In situ measurements of background aerosol and subvisible cirrus in the tropical tropopause region

Abstract

[1] In situ aerosol measurements were performed in the Indian Ocean Intertropical Convergence Zone (ITCZ) region during the Airborne Polar Experiment-Third European Stratospheric Experiment on Ozone (APE-THESEO) field campaign based in Mahe, Seychelles between 24 February and 6 March 1999. These are measurements of particle size distributions with a laser optical particle counter of the Forward Scattering Spectrometer Probe (FSSP)-300 type operated on the Russian M-55 high- altitude research aircraft Geophysica in the tropical upper troposphere and lower stratosphere up to altitudes of 21 km. On 24 and 27 February 1999, ultrathin layers of cirrus clouds were penetrated by Geophysica directly beneath the tropical tropopause at 17 km pressure altitude and temperatures below 190 K. These layers also were concurrently observed by the Ozone Lidar Experiment (OLEX) lidar operating on the lower- flying German DLR Falcon research aircraft. The encountered ultrathin subvisual cloud layers can be characterized as (1) horizontally extending over several hundred kilometers, (2) persisting for at least 3 hours (but most likely much longer), and (3) having geometrical thicknesses of 100-400 m. These cloud layers belong to the geometrically and optically thinnest ever observed. In situ particle size distributions covering diameters between 0.4 and 23 mum obtained from these layers are juxtaposed with those obtained inside cloud veils around cumulonimbus (Cb) anvils and also with background aerosol measurements in the vicinity of the clouds. A significant number of particles with size diameters around 10 mum were detected inside these ultrathin subvisible cloud layers. The cloud particle size distribution closely resembles a background aerosol onto which a modal peak between 2 and 17 mum is superimposed. Measurements of particles with sizes above 23 mum could not be obtained since no suitable instrument was available on Geophysica. During the flight of 6 March 1999, upper tropospheric and lower stratospheric background aerosol was measured in the latitude band between 4degreesS and 19degreesS latitude. The resulting particle number densities along the 56th meridian exhibit very little latitudinal variation. The concentrations for particles with sizes above 0.5 m m encountered under these background conditions varied between 0.1 and 0.3 particles/cm(3) of air in altitudes between 17 and 21 km

Tropical tropopause ice clouds: a dynamic approach to the mystery of low crystal numbers

“…A step forward in this discussion was made recently: Krämer et al (2009) observed ice crystal concentrations much lower than expected (most often < 0.1 cm −3 ), but consistent with the high supersaturations measured. These observations turned the "supersaturation" into a "nucleation puzzle", which is supported by earlier measurements of low ice crystal numbers (McFarquhar et al, 2000;Thomas et al, 2002;Lawson et al, 2008). The "nucleation puzzle" is currently being intensely discussed and other nucleation pathways suppressing, modifying or replacing homogeneous freezing have been proposed (Kärcher and Koop, 2005;Gensch et al, 2008;Murray, 2008;Zobrist et al, 2008;Krämer et al, 2009;Spichtinger and Gierens, 2009c;Murray et al, 2010;Jensen et al, 2013).…”

Supersaturation, dehydration, and denitrification in Arctic cirrus

“…The distribution extends down to the sizes of the freezing aerosol particles (0.5-1 µm), indicative of ongoing freezing. Interestingly, this feature is seen in young midlatitude (Schröder et al, 2000) and thin tropical cirrus (Thomas et al, 2002) ice crystal size distributions. However, difficulties in www.atmos-chem-phys.org/acp/0000/0001/ Atmos.…”

A comparison of light backscattering and particle size distribution measurements in tropical cirrus clouds

“…In principle the histogram may consist of up to 40 unequal size bins, but this resolution is usually reduced to below 10 bins (7 in the present study) to overcome ambiguities linked to the relationship between light scattering intensity and particle size, which is not always univocal, and to increase the counting statistics in the single bin. The uncertainties of the number concentrations reported by the FSSP are determined by the uncertainty of the sample volume (approximately 20%) and mostly by poor counting statistics at low number densities like those encountered in thin or subvisual cirrus clouds (Thomas et al, 2002). To increase counting statistics, data are averaged over 10 s in cirrus clouds, or to several minutes in clear air.…”

“…and modal radius of few tens of µm on a similar thin subvisible cirrus layer over the tropical central Pacific. Size distributions of cirrus clouds in the upper tropical troposphere, exhibiting size diameters with pronounced peak at 10 µm, have been reported by Thomas et al (2002), presenting observations taken on board the high altitude research aircraft M55 Geophysica. Ultrathin tropical tropospheric clouds (UTTC) were observed directly beneath the tropical tropopause, characterised by a very low ice crystal number concentration (0.05 cm −3 ), and a size diameters modal peak of 10 to 12 µm .…”