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-O3 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 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 3×10−4g 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. Utilizing total aerosol number concentration measurements from a four channel condensation particle counter during two separate campaigns, it can be shown that the fraction of ice particles to the number of aerosol particles remaining ranges from 1:300 to 1:30 000 for tropical upper tropospheric ice clouds with ambient temperatures below −75°C.
Highlights
Cirrus clouds play a significant role in regulating the radiation balance of the Earth-atmosphere system and are, an important component of the Earth’s climate system
Incidences of shattering have been removed from the Cloud Imaging Probe (CIP) dataset by using a threshold interarrival time of 10−5 s (Field et al, 2006). This method assumes that shattering of a large ice crystal causes a burst of small particles with very short interarrival times, which can be distinguished from the longer interarrival times of real cloud particles in a cirrus cloud
30 November 2005 (evFenigt u1rein1F. i(ga.)6I)c.eTchreysictael csriyzsetadlissitzriebuditsiotrniboubtisoenrvheads ibneetnhecotrmoppiocsaeldsftrroatmosFpShSerPe,an0d.7CkImP daabtoav. eFor the CIP data the maximum and minimum dimension are shown. (b) Interarrival time distribution for the ice crystals observed by the CIP for this time period, indicating the crystals wthheiclhocaarel tprroopdoupcaeudseb,yosvheartttehreinTg iawniditshlaenredfsoroenhNavoevbemeebnerre3m0o,v2e0d0f5ro(mevtehnet d1atian sFeitg. u(cre) I6c)e. cTrhyestal images recorded by the CIP instrument fiocrethcrisystitmalesipzeeridoids.tr(idb)uEtioxnamhpalseboefelnarcgoemr ipcoescerdysfrtaolms oFbSseSrPveadndonC2I8P Ndaotvae.mFboerrth2e00C5IPatd1a0takmthealtitude
Summary
Cirrus clouds play a significant role in regulating the radiation balance of the Earth-atmosphere system and are, an important component of the Earth’s climate system. No significant differences in the ice water content and ice crystal shape were observed (Gayet et al, 2004) This is in agreement with mid-latitude Northern Hemisphere measurements presented by Strom et al (1997). They observed a median ice crystal concentration of 2.6 cm−3, while the crystal number distribution peaks at diameters below 10 μm. Size distributions of cirrus clouds in the upper tropical troposphere, exhibiting a pronounced peak at 10 μm, have been reported by Thomas et al (2002) They found very thin layers of subvisible cirrus clouds in the outflow of a cumulonimbus cloud, with ice crystal number concentrations ranging between 0.04 and 0.87 cm−3, depending on the exact position within the cirrus layer. Vertical profiles of the total and non-volatile aerosol number concentration are shown and a relationship between the number of measured ice cloud particles with respect to the available aerosol particles larger than 10 nm is presented
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
