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Clouds and Precipitation Response to Anthropogenic Changes in the Natural Environment

Final Report Summary - CAPRI (Clouds and Precipitation Response to Anthropogenic Changes in the Natural Environment)

In this project we proposed a new approach for studying basic cloud physics, anthropogenic effects on clouds and the derived climate impacts, approaching the challenge from both scientific ends: reductionism and a systems approach. During this project, using observational, numerical and theoretical tools we have shown that our new methodology produces exciting and novel insights when applied on large range of spatial and temporal scales problems. The reductionist, process-level approach aims in resolving the basic physics of processes and the feedbacks between them. On the other side, the system’s approach seeks for an emergent behavior as the output of competition and synergism between processes and feedbacks. To do so often a tailor-made “toy” model is designed such that it can represent basic rules that describe aspects of the emergent behavior. In our study we show that such approach is applicable in many scales and levels of complexity. We combine the emergent behavior together with a detailed process-level understanding analysis from the scales of the smallest clouds with the weakest optical signature (10's of meters) to the oceanic atmosphere over an algae bloom (1000's of kilometers).
Specifically, using our multi-scale hybrid (system and process-level) approach allowed us to propose a new key insight linking the problem of cloud invigoration by aerosols to the concept of aerosol-limited clouds (Koren et al, 2014, Science). Our new perspective offers a unified physical mechanism describing clouds response to changes in aerosol amounts from a very clean environment (natural oceanic atmosphere) to a very polluted one. We show that in a clean environment the clouds are limited by the droplets surface area and hence cannot consume efficiently the available water vapor. As the environmental conditions change towards moderately polluted the cloud condenses more water, releases more latent heat and advance its ability to use the thermodynamic potential for development. This trend was shown in observational data of cloud fields over the southern Oceans and explained by numerical simulations. Moreover, we showed, using single cloud model simulations together with cloud fields LES that the outcome of the competitions between nonlinear processes can turn from invigoration to suppression (due to processes like entrainment and drag forces) and it depends on the thermodynamic conditions and the aerosol properties. Our study suggests that there is an optimal aerosol concentration for cloud development and rain production (Dagan et al 2015a,b, ACP, GRL), which depends on the thermodynamic conditions. For environment that permits only small, shallow clouds the optimal concentration would be relatively small compared to an environment that supports deeper clouds. In addition we have shown how the clouds impact their environmental conditions and change them, driving a change in time of this optimal concentration value (Dagan et al., 2016, Sci. Rep).
In the case of cloud-field scale, we showed that describing the system from an “ecological” point of view, seeking for the emergent behavior between the clouds (pray), rain (predator) and aerosol (nutrients) can shad a new light on the mechanisms for self organization of clouds within a field (Feingold et al., 2013, Nonlin. Processes Geophys) and offers a new perspective on the rigidity of the structure of a cloud field (Koren et al., 2013, Sci. Rep; Koren et al., 2017, Chaos).
In a similar manner, dynamical problems with comparable scales in the ocean were tackled, using our hybrid approach. We studied how delicate processes that occur in the mixed layer near the ocean’s surface affect aerosol fluxes and properties in the atmosphere. Analysis of in-situ data that was collected in a cruise form the Azores islands to Iceland during the summer of 2012, in conjunction with satellite observations, allowed us to quantify the impact of oceanic and atmospheric factors on properties of aerosols emitted from the sea surface (Lehahn et al., 2014, GRL). As part of this study we developed and used multi-satellite Lagrangian diagnostic, for decoupling the contributions of physical and biological processes to estimate the role of viruses in regulating phytoplankton bloom (Lehahn et al., 2014, Current Biology). Complimentary part of this project was a laboratory work that explored the emission of biological aerosol into the atmosphere (in a controlled environment) and the possible infection of phytoplankton by aerosolized marine viruses (Sharoni et al., 2015; PNAS). As mixed layer processes impact the properties of the emitted aerosols, they can therefore regulate clouds processes. Such data over the pristine oceans together with controlled laboratory measurements enabled the discovery of new mechanisms relating biological activity and physical processes related to sea-atmosphere interface.
Using observational tools together with numerical models and theoretical considerations we discovered the properties of small convective clouds. Those small clouds that are usually overlooked were discovered to have a significant climatic effect. We measured them using a new retrieval that we developed, studied their properties (Hirsch et al., 2014, ACP), estimated their climatic radiative effect (Hirsch et al., 2015, ERL) and discovered their formation mechanism using a new cloud model we developed for this purpose (Hirsch et al., 2017, ERL).
This project discoveries opened new horizons for our scientific community. The new insights together with the new methods and new tools will be used in the future by our research groups as well as by other similar groups in the world for keep improving our ability to predict the role of clouds in the climate system and the anthropogenic impact on them.