The net warming effect of cirrus clouds has driven part of the geoengineering research toward the idea of decreasing their occurrence frequency by seeding them with efficient ice nucleating particles. We study responses of cirrus clouds to simplified global seeding strategies in terms of their radiative fluxes with the help of the ECHAM-HAM general circulation model. Our cirrus scheme takes into account the competition between homogeneous and heterogeneous freezing, preexisting ice crystals, and the full spectrum of updraft velocities. While we find that the cirrus cloud radiative effect evaluated from our model is positive and large enough (5.7 W/m 2 ) to confirm their geoengineering potential, none of the seeding strategies achieves a significant cooling due to complex microphysical mechanisms limiting their climatic responses. After globally uniform seeding is applied, we observe an increase in cirrus cloud cover, a decrease in ice crystal number concentration, and a decrease in ice crystal radius. An analysis of their respective radiative contributions points to the ice crystal radius decrease as the main factor limiting seeding effectiveness.

The net radiative effects of tropical clouds are determined by the evolution of thick, freshly detrained anvil clouds into thin anvil clouds. Thick anvil clouds reduce Earth's energy balance and cool the climate, while thin anvil clouds warm the climate. To determine role of these clouds in climate change we need to understand how interactions of their microphysical and macrophysical properties control their radiative properties. We explore anvil cloud evolution using a cloud‐resolving model in three‐simulation setups of increasing complexity to disentangle the impacts of the various components of diabatic heating and their interaction with cloud‐scale motions. The first phase of evolution and rapid cloud spreading is dominated by latent heating within convective updrafts. After the convective detrainment stops, most of the spreading and thinning of the anvil cloud is driven by cloud radiative processes and latent heating. The combination of radiative cooling at cloud top, latent cooling due to sublimation at cloud base, latent heating due to deposition and radiative heating in between leads to a sandwich‐like, cooling‐heating‐cooling structure. The heating sandwich promotes the development of two within‐anvil convective layers and a double cell circulation, dominated by strong outflow at 12‐km altitude with inflow above and below. Our study reveals how small‐scale processes including convective, microphysical processes, latent and radiative heating interact within the anvil cloud system. The absence or a different representation of only one component results in a significantly different cloud evolution with large impacts on cloud radiative effects.

Cirrus clouds impact the planetary energy balance and upper-tropospheric water vapor transport and are therefore relevant for climate. In this study cirrus clouds at temperatures colder than −40°C simulated by the ECHAM–Hamburg Aerosol Module (ECHAM-HAM) general circulation model are compared to Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations ( CALIPSO) satellite data. The model captures the general cloud cover pattern and reproduces the observed median ice water content within a factor of 2, while extinction is overestimated by about a factor of 3 as revealed by temperature-dependent frequency histograms. Two distinct types of cirrus clouds are found: in situ–formed cirrus dominating at temperatures colder than −55°C and liquid-origin cirrus dominating at temperatures warmer than −55°C. The latter cirrus form in anvils of deep convective clouds or by glaciation of mixed-phase clouds, leading to high ice crystal number concentrations. They are associated with extinction coefficients and ice water content of up to 1 km−1 and 0.1 g m−3, respectively, while the in situ–formed cirrus are associated with smaller extinction coefficients and ice water content. In situ–formed cirrus are nucleated either heterogeneously or homogeneously. The simulated homogeneous ice crystals are similar to liquid-origin cirrus, which are associated with high ice crystal number concentrations. On the contrary, heterogeneously nucleated ice crystals appear in smaller number concentrations. However, ice crystal aggregation and depositional growth smooth the differences between several formation mechanisms, making the attribution to a specific ice nucleation mechanism challenging.

Abstract. The complex microphysical details of cirrus seeding with ice nucleating particles (INPs) in numerical simulations are often mimicked by increasing ice crystal sedimentation velocities. So far it has not been tested whether these results are comparable to geoengineering simulations in which cirrus clouds are seeded with INPs. We compare simulations where the ice crystal sedimentation velocity is increased at temperatures colder than − 35 • C with simulations of cirrus seeding with INPs using the ECHAM-HAM general circulation model. The radiative flux response of the two methods shows a similar behaviour in terms of annual and seasonal averages. Both methods decrease surface temperature but increase precipitation in response to a decreased atmospheric stability. Moreover, simulations of seeding with INPs lead to a decrease in liquid clouds, which counteracts part of the cooling due to changes in cirrus clouds. The liquid cloud response is largely avoided in a simulation where seeding occurs during night only. Simulations with increased ice crystal sedimentation velocity, however, lead to counteracting mixed-phase cloud responses. The increased sedimentation velocity simulations can counteract up to 60 % of the radiative effect of CO 2 doubling with a maximum net top-ofthe-atmosphere forcing of −2.2 W m −2 . They induce a 30 % larger surface temperature response, due to their lower altitude of maximum diabatic forcing compared with simulations of seeding with INPs.

We explore the importance of the life cycle of detrained tropical anvil clouds in producing a weak net cloud radiative effect (NCRE) by tropical convective systems. We simulate a horizontally homogeneous elevated ice cloud in a 2‐D framework using the System for Atmospheric Modeling cloud‐resolving model. The initially thick cloud produces a negative NCRE, which is later canceled by a positive NCRE as the cloud thins and rises. Turning off interactive cloud radiation reveals that cloud radiative heating and in‐cloud convection are fundamental in driving net radiative neutrality. In‐cloud convection acts to thin initially thick anvil clouds and loft and maintain thin cirrus. The maintenance of anvil clouds is tied to the recycling of water vapor and cloud ice through sublimation, nucleation, and deposition as air parcels circulate vertically within the cloud layer. Without interactive radiation, the cloud sediments and sublimates away, producing a large negative NCRE. The specification of cloud microphysics substantially influences the cloud's behavior and life cycle , but the tendency of the life cycle to produce compensating cloud radiative effects is robust to substantial changes in the microphysics. Our study shows that small‐scale processes within upper level ice clouds likely have a strong influence on the NCRE associated with tropical convective cloud systems.