Evidence for ice particles in the tropical stratosphere from in-situ measurements

Abstract

Abstract. In-situ ice crystal size distribution measurements are presented within the tropical troposphere and lower stratosphere. The measurements were performed using a combination of a Forward Scattering Spectrometer Probe (FSSP-100) and a Cloud Imaging Probe (CIP), which were installed on the Russian high altitude research aircraft M55 "Geophysica" during the SCOUT-O 3 campaign in Darwin, Australia. One of the objectives of the campaign was to characterise the Hector convective system, which appears on an almost daily basis during the pre-monsoon season over the Tiwi Islands, north of Darwin. In total 90 encounters with ice clouds, between 10 and 19 km altitude were selected from the dataset and were analysed. Six of these encounters were observed in the lower stratosphere, up to 1.4 km above the local tropopause. Concurrent lidar measurements on board "Geophysica" indicate that these ice clouds were a result of overshooting convection. Large ice crystals, with a maximum dimension up to 400 µm, were observed in the stratosphere. The stratospheric ice clouds included an ice water content ranging from 7.7×10 −5 to 8.5×10 −4 g m −3 and were observed at ambient relative humidities (with respect to ice) between 75 and 157%. Three modal lognormal size distributions were fitted to the average size distributions for different potential temperature intervals, showing that the Correspondence to: S. Borrmann (stephan.borrmann@mpic.de) shape of the size distribution of the stratospheric ice clouds are similar to those observed in the upper troposphere.In the tropical troposphere the effective radius of the ice cloud particles decreases from 100 µm at about 10 km altitude, to 3 µm at the tropopause, while the ice water content decreases from 0.04 to 10 −5 g m −3 . No clear trend in the number concentration was observed with altitude, due to the thin and inhomogeneous characteristics of the observed cirrus clouds.The ice water content calculated from the observed ice crystal size distribution is compared to the ice water content derived from two hygrometer instruments. This independent measurement of the ice water content agrees within the combined uncertainty of the instruments for ice water contents exceeding 3×10 −4 g m −3 .Stratospheric residence times, calculated based on gravitational settling, and evaporation rates show that the ice crystals observed in the stratosphere over the Hector storm system had a high potential of humidifying the stratosphere locally.

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

“…De Reus et al (2009) reported cirrus measurements showing a peak in the ice crystal number size distribution at about 10 µm diameter, while Lawson et al (2008) reported subvisible cirrus observations from Costa Rica with average values of ice particle number concentration (0.066 cm −3 ), and effective radius (8.82 µm) similar to those by McFarquhar et al (2004).…”

“…De Reus et al (2009) found such effect more marked with increasing IWC, and r eff , i.e. given the general trends of these two quantities with altitude, for thick, low cirrus clouds (Heymsfield and McFarquhar, 2002).…”

“…In a recent work based on the same Geophysica cirrus clouds measurements during the SCOUT-Darwin campaign, De Reus et al (2009) shows that the shapes of the size distributions display remarkably similar features, in the FSSP detection range, once normalized for the total number of particles. This again confirms N as the main parameter governing the cirrus scattering properties at optical wavelenghts.…”

Evidence for heterogeneous chlorine activation in the tropical UTLS

“…Substantial mixing ratios of several hundred ppt inorganic chlorine have been observed in the marine boundary layer (Pszenny et al, 1993), but the supposition of rapid transport to the TTL seems unwarranted because of loss by wet deposition (e.g. Crutzen and Lawrence, 2000;Mari et al, 2000). A contribution of chlorine from sea salt aerosol Fig.…”

The impact of overshooting deep convection on local transport and mixing in the tropical upper troposphere/lower stratosphere (UTLS)

“…Deep convective clouds may even overshoot into the stratosphere, injecting trace gases, water vapour, and even ice crystals directly (Corti et al, 2008;de Reus et al, 2009). There has been an ongoing debate about whether these injections lead to dehydration or hydration of the stratospheric air.…”