scholarly journals Extracting Microphysical Impacts in Large-Eddy Simulations of Shallow Convection

2014 ◽  
Vol 71 (12) ◽  
pp. 4493-4499 ◽  
Author(s):  
Wojciech W. Grabowski
Keyword(s):  
Traditional Approach ◽  
Cloud Droplet ◽  
Cloud Microphysics ◽  
Natural Variability ◽  
Large Eddy Simulations ◽  
Shallow Convection ◽  
Liquid Water Path ◽  
Large Eddy ◽  
The Mean ◽  
The Impact

Abstract A simple methodology is proposed to extract impacts of cloud microphysics on macrophysical cloud-field properties in large-eddy simulations of shallow convection. These impacts are typically difficult to assess because of natural variability of the simulated cloud field. The idea is to use two sets of thermodynamic variables driven by different microphysical schemes or by a single scheme with different parameters as applied here. The first set is coupled to the dynamics as in the standard model, and the second set is applied diagnostically—that is, driven by the flow but without the feedback on the flow dynamics. Having the two schemes operating in the same flow pattern allows for extracting the impact with high confidence. For illustration, the method is applied to simulations of precipitating shallow convection applying a simple bulk representation of warm-rain processes. Because of natural variability, the traditional approach provides an uncertain estimate of the impact of cloud droplet concentration on the mean cloud-field rainfall even with an ensemble of simulations. In contrast, the impact is well constrained while applying the new methodology. The method can even detect minuscule changes of the mean cloud cover and liquid water path despite their large temporal fluctuations and different evolutions within the ensemble.

scholarly journals Large-Eddy Simulations of a Drizzling, Stratocumulus-Topped Marine Boundary Layer

2009 ◽  
Vol 137 (3) ◽  
pp. 1083-1110 ◽  
Author(s):  
Andrew S. Ackerman ◽  
Margreet C. vanZanten ◽  
Bjorn Stevens ◽  
Verica Savic-Jovcic ◽  
Christopher S. Bretherton ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Marine Boundary Layer ◽  
Cloud Water ◽  
Large Eddy Simulations ◽  
Cloud Base ◽  
Entrainment Rate ◽  
Liquid Water Path ◽  
Average Maximum ◽  
Large Eddy ◽  
The Mean

Abstract Cloud water sedimentation and drizzle in a stratocumulus-topped boundary layer are the focus of an intercomparison of large-eddy simulations. The context is an idealized case study of nocturnal stratocumulus under a dry inversion, with embedded pockets of heavily drizzling open cellular convection. Results from 11 groups are used. Two models resolve the size distributions of cloud particles, and the others parameterize cloud water sedimentation and drizzle. For the ensemble of simulations with drizzle and cloud water sedimentation, the mean liquid water path (LWP) is remarkably steady and consistent with the measurements, the mean entrainment rate is at the low end of the measured range, and the ensemble-average maximum vertical wind variance is roughly half that measured. On average, precipitation at the surface and at cloud base is smaller, and the rate of precipitation evaporation greater, than measured. Including drizzle in the simulations reduces convective intensity, increases boundary layer stratification, and decreases LWP for nearly all models. Including cloud water sedimentation substantially decreases entrainment, decreases convective intensity, and increases LWP for most models. In nearly all cases, LWP responds more strongly to cloud water sedimentation than to drizzle. The omission of cloud water sedimentation in simulations is strongly discouraged, regardless of whether or not precipitation is present below cloud base.

scholarly journals Broadening of Cloud Droplet Spectra through Eddy Hopping: Turbulent Entraining Parcel Simulations

2018 ◽  
Vol 75 (10) ◽  
pp. 3365-3379 ◽  
Author(s):  
Gustavo C. Abade ◽  
Wojciech W. Grabowski ◽  
Hanna Pawlowska
Keyword(s):  
Droplet Size ◽  
Turbulent Mixing ◽  
Cloud Condensation Nuclei ◽  
Cloud Droplet ◽  
Cloud Microphysics ◽  
Droplet Growth ◽  
Size Spectrum ◽  
Entrainment Rate ◽  
The Mean ◽  
The Impact

This paper discusses the effects of cloud turbulence, turbulent entrainment, and entrained cloud condensation nuclei (CCN) activation on the evolution of the cloud droplet size spectrum. We simulate an ensemble of idealized turbulent cloud parcels that are subject to entrainment events modeled as a random process. Entrainment events, subsequent turbulent mixing inside the parcel, supersaturation fluctuations, and the resulting stochastic droplet activation and growth by condensation are simulated using a Monte Carlo scheme. Quantities characterizing the turbulence intensity, entrainment rate, CCN concentration, and the mean fraction of environmental air entrained in an event are all specified as independent external parameters. Cloud microphysics is described by applying Lagrangian particles, the so-called superdroplets. These are either unactivated CCN or cloud droplets that grow from activated CCN. The model accounts for the addition of environmental CCN into the cloud by entraining eddies at the cloud edge. Turbulent mixing of the entrained dry air with cloudy air is described using the classical linear relaxation to the mean model. We show that turbulence plays an important role in aiding entrained CCN to activate, and thus broadening the droplet size distribution. These findings are consistent with previous large-eddy simulations (LESs) that consider the impact of variable droplet growth histories on the droplet size spectra in small cumuli. The scheme developed in this work is ready to be used as a stochastic subgrid-scale scheme in LESs of natural clouds.

The Strong Impact of Weak Horizontal Convergence on Continental Shallow Convection

2020 ◽  
Vol 77 (9) ◽  
pp. 3119-3137
Author(s):  
Marcin J. Kurowski ◽  
Wojciech W. Grabowski ◽  
Kay Suselj ◽  
João Teixeira
Keyword(s):  
Large Scale ◽  
Cloud Microphysics ◽  
Cloud Base ◽  
Small Scale ◽  
Strong Impact ◽  
Shallow Convection ◽  
Liquid Water Path ◽  
Strong Response ◽  
The Impact ◽  
Benchmark Solutions

Abstract Idealized large-eddy simulation (LES) is a basic tool for studying three-dimensional turbulence in the planetary boundary layer. LES is capable of providing benchmark solutions for parameterization development efforts. However, real small-scale atmospheric flows develop in heterogeneous and transient environments with locally varying vertical motions inherent to open multiscale interactive dynamical systems. These variations are often too subtle to detect them by state-of-the-art remote and in situ measurements, and are typically excluded from idealized simulations. The present study addresses the impact of weak [i.e., O(10−6) s−1] short-lived low-level large-scale convergence/divergence perturbations on continental shallow convection. The results show a strong response of shallow nonprecipitating convection to the applied weak large-scale dynamical forcing. Evolutions of CAPE, mean liquid water path, and cloud-top heights are significantly affected by the imposed convergence/divergence. In contrast, evolving cloud-base properties, such as the area coverage and mass flux, are only weakly affected. To contrast those impacts with microphysical sensitivity, the baseline simulations are perturbed assuming different observationally based cloud droplet number concentrations and thus different rainfall. For the tested range of microphysical perturbations, the imposed convergence/divergence provides significantly larger impact than changes in the cloud microphysics. Simulation results presented here provide a stringent test for convection parameterizations, especially important for large-scale models progressing toward resolving some nonhydrostatic effects.

scholarly journals Indirect Impact of Atmospheric Aerosols in Idealized Simulations of Convective–Radiative Quasi Equilibrium. Part II: Double-Moment Microphysics

2011 ◽  
Vol 24 (7) ◽  
pp. 1897-1912 ◽  
Author(s):  
Wojciech W. Grabowski ◽  
Hugh Morrison
Keyword(s):  
Atmospheric Aerosols ◽  
Cloud Condensation Nuclei ◽  
Cloud Droplet ◽  
Bowen Ratio ◽  
Cloud Microphysics ◽  
Quasi Equilibrium ◽  
Double Moment ◽  
The Mean ◽  
The Difference ◽  
The Impact

Abstract This paper extends the previous cloud-resolving modeling study concerning the impact of cloud microphysics on convective–radiative quasi equilibrium (CRQE) over a surface with fixed characteristics and prescribed solar input, both mimicking the mean conditions on earth. The current study applies sophisticated double-moment warm-rain and ice microphysics schemes, which allow for a significantly more realistic representation of the impact of aerosols on precipitation processes and on the coupling between clouds and radiative transfer. Two contrasting cloud condensation nuclei (CCN) characteristics are assumed, representing pristine and polluted conditions, as well as contrasting representations of the effects of entrainment and mixing on the mean cloud droplet size. In addition, four sets of sensitivity simulations are also performed with changes that provide a reference for the main simulation set. As in the previous study, the CRQE mimics the estimates of globally and annually averaged water and energy fluxes across the earth’s atmosphere. There are some differences from the previous study, however, consistent with the slightly lower water vapor content in the troposphere and significantly reduced lower-tropospheric cloud fraction in current simulations. There is also a significant reduction of the difference between the pristine and polluted cases, from ∼20 to ∼4 W m−2 at the surface from ∼20 to ∼9 W m−2 at the top of the atmosphere (TOA). The difference between the homogeneous and extremely inhomogeneous mixing scenarios, ∼20 W m−2 in the previous study, is reduced to a mere 2 (1) W m−2 at the surface (TOA). An unexpected difference between the previous and current simulations is the lower Bowen ratio of the surface heat flux, the partitioning of the total flux into sensible and latent components. It is shown that most of the change comes from the difference in the representation of rain evaporation in the subcloud layer in the single- and double-moment microphysics schemes. The difference affects the mean air temperature and humidity near the surface, and thus the Bowen ratio. The differences between the various simulations are discussed, contrasting the process-level approach with the impact of cloud microphysics on the quasi-equilibrium state with a more appropriate system dynamics approach. The key distinction is that the latter includes the interactions among all the processes in the modeled system.

scholarly journals Modeling Condensation in Shallow Nonprecipitating Convection

2015 ◽  
Vol 72 (12) ◽  
pp. 4661-4679 ◽  
Author(s):  
Wojciech W. Grabowski ◽  
Dorota Jarecka
Keyword(s):  
Cloud Droplet ◽  
Cloud Microphysics ◽  
Cloud Water ◽  
Cloud Base ◽  
Shallow Convection ◽  
Cloud Dynamics ◽  
Wide Range ◽  
High Vertical Resolution ◽  
The Impact ◽  
Theoretical Considerations

Abstract Two schemes for modeling condensation in warm nonprecipitating clouds are compared. The first one is the efficient bulk condensation scheme where cloudy volumes are always at saturation and cloud water evaporates instantaneously to maintain saturation. The second one is the comprehensive bin condensation scheme that predicts the evolution of the cloud droplet spectrum and allows sub- and supersaturations in cloudy volumes. The emphasis is on the impact of the two schemes on cloud dynamics. Theoretical considerations show that the bulk condensation scheme provides more buoyancy than the bin scheme, but the effect is small, with the potential density temperature difference around 0.1 K for 1% supersaturation. The 1D advection–condensation tests document the high-vertical-resolution requirement for the bin scheme to resolve the cloud-base supersaturation maximum and CCN activation, which is difficult to employ in 3D cloud simulations. Simulations of shallow convection cloud fields are executed applying bulk and bin schemes, with the mean droplet concentrations in the bin scheme covering a wide range, from about 5 to over 4000 cm−3. Simulations employ the microphysical piggybacking methodology to extract impacts with high confidence. They show that the differences in cloud fields simulated with bulk and bin schemes come not from small differences in the condensation but from more significant differences in the evaporation of cloud water near cloud edges as a result of entrainment and mixing with the environment. The latter makes the impact of cloud microphysics on simulated macroscopic cloud field properties even more difficult to assess because of highly uncertain subgrid-scale parameterizations.

Indirect Impact of Atmospheric Aerosols in Idealized Simulations of Convective–Radiative Quasi Equilibrium

2006 ◽  
Vol 19 (18) ◽  
pp. 4664-4682 ◽  
Author(s):  
Wojciech W. Grabowski
Keyword(s):  
Solar Energy ◽  
Indirect Effect ◽  
Cloud Microphysics ◽  
Effective Radius ◽  
Future Research ◽  
Quasi Equilibrium ◽  
Shallow Convection ◽  
Cloud Droplets ◽  
The Mean ◽  
The Impact

Abstract This paper discusses a cloud-resolving modeling study concerning the impact of warm-rain microphysics on convective–radiative quasi equilibrium with fixed surface characteristics and prescribed solar input, both mimicking the mean conditions on earth. Two limits of the concentration of cloud droplets, either 100 cm−3 (referred to as “pristine”) or 1000 cm−3 (referred to as “polluted”), are considered. In addition, three formulations of the effective radius of water droplets in diluted cloudy volumes are used, corresponding to the homogeneous, intermediate, and extremely inhomogeneous mixing scenarios. The assumed concentration of cloud droplets, together with the assumed mixing scenario, affects the local value of the effective radius of cloud droplets (the first indirect aerosol effect, also known as the Twomey effect) and the transfer of cloud water into drizzle and rain, which can affect the mean cloudiness and the hydrologic cycle (the second indirect effect). The convective–radiative quasi equilibrium mimics the estimates of globally and annually averaged water and energy fluxes across the earth’s atmosphere to within less than 10 W m−2. As on earth, the model cloudiness is dominated by shallow convection. It is found that the impact of warm microphysics is dominated by the first indirect effect, whereas the second indirect effect has a smaller impact. The assumed droplet concentration and mixing scenario impact the mean “planetary” albedo and, thus, the amount of solar energy reaching the surface, with all other components of atmospheric energy and water budgets virtually the same in all simulations. The weak second indirect effect highlights the difference between the impact of cloud microphysics on a single cloud and the impact on an ensemble of clouds, with only the latter including the feedbacks between clouds and their environment. The formulation of the effective radius in the diluted cloudy volumes turns out to be of critical importance, with the amount of solar energy reaching the surface being the same in the pristine case assuming the homogeneous mixing scenario and in the polluted case with the extremely inhomogeneous mixing. This result emphasizes the essential role of poorly understood microphysical transformations within diluted convective clouds, which strongly impact the magnitude of the first indirect (Twomey) effect. Implications for future research in this area are discussed.

scholarly journals The relative importance of macrophysical and cloud albedo changes for aerosol induced radiative effects in stratocumulus

2016 ◽  
Author(s):  
Daniel P. Grosvenor ◽  
Paul R. Field ◽  
Adrian A. Hill ◽  
Benjamin J. Shipway
Keyword(s):  
Mean Value ◽  
Cloud Microphysics ◽  
Liquid Water Path ◽  
Frequency Distributions ◽  
Cloud Albedo ◽  
Aerosol Loading ◽  
Stratocumulus Clouds ◽  
Order Of Magnitude ◽  
The Mean ◽  
The Impact

Abstract. Aerosol-cloud interactions are explored using 1 km resolution simulations of SE Pacific stratocumulus clouds that include realistic meteorology along with newly implemented cloud microphysics and sub-grid cloud schemes. The model was critically assessed against observations of Liquid Water Path (LWP), broadband fluxes, cloud fraction (fc), droplet number concentrations (Nd) and radar reflectivities. Aerosol loading sensitivity tests showed that at low aerosol loadings, changes to aerosol affected shortwave fluxes equally through changes to cloud macrophysical charateristics (LWP, fc) and cloud albedo changes due solely to Nd changes. However, at high aerosol loadings, only the Nd albedo change was important. Evidence was also provided to show that a treatment of sub-grid clouds is as important as order of magnitude changes in aerosol loading for the accurate simulation of stratocumulus at this grid resolution. Overall, the control model demonstrated a credible ability to reproduce observations suggesting that many of the important physical processes for accurately simulating these clouds are represented within the model and giving some confidence in the predictions of the model concerning stratocumulus and the impact of aerosol. For example, the control run was able to reproduce the shape and magnitude of the observed diurnal cycle of domain mean LWP to within ~ 10 g m−2 for the nighttime, but with an overestimate for the daytime of up to 30 g m−2. The latter was attributed to the uniform aerosol fields imposed on the model, which meant that the model failed to include the low Nd mode that was observed further offshore, preventing the LWP removal through precipitation that likely occurred in reality. The boundary layer was too low by around 260 m, which was attributed to the driving global model analysis. The shapes and sizes of the observed bands of clouds and open-cell-like regions of low areal cloud cover were qualitatively captured. The daytime fc frequency distribution was reproduced to within fc = 0.04 for fc > ~ 0.7 as was the domain mean nighttime fc (at a single time) to within fc = 0.02. Frequency distributions of shortwave top-of-the-atmosphere (TOA) fluxes from satellite were well represented by the model with only a slight underestimate of the mean by 15 %; this was attributed to near--shore aerosol concentrations that were too low for the particular times of the satellite overpasses. TOA longwave flux distributions were close to those from satellite with agreement of the mean value to within 0.4 %. From comparisons of Nd distributions to those from satellite it was found that the Nd mode from the model agreed with the higher of the two observed modes to within ~ 15 %.

scholarly journals The relative importance of macrophysical and cloud albedo changes for aerosol-induced radiative effects in closed-cell stratocumulus: insight from the modelling of a case study

2017 ◽  
Vol 17 (8) ◽  
pp. 5155-5183 ◽  
Author(s):  
Daniel P. Grosvenor ◽  
Paul R. Field ◽  
Adrian A. Hill ◽  
Benjamin J. Shipway
Keyword(s):  
Mean Value ◽  
Cloud Microphysics ◽  
Liquid Water Path ◽  
Frequency Distributions ◽  
Cloud Albedo ◽  
Closed Cell ◽  
The Mean ◽  
Wave Flux ◽  
The Impact

Abstract. Aerosol–cloud interactions are explored using 1 km simulations of a case study of predominantly closed-cell SE Pacific stratocumulus clouds. The simulations include realistic meteorology along with newly implemented cloud microphysics and sub-grid cloud schemes. The model was critically assessed against observations of liquid water path (LWP), broadband fluxes, cloud fraction (fc), droplet number concentrations (Nd), thermodynamic profiles, and radar reflectivities.Aerosol loading sensitivity tests showed that at low aerosol loadings, changes to aerosol affected shortwave fluxes equally through changes to cloud macrophysical characteristics (LWP, fc) and cloud albedo changes due solely to Nd changes. However, at high aerosol loadings, only the Nd albedo change was important. Evidence was also provided to show that a treatment of sub-grid clouds is as important as order of magnitude changes in aerosol loading for the accurate simulation of stratocumulus at this grid resolution.Overall, the control model demonstrated a credible ability to reproduce observations, suggesting that many of the important physical processes for accurately simulating these clouds are represented within the model and giving some confidence in the predictions of the model concerning stratocumulus and the impact of aerosol. For example, the control run was able to reproduce the shape and magnitude of the observed diurnal cycle of domain mean LWP to within  ∼  10 g m−2 for the nighttime, but with an overestimate for the daytime of up to 30 g m−2. The latter was attributed to the uniform aerosol fields imposed on the model, which meant that the model failed to include the low-Nd mode that was observed further offshore, preventing the LWP removal through precipitation that likely occurred in reality. The boundary layer was too low by around 260 m, which was attributed to the driving global model analysis. The shapes and sizes of the observed bands of clouds and open-cell-like regions of low areal cloud cover were qualitatively captured. The daytime fc frequency distribution was reproduced to within Δfc = 0.04 for fc >  ∼ 0.7 as was the domain mean nighttime fc (at a single time) to within Δfc = 0.02. Frequency distributions of shortwave top-of-the-atmosphere (TOA) fluxes from the satellite were well represented by the model, with only a slight underestimate of the mean by 15 %; this was attributed to near–shore aerosol concentrations that were too low for the particular times of the satellite overpasses. TOA long-wave flux distributions were close to those from the satellite with agreement of the mean value to within 0.4 %. From comparisons of Nd distributions to those from the satellite, it was found that the Nd mode from the model agreed with the higher of the two observed modes to within  ∼  15 %.

scholarly journals MBL drizzle properties and their impact on cloud property retrievals

2015 ◽  
Vol 8 (4) ◽  
pp. 4307-4323
Author(s):  
P. Wu ◽  
X. Dong ◽  
B. Xi
Keyword(s):  
Liquid Water ◽  
Cloud Droplet ◽  
Number Concentration ◽  
Effective Radius ◽  
Liquid Water Path ◽  
Water Path ◽  
Cloud Property ◽  
Annual Means ◽  
Mean Differences ◽  
The Impact

Abstract. In this study, we retrieve and document drizzle properties, and investigate the impact of drizzle on cloud property retrievals from ground-based measurements at the ARM Azores site from June 2009 to December 2010. For the selected cloud and drizzle samples, the drizzle occurrence is 42.6% with a maximum of 55.8% in winter and a minimum of 35.6% in summer. The annual means of drizzle liquid water path LWPd, effective radius rd, and number concentration Nd for the rain (virga) samples are 5.48 (1.29) g m−2, 68.7 (39.5) μm, and 0.14 (0.38) cm−3. The seasonal mean LWPd values are less than 4% of the MWR-retrieved LWP values. The annual mean differences in cloud-droplet effective radius with and without drizzle are 0.12 and 0.38 μm, respectively, for the virga and rain samples. Therefore, we conclude that the impact of drizzle on cloud property retrievals is insignificant at the ARM Azores site.

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