Optically thin clouds in the trades

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

10.5194/acp-2020-1160 ◽

2020 ◽

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 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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Total cloud cover from satellite observations and climate models

Atmospheric Research ◽

10.1016/j.atmosres.2012.01.005 ◽

2012 ◽

Vol 107 ◽

pp. 161-170 ◽

Cited By ~ 18

Author(s):

P. Probst ◽

R. Rizzi ◽

E. Tosi ◽

V. Lucarini ◽

T. Maestri

Keyword(s):

Cloud Cover ◽

Climate Models ◽

Satellite Observations ◽

Total Cloud Cover

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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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Variability and trend of diurnal temperature range in China and their relationship to total cloud cover and sunshine duration

Annales Geophysicae ◽

10.5194/angeo-31-795-2013 ◽

2013 ◽

Vol 31 (5) ◽

pp. 795-804 ◽

Cited By ~ 36

Author(s):

X. Xia

Keyword(s):

Diurnal Temperature Range ◽

Cloud Cover ◽

Sunshine Duration ◽

Radiative Effect ◽

Total Cloud Cover ◽

Diurnal Temperature ◽

Temperature Range ◽

Clear Sky ◽

Sky Conditions

Abstract. This study aims to investigate the effect of total cloud cover (TCC) and sunshine duration (SSD) in the variation of diurnal temperature range (DTR) in China during 1954–2009. As expected, the inter-annual variation of DTR was mainly determined by TCC. Analysis of trends of 30-year moving windows of DTR and TCC time series showed that TCC changes could account for that of DTR in some cases. However, TCC decreased during 1954–2009, which did not support DTR reduction across China. DTRs under sky conditions such as clear, cloudy and overcast showed nearly the same decreasing rate that completely accounted for the overall DTR reduction. Nevertheless, correlation between SSD and DTR was weak and not significant under clear sky conditions in which aerosol direct radiative effect should be dominant. Furthermore, 30–60% of DTR reduction was associated with DTR decrease under overcast conditions in south China. This implies that aerosol direct radiative effect appears not to be one of the main factors determining long-term changes in DTR in China.

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How Well Are Clouds Simulated over Greenland in Climate Models? Consequences for the Surface Cloud Radiative Effect over the Ice Sheet

Journal of Climate ◽

10.1175/jcli-d-18-0023.1 ◽

2018 ◽

Vol 31 (22) ◽

pp. 9293-9312 ◽

Cited By ~ 6

Author(s):

A. Lacour ◽

H. Chepfer ◽

N. B. Miller ◽

M. D. Shupe ◽

V. Noel ◽

...

Keyword(s):

Cloud Cover ◽

Climate Models ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Ground Level ◽

Radiative Effect ◽

Cloud Properties ◽

Cloud Radiative Effect ◽

Primary Driver ◽

Cloud Radiative Effects

Using lidar and radiative flux observations from space and ground, and a lidar simulator, we evaluate clouds simulated by climate models over the Greenland ice sheet, including predicted cloud cover, cloud fraction profile, cloud opacity, and surface cloud radiative effects. The representation of clouds over Greenland is a central concern for the models because clouds impact ice sheet surface melt. We find that over Greenland, most of the models have insufficient cloud cover during summer. In addition, all models create too few nonopaque, liquid-containing clouds optically thin enough to let direct solar radiation reach the surface (−1% to −3.5% at the ground level). Some models create too few opaque clouds. In most climate models, the cloud properties biases identified over all Greenland also apply at Summit, Greenland, proving the value of the ground observatory in model evaluation. At Summit, climate models underestimate cloud radiative effect (CRE) at the surface, especially in summer. The primary driver of the summer CRE biases compared to observations is the underestimation of the cloud cover in summer (−46% to −21%), which leads to an underestimated longwave radiative warming effect (CRELW = −35.7 to −13.6 W m−2 compared to the ground observations) and an underestimated shortwave cooling effect (CRESW = +1.5 to +10.5 W m−2 compared to the ground observations). Overall, the simulated clouds do not radiatively warm the surface as much as observed.

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Exploring Perturbed Physics Ensembles in a Regional Climate Model

Journal of Climate ◽

10.1175/jcli-d-11-00275.1 ◽

2012 ◽

Vol 25 (13) ◽

pp. 4582-4599 ◽

Cited By ~ 42

Author(s):

Omar Bellprat ◽

Sven Kotlarski ◽

Daniel Lüthi ◽

Christoph Schär

Keyword(s):

Regional Climate Model ◽

Annual Cycle ◽

Cloud Cover ◽

Climate Models ◽

Climate Model ◽

Regional Climate ◽

Total Cloud Cover ◽

Observational Uncertainty ◽

Perturbed Physics ◽

Perturbed Physics Ensembles

Abstract Perturbed physics ensembles (PPEs) have been widely used to assess climate model uncertainties and have provided new estimates of climate sensitivity and parametric uncertainty in state-of-the-art climate models. So far, mainly global climate models were used to generate PPEs, and little work has been conducted with regional climate models. This paper discusses the parameter uncertainty in two PPEs of a regional climate model driven by reanalysis data for the present climate over Europe. The uncertainty is evaluated for the variables of 2-m temperature, precipitation, and total cloud cover, with a focus on the annual cycle, interannual variability, and selected extremes. The authors show that the simulated spread of the PPEs encompasses the observations at a regional scale in terms of the annual cycle and the interannual variability, provided observational uncertainty is taken into account. To rank the PPEs a new skill metric is proposed, which takes into account observational uncertainty and natural variability. The metric is a generalization of the climate prediction index (CPI) and is compared to metrics used in other studies. The consideration of observational uncertainty is particularly important for total cloud cover and reveals that current observations do not allow for a systematic evaluation of high precipitation intensities over the entire European domain. The skill framework is additionally used to identify important model parameters, which are of interest for an objective model calibration.

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On clocks and clouds

Atmospheric Chemistry and Physics ◽

10.5194/acp-14-6729-2014 ◽

2014 ◽

Vol 14 (13) ◽

pp. 6729-6738 ◽

Cited By ~ 6

Author(s):

M. K. Witte ◽

P. Y. Chuang ◽

G. Feingold

Keyword(s):

Vertical Direction ◽

Trade Wind ◽

Cumulus Cloud ◽

Initial Value ◽

Cumulus Clouds ◽

Mature Phase ◽

Cloud Properties ◽

Large Eddy ◽

Multiple Pulses

Abstract. Cumulus clouds exhibit a life cycle that consists of (a) the growth phase (increasing size, most notably in the vertical direction); (b) the mature phase (growth ceases; any precipitation that develops is strongest during this period); and (c) the dissipation phase (cloud dissipates because of precipitation and/or entrainment; no more dynamical support). Although radar can track clouds over time and give some sense of the age of a cloud, most aircraft in situ measurements lack temporal context. We use large eddy simulations of trade wind cumulus cloud fields from cases during the Barbados Oceanographic and Meteorological Experiment (BOMEX) and Rain In Cumulus over the Ocean (RICO) campaigns to demonstrate a potential cumulus cloud "clock." We find that the volume-averaged total water mixing ratio rt is a useful cloud clock for the 12 clouds studied. A cloud's initial rt is set by the subcloud mixed-layer mean rt and decreases monotonically from the initial value due primarily to entrainment. The clock is insensitive to aerosol loading, environmental sounding and extrinsic cloud properties such as lifetime and volume. In some cases (more commonly for larger clouds), multiple pulses of buoyancy occur, which complicate the cumulus clock by replenishing rt. The clock is most effectively used to classify clouds by life phase.

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On clocks and clouds

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-13-23461-2013 ◽

2013 ◽

Vol 13 (9) ◽

pp. 23461-23490

Author(s):

M. K. Witte ◽

P. Y. Chuang ◽

G. Feingold

Keyword(s):

Vertical Direction ◽

Trade Wind ◽

Cumulus Cloud ◽

Initial Value ◽

Cumulus Clouds ◽

Mature Phase ◽

Cloud Properties ◽

Large Eddy ◽

Multiple Pulses

Abstract. Cumulus clouds exhibit a life cycle that consists of: (a) the growth phase (increasing size, most notably in the vertical direction); (b) the mature phase (growth ceases; any precipitation that develops is strongest during this period); and (c) the dissipation phase (cloud dissipates because of precipitation and/or entrainment; no more dynamical support). Although radar can track clouds over time and give some sense of the age of a cloud, most aircraft in situ measurements lack temporal context. We use large eddy simulations of trade wind cumulus cloud fields from cases during the Barbados Oceanographic and Meteorological Experiment (BOMEX) and Rain In Cumulus over the Ocean (RICO) campaigns to demonstrate a potential cumulus cloud "clock". We find that the volume-averaged total water mixing ratio rt is a useful cloud clock for the 12 clouds studied. A cloud's initial rt is set by the subcloud mixed-layer mean rt and decreases monotonically from the initial value due primarily to entrainment. The clock is insensitive to aerosol loading, environmental sounding and extrinsic cloud properties such as lifetime and volume. In some cases (more commonly for larger clouds), multiple pulses of buoyancy occur, which complicate the cumulus clock by replenishing rt. The clock is most effectively used to classify clouds by life phase.

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