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Marine Stratocumulus Cloud Cover and Climate

Periodic Reporting for period 1 - MSCCC (Marine Stratocumulus Cloud Cover and Climate)

Reporting period: 2016-04-01 to 2018-03-31

Global climate change is forced by the balance of the warming due to anthropogenic greenhouse gases and the cooling due to anthropogenic aerosol pollution particles. Among these, the cloud-mediated aerosol radiative forcing is by far the main component of the uncertainty. Marine Stratocumulus Clouds (MSC) play a decisive role in the climate system due to their very large net effect on the Earth's radiative energy budget. The two main cloud regimes of MSC are open and closed cells. The less cloudy open cells are leaving much of the dark ocean unobscured, while the fully cloudy closed cells in turn are causing a very large cloud radiative effect in the solar radiation spectrum. Anthropogenic perturbation such as CO2 warming, but in particular also anthropogenic aerosol pollution, may delay the transition between open and closed cell regimes. The main hypothesis of the Marine Stratocumulus Cloud Cover and Climate (MSCCC) is that anthropogenic aerosols exert a substantial radiative forcing via their potential to impede or delay the transition from closed to open cells regime. This hypothesis was addresses via analysis of observational data and model simulations of different scales.

The objectives of the study were (1) developing novel methodologies for satellite retrievals of properties related to marine stratocumulus clouds, (2) to employ the developed methodologies to seek an in-depth understanding of the processes relevant for the transition between marine stratocumulus cloud cover regimes, (3) to test global climate model parameterizations responsible for marine stratocumulus cloud cover changes, (4) to quantify the contribution of the delay in marine stratocumulus transitions to the climate forcing, and (5) to obtain new skills and knowledge in climate modelling, gain scientific and public recognition of my work, and gather experience in teaching and student mentoring.

The results and insights from MSCCC are of immediate relevance to society via improved physical understanding that leads to a more reliable projections of future climate change. The results further are highly relevant to the research on climate geo-engineering via marine cloud brightening, although it remains an open question whether this may be considered beneficial for society.
The progress in the MSCCC project can be divided into the following:
(1) Observations of marine stratocumulus clouds (MSC): A novel methodology for retrieving MSC geometrical and thermodynamical properties from satellites was developed and validated against in-situ observations. Using the methodology, I showed that MSC are less likely to breakup when the cloud are decoupled from the ocean surface. This challenges the generally accepted mechanism of transitions between closed and open cells of MSC. The results are published open-access (doi:10.1029/2018GL078122).
Together with a master student that I was advising, the synoptic conditions in which MSC over the north Atlantic ocean are affected by European air pollution were diagnosed. The results demonstrates the co-variability between synoptic conditions and cloud radiative forcing due to anthropogenic air pollution, and allows a long term quantification of the forcing exerted by aerosol-cloud interaction in the region of the north Atlantic ocean. This work was accepted as a successful master project at the University of Leipzig.

(2) Modeling of marine stratocumulus clouds: In order to understand the process level in which global climate models simulate MSC, simulations of cases in which MSC were observed to be affected by anthropogenic aerosols were performed using the ECHAM6-HAM2 global climate model (GCM). The results revealed that cloud cover and cloud properties differ significantly between polluted and clean simulations, and that the highly simplified model parameterization is able to capture the complex physical processes. The results are under preparation for publication.
In addition to the GCM simulations, large eddy simulations (LES) with resolved cloud processes were performed as well. I used a novel approach in which LES simulations are driven by re-analysis data. Initial results show that, for the same governing large scale conditions (i.e. meteorology and sea surface temperature), changes in aerosol concentrations affect the timing of the closed-to-open cells transitions, so that more polluted clouds survive longer before breaking up. This validates the main hypothesis of MSCCC.

(3) In addition to the MSCCC project, I also contributed to collaborative projects led by colleagues. Among those (1) a novel methodology for satellite retrieval of ice particle concentration (doi:10.5194/acp-2018-20) (2) cirrus clouds formation mechanism classification (doi:10.5194/acp-18-6157-2018) (3) characterizing entrainment in GCMs, and (4) response of cloud liquid water path to aerosols and the related cloud radiative effect.

(4) Along the research work, I also took part in teaching bachelor and master students (Dynamic meteorology and Climate dynamics courses), advising a Master project, as well as giving a public lecture on climate related topics. Results of my research were presented in international conferences and scietific meetings.
Marine stratocumulus clouds (MSC) are of large interest for climate research due to their very large cloud radiative effect. At the same time, their feedback to global warming is one of the most uncertain aspects in climate sensitivity quantification by climate models. An improved understanding of stratocumulus is thus necessary for advances in climate change research. Evaluating and improving the cloud cover controlling parameterizations in global climate models will provide a reliable assessment of the MSC representation in global climate models. The latter is crucial in order to narrow the uncertainty in future climate projections. I expect the methodologies that I have developed to be widely used by other researchers to study boundary layer clouds. I further plan to use these methodologies to derive a novel datasets of cloud thermodynamic properties and surface fluxes from satellite observations. The dataset is planned to be published in an open-access database with a digital object identifier.
Satellite image illustrating MSCCC hypothesis