Cloud-top entrainment analyzed with a Lagrangian parcel tracking model in large-eddy simulations

2010 Fall.Includes bibliographical references.Despite decades of research, cloud-top entrainment has not been described with firm evidence. This leads to insufficient understanding of the physics of marine stratocumulus clouds. A Lagrangian Parcel Tracking Model (LPTM) was implemented in a large-eddy simulation model for detailed and direct analysis of the entrained air parcel following the parcel trajectory. The scalar advection scheme of the host model was replaced by a monotonic multidimensional odd-order conservative advection scheme. Tests with an idealized scalar field and stratocumulus turbulence suggested that the fifth-order scheme is optimal. Evaluation of the LPTM was performed with stratocumulus simulations. Parcel statistics agreed with Eulerian statistics, and the parcel paths agreed with the theoretical parcel paths. The Lagrangian budget equation for a scalar, however, generally does not hold for a simulated turbulence field, since the fractal nature of turbulence may cause numerical errors. Two large-eddy simulations were performed with grid spacing of O (5 m). The power spectra of these runs showed relatively good agreement with the energy cascade slope. A comparison with low-resolution simulations suggested that horizontal refinement is necessary for better representation of entrainment and microphysical processes. The LPTM with the high-resolution stratocumulus simulation showed that the location of entrainment is in cloud holes, which are drier downdraft regions. Parcels in the inversion layer, subsiding from the free atmosphere, are entrained in to the mixed layer. They are cooled and moistened by radiation, evaporation, and mixing. A mixing fraction analysis shows that the coolings during entrainment due to radiation and evaporation are comparable. The largest contribution to buoyancy reduction is the cooling due to mixing, for our simulation. The analysis also shows that buoyancy reversal occurs for the entrained parcels. Radiative cooling and cloud-top entrainment instability (CTEI) interact such that the radiative cooling forces larger saturation mixing fractions while CTEI forces smaller values. Additional simulations suggest that radiative cooling produces a negative feedback on the entrainment rate, which is strong enough to control turbulence and hide CTEI. Under such conditions, cloud breakup due to CTEI is unlikely

Cloud droplet growth in shallow cumulus clouds considering 1-D and 3-D thermal radiative effects

The effect of 1-D and 3-D thermal radiation on cloud droplet growth in shallow cumulus clouds is investigated using large eddy simulations with size-resolved cloud microphysics. A two-step approach is used for separating microphysical effects from dynamical feedbacks. In step one, an offline parcel model is used to describe the onset of rain. The growth of cloud droplets to raindrops is simulated with bin-resolved microphysics along previously recorded Lagrangian trajectories. It is shown that thermal heating and cooling rates can enhance droplet growth and raindrop production. Droplets grow to larger size bins in the 10-30 mu m radius range. The main effect in terms of raindrop production arises from recirculating parcels, where a small number of droplets are exposed to strong thermal cooling at cloud edge. These recirculating parcels, comprising about 6%-7% of all parcels investigated, make up 45% of the rain for the no-radiation simulation and up to 60% when 3-D radiative effects are considered. The effect of 3-D thermal radiation on rain production is stronger than that of 1-D thermal radiation. Three-dimensional thermal radiation can enhance the rain amount up to 40% compared to standard droplet growth without radiative effects in this idealized framework. In the second stage, fully coupled large eddy simulations show that dynamical effects are stronger than microphysical effects, as far as the production of rain is concerned. Three-dimensional thermal radiative effects again exceed one-dimensional thermal radiative effects. Small amounts of rain are produced in more clouds (over a larger area of the domain) when thermal radiation is applied to microphysics. The dynamical feedback is shown to be an enhanced cloud circulation with stronger subsiding shells at the cloud edges due to thermal cooling and stronger updraft velocities in the cloud center. It is shown that an evaporation-circulation feedback reduces the amount of rain produced in simulations where 3-D thermal radiation is applied to microphysics and dynamics, in comparison to where 3-D thermal radiation is only applied to dynamics

Mesoscale organization, entrainment, and the properties of a closed-cell stratocumulus cloud

Closed-cell mesoscale organization and its relationship to entrainment and the properties of a low, nonprecipitating stratocumulus cloud is investigated. Large eddy simulations were run over 10 periodic diurnal cycles during which mesoscale organization could fully develop and approach a quasi-steady state on five domains sized from 2.4 km × 2.4 km to 38.4 km × 38.4 km. The four smaller domains hosted a single cell with an aspect ratio that increased with domain size. On the largest domain, mesoscale organization consisted of a cell population that evolved over the course of the diurnal cycle. It is found that with increasing cell aspect ratio, entrainment weakens and the boundary layer becomes shallower, cooler, moister, and more decoupled. This causes an increase in cloud water path and cloud radiative effect up to a cell aspect ratio of 16. With further increase in cell aspect ratio, circulation on the cell scale becomes less effective in supplying moisture to the cloud and in producing turbulent kinetic energy (TKE). This mechanism can explain scale saturation in closed-cell mesoscale organization. The simulations support a maximum stable aspect ratio of closed-cell mesoscale organization between 32 and 64, consistent with the observational limit of [asymp]40. The simulations show furthermore that entrainment does not, in general, scale with buoyant production of TKE. Instead, entrainment correlates with the vertical component of TKE. This implies vertical motion as a driver of entrainment, and a convective velocity scale based on the vertical component of TKE rather than on buoyant production of TKE. Key Points Entrainment weakens and the boundary layer becomes shallower, cooler, moister, and more decoupled with increasing closed cell aspect ratio Reduced cell scale moisture transport and TKE production at large aspect ratios can explain closed cell scale saturation Entrainment is driven by the vertical component of TKE rather than by TKE productio

Stratocumulus to cumulus transition by drizzle

The stratocumulus to cumulus transition (SCT) is typically considered to be a slow, multiday process, caused primarily by dry air entrainment associated with overshooting cumulus, with minor influence of drizzle. This study revisits the role of drizzle in the SCT with large eddy simulations coupled with a two‐moment bulk microphysics scheme that includes a budget on aerosol (Na) and cloud droplet number concentrations (Nc). We show a strong precipitation‐induced modulation of the SCT by drizzle initiated in penetrative cumulus under stratocumulus. Lagrangian SCT simulations are initiated with various, moderate Na&nbsp;(100&ndash;250 cm-3), which produce little to no drizzle from the stratocumulus. As expected, drizzle formation in cumuli is regulated by cloud depth and N, with stronger dependence on cloud depth, so that, for the current case, drizzle is generated in all simulations once cumulus clouds become sufficiently deep. The drizzle generated in the cumuli washes out stratocumulus cloud water and much of the aerosol, and a cumulus state appears for approximately 10 h. With additional simulations with a fixed Nc&nbsp;(100 cm-3), we show that prediction of Nc&nbsp;is necessary for this fast SCT since it is a result of a positive feedback of collision‐coalescence‐induced aerosol depletion that enhances drizzle formation. A fixed Nc&nbsp;does not permit this feedback, and thus results in weak influence of drizzle on the SCT. Simulations with fixed droplet concentrations that bracket the time varying aerosol/drop concentrations are therefore not representative of the role of drizzle in the SCT.</p

Wind speed response of marine non-precipitating stratocumulus clouds over a diurnal cycle in cloud-system resolving simulations

Observed and projected trends in large-scale wind speed over the oceans prompt the question: how do marine stratocumulus clouds and their radiative properties respond to changes in large-scale wind speed? Wind speed drives the surface fluxes of sensible heat, moisture, and momentum and thereby acts on cloud liquid water path (LWP) and cloud radiative properties. We present an investigation of the dynamical response of non-precipitating, overcast marine stratocumulus clouds to different wind speeds over the course of a diurnal cycle, all else equal. In cloud-system resolving simulations, we find that higher wind speed leads to faster boundary layer growth and stronger entrainment. The dynamical driver is enhanced buoyant production of turbulence kinetic energy (TKE) from latent heat release in cloud updrafts. LWP is enhanced during the night and in the morning at higher wind speed, and more strongly suppressed later in the day. Wind speed hence accentuates the diurnal LWP cycle by expanding the morning–afternoon contrast. The higher LWP at higher wind speed does not, however, enhance cloud top cooling because in clouds with LWP ⪆ 50 g m−2, longwave emissions are insensitive to LWP. This leads to the general conclusion that in sufficiently thick stratocumulus clouds, additional boundary layer growth and entrainment due to a boundary layer moistening arises by stronger production of TKE from latent heat release in cloud updrafts, rather than from enhanced longwave cooling. We find that large-scale wind modulates boundary layer decoupling. At nighttime and at low wind speed during daytime, it enhances decoupling in part by faster boundary layer growth and stronger entrainment and in part because shear from large-scale wind in the sub-cloud layer hinders vertical moisture transport between the surface and cloud base. With increasing wind speed, however, in decoupled daytime conditions, shear-driven circulation due to large-scale wind takes over from buoyancy-driven circulation in transporting moisture from the surface to cloud base and thereby reduces decoupling and helps maintain LWP. The total (shortwave + longwave) cloud radiative effect (CRE) responds to changes in LWP and cloud fraction, and higher wind speed translates to a stronger diurnally averaged total CRE. However, the sensitivity of the diurnally averaged total CRE to wind speed decreases with increasing wind speed

Quantifying albedo susceptibility biases in shallow clouds

The evaluation of radiative forcing associated with aerosol&ndash;cloud interactions remains a significant source of uncertainty in future climate projections. The problem is confounded by the fact that aerosol particles influence clouds locally and that averaging to larger spatial and/or temporal scales carries biases that depend on the heterogeneity and spatial correlation of the interacting fields and the nonlinearity of the responses. Mimicking commonly applied satellite data analyses for calculation of albedo susceptibility So, we quantify So&nbsp;aggregation biases using an ensemble of 127&nbsp;large eddy simulations of marine stratocumulus. We explore the cloud field properties that control this spatial aggregation bias and quantify the bias for a large range of shallow stratocumulus cloud conditions manifesting a variety of morphologies and ranges of cloud fractions. We show that So&nbsp;spatial aggregation biases can be on the order of hundreds of percent, depending on the methodology. Key uncertainties emanate from the typically applied adiabatic drop concentration&nbsp;Nd retrieval, the correlation between aerosol and cloud fields, and the extent to which averaging reduces the variance in cloud albedo&nbsp;Ac and&nbsp;Nd. So biases are more often positive than negative and are highly correlated with biases in the liquid water path adjustment. Temporal aggregation biases are shown to offset spatial aggregation biases. Both spatial and temporal biases have significant implications for observationally based assessments of aerosol indirect effects and our inferences of underlying aerosol&ndash;cloud&ndash;radiation effects. &nbsp;</p