21 km between 13 November 1996 and 14 January 1997, inside and outside of the polar vortex during the Airborne Polar Experiment (APE) field campaign. These are measurements of particle size distributions with a laser optical particle counter of the FSSP-300 type operated during 9 flights on the Russian M-55 high-altitude research aircraft Geophysika. For specific flights, the FSSP-300 measurements are compared with balloon-borne data (launched from Kiruna, Sweden). It was found that the stratospheric aerosol content reached levels well below the background concentrations measured by the NASA operated ER-2 in 1988/89 in the northern hemisphere. During the APE campaign, no PSC particle formation was observed at flight altitudes although the temperatures were below the NAT condensation point during one flight. The measured correlations between ozone and aerosol give an indication of the subsidence inside the 1996/97 polar vortex. Despite the lower aerosol content in the winter 1996/97 compared to the 1989 background, the heterogeneous reactivity of the aerosol (as calculated from the measured data with additional model input) is comparable. This is due to the dependency of the reactive uptake coefficients on the atmospheric water vapor content. Under the described assumptions the reaction rates on the background aerosol are significantly smaller than for competing gas phase chlorine activation, as can be expected for stratospheric background conditions especially inside the polar vortex.

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 μm3u200acm−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...

Scientific Applications of Satellite Data within the Geophysics Research Community

This report is made on behalf of the Geophysica community, which comprises about 30 research groups across Europe, and forms part of the APERCU initiative for the exploitation of ENVISAT data.