Shallow Cumulus Cloud Feedback in Large Eddy Simulations – Bridging the Gap to Storm Resolving Models

Abstract. The response of shallow trade cumulus clouds to global warming is a leading source of uncertainty to interpretations and projections of the Earth's changing climate. A setup based on the Rain In Cumulus over the Ocean field campaign is used to simulate a shallow trade wind cumulus field with the Icosahedral Non-hydrostatic Large Eddy Model in a control and a perturbed 4 K warmed climate, while degrading horizontal resolution from 100 m to 5 km. As the resolution is coarsened the basic state cloud fraction increases substantially, especially at cloud base, lateral mixing is weaker and cloud tops reach higher. Nevertheless, the overall vertical structure of the cloud layer is surprisingly robust across resolutions. In a warmer climate, cloud cover reduces, alone constituting a positive shortwave cloud feedback: the strength correlates with the amount of basic state cloud fraction, thus is stronger at coarser resolutions. Cloud thickening, resulting from more water vapor availability for condensation in a warmer climate, acts as a compensating feedback, but unlike the cloud cover reduction it is largely resolution independent. Therefore, refining the resolution leads to convergence to a near-zero shallow cumulus feedback. This dependence holds in experiments with enhanced realism including precipitation processes or warming along a moist adiabat instead of uniform warming. Insofar as these findings carry over to other models, they suggest that storm resolving models may exaggerate the trade wind cumulus cloud feedback.

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Shallow cumulus cloud feedback in large eddy simulations – bridging the gap to storm-resolving models

Atmospheric Chemistry and Physics ◽

10.5194/acp-21-3275-2021 ◽

2021 ◽

Vol 21 (5) ◽

pp. 3275-3288

Author(s):

Jule Radtke ◽

Thorsten Mauritsen ◽

Cathy Hohenegger

Keyword(s):

Cloud Cover ◽

Horizontal Resolution ◽

Cloud Fraction ◽

Cloud Feedback ◽

Cloud Base ◽

Trade Wind ◽

Cumulus Cloud ◽

Shallow Cumulus ◽

Large Eddy ◽

Carry Over

Abstract. The response of shallow trade cumulus clouds to global warming is a leading source of uncertainty in projections of the Earth's changing climate. A setup based on the Rain In Cumulus over the Ocean field campaign is used to simulate a shallow trade wind cumulus field with the Icosahedral Nonhydrostatic Large Eddy Model in a control and a perturbed 4 K warmer climate, while degrading horizontal resolution from 100 m to 5 km. As the resolution is coarsened, the base-state cloud fraction increases substantially, especially near cloud base, lateral mixing is weaker, and cloud tops reach higher. Nevertheless, the overall vertical structure of the cloud layer is surprisingly robust across resolutions. In a warmer climate, cloud cover reduces, alone constituting a positive shortwave cloud feedback: the strength correlates with the amount of base-state cloud fraction and thus is stronger at coarser resolutions. Cloud thickening, resulting from more water vapour availability for condensation in a warmer climate, acts as a compensating feedback, but unlike the cloud cover reduction it is largely resolution independent. Therefore, refining the resolution leads to convergence to a near-zero shallow cumulus feedback. This dependence holds in experiments with enhanced realism including precipitation processes or warming along a moist adiabat instead of uniform warming. Insofar as these findings carry over to other models, they suggest that storm-resolving models may exaggerate the trade wind cumulus cloud feedback.

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Mass flux estimates and their relationship to cloud-base cloudiness during the EUREC4A campaign

10.5194/egusphere-egu21-4804 ◽

2021 ◽

Author(s):

Raphaela Vogel ◽

Sandrine Bony ◽

Anna Lea Albright ◽

Bjorn Stevens ◽

Geet George ◽

...

Keyword(s):

Mass Flux ◽

Cloud Cover ◽

Climate Models ◽

Cloud Fraction ◽

Cloud Feedback ◽

Surface Fluxes ◽

Cloud Base ◽

Entrainment Rate ◽

Cumulus Cloud ◽

Total Cloud Cover

The trade-cumulus cloud feedback in climate models is mostly driven by changes in cloud-base cloudiness, which can largely be attributed to model differences in the strength of lower-tropospheric mixing. Using observations from the recent EUREC4A field campaign, we test the hypothesis that enhanced lower-tropospheric mixing dries the lower cloud layer and reduces near-base cloudiness. The convective mass flux at cloud base is used as a proxy for the strength of convective mixing and is estimated as the residual of the subcloud layer mass budget, which is derived from dropsondes intensively launched along a circle of ~200 km diameter. The cloud-base cloud fraction is measured with horizontally-pointing lidar and radar from an aircraft flying near cloud base within the circle area. Additional airborne, ground- and ship-based radar, lidar and in-situ measurements are used to estimate the total cloud cover, the surface fluxes and to validate the consistency of the approach.Preliminary mass flux estimates have reasonable mean values of about 15 mm/s. 3- circle (i.e. 3h) averaged estimates range between 0-40 mm/s and reveal substantial day-to-day and daily variability. The day-to-day variability in the mass flux is mostly due to variability in the mesoscale vertical velocity, whereas the entrainment rate mostly explains variability on the daily timescale, consistent with previous large-eddy simulations. We find the mass flux to be positively correlated to both the cloud-base cloud fraction and the total cloud cover (R=0.55 and R~0.4, respectively). Other indicators of lower-tropospheric mixing due to convection and mesoscale circulations also suggest positive relationships between mixing and cloudiness. Implications of these analyses for testing the hypothesized mechanism of positive trade-cumulus cloud feedback will be discussed.

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Aerosol effects on shallow cumulus cloud fields in idealised and realistic simulations

10.5194/egusphere-egu2020-18231 ◽

2020 ◽

Author(s):

George Spill ◽

Philip Stier ◽

Paul Field ◽

Guy Dagan

Keyword(s):

Large Scale ◽

Trade Wind ◽

Quasi Equilibrium ◽

Cumulus Cloud ◽

Shallow Convection ◽

Shallow Cumulus ◽

Synoptic Conditions ◽

Large Eddy ◽

Transient Forcing ◽

Periodic Domains

Shallow cumulus clouds interact with their environment in myriad significant ways, and yet their behavour is still poorly understood, and is responsible for much uncertainty in climate models. Improving our understanding of these clouds is therefore an important part of improving our understanding of the climate system as a whole.Modelling studies of shallow convection have traditionally made use of highly idealised simulations using large-eddy models, which allow for high resolution, detailed simulations. However, this idealised nature, with periodic boundaries and constant forcing, and the quasi-equilibrium cloud fields produced, means that they do not capture the effect of transient forcing and conditions found in the real atmosphere, which contains shallow cumulus cloud fields unlikely to be in equilibrium.&#160;Simulations with more realistic nested domains and forcings have previously been shown to have significant persistent responses differently to aerosol perturbations, in contrast to many large eddy simulations in which perturbed runs tend to reach a similar quasi-equilibrium.&#160;Here, we further this investigation by using a single model to present a comparison of familiar idealised simulations of trade wind cumuli in periodic domains, and simulations with a nested domain, whose boundary conditions are provided by a global driving model, able to simulate transient synoptic conditions.&#160;The simulations are carried out using the Met Office Unified Model (UM), and are based on a case study from the Rain In Cumulus over the Ocean (RICO) field campaign. Large domains of 500km are chosen in order to capture large scale cloud field behaviour. A double-moment interactive microphysics scheme is used, along with prescribed aerosol profiles based on RICO observations, which are then perturbed.We find that the choice between realistic nested domains with transient forcing and idealised periodic domains with constant forcing does indeed affect the nature of the response to aerosol perturbations, with the realistic simulations displaying much larger persistent changes in domain mean fields such as liquid water path and precipitation rate.&#160;

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The vertical structure and spatial variability of lower-tropospheric water vapor and clouds in the trades

Atmospheric Chemistry and Physics ◽

10.5194/acp-20-6129-2020 ◽

2020 ◽

Vol 20 (10) ◽

pp. 6129-6145

Author(s):

Ann Kristin Naumann ◽

Christoph Kiemle

Keyword(s):

Water Vapor ◽

Cloud Fraction ◽

Airborne Lidar ◽

Cloud Base ◽

Trade Wind ◽

Model Resolution ◽

Grid Spacing ◽

Model Bias ◽

Shallow Cumulus ◽

Vapor Distribution

Abstract. Horizontal and vertical variability of water vapor is omnipresent in the tropics, but its interaction with cloudiness poses challenges for weather and climate models. In this study we compare airborne lidar measurements from a summer and a winter field campaign in the tropical Atlantic with high-resolution simulations to analyze the water vapor distributions in the trade wind regime, its covariation with cloudiness, and their representation in simulations. Across model grid spacing from 300 m to 2.5 km, the simulations show good skill in reproducing the water vapor distribution in the trades as measured by the lidar. An exception to this is a pronounced moist model bias at the top of the shallow cumulus layer in the dry winter season which is accompanied by a humidity gradient that is too weak at the inversion near the cloud top. The model's underestimation of water vapor variability in the cloud and subcloud layer occurs in both seasons but is less pronounced than the moist model bias at the inversion. Despite the model's insensitivity to resolution from hecto- to kilometer scale for the distribution of water vapor, cloud fraction decreases strongly with increasing model resolution and is not converged at hectometer grid spacing. The observed cloud deepening with increasing water vapor path is captured well across model resolution, but the concurrent transition from cloud-free to low cloud fraction is better represented at hectometer resolution. In particular, in the wet summer season the simulations with kilometer-scale resolution overestimate the observed cloud fraction near the inversion but lack condensate near the observed cloud base. This illustrates how a model's ability to properly capture the water vapor distribution does not necessarily translate into an adequate representation of shallow cumulus clouds that live at the tail of the water vapor distribution.

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Optically thin clouds in the trades

10.5194/acp-2021-453 ◽

2021 ◽

Author(s):

Theresa Mieslinger ◽

Bjorn Stevens ◽

Tobias Kölling ◽

Manfred Brath ◽

Martin Wirth ◽

...

Keyword(s):

Cloud Cover ◽

Climate Models ◽

Trade Wind ◽

Cumulus Cloud ◽

Total Cloud Cover ◽

Clear Sky ◽

Cloud Radiative Effect ◽

Large Eddy ◽

Lidar Measurements ◽

Cloud Mask

Abstract. We develop a new method to describe the total cloud cover including optically thin clouds in trade wind cumulus cloud fields. Climate models as well as Large Eddy Simulations commonly underestimate the cloud cover, while estimates from observations largely disagree on the cloud cover in the trades. Currently, trade wind clouds contribute significantly to the uncertainty in climate sensitivity estimates derived from model perturbation studies. To simulate clouds well and especially how they change in a future climate we have to know how cloudy it is. In this study we develop a method to quantify the cloud cover from a clear-sky perspective. Using well-known radiative transfer relations we retrieve the clear-sky contribution in high-resolution satellite observations of trade cumulus cloud fields during EUREC4A. Knowing the clear-sky part, we can investigate the remaining cloud-related contributions consisting of areas detected by common cloud masking algorithms and those undetected areas related to optically thin clouds. We find that the cloud-mask cloud cover underestimates the total cloud cover by a factor of 2. Lidar measurements on board the HALO aircraft support our findings by showing a high abundance of optically thin clouds during EUREC4A. Mixing the undetected optically thin clouds into the clear-sky signal can cause an underestimation of the cloud radiative effect of up to −32 %. We further discuss possible artificial correlations in aersol-cloud cover interaction studies that might arise from undetected optically thin clouds. Our analysis suggests that the known underestimation of trade wind cloud cover and simultaneous overestiamtion of cloud brightness in models is even higher than assumed so far.

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On the lifecycle of a shallow cumulus cloud: Is it a bubble or plume, active or forced?

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-20-0361.1 ◽

2021 ◽

Author(s):

David M. Romps ◽

Rusen Öktem ◽

Satoshi Endo ◽

Andrew M. Vogelmann

Keyword(s):

Cloud Cover ◽

Large Eddy Simulations ◽

The Other ◽

Doppler Lidar ◽

Cumulus Cloud ◽

Cumulus Clouds ◽

Shallow Cumulus ◽

Large Eddy ◽

Stereo Cameras ◽

Using Data

AbstractA cloud’s lifecycle determines how its mass flux translates into cloud cover, thereby setting Earth’s albedo. Here, an attempt is made to quantify the most basic aspects of the lifecycle of a shallow cumulus cloud: the degree to which it is a bubble or plume, and active or forced. Quantitative measures are proposed for these properties, which are then applied to hundreds of shallow cumulus clouds in Oklahoma using data from stereo cameras, a Doppler lidar, and large-eddy simulations. The observed clouds are intermediate between active and forced, but behave more like bubbles than plumes. The simulated clouds, on the other hand, are more active and plume-like, suggesting room for improvement in the modeling of shallow cumulus.

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Reconciling Differences Between Large‐Eddy Simulations and Doppler Lidar Observations of Continental Shallow Cumulus Cloud‐Base Vertical Velocity

Geophysical Research Letters ◽

10.1029/2019gl084893 ◽

2019 ◽

Vol 46 (20) ◽

pp. 11539-11547 ◽

Cited By ~ 3

Author(s):

Satoshi Endo ◽

Damao Zhang ◽

Andrew M. Vogelmann ◽

Pavlos Kollias ◽

Katia Lamer ◽

...

Keyword(s):

Vertical Velocity ◽

Large Eddy Simulations ◽

Cloud Base ◽

Doppler Lidar ◽

Cumulus Cloud ◽

Shallow Cumulus ◽

Large Eddy ◽

Lidar Observations

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The vertical Structure and spatial Variability of lower tropospheric Water Vapor and Clouds in the Trades

10.5194/acp-2019-1015 ◽

2019 ◽

Cited By ~ 1

Author(s):

Ann Kristin Naumann ◽

Christoph Kiemle

Keyword(s):

Water Vapor ◽

Winter Season ◽

Cloud Fraction ◽

Airborne Lidar ◽

Cloud Base ◽

Trade Wind ◽

Model Resolution ◽

Grid Spacing ◽

Shallow Cumulus ◽

Vapor Distribution

Abstract. Horizontal and vertical variability of water vapor is omnipresent in the tropics but its interaction with cloudiness poses challenges for weather and climate models. In this study we compare airborne lidar measurements from a summer and a winter field campaign in the tropical Atlantic with high-resolution simulations to analyse the water vapor distributions in the trade wind regime, its covariation with cloudiness and their representation in simulations. Across model grid spacing from 300 m to 2.5 km, the simulations show good skill in reproducing the water vapor distribution in the trades as measured by the lidar. An exception to this is a pronounced moist model bias at the top of the shallow cumulus layer in the dry winter season which is accompanied by a too weak humidity inversion at the cloud top. The model's underestimation of water vapor variability in the cloud and subcloud layer occurs in both seasons but is less pronounced. Despite the model's insensitivity to resolution from hecto- to kilometer scale for the distribution of water vapor, cloud fraction decreases strongly with increasing model resolution and is not converged at hectometer grid spacing. The observed cloud deepening with increasing water vapor path is captured well across model resolution but the concurrent transition from cloud-free to low cloud fraction is better represented at hectometer resolution. In particular, in the wet summer season the simulations with kilometer-scale resolution overestimate the observed cloud fraction near the inversion but lack condensate near the observed cloud base. This illustrates how a model's ability to properly capture the water vapor distribution does not need to translate into an adequate representation of shallow cumulus clouds that live at the tail of the water vapor distribution.

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Climatology of stratocumulus cloud morphologies: microphysical properties and radiative effects

Atmospheric Chemistry and Physics ◽

10.5194/acp-14-6695-2014 ◽

2014 ◽

Vol 14 (13) ◽

pp. 6695-6716 ◽

Cited By ~ 45

Author(s):

A. Muhlbauer ◽

I. L. McCoy ◽

R. Wood

Keyword(s):

Physical Properties ◽

Global Scale ◽

Cloud Fraction ◽

Cloud Base ◽

Trade Wind ◽

Radiative Effects ◽

Regional Variability ◽

Microphysical Properties ◽

Low Clouds ◽

Low Cloud

Abstract. An artificial neural network cloud classification scheme is combined with A-train observations to characterize the physical properties and radiative effects of marine low clouds based on their morphology and type of mesoscale cellular convection (MCC) on a global scale. The cloud morphological categories are (i) organized closed MCC, (ii) organized open MCC and (iii) cellular but disorganized MCC. Global distributions of the frequency of occurrence of MCC types show clear regional signatures. Organized closed and open MCCs are most frequently found in subtropical regions and in midlatitude storm tracks of both hemispheres. Cellular but disorganized MCC are the predominant type of marine low clouds in regions with warmer sea surface temperature such as in the tropics and trade wind zones. All MCC types exhibit a pronounced seasonal cycle. The physical properties of MCCs such as cloud fraction, radar reflectivity, drizzle rates and cloud top heights as well as the radiative effects of MCCs are found highly variable and a function of the type of MCC. On a global scale, the cloud fraction is largest for closed MCC with mean cloud fractions of about 90%, whereas cloud fractions of open and cellular but disorganized MCC are only about 51% and 40%, respectively. Probability density functions (PDFs) of cloud fractions are heavily skewed and exhibit modest regional variability. PDFs of column maximum radar reflectivities and inferred cloud base drizzle rates indicate fundamental differences in the cloud and precipitation characteristics of different MCC types. Similarly, the radiative effects of MCCs differ substantially from each other in terms of shortwave reflectance and transmissivity. These differences highlight the importance of low-cloud morphologies and their associated cloudiness on the shortwave cloud forcing.

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