Multi-thermals and high concentrations of secondary ice: A modelling study of convective clouds during the ICE-D campaign

This paper examines the mechanisms responsible for the production of ice in convective clouds influenced by mineral dust. Observations were made in the Ice in Clouds Experiment – Dust (ICE-D) field campaign which took place in the vicinity of Cape Verde during August 2015. Measurements made with instruments on the FAAM aircraft through the clouds on 21 August showed that ice particles were observed in high concentrations at temperatures greater than about −8 °C. Sensitivity studies were performed using existing parametrisation schemes in a cloud model to explore the impact of the freezing onset temperature, the efficiency of freezing, mineral dust as efficient ice nuclei, and multi-thermals on secondary ice production by the rime-splintering process. The simulation with the default Morrison microphysics scheme (Morrison et al., 2005) that involved a single thermal produced a concentration of secondary ice that was much lower than the observed value of total ice number concentration. Relaxing the onset temperature to a higher value, enhancing the freezing efficiency, or combinations of these, increased the secondary ice particle concentration, but not by a sufficient amount. Simulations that involved only dust particles as ice nucleating particles produced a lower concentration of secondary ice particles, since the freezing onset temperature is low. The simulations implicate that a higher concentration of ice nucleating particles with a higher freezing onset temperature may explain some of the observed high concentrations of secondary ice. However, a simulation with two thermals that used the original Morrison scheme without enhancement or relaxation produced the greatest concentration of secondary ice particles. It did so because of the increased time that graupel particles were exposed to significant cloud liquid water content in the Hallett-Mossop temperature zone. The forward-facing camera and measurements of the vertical wind in repeated passes of the same cloud suggested that these tropical clouds contained multiple thermals. Hence, in a similar way to other convective clouds observed elsewhere in the world, it is likely that multi-thermals play an important role in producing very high concentrations of secondary ice particles in some tropical clouds.

Link between opaque cloud properties and atmospheric dynamics in observations and simulations of current climate in the Tropics, and impact on future predictions

<p><span>Climate models predict a weakening of the tropical atmospheric circulation, more specifically a slowdown of Hadley and Walker circulations. Many climate models predict that global warming will have a major impact on cloud properties, including their geographic and vertical distribution. Climate feedbacks from clouds, which amplify warming when positive, are today the main source of uncertainty in climate forecasts. Tropical clouds play a key role in the redistribution of solar energy and their evolution will likely affect climate. Therefore, it is crucial to better understand how tropical clouds will evolve in a changing climate. Among cloud properties, the vertical distribution is sensitive to climate change. Active sensors integrated into satellites, such as CALIOP (Cloud-Aerosol LIdar with Orthogonal Polarization), make it possible to obtain a detailed vertical distribution of clouds. CALIOP measurements and calibration are more stable over time and more precise than passive remote sensing satellite detectors. CALIOP observations can be simulated in the atmospheric conditions predicted by climate models using lidar simulators such as COSP (</span><span>CFMIP Observation Simulator Package). Moreover, </span><span>cloud properties directly drive the Cloud Radiative Effect (CRE). Understanding how models predict cloud vertical distribution will evolve in the future has implications for how models predict the Cloud Radiative Effect (CRE) at the Top of the Atmosphere (TOA) will evolve in the future. </span></p><p><span>The purpose of our study is to compare, firstly, based on satellite observations (GOCCP) and reanalyzes (ERA5), we will establish the relationship between atmospheric dynamic circulation, opaque cloud properties and TOA CRE. Then, we will compare this observed relationship with the one found in climate model simulations of current climate conditions (CESM1 and IPSL-CM6). Finally, we will identify how model biases in present climate conditions influence the cloud feedback spread between models in a warmer climate.</span></p>

Hemispheric TOA SW radiation budgets under changed atmospheric radiation conditions

<p>Under present day conditions the observations approximately show a hemispheric symmetry of the top of atmosphere (TOA)  short wave (SW) reflection despite the asymmetry of surface SW reflection. This has been confirmed by climate models. With models in an aqua planet setup, Voigt et al. (2014) found that tropical clouds largely compensate surface SW hemispheric asymmetries, however to a different degree in dependence on the convection scheme.</p><p>In this study, we question, whether there is also a hemispheric symmetry of TOA SW radiation under changed atmospheric radiation conditions. For that reason, we analyze experiments performed with a set of fully coupled general circulation models. The experiments were performed with either a) hemispheric asymmetric incoming radiation, b) increased atmospheric CO2 concentrations, c) increased atmospheric CO2 concentrations combined with increased stratospheric aerosol burden, or d) increased atmospheric CO2 concentration in conjunction with increased ocean albedo.</p><p>We show that generally, a hemispheric symmetry of TOA SW radiation does not occur. Overall, among the group of models, the hemispheric TOA SW radiation budgets are roughly similar for the distinct experiments, although the models utilyze different convection schemes.  We discuss the role of surface and atmospheric feedbacks in the different experiments, especially of tropical and extratropical clouds.</p><p>Reference:<br>Voigt, A., B. Stevens, J. Bader, and T. Mauritsen, 2014: Compensation of Hemispheric Albedo Asymmetries by Shifts of the ITCZ and Tropical Clouds. J. Climate, 27, 1029–1045, https://doi.org/10.1175/JCLI-D-13-00205.1.</p>