Abstract
Abstract. Eight years of upper-tropospheric (UT) ice crystal measurements with the backscatter cloud probe (BCP), installed on commercial aircraft operated as part of the In-Service Aircraft for a Global Observing System (IAGOS), have been analyzed to assess the frequency and characteristics of extreme ice crystal events (EIEs), defined in this study as encounters with clouds that have number concentrations exceeding 5000 L−1. A total of 3196 events, in clouds of horizontal extent ≥ 2.5 km, were identified during the period from December 2011 to March 2020 in the latitude band between 30∘ S and 30∘ N. Regions of anthropogenic sources of carbon monoxide, with particles that can alter cloud microphysics, were attributed to these EIEs in UT clouds using the SOFT-IO model. The evaluation of low- and upper-level kinematic variables from the European Centre for Medium-Range Weather Forecasts (ERA5) reanalysis, combined with spatial distributions of aerosol optical depth and regions of biomass burning, highlights the physical mechanisms by which the particles are lofted to flight levels in regions of deep convection. The maps of lightning frequency, derived from the World Wide Lightning Location Network (WWLLN), provide additional evidence of the role of deep convection in transporting aerosol particles, cloud hydrometeors and carbon monoxide to aircraft cruising altitudes. The evaluation of aerosol particle mass concentrations and composition from the Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2) contributes additional evidence for a link between regions of EIEs and surface emissions of dust, black carbon (BC), organic carbon (OC) and sulfate particles. Given the composition of the source aerosols and the role of deep convection in their transport to the UT, the sampled ice clouds likely originate from the homogeneous or heterogeneous freezing of droplets formed on these particles, as has been reported in previous studies. The results from this study, which have been obtained from a large sample of measurements, have ramifications related to satellite measurement validation, weather forecasting and climate change. In addition, over 2000 of the randomly sampled clouds had derived ice water contents larger than 1 g m−3, a concentration that is considered potentially hazardous to commercial aircraft operations.
Highlights
The studies reported by Krämer et al (2016, 2020) provide a comprehensive description of the origins and microphysical properties of cirrus clouds based on 150 flights with nine research aircraft that accumulated 168 h of cloud measurements
The four objectives of the study are to (1) document the frequency of extreme ice crystal events (EIEs) by geographic region within the tropical latitude band most impacted by biomass burning (BB) and urban pollution (UP) emissions, (2) evaluate the seasonal variations of EIEs as related to dry and rainy periods, (3) identify regional sources of carbon monoxide (CO) and aerosols associated with the EIEs, and (4) show that there is sufficient convection to transport aerosol and the cloud particles that form on them up to cruising altitude
We present results as statistics derived from the 9 years of accumulated measurements, first from only the backscatter cloud probe (BCP) cloud measurements, only the CO in situ measurements accompanied by the anomalous CO extracted from the back-trajectory and attribution model, and an evaluation of the cloud and CO data together
Summary
The studies reported by Krämer et al (2016, 2020) provide a comprehensive description of the origins and microphysical properties of cirrus clouds based on 150 flights with nine research aircraft that accumulated 168 h of cloud measurements These data were acquired during multiple field campaigns in the mid-latitudes and tropics over a period of 19 years (1999–2017). Statistics were gathered on the number concentration (Nice), ice water content (IWC) and mass mean radius (Rice) as a function of ambient temperature and compared to 10 years of cirrus ice number concentrations derived from satellite measurements This extensive data set serves to underscore the importance of aircraft measurements to document cirrus properties, and to provide the detailed information about those cloud characteristics that cannot be extracted from the lower-spatial-resolution satellite data. A much wider ranging data set is needed, temporally and geographically, to provide corroborative data to extend the data sets beyond limited research field campaigns
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