Idealised studies of aerosol effects on precipitation – from aqua-planets to global km-scale models

(2023)

Authors:

Philip Stier, Andrew Williams, Ross Herbert, Philipp Weiss, Guy Dagan, Duncan Watson-Parris

Abstract:

Aerosol effects on precipitation can be broadly categorized into radiatively and microphysically mediated effects – all of which remain highly uncertain. Their assessment in atmospheric models generally relies on the simulation of a complex chain of microphysical process growing aerosols into radiatively active size ranges and into size ranges suitable to act as cloud condensation nuclei, subsequently perturbing radiative fluxes, diabatic heating, cloud microphysical processes and ultimately precipitation formation.  The associated uncertainties along each step in these complex process chains remain significant and make it difficult to disentangle uncertainties in aerosol and cloud processes. Here we present results from a hierarchy of highly idealised model simulations in which aerosols are prescribed as fixed plumes of radiative properties, with an optional associated semi-empirical scaling of droplet number perturbations. These idealised simulations provide fascinating insights into the physical processes underlying aerosol effects on precipitation and into the interaction of local perturbations with the larger scale dynamics. Idealised aqua-planet general circulation model simulations reveal that the response of regional precipitation to idealised and realistic aerosol radiative perturbations can be well explained in an energetic framework (because associated changes in the net diabatic heating needs to be balanced by latent heat release, surface or top-of-atmosphere fluxes or compensated for by energy divergence/convergence). Extending this framework by adding land and realistic sea surface temperatures in an AMIP setup, we probe the regional sensitivity of precipitation changes to absorbing aerosol perturbations across the globe. Our results confirm the findings from the aqua-planet studies that that the local precipitation response to aerosol absorption is opposite in sign between the tropics and the extratropics and we show that this contrasting response can be understood in terms of different mechanisms by which the large-scale circulation responds to heating in the extratropics and in the tropics. Finally, we apply our framework in cloud resolving km-scale model simulations regionally and globally, which highlights the importance of radiative perturbations as well as a complex interplay of aerosol effects with the diurnal cycle of precipitation. 

Invertible neural networks for satellite retrievals of aerosol optical depth

(2023)

Authors:

Paolo Pelucchi, Jorge Vicent, J Emmanuel Johnson, Philip Stier, Gustau Camps-Valls

Abstract:

The retrieval of atmospheric aerosol properties from satellite remote sensing is a complex and under-determined inverse problem. Traditional retrieval algorithms, based on radiative transfer models, must make approximations and assumptions to reach a unique solution or repeatedly use the expensive forward models to be able to quantify uncertainty. The recently introduced Invertible Neural Networks (INNs), a machine learning method based on Normalizing Flows, appear particularly suited for tackling inverse problems. They simultaneously model both the forward and the inverse branches of the problem, and their generative aspect allows them to efficiently provide non-parametric posterior distributions for the retrieved parameters, which can be used to quantify the retrieval uncertainty. So far INNs have successfully been applied to low-dimensional idealised inverse problems and even to some simpler scientific retrieval problems. Still, satellite aerosol retrievals present particular challenges, such as the high variability of the surface reflectance signal and the often comparatively small aerosol signal in the top-of-the-atmosphere (TOA) measurements.In this study, we investigate the use of INNs for retrieving aerosol optical depth (AOD) and its uncertainty estimates at the pixel level from MODIS TOA reflectance measurements. The models are trained with custom synthetic datasets of TOA reflectance-AOD pairs made by combining the MODIS Dark Target algorithm’s atmospheric look-up tables and a MODIS surface reflectance product. The INNs are found to perform emulation and inversion of the look-up tables successfully. We initially train models adapted to different surface types by focusing our application on limited regional and seasonal contexts. The models are applied to real measurements from the MODIS sensor, and the generated AOD retrievals and posterior distributions are compared to the corresponding Dark Target and AERONET retrievals for evaluation and discussion.

The Diurnal Cycle of the Cloud Radiative Effect of Deep Convective Clouds over Africa from a Lagrangian Perspective

(2023)

Authors:

William Jones, Martin Stengel, Philip Stier

Abstract:

Tropical deep connective clouds (DCCs) have large top of atmosphere (ToA) cloud radiative effects (CREs) in both the shortwave (SW) and longwave (LW), which both have average magnitudes of greater than 100 Wm-2. Due to the opposite sign of the two components, the overall ToA CRE is generally assumed to average to approximately 0 Wm-2. Although there are a number of mechanisms that contribute to this balance, the fact that the daytime only SW CRE balances with the LW CRE indicates that the diurnal lifecycle of DCCs is a key component of this balance. Understanding how the diurnal cycle of DCCs influences their CRE is vital for understanding how any changes in their diurnal cycle of these clouds may influence the climate. A year-long dataset of retrieved cloud properties and derived broadband radiative fluxes has been produced by the ESA Cloud CCI project using temporally highly resolved satellite observations. Using a novel method, we are able to detect and track both isolated DCCs and large, mesoscale convective systems (MCSs) over their entire lifecycle. We explicitly retrieve the cloud properties and CREs of DCCs over Africa, and how these properties change over the lifecycle of approximately 100,000 observed clouds. We find that the mean anvil SW CRE greatly varies depending on the initiation time of day and the lifetime of the DCC, whereas the LW CRE is consistent throughout the diurnal cycle and varies primarily with cloud top temperature. As a result of our study we can confirm that the mean observed ToA CRE of all DCCs (integrated over area and lifetime) is indeed approximately 0 Wm-2, but very few DCCs individually have mean CREs near this value. Instead, we find that DCCs occurring during the daytime have a large cooling effect, and those at nighttime have a warming effect, resulting in a bimodal distribution. While MCSs make the largest contribution to the overall effect due to their large areas and lifetimes, because they tend to exist during both nighttime and daytime the overall magnitude of their ToA CREs tend to be smaller than those of isolated DCCs. As a result, factors which influence the diurnal cycle of deep convection – such as changes in CAPE generation or convective inhibition – may have a more important influence on the properties of isolated DCCs rather than larger MCSs.

Reducing aerosol forcing uncertainty by combining models with satellite and within-the-atmosphere observations: a three-way street

Reviews of Geophysics American Geophysical Union 61:2 (2023) e2022RG000796

Authors:

Ralph A Kahn, Elisabeth Andrews, Charles A Brock, Mian Chin, Graham Feingold, Andrew Gettelman, Robert C Levy, Daniel M Murphy, Athanasios Nenes, Jeffrey R Pierce, Thomas Popp, Jens Redemann, Andrew M Sayer, Arlindo da Silva, Larisa Sogacheva, Philip Stier

Abstract:

Aerosol forcing uncertainty represents the largest climate forcing uncertainty overall. Its magnitude has remained virtually undiminished over the past 20 years despite considerable advances in understanding most of the key contributing elements. Recent work has produced modest increases only in the confidence of the uncertainty estimate itself. This review summarizes the contributions toward reducing the uncertainty in the aerosol forcing of climate made by satellite observations, measurements taken within the atmosphere, as well as modeling and data assimilation. We adopt a more measurement-oriented perspective than most reviews of the subject in assessing the strengths and limitations of each; gaps and possible ways to fill them are considered. Currently planned programs supporting advanced, global-scale satellite and surface-based aerosol, cloud, and precursor gas observations, climate modeling, and intensive field campaigns aimed at characterizing the underlying physical and chemical processes involved, are all essential. But in addition, new efforts are needed: (1) to obtain systematic aircraft in situ measurements capturing the multi-variate probability distribution functions of particle optical, microphysical, and chemical properties (and associated uncertainty estimates), as well as co-variability with meteorology, for the major aerosol airmass types; (2) to conceive, develop, and implement a suborbital (aircraft plus surface-based) program aimed at systematically quantifying the cloud-scale microphysics, cloud optical properties, and cloud related vertical velocities associated with aerosol-cloud interactions; and (3) to focus much more research on integrating the unique contributions satellite observations, suborbital measurements, and modeling, in order to reduce the uncertainty in aerosol climate forcing.

Satellite observations of smoke–cloud–radiation interactions over the Amazon rainforest

Atmospheric Chemistry and Physics Copernicus Publications 23:7 (2023) 4595-4616

Authors:

Ross Herbert, Philip Stier

Abstract:

The Amazon rainforest routinely experiences intense and long-lived biomass burning events that result in smoke plumes that cover vast regions. The spatial and temporal extent of the plumes and the complex pathways through which they interact with the atmosphere have proved challenging to measure for purposes of gaining a representative understanding of smoke impacts on the Amazonian atmosphere. In this study, we use multiple collocated satellite sensors on board AQUA and TERRA platforms to study the underlying smoke–cloud–radiation interactions during the diurnal cycle. An 18-year time series for both morning and afternoon overpasses is constructed, providing collocated measurements of aerosol optical depth (AOD; column-integrated aerosol extinction), cloud properties, top-of-atmosphere radiative fluxes, precipitation, and column water vapour content from independent sources.
The long-term time series reduces the impact of interannual variability and provides robust evidence that smoke significantly modifies the Amazonian atmosphere. Low loadings of smoke (AOD ≤ 0.4) enhance convective activity, cloudiness, and precipitation, but higher loadings (AOD > 0.4) strongly suppress afternoon convection and promote low-level cloud occurrence. Accumulated precipitation increases with convective activity but remains elevated under high smoke loadings, suggesting fewer but more intense convective cells. Contrasting morning and afternoon cloud responses to smoke are observed, in line with recent simulations. Observations of top-of-atmosphere radiative fluxes support the findings and show that the response of low-level cloud properties and cirrus coverage to smoke results in a pronounced and consistent increase in top-of-atmosphere outgoing radiation (cooling) of up to 50 W m−2 for an AOD perturbation of +1.0.
The results demonstrate that smoke strongly modifies the atmosphere over the Amazon via widespread changes to the cloud field properties. Rapid adjustments work alongside instantaneous radiative effects to drive a stronger cooling effect from smoke than previously thought, whilst contrasting morning and afternoon responses of liquid and ice water paths highlight a potential method for constraining aerosol impacts on climate. Increased drought susceptibility, land use change, and deforestation will have important and widespread impacts on the region over the coming decades. Based on this analysis, we anticipate that further increases in anthropogenic fire activity will associated with an overall reduction in regional precipitation and a negative forcing (cooling) on the Earth's energy budget.