Henri Ovarlez

Water vapour measurements inside cirrus clouds in Northern and Southern hemispheres during INCA

Water vapour data inside cirrus clouds from in-situ measurements with an aircraft-borne frost-point hygrometer are analysed. These data have been obtained during two field campaigns, performed in the Southern and Northern hemisphere mid latitudes. There were many occurrences of ice supersaturation inside the investigated cirrus, with a higher frequency of occurrences in the Southern Hemisphere. The source of the differences in the humidity data from the two hemispheres is not clear, and it is speculated that these differences may be related to different levels of pollution. A distribution law for the relative humidity inside cirrus clouds is inferred.

Stratospheric water vapor content evolution during EASOE

During EASOE, 7 in situ stratospheric water vapor content measurements were performed, in the polar region, with a balloon-borne frost-point hygrometer, from November 1991 until March 1992. The comparison between the frost-point and the air temperature profiles shows on 11 December the occurrence of PSC formation conditions when a PSC was observed by lidars. Outside the vortex the mean water vapor content seems to be very stable, 4 to 5 ppmv from 16 to 25 km. But is very changeable below 16 km, due to the variability in the origin of the air masses where the measurements occur, as these air parcels come sometimes from the tropics, then bringing dry air. Inside the vortex, a vertical descent of the air masses is observed, and is estimated to be 4 km around 25 km at the end of March, before the vortex breakdown. The rehydration of the vortex induced by this descent, from February to March, is shown.

Comparison of water vapor measurements from POLINAT 2 with ECMWF analyses in high‐humidity conditions

During the Pollution From Aircraft in the North Atlantic Flight Corridor (POLINAT 2) experiment, water vapor content was measured during 12 flights of the Deutsches Zentrum fuer Luft und Raumfahrt Falcon research aircraft by means of a frost-point hygrometer. The last three flights were coordinated with the flights of the NASA DC8 aircraft, taking part in the Subsonic Assessment Ozone and Nitrogen Oxide Experiment (SONEX). The measurements were generally performed in high-humidity and cloudy environments. Synthesized profiles of humidity and cloud cover along the flight tracks were retrieved from the European Centre for Medium-Range Weather Forecasts analyses. It is shown that the measurements of supersaturation are generally correlated to the occurrence of clouds as indicated by the model cloud cover product. However, there are large discrepancies between the mixing ratio from the analyses and the measurements: the model underestimates the water vapor content. Some reasons for this underestimation are put forward, and the consequences for the investigation of the occurrence of contrail formation and persistence, when using such analyses, are pointed out.

Large nitric acid particles at the top of an Arctic stratospheric cloud

measurements approximately 200 km upwind of the in situ measurements indicate a similar vertical structure for the cloud. These in situ measurements represent, to our knowledge, the most comprehensive in situ observations of all phases of polar stratospheric cloud particles, while the large particles at cloud top have not been previously observed and may have implications for producing particles large enough to remove reactive nitrogen from the polar stratosphere. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0320 Atmospheric Composition and Structure: Cloud physics and chemistry; 0340 Atmospheric Composition and Structure: Middle atmosphere— composition and chemistry; KEYWORDS: polar stratospheric clouds, in situ stratospheric cloud measurements, Arctic stratospheric clouds, polar stratospheric cloud composition, balloon-borne aerosol measurements, large polar stratospheric cloud particles

In situ mountain-wave polar stratospheric cloud measurements: Implications for nitric acid trihydrate formation

0.2 cm 3 , median radii of 1 to 2 mm and volumes up to 1 mm 3 cm 3 . A comparison between optical PSC data and optical simulations based on the measured particle size distribution indicates that the NAT particles were aspherical with an aspect ratio of 0.5. The NAT particle properties have been compared to another PSC observation on 25 January 2000, where NAT particle number densities were about an order of magnitude higher. In both cases, microphysical modeling indicates that the NAT particles have formed on ice particles in the mountain-wave events. Differences in the NAT particle number density can be explained by the meteorological conditions. We suggest that the higher NAT number density on 25 January can be caused by stronger wave activity observed on that day, larger cooling rates and therefore higher NAT supersaturation. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0320 Atmospheric Composition and Structure: Cloud physics and chemistry; 0340 Atmospheric Composition and Structure: Middle atmosphere— composition and chemistry; KEYWORDS: polar stratospheric cloud (PSC), nitric acid trihydrate (NAT), ozone, gravity wave, PSC formation

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.

In situ Measurement of H2O and CH4 with Telecommunication Laser Diodes in the Lower Stratosphere: Dehydration and Indication of a Tropical Air Intrusion at Mid-Latitudes

Telecommunication laser diodes emitting near 1.39 μ m and 1.65 μ m in combination with direct-differential absorption spectroscopy are efficient tools to monitor in situ stratospheric H2O andCH4 with a good precision error (a few percents), a high temporal resolution (ranging from 10 ms to 1 s), a large dynamic range in the concentration measurements (four orders of magnitude) and a high selectivity in the analyte species. To illustrate the capability of laser probing technique, we report balloonborne H2Oand CH4 simultaneous measurements obtained on October 2001 atmidlatitudes (43° N). The H2O vertical profile achieved with the lasersensor in the lower stratosphere is compared with the H2O data yielded by a balloonborne frost-point hygrometer. The total hydrogen mixing ratio in the lower stratosphere, 2[CH4] + [H2O], appears to beconstant at 7.5 ± 0.1 ppmv. Nevertheless, an unexpected largedehydration of ∼0.5 ppmv was detected by both the laser sensor and thehygrometer between 16 km and 23 km. We suspect the occurrence of a tropicalair intrusion into mid-latitudes. We support this interpretation using a high-resolution advection model for potential vorticity.

Optical and physical properties of stratospheric aerosols from balloon measurements in the visible and near-infrared domains. II. Comparison of extinction, reflectance, polarization, and counting measurements

The physical properties of stratospheric aerosols can be retrieved from optical measurements involving extinction, radiance, polarization, and counting. We present here the results of measurements from the balloonborne instruments AMON, SALOMON, and RADIBAL, and from the French Laboratoire de Météorologie Dynamique and the University of Wyoming balloonborne particle counters. A cross comparison of the measurements was made for observations of background aerosols conducted during the polar winters of February 1997 and January-February 2000 for various altitudes from 13 to 19 km. On the one band, the effective radius and the total amount of background aerosols derived from the various sets of data are similar and are in agreement with pre-Pinatubo values. On the other hand, strong discrepancies occur in the shapes of the bimodal size distributions obtained from analysis of the raw measurement of the various instruments. It seems then that the log-normal assumption cannot fully reproduce the size distribution of background aerosols. The effect ofthe presence of particular aerosols on the measurements is discussed, and a new strategy for observations is proposed.

Ultraviolet–visible bulk optical properties of randomly distributed soot

The presence of soot in the lower stratosphere was recently established by in situ measurements. To isolate their contribution to optical measurements from that of background aerosol, the soots bulk optical properties must be determined. Laboratory measurements of extinction and polarization of randomly distributed soot were conducted. For all soot, measurements show a slight reddening extinction between 400 and 700 nm and exhibit a maximum of 100% polarization at a scattering angle of 75 +/- 5 degrees. Such results cannot be reproduced by use of Mie theory assumptions. The different optical properties of soot and background stratospheric aerosol could allow isolation of soot in future analyses of stratospheric measurements.

Superpressure Balloon Flights in the Tropical Boundary Layer

Abstract Two series of quasi-constant-level pressurized balloons have been flown within the tropical planetary boundary layer, preliminary to a larger experiment for exploring the low-level flow over the Indian Ocean. The first flight series from a South Pacific island indicated a very short life expectancy of the equipment due to the high probability of the vehicle being overloaded by rain drops or dew and the catastrophic effect of immersion. A second series of flights from Ascension Island in the South Atlantic was performed to test a new and completely self-enclosed balloon design, as well as a metalized reflecting screen for radiative control of the envelope temperature. This new design demonstrated acceptable vertical stability, thus proving the feasibility of long-duration quasi-horizontal balloon flights at levels as low as 500 m above the sea surface.