High concentrations of ice crystals in upper-tropospheric tropical clouds: is there a link to biomass and fossil fuel combustion?

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.

Extremely High Concentrations of Ice Particles in East Asian Dust‐Infused Baroclinic Storm (DIBS) Cirrus Shield: Dominant Role of Dust Ice Nucleation Effect

“…Our study attributes the extremely high ice number concentrations in DIBS to the deposition ice nucleation of dust particles. In previous studies, they concluded that these extreme ice events (see Section 3.2) were most likely to occur in the tropics and were formed via the freezing of droplets on ascent to the upper‐tropospheric clouds from mixed‐phase regime rather than formed by deposition ice nucleation in the cirrus regime (Beswick et al., 2015; Krämer et al., 2016, 2020; Raga et al., 2022). In addition, the highest concentrations that have been observed by the deposition heterogeneous ice nucleation pathway is around 1 cm −3 (Raga et al., 2022).…”

“…Raga et al. (2022) referred to the ice clouds when ice crystal concentrations exceeded 5 cm −3 as “extreme ice events” (EIEs), falling in the 85th percentile of all upper‐tropospheric ice clouds from their in situ measurements of 3196 events. Beswick et al.…”

“…In previous studies, they concluded that these extreme ice events (see Section 3.2) were most likely to occur in the tropics and were formed via the freezing of droplets on ascent to the upper-tropospheric clouds from mixed-phase regime rather than formed by deposition ice nucleation in the cirrus regime (Beswick et al, 2015;Krämer et al, 2016Krämer et al, , 2020Raga et al, 2022). In addition, the highest concentrations that have been observed by the deposition heterogeneous ice nucleation pathway is around 1 cm −3 (Raga et al, 2022). This study demonstrated that cirrus clouds could be formed by direct deposition nucleation with extremely high ice number concentrations, comparable to other ice formation mechanisms in cirrus regions.…”

“…Such high ice number concentrations rarely happen in cirrus clouds. Raga et al (2022) referred to the ice clouds when ice crystal concentrations exceeded 5 cm −3 as "extreme ice events" (EIEs), falling in the 85th percentile of all upper-tropospheric ice clouds from their in situ measurements of 3196 events. Beswick et al (2015) identified extreme ice events with N ice exceeded 10 cm −3 , which was 300 times higher than the median values and 30 times higher than the 90th percentile reported by Krämer et al (2020).…”

Recent Trends in Transport of Surface Carbonaceous Aerosols to the Upper‐Troposphere‐Lower‐Stratosphere Linked to Expansion of the Asian Summer Monsoon Anticyclone

“…Hence, for better understanding effects of aerosol on ASM climate change, it is important to include, not only change in anthropogenic surface emissions, but also change in emissions and transport of natural aerosols that could also contribute to feedback processes spurred on by radiative forcing from anthropogenic emissions (Christian et al., 2019; Fadnavis et al., 2013; Lau et al., 2006; Li et al., 2016). Transport of surface pollutants to ATAL and STEX are well known to be strongly affected by variability of the ASMA, and jetstream at UTLS levels in the subtropics and extratropics over a wide range of time scales from intraseasonal to inter‐decadal (Bian et al., 2020; Gayatry et al., 2020; Pan et al., 2016; Park et al., 2007; Raga et al., 2022; Yuan et al., 2019). In this study, we focus on identifying basic mechanisms in the long‐term trends in transport of CA from natural and anthropogenic sources to the UTLS over the greater ASM region.…”