scholarly journals The effects of turbulent collision-coalescence on precipitation formation and precipitation-dynamical feedbacks in simulations of stratocumulus and shallow cumulus convection

2014 ◽  
Vol 14 (2) ◽  
pp. 2277-2306
Author(s):  
C. N. Franklin
Keyword(s):  
Liquid Water ◽  
Cloud Droplet ◽  
Shallow Convection ◽  
Aerosol Effect ◽  
Shallow Cumulus ◽  
Turbulence Effects ◽  
Double Moment ◽  
Sensitivity Studies ◽  
Microphysical Processes ◽  
The Impact

Abstract. A double moment warm rain scheme that includes the effects of turbulence on droplet collision rates has been implemented in a large-eddy model to investigate the impact of turbulence effects on clouds and precipitation. Simulations of shallow cumulus and stratocumulus show that different precipitation-dynamical feedbacks occur in these regimes when the effects of turbulence are included in the microphysical processes. In both cases, inclusion of turbulent microphysics increases precipitation due to a more rapid conversion of cloud water to rain. In the shallow convection case, the greater water loading and latent heating in the upper cloud levels reduces the buoyancy production of turbulent kinetic energy and the entrainment. The stratocumulus case on the other hand shows a positive precipitation feedback, with enhanced rainwater producing greater evaporation, stronger circulations and more turbulence. Sensitivity studies where the cloud droplet number was varied show that greater number concentrations suppress the stratocumulus precipitation leading to larger liquid water paths. This positive second indirect aerosol effect was produced in all of the simulations except for the case using the turbulent microphysics with the highest droplet number, which suggests a limit on the amount of liquid water that can be produced. While the sign of the second indirect effect is negative in the shallow convection case whether the effects of turbulence are considered or not, the magnitude of the effect is doubled when the turbulent microphysics are used.

scholarly journals The effects of turbulent collision–coalescence on precipitation formation and precipitation-dynamical feedbacks in simulations of stratocumulus and shallow cumulus convection

2014 ◽  
Vol 14 (13) ◽  
pp. 6557-6570 ◽  
Author(s):  
C. N. Franklin
Keyword(s):  
Cloud Droplet ◽  
Shallow Convection ◽  
Microphysics Scheme ◽  
Droplet Collision ◽  
Shallow Cumulus ◽  
Turbulence Effects ◽  
Double Moment ◽  
Sensitivity Studies ◽  
Microphysical Processes ◽  
The Impact

Abstract. A double moment warm rain scheme that includes the effects of turbulence on droplet collision rates has been implemented in a large-eddy model to investigate the impact of turbulence effects on clouds and precipitation. Simulations of shallow cumulus and stratocumulus show that different precipitation-dynamical feedbacks occur in these regimes when the effects of turbulence are included in the microphysical processes. In both cases inclusion of turbulent microphysics increases precipitation due to a more rapid conversion of cloud water to rain. In the shallow convection case, the greater water loading in the upper cloud levels reduces the buoyancy production of turbulent kinetic energy and the entrainment. The stratocumulus case on the other hand shows a weak positive precipitation feedback, with enhanced rainwater producing greater evaporation, stronger circulations and more turbulence. Sensitivity studies in which the cloud droplet number was varied show that greater number concentrations suppress the stratocumulus precipitation leading to larger liquid water paths. This positive second indirect aerosol effect shows no sensitivity to whether or not the effects of turbulence on droplet collision rates are included. While the sign of the second indirect effect is negative in the shallow convection case whether the effects of turbulence are considered or not, the magnitude of the effect is doubled when the turbulent microphysics are used. It is found that for these two different cloud regimes turbulence has a larger effect than cloud droplet number and the use of a different bulk microphysics scheme on producing rainfall in shallow cumuli. However, for the stratocumulus case examined here, the effects of turbulence on rainfall are not statistically significant and instead it is the cloud droplet number concentration or the choice of bulk microphysics scheme that has the largest control on the rain water.

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.

scholarly journals Vertical redistribution of moisture and aerosol in orographic mixed-phase clouds

2020 ◽  
Vol 20 (13) ◽  
pp. 7979-8001
Author(s):  
Annette K. Miltenberger ◽  
Paul R. Field ◽  
Adrian H. Hill ◽  
Andrew J. Heymsfield
Keyword(s):  
Cloud Droplet ◽  
Wave Structure ◽  
Specific Humidity ◽  
Dust Particles ◽  
Microphysical Properties ◽  
Microphysical Processes ◽  
The Right ◽  
A Minor ◽  
Mixed Phase Clouds ◽  
The Impact

Abstract. Orographic wave clouds offer a natural laboratory to investigate cloud microphysical processes and their representation in atmospheric models. Wave clouds impact the larger-scale flow by the vertical redistribution of moisture and aerosol. Here we use detailed cloud microphysical observations from the Ice in Clouds Experiment – Layer Clouds (ICE-L) campaign to evaluate the recently developed Cloud Aerosol Interacting Microphysics (CASIM) module in the Met Office Unified Model (UM) with a particular focus on different parameterizations for heterogeneous freezing. Modelled and observed thermodynamic and microphysical properties agree very well (deviation of air temperature <1 K; specific humidity <0.2 g kg−1; vertical velocity <1 m s−1; cloud droplet number concentration <40 cm−3), with the exception of an overestimated total condensate content and too long a sedimentation tail. The accurate reproduction of the environmental thermodynamic and dynamical wave structure enables the model to reproduce the right cloud in the right place and at the right time. All heterogeneous freezing parameterizations except Atkinson et al. (2013) perform reasonably well, with the best agreement in terms of the temperature dependency of ice crystal number concentrations for the parameterizations of DeMott et al. (2010) and Tobo et al. (2013). The novel capabilities of CASIM allowed testing of the impact of assuming different soluble fractions of dust particles on immersion freezing, but this is found to only have a minor impact on hydrometeor mass and number concentrations. The simulations were further used to quantify the modification of moisture and aerosol profiles by the wave cloud. The changes in both variables are on order of 15 % of their upstream values, but the modifications have very different vertical structures for the two variables. Using a large number of idealized simulations we investigate how the induced changes depend on the wave period (100–1800 s), cloud top temperature (−15 to −50 ∘C), and cloud thickness (1–5 km) and propose a conceptual model to describe these dependencies.

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 Impacts of cloud and precipitation processes on maritime shallow convection as simulated by an LES model with bin microphysics

2014 ◽  
Vol 14 (13) ◽  
pp. 19837-19873 ◽  
Author(s):  
W. W. Grabowski ◽  
L.-P. Wang ◽  
T. V. Prabha
Keyword(s):  
Remote Sensing ◽  
Cloud Droplet ◽  
Convective Cloud ◽  
Collision Kernel ◽  
Small Scale ◽  
Shallow Convection ◽  
Precipitation Processes ◽  
Macrophysical Properties ◽  
Bin Microphysics ◽  
The Impact

Abstract. This paper discusses impacts of cloud and precipitation processes on macrophysical properties of shallow convective clouds as simulated by a large-eddy model applying warm-rain bin microphysics. Simulations with and without collision-coalescence are considered with CCN concentrations of 30, 60, 120, and 240 mg−1. Simulations with collision-coalescence include either the traditional gravitational collision kernel or a novel kernel that includes enhancements due to the small-scale cloud turbulence. Simulations with droplet collisions were discussed in Wyszogrodzki et al. (2013) focusing on the impact of the turbulent collision kernel. The current paper expands that analysis and puts model results in the context of previous studies. Despite a significant increase of the drizzle/rain with the decrease of CCN concentration, enhanced by the impact of the small-scale turbulence, impacts on the macroscopic cloud field characteristics are relatively minor. We document a clear feedback between cloud-scale processes and the mean environmental profiles that increases with the amount of drizzle/rain. Model results show a systematic shift in the cloud top height distributions, with an increasing contributions of deeper clouds and an overall increase of the number of cloudy columns for stronger precipitating cases. We argue that this is consistent with the explanation suggested in Wyszogrodzki et al. (2013) namely, the increase of drizzle/rain leading to a more efficient condensate off-loading in the upper parts of the cloud field. An additional effect involves suppressing cloud droplet evaporation near cloud edges in low-CCN simulations as documented in previous studies. We pose a question whether the effects of cloud turbulence on drizzle/rain formation can be corroborated by remote sensing observations, for instance, from space. Although a clear signal is extracted from model results, we argue that the answer is negative due to uncertainties caused by the temporal variability of the shallow convective cloud field, sampling and spatial resolution of the satellite data, and overall accuracy of remote sensing retrievals.

Influence of horizontal resolution and complexity of aerosol-cloud interactions on marine stratocumulus and stratocumulus-to-cumulus transition in HadGEM-GC31

2020 ◽  
Author(s):  
Annica M. L. Ekman ◽  
Eva Nygren ◽  
Gunilla Svensson ◽  
Nicolas Bellouin
Keyword(s):  
Southern Hemisphere ◽  
Liquid Water ◽  
Cloud Droplet ◽  
Background Level ◽  
Specific Model ◽  
Horizontal Resolution ◽  
Marine Stratocumulus ◽  
Aerosol Cloud ◽  
Model Version ◽  
The Impact

&lt;p&gt;We evaluate the impact of horizontal model resolution (~135, 60 and 25 km, respectively) and different levels of complexity of the aerosol-cloud interaction parameterization (interactive versus non-interactive) on springtime subtropical marine stratocumulus properties and stratocumulus-to-cumulus transition (SCT) using the atmosphere-only version of HadGEM-GC31. Higher resolution and non-interactive aerosols resulted in small, but significantly higher, liquid water contents and lower precipitation rates, in particular over the southern hemisphere. Higher resolution also resulted in a significantly stronger shortwave (SW) cloud radiative effect (CRE). Over the southern hemisphere, non-interactive aerosols also resulted in a stronger SW CRE, but over the northern hemisphere non-significant changes or a weaker SW CRE was obtained compared to the simulation using interactive aerosols. In general, no significant changes in the all-sky SW radiation was obtained. Only the model version with lowest resolution showed a weak tendency of a faster SCT than the other model versions. We conclude that a change in the complexity of the aerosol-cloud parameterization may significantly affect the SW CRE of marine stratocumuli, at least regionally, but the sign and magnitude of the impact will be dependent on the background level as well as the relative change in liquid water and the absolute change in cloud droplet number concentration of the specific model version.&lt;/p&gt;

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.

The Sensitivity of Simulated Shallow Cumulus Convection and Cold Pools to Microphysics

2015 ◽  
Vol 72 (9) ◽  
pp. 3340-3355 ◽  
Author(s):  
Zhujun Li ◽  
Paquita Zuidema ◽  
Ping Zhu ◽  
Hugh Morrison
Keyword(s):  
Radiative Forcing ◽  
Cloud Cover ◽  
Trade Wind ◽  
Shallow Convection ◽  
Shallow Marine ◽  
Cold Pools ◽  
Double Moment ◽  
Microphysical Parameterization ◽  
Rain Rates ◽  
The Impact

Abstract The sensitivity of nested WRF simulations of precipitating shallow marine cumuli and cold pools to microphysical parameterization is examined. The simulations differ only in their use of two widely used double-moment rain microphysical schemes: the Thompson and Morrison schemes. Both simulations produce similar mesoscale variability, with the Thompson scheme producing more weak cold pools and the Morrison scheme producing more strong cold pools, which are associated with more intense shallow convection. The most robust difference is that the cloud cover and LWP are significantly larger in the Morrison simulation than in the Thompson simulation. One-dimensional kinematic simulations confirm that dynamical feedbacks do not mask the impact of microphysics. These also help elucidate that a slower autoconversion process along with a stronger accretion process explains the Morrison scheme’s higher cloud fraction for a similar rain mixing ratio. Differences in the raindrop terminal fall speed parameters explain the higher evaporation rate of the Thompson scheme at moderate surface rain rates. Given the implications of the cloud-cover differences for the radiative forcing of the expansive trade wind regime, the microphysical scheme should be considered carefully when simulating precipitating shallow marine cumulus.

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.

scholarly journals Untangling Microphysical Impacts on Deep Convection Applying a Novel Modeling Methodology. Part II: Double-Moment Microphysics

2016 ◽  
Vol 73 (9) ◽  
pp. 3749-3770 ◽  
Author(s):  
Wojciech W. Grabowski ◽  
Hugh Morrison
Keyword(s):  
Large Scale ◽  
Cloud Condensation Nuclei ◽  
Deep Convection ◽  
Cloud Droplet ◽  
Cloud Base ◽  
Microphysics Scheme ◽  
Freezing Level ◽  
Modeling Methodology ◽  
Double Moment ◽  
The Impact

Abstract The suggested impact of pollution on deep convection dynamics, referred to as the convective invigoration, is investigated in simulations applying microphysical piggybacking and a comprehensive double-moment bulk microphysics scheme. The setup follows the case of daytime convective development over land based on observations during the Large-Scale Biosphere–Atmosphere (LBA) experiment in Amazonia. In contrast to previous simulations with single-moment microphysics schemes and in agreement with results from bin microphysics simulations by others, the impact of pollution simulated by the double-moment scheme is large for the upper-tropospheric convective anvils that feature higher cloud fractions in polluted conditions. The increase comes from purely microphysical considerations: namely, the increased cloud droplet concentrations in polluted conditions leading to the increased ice crystal concentrations and, consequently, smaller fall velocities and longer residence times. There is no impact on convective dynamics above the freezing level and thus no convective invigoration. Polluted deep convective clouds precipitate about 10% more than their pristine counterparts. The small enhancement comes from smaller supersaturations below the freezing level and higher buoyancies inside polluted convective updrafts with velocities between 5 and 10 m s−1. The simulated supersaturations are large, up to several percent in both pristine and polluted conditions, and they call into question results from deep convection simulations applying microphysical schemes with saturation adjustment. Sensitivity simulations show that the maximum supersaturations and the upper-tropospheric anvil cloud fractions strongly depend on the details of small cloud condensation nuclei (CCN) that can be activated in strong updrafts above the cloud base.

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