Shallow cumulus properties as captured by adiabatic fraction in high-resolution LES simulations

Abstract Shallow convective clouds are important players in Earth’s energy budget and hydrological cycle, and are abundant in the tropical and subtropical belts. They greatly contribute to the uncertainty in climate predictions, due to their unresolved, complex processes that include coupling between the dynamics and microphysics. Analysis of cloud structure can be simplified by considering cloud motions as a combination of moist adiabatic motions like adiabatic updrafts and turbulent motions leading to deviation from adiabaticity. In this work, we study the sizes and occurrence of adiabatic regions in shallow cumulus clouds during their growth and mature stages, and use the adiabatic fraction (AF) as a continuous metric to describe cloud processes and properties from the core to the edge. To do so, we simulate isolated trade wind cumulus clouds of different sizes using the System of Atmospheric Modeling (SAM) model in high-resolution (10 m) with the Hebrew University spectral bin microphysics (SBM). The fine features in the cloud’s dynamics and microphysics, including small near-adiabatic volumes and a thin transition zone at the edge of the cloud (∼20-40 m in width) are captured. The AF is shown to be an efficient measure for analyzing cloud properties and key processes determining the droplets-size-distribution formation and shape during the cloud evolution. Physical processes governing the properties of droplets size distributions at different cloud regions (e.g. core, edge) are analyzed in relation to AF.

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The adiabaticity of warm cumulus clouds simulated in high-resolution

10.5194/egusphere-egu21-10009 ◽

2021 ◽

Author(s):

Eshkol Eytan ◽

Ilan Koren ◽

Alexander Khain ◽

Orit Altaratz ◽

Mark Pinsky ◽

...

Keyword(s):

High Resolution ◽

Environmental Parameters ◽

Liquid Water Content ◽

Cumulus Clouds ◽

Local Boundary ◽

Aerosol Loading ◽

Hebrew University ◽

Key Variables ◽

Microphysical Processes ◽

Bin Microphysics

The strong coupling between dynamic, thermodynamic, and microphysical processes and the numerous environmental parameters on which they depend makes clouds a highly complex system. Adiabatic regions (i.e., undiluted core) in the cloud allow to approximate in a simple way thermodynamic and microphysical profiles and provide local boundary conditions (i.e. core is a source of adiabatic values in each level). Mixing of the cloud with its environment affects both the cloud and the environmental properties. While environmental humidity, temperature and aerosol loading affect the clouds&#8217; buoyancy and droplets size distribution (DSD), clouds simultaneously affect their surrounding via detrainment of droplets, humid air, and processed aerosols. Mixing occurs within a large spectrum of scales and leads to deviation of parts of the cloud from adiabaticity. The level of adiabaticity can be represented continuously by the adiabatic fraction (AF; defined as the ratio of the liquid water content to the theoretical adiabatic value). In this work we used the System of Atmosphere Modeling (SAM) with the Hebrew University Spectral Bin Microphysics to simulate a few isolated non-precipitating trade cumulus clouds (in different sizes and aerosol loading) in high resolution (10m). Passive tracer was added to all the simulations. We found cloudy volumes that contain both high tracer concentration and high AF (up to the clouds&#8217; top), compared these two measures of mixing, and discuss their differences. The accuracy of AF calculations, based on different known methods is tested. For example, we show that the saturation adjustment assumption that is often used in AF calculations can lead to an underestimation of AF in pristine environments. This will mask microphysical effects and cause biases when comparing the adiabaticity of clouds under different aerosols loading. We show that the space spanned by the AF versus height in the cloud is a good measure for describing changes in cloud&#8217;s key variables in space and time (like temperature, updraft, and DSD properties). This space of AF vs height demonstrates how certain processes (e.g. in-cloud nucleation, mixing, evaporation, etc.) dominate different regions in the cloud (core, edge), and cause different dependence of the DSD on AF under different aerosols loading.

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EVALUATION OF CLOUDS, RADIATION, AND PRECIPITATION IN CMIP6 MODELS USING GLOBAL WEATHER STATES DERIVED FROM ISCCP-H CLOUD PROPERTY DATA

Journal of Climate ◽

10.1175/jcli-d-21-0076.1 ◽

2021 ◽

pp. 1-42

Author(s):

George Tselioudis ◽

William. B. Rossow ◽

Christian Jakob ◽

Jasmine Remillard ◽

Derek Tropf ◽

...

Keyword(s):

Cumulus Clouds ◽

Cloud Optical Depth ◽

Cloud Properties ◽

Shallow Cumulus ◽

Property Data ◽

Ensemble Mean ◽

Stratocumulus Clouds ◽

Model Cloud ◽

Adequate Representation ◽

Cloud Top Pressure

AbstractA clustering methodology is applied to cloud optical depth cloud top pressure (TAU-PC) histograms from the new, 1-degree resolution, ISCCP-H dataset, to derive an updated global Weather State (WS) dataset. Then, PC-TAU histograms from current-climate CMIP6 model simulations are assigned to the ISCCP-H WSs along with their concurrent radiation and precipitation properties, to evaluate model cloud, radiation, and precipitation properties in the context of the Weather States. The new ISCCP-H analysis produces WSs that are very similar to those previously found in the lower resolution ISCCP-D dataset. The main difference lies in the splitting of the ISCCP-D thin stratocumulus WS between the ISCCP-H shallow cumulus and stratocumulus WSs, which results in the reduction by one of the total WS number. The evaluation of the CMIP6 models against the ISCCP-H Weather States, shows that, in the ensemble mean, the models are producing an adequate representation of the frequency and geographical distribution of the WSs, with measurable improvements compared to the WSs derived for the CMIP5 ensemble. However, the frequency of shallow cumulus clouds continues to be underestimated, and, in some WSs the good agreement of the ensemble mean with observations comes from averaging models that significantly overpredict and underpredict the ISCCP-H WS frequency. In addition, significant biases exist in the internal cloud properties of the model WSs, such as the model underestimation of cloud fraction in middle-top clouds and secondarily in midlatitude storm and stratocumulus clouds, that result in an underestimation of cloud SW cooling in those regimes.

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Statistical analysis of an LES shallow cumulus cloud ensemble using a cloud tracking algorithm

Atmospheric Chemistry and Physics ◽

10.5194/acp-12-1101-2012 ◽

2012 ◽

Vol 12 (2) ◽

pp. 1101-1119 ◽

Cited By ~ 35

Author(s):

J. T. Dawe ◽

P. H. Austin

Keyword(s):

Thermodynamic Properties ◽

Cloud Base ◽

Cumulus Cloud ◽

Cumulus Clouds ◽

Base Area ◽

Eddy Simulation ◽

Cloud Properties ◽

Shallow Cumulus ◽

Cloud Height ◽

Upper Level

Abstract. A technique for the tracking of individual clouds in a Large Eddy Simulation (LES) is presented. We use this technique on an LES of a shallow cumulus cloud field based upon the Barbados Oceanographic and Meteorological Experiment (BOMEX) to calculate statistics of cloud height, lifetime, and other physical properties for individual clouds in the model. We also examine the question of nature versus nurture in shallow cumulus clouds: do properties at cloud base determine the upper-level properties of the clouds (nature), or are cloud properties determined by the environmental conditions they encounter (nurture). We find that clouds which ascend through an environment that has been pre-moistened by previous cloud activity are no more likely to reach the inversion than clouds that ascend through a drier environment. Cloud base thermodynamic properties are uncorrelated with upper-level cloud properties, while mean fractional entrainment and detrainment rates display moderate correlations with cloud properties up to the inversion. Conversely, cloud base area correlates well with upper-level cloud area and maximum cloud height. We conclude that cloud thermodynamic properties are primarily influenced by entrainment and detrainment processes, cloud area and height are primarily influenced by cloud base area, and thus nature and nurture both play roles in the dynamics of BOMEX shallow cumulus clouds.

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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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Horizontal geometry of trade-wind cumuli – aircraft observations from shortwave infrared imager versus radar profiler

10.5194/amt-2021-318 ◽

2021 ◽

Author(s):

Henning Dorff ◽

Heike Konow ◽

Felix Ament

Keyword(s):

High Resolution ◽

Size Distribution ◽

Wind Field ◽

Trade Wind ◽

Size Spectrum ◽

Lower Sensitivity ◽

Cumulus Clouds ◽

Research Aircraft ◽

Shortwave Infrared ◽

Along Track

Abstract. This study elaborates how aircraft-based horizontal geometries of trade-wind cumulus clouds differ whether a one-dimensional (1D) profiler or a two-dimensional (2D) imager is used. While nadir profiling devices are limited to 1D realisation of the cloud transect size with limited representativeness of horizontal cloud extension, 2D imagers enhance our perspectives by mapping the horizontal cloud field. Both require high-resolution to detect the lower end of the cloud size spectrum. In this regard, the payload aboard the High Altitude and Long Range Research Aircraft (HALO) achieves a comparison and also a synergy of both measurement systems. Using the NARVAL-II campaign, we combine HALO observations from a 35.2 GHz cloud and precipitation radar (1D) and from the hyperspectral 2D imager specMACS, having a 30 times higher along-track resolution and compare their cloud masks. We examine cloud size distributions in terms of sensitivity to sample size, resolution and the considered field of view (2D or 1D). This specifies impacts on horizontal cloud sizes derived from the across-track perspective of the high-resolution imager in comparison to the radar curtain. We assess whether and how the trade-wind field amplifies uncertainties in cloud geometry observations along 1D transects through directional cloud elongation. Our findings reveal that each additional dimension, no matter of the device, causes a significant increase of observed clouds. The across-track field yields the highest increase in the cloud sample. The radar encounters difficulties to characterize the trade-wind cumuli size distribution. More than 60 % of clouds are subgrid scale for the radar. While the radar cannot resolve clouds shorter than 200 m and has a lower sensitivity, the amount of small invisible clouds leads to deviations in the size distribution. Double power law characteristics in the imager based cloud size distribution do not occur in radar observations. Along-track measurements do not necessarily cover the predominant cloud extent and inferred geometries lack of representativeness. Trade-wind cumuli show horizontal patterns similar to ellipses with a mean aspect ratio of 3 : 2. Instead of circular estimations based on the 1D transect, elliptic fits maintain the cloud area size distribution. Increasing wind speed tends to stretch clouds more and tilts them into the wind field, which makes transect measurements more representative along this axis.

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Identifying the radiative ‘twilight zone’ surrounding shallow cumulus clouds from high-resolution ASTER observations

10.5194/egusphere-egu2020-19086 ◽

2020 ◽

Author(s):

Theresa Mieslinger ◽

Manfred Brath ◽

Stefan A. Buehler ◽

Bjorn Stevens

Keyword(s):

High Resolution ◽

Satellite Images ◽

Twilight Zone ◽

Radiative Effect ◽

Cumulus Clouds ◽

Tropical Ocean ◽

Clear Sky ◽

Shallow Cumulus ◽

Precise Knowledge ◽

High Uncertainty

The uncertain radiative effect of shallow cumulus clouds over tropical ocean significantly contributes to the high uncertainty in climate sensitivity estimates. Radiances corresponding to clear-sky and cloudy areas can be observed in moderate resolution satellite images. To observe the radiance originating from very small clouds and from the transition zone surrounding shallow cumulus clouds, the &#8216;twilight zone&#8217;, high-resolution data is required. Twilight zone radiances can be higher than clear-sky radiances due to unresolved cloud fragments and/or humidified aerosols. The area of the twilight zone depends on the resolution of the underlying data. If we think of the twilight zone in terms of partially cloudy pixels, such an area results in a high uncertainty in cloud and aerosol retrievals, as they are based on cloudy and clear-sky assumptions respectively. A precise knowledge of radiances from clouds and their twilight zone is decisive in terms of the total cloud reflectance and subsequently the shallow cumulus cloud radiative effect, which climate models struggle to properly simulate.We therefore investigate the abundance and importance of such a twilight zone from high-resolution satellite images from ASTER recoded previously and during the EUREC4A field campaign. We use radiative transfer simulations to model the contribution of background clear-sky radiances including aerosols. Subtracting known clear-sky radiances from observed cloud field radiances leaves us with a precise knowledge of non-clear-sky radiances originating from shallow cumulus clouds and their surrounding twilight zone.

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The Importance of the Shape of Cloud Droplet Size Distributions in Shallow Cumulus Clouds. Part I: Bin Microphysics Simulations

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-15-0382.1 ◽

2017 ◽

Vol 74 (1) ◽

pp. 249-258 ◽

Cited By ~ 18

Author(s):

Adele L. Igel ◽

Susan C. van den Heever

Keyword(s):

Droplet Size ◽

Cloud Droplet ◽

Relative Width ◽

Size Distributions ◽

Cumulus Clouds ◽

Microphysics Scheme ◽

Droplet Concentration ◽

Shallow Cumulus ◽

Bin Microphysics ◽

Droplet Size Distributions

Abstract In this two-part study, the relationships between the width of the cloud droplet size distribution and the microphysical processes and cloud characteristics of nonprecipitating shallow cumulus clouds are investigated using large-eddy simulations. In Part I, simulations are run with a bin microphysics scheme and the relative widths (standard deviation divided by mean diameter) of the simulated cloud droplet size distributions are calculated. They reveal that the value of the relative width is higher and less variable in the subsaturated regions of the cloud than in the supersaturated regions owing to both the evaporation process itself and enhanced mixing and entrainment of environmental air. Unlike in some previous studies, the relative width is not found to depend strongly on the initial aerosol concentration or mean droplet concentration. Nonetheless, local values of the relative width are found to positively correlate with local values of the droplet concentrations, particularly in the supersaturated regions of clouds. In general, the distributions become narrower as the local droplet concentration increases, which is consistent with the difference in relative width between the supersaturated and subsaturated cloud regions and with physically based expectations. Traditional parameterizations for the relative width (or shape parameter, a related quantity) of cloud droplet size distributions in bulk microphysics schemes are based on cloud mean values, but the bin simulation results shown here demonstrate that more appropriate parameterizations should be based on the relationship between the local values of the relative width and the cloud droplet concentration.

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Shallow Cumulus Mixing and Subcloud-Layer Responses to Variations in Aerosol Loading

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-13-0352.1 ◽

2014 ◽

Vol 71 (7) ◽

pp. 2581-2603 ◽

Cited By ~ 22

Author(s):

Robert B. Seigel

Keyword(s):

Potential Temperature ◽

Domain Size ◽

Aerosol Concentration ◽

Trade Wind ◽

Cold Pool ◽

Cumulus Clouds ◽

Core Mass ◽

Shallow Cumulus ◽

Rise Rate ◽

Cloud Cores

Abstract The sensitivity of lightly precipitating trade wind shallow cumulus to both aerosol concentration and domain size is investigated using large-eddy simulations (LESs). The mean states of liquid water potential temperature, total water, and velocity field exhibit negligible change between all LES runs, offering the perfect opportunity to investigate microphysical–dynamical interactions solely due to variations in aerosol concentration and not changes in meteorology. As aerosol concentration increases, two cloud population responses are found: 1) cloud and cloud-core widths decrease while their strength increases and 2) cloud and core numbers increase. The narrowing of the polluted clouds is caused by enhanced evaporation rates surrounding the cloud cores, which in turn shrinks the diameter of the cumulus toroidal circulation. The more narrow toroidal circulation in polluted clouds has a faster rise rate and imparts weaker dynamical entrainment on the cloud cores, resulting in stronger clouds as aerosol concentration increases. The reduction in cloud number for more pristine conditions occurs from greater cold pool coverage, reducing the likelihood of subcloud-layer thermals reaching their lifting condensation level. The increase in cloud number as aerosol concentration increases is compensated by narrower and stronger clouds, resulting in a cumulus-core mass flux that appears to be unaffected by aerosol concentration variability. For the weakly precipitating case studied here, the trends in the response of the cumulus clouds to aerosol concentration are found to be insensitive to domain size.

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Revisiting adiabatic fraction estimations in cumulus clouds: high-resolution simulations with a passive tracer

Atmospheric Chemistry and Physics ◽

10.5194/acp-21-16203-2021 ◽

2021 ◽

Vol 21 (21) ◽

pp. 16203-16217

Author(s):

Eshkol Eytan ◽

Ilan Koren ◽

Orit Altaratz ◽

Mark Pinsky ◽

Alexander Khain

Keyword(s):

High Resolution ◽

Accurate Description ◽

Cloud Base ◽

Passive Tracer ◽

Cumulus Clouds ◽

Microphysics Scheme ◽

Convective Clouds ◽

Open Questions ◽

Bin Microphysics ◽

Normalized Concentration

Abstract. The process of mixing in warm convective clouds and its effects on microphysics are crucial for an accurate description of cloud fields, weather, and climate. Still, they remain open questions in the field of cloud physics. Adiabatic regions in the cloud could be considered non-mixed areas and therefore serve as an important reference to mixing. For this reason, the adiabatic fraction (AF) is an important parameter that estimates the mixing level in the cloud in a simple way. Here, we test different methods of AF calculations using high-resolution (10 m) simulations of isolated warm cumulus clouds. The calculated AFs are compared with a normalized concentration of a passive tracer, which is a measure of dilution by mixing. This comparison enables the examination of how well the AF parameter can determine mixing effects and the estimation of the accuracy of different approaches used to calculate it. Comparison of three different methods to derive AF, with the passive tracer, shows that one method is much more robust than the others. Moreover, this method's equation structure also allows for the isolation of different assumptions that are often practiced when calculating AF such as vertical profiles, cloud-base height, and the linearity of AF with height. The use of a detailed spectral bin microphysics scheme allows an accurate description of the supersaturation field and demonstrates that the accuracy of the saturation adjustment assumption depends on aerosol concentration, leading to an underestimation of AF in pristine environments.

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