scholarly journals Response of a Subtropical Stratocumulus-Capped Mixed Layer to Climate and Aerosol Changes

2009 ◽  
Vol 22 (1) ◽  
pp. 20-38 ◽  
Author(s):  
Peter Caldwell ◽  
Christopher S. Bretherton
Keyword(s):  
Boundary Layer ◽  
Mixed Layer ◽  
Liquid Water ◽  
Static Stability ◽  
Climate Change Scenarios ◽  
Layer Model ◽  
Liquid Water Path ◽  
Layer Depth ◽  
Water Path ◽  
Droplet Concentration

Abstract In this paper, an idealized framework based on a cloud-topped mixed layer model is developed for investigating feedbacks between subtropical stratocumulus (Sc) and global warming. The two principal control parameters are Sc-region sea surface temperature (SST) and intertropical convergence zone (ITCZ) SST (which controls the temperature and mean subsidence profiles above the Sc). The direct effect of CO2 doubling (leaving all other parameters fixed) is tested and found to somewhat reduce liquid water path; discussion of this effect on the SST-change simulations is included. The presence of a cold boundary layer is found to significantly affect the temperature and subsidence rate just above cloud top by enhancing lower-tropospheric diabatic cooling in this region. A simple representation of this effect (easily generalizable to a more realistic boundary layer model) is developed. Steady-state solutions are analyzed as a function of local and ITCZ SST. Two climate change scenarios are considered. The first scenario is an equal increase of local and ITCZ SSTs. In this case, predicted boundary layer depth and cloud thickness increase. This is found in a simplified context to result from subsidence and entrainment decreases due to increased static stability in a warmer climate. In the second case, local SST change is diagnosed from a surface energy balance under the assumption that ocean heat transport remains unchanged. In this case, predicted boundary layer depth decreases. Cloud continues to thicken with rising ITCZ SST, but at a rate much reduced in comparison to the equal-warming scenario. This cloud shading feedback keeps SST in the Sc region nearly constant as the ITCZ SST increases. Model sensitivity to aerosol indirect effects is also considered by varying the assumed droplet concentration. The resulting change in liquid water path is small, suggesting a weaker dependence on second indirect effect than found in previous studies.

scholarly journals The sensitivity of stratocumulus-capped mixed layers to cloud droplet concentration: do LES and mixed-layer models agree?

2010 ◽  
Vol 10 (9) ◽  
pp. 4097-4109 ◽  
Author(s):  
J. Uchida ◽  
C. S. Bretherton ◽  
P. N. Blossey
Keyword(s):  
Boundary Layer ◽  
Mixed Layer ◽  
Liquid Water ◽  
Indirect Effect ◽  
Cloud Droplet ◽  
Model Potential ◽  
Entrainment Rate ◽  
Liquid Water Path ◽  
Water Path ◽  
Droplet Concentration

Abstract. The sensitivity of a stratocumulus-capped mixed layer to a change in cloud droplet concentration is evaluated with a large-eddy simulation (LES) and a mixed layer model (MLM). The strength of the second aerosol indirect effect simulated by the two model types agrees within 50% for cases in which the LES-simulated boundary layer remains well mixed, if the MLM entrainment closure includes the effects of cloud droplet sedimentation. To achieve this agreement, parameters in the MLM entrainment closure and the drizzle parameterization must be retuned to match the LES. This is because the LES advection scheme and microphysical parameterization significantly bias the entrainment rate and precipitation profile compared to observational best guesses. Before this modification, the MLM simulates more liquid water path and much more drizzle at a given droplet concentration than the LES and is more sensitive to droplet concentration, even undergoing a drizzle-induced boundary layer collapse at low droplet concentrations. After this modification, both models predict a comparable decrease of cloud liquid water path as droplet concentration increases, cancelling 30–50% of the Twomey effect for our case. The agreement breaks down at the lowest simulated droplet concentrations, for which the boundary layer in the LES is not well mixed. Our results highlight issues with both types of model. Potential LES biases due to inadequate resolution, subgrid mixing and parameterized microphysics must be carefully considered when trying to make a quantitative inference of the second indirect effect from an LES of a stratocumulus-topped boundary layer. On the other hand, even slight internal decoupling of the boundary layer invalidates the central assumption of an MLM, substantially limiting the range of conditions that MLM-predicted sensitivities to droplet concentration are meaningful.

scholarly journals How important is the vertical structure for the representation of aerosol impacts on the diurnal cycle of marine stratocumulus?

2009 ◽  
Vol 9 (2) ◽  
pp. 5465-5503 ◽  
Author(s):  
I. Sandu ◽  
J.-L. Brenguier ◽  
O. Thouron ◽  
B. Stevens
Keyword(s):  
Boundary Layer ◽  
Mixed Layer ◽  
Diurnal Cycle ◽  
Liquid Water ◽  
Vertical Stratification ◽  
Layer Model ◽  
Liquid Water Path ◽  
Marine Stratocumulus ◽  
Water Path ◽  
Mixed Layer Model

Abstract. Large Eddy Simulations (LES) are performed to examine the impact of hygroscopic aerosols on the diurnal cycle of marine stratocumulus clouds, under varying meteorological forcing conditions. When the cloud condensation nuclei concentration increase is sufficient to inhibit drizzle formation in the cloud layer, the precipitating and the non-precipitating cloud layers exhibit contrasting evolutions, with noticeable differences in liquid water path. Aerosol induced modifications of the droplet sedimentation and drizzle precipitation result in noticeable changes of the entrainment velocity at cloud top, but also in significant changes of the vertical stratification in the boundary layer. This set of simulations is then used to evaluate whether a model which does not explicitly represent the effects of the interactions occurring within the boundary layer on its vertical stratification (i.e. such as a mixed layer model) is capable of reproducing at least the sign, if not the amplitude, of these aerosol impacts on the liquid water path. It is shown that the deviations from the mixed layer idealized state are crucial ingredients of the aerosol impacts so that a mixed layer model is unable to even replicate the sign of the liquid water path changes.

scholarly journals The sensitivity of stratocumulus-capped mixed layers to cloud droplet concentration: do LES and mixed-layer models agree?

2009 ◽  
Vol 9 (6) ◽  
pp. 25853-25883
Author(s):  
J. Uchida ◽  
C. S. Bretherton ◽  
P. N. Blossey
Keyword(s):  
Boundary Layer ◽  
Mixed Layer ◽  
Liquid Water ◽  
Indirect Effect ◽  
Cloud Droplet ◽  
Model Potential ◽  
Entrainment Rate ◽  
Liquid Water Path ◽  
Water Path ◽  
Droplet Concentration

Abstract. The sensitivity of a stratocumulus-capped mixed layer to a change in cloud droplet concentration is evaluated with a large-eddy simulation (LES) and a mixed layer model (MLM), to see if the two model types agree on the strength of the second aerosol indirect effect. Good agreement can be obtained if the MLM entrainment closure explicitly reduces entrainment efficiency proportional to the rate of cloud droplet sedimentation at cloud top for cases in which the LES-simulated boundary layer remains well mixed, with a single peak in the vertical profile of vertical velocity variance. To achieve this agreement, the MLM entrainment closure and the drizzle parameterization must be modified from their observationally-based defaults. This is because the LES advection scheme and microphysical parameterization significantly bias the entrainment rate and precipitation profile compared to observational best guesses. Before this modification, the MLM simulates more liquid water path and much more drizzle at a given droplet concentration than the LES and is more sensitive to droplet concentration, even undergoing a drizzle-induced boundary layer collapse at low droplet concentrations. After this modification, both models predict a similar decrease of cloud liquid water path as droplet concentration increases, cancelling 30–50% of the Twomey effect for our case. The agreement breaks down at the lowest simulated droplet concentrations, for which the boundary layer in the LES is not well mixed. Our results highlight issues with both types of model. Potential LES biases due to inadequate resolution, subgrid mixing and microphysics must be carefully considered when trying to make a quantitative inference of the second indirect effect from an LES of a stratocumulus-topped boundary layer. On the other hand, even slight internal decoupling of the boundary layer invalidates MLM-predicted sensitivity to droplet concentrations.

scholarly journals Large Eddy Simulation of the Diurnal Cycle in Southeast Pacific Stratocumulus

2009 ◽  
Vol 66 (2) ◽  
pp. 432-449 ◽  
Author(s):  
Peter Caldwell ◽  
Christopher S. Bretherton
Keyword(s):  
Boundary Layer ◽  
Diurnal Cycle ◽  
Liquid Water ◽  
Cloud Base ◽  
Liquid Water Path ◽  
Water Path ◽  
Diurnal Cycles ◽  
Eddy Simulation ◽  
Droplet Concentration ◽  
Large Eddy

Abstract This paper describes a series of 6-day large eddy simulations of a deep, sometimes drizzling stratocumulus-topped boundary layer based on forcings from the East Pacific Investigation of Climate (EPIC) 2001 field campaign. The base simulation was found to reproduce the observed mean boundary layer properties quite well. The diurnal cycle of liquid water path was also well captured, although good agreement appears to result partially from compensating errors in the diurnal cycles of cloud base and cloud top due to overentrainment around midday. At all times of the day, entrainment is found to be proportional to the vertically integrated buoyancy flux. Model stratification matches observations well; turbulence profiles suggest that the boundary layer is always at least somewhat decoupled. Model drizzle appears to be too sensitive to liquid water path and subcloud evaporation appears to be too weak. Removing the diurnal cycle of subsidence had little effect on simulated liquid water path. Simulations with changed droplet concentration and drizzle susceptibility showed large liquid water path differences at night, but differences were quite small at midday. Droplet concentration also had a significant impact on entrainment, primarily through droplet sedimentation feedback rather than through drizzle processes.

scholarly journals Factors Controlling Low-Cloud Evolution over the Eastern Subtropical Oceans: A Lagrangian Perspective Using the A-Train Satellites

2015 ◽  
Vol 73 (1) ◽  
pp. 331-351 ◽  
Author(s):  
Ryan Eastman ◽  
Robert Wood
Keyword(s):  
Boundary Layer ◽  
Boundary Layers ◽  
Liquid Water ◽  
Cloud Cover ◽  
Liquid Water Path ◽  
Water Path ◽  
Cloud Properties ◽  
Droplet Concentration ◽  
Low Cloud ◽  
Cloud Evolution

Abstract A Lagrangian technique is developed to sample satellite data to quantify and understand factors controlling temporal changes in low-cloud properties (cloud cover, areal-mean liquid water path, and droplet concentration). Over 62 000 low-cloud scenes over the eastern subtropical/tropical oceans are sampled using the A-Train satellites. Horizontal wind fields at 925 hPa from the ERA-Interim are used to compute 24-h, two-dimensional, forward, boundary layer trajectories with trajectory locations starting on the CloudSat/CALIPSO track. Cloud properties from MODIS and AMSR-E are sampled at the trajectory start and end points, allowing for direct measurement of the temporal cloud evolution. The importance of various controls (here, boundary layer depth, lower-tropospheric stability, and precipitation) on cloud evolution is evaluated by comparing cloud evolution for different initial values of these controls. Viewing angle biases are removed and cloud anomalies (diurnal and seasonal cycles removed) are used throughout to quantify cloud evolution relative to the climatological-mean evolution. Cloud property anomalies show temporal changes similar to those expected for a stochastic red noise process, with linear relationships between initial anomalies and their mean 24-h changes. This creates a potential bias when comparing the evolutions of sets of trajectories with different initial anomalies; three methods are introduced and evaluated to account for this. Results provide statistically robust observational support for theoretical/modeling studies by showing that low clouds in deep boundary layers and under weak inversions are prone to break up. Precipitation shows a more complex and less statistically significant relationship with cloud breakup. Cloud cover in shallow precipitating boundary layers is more persistent than in deep precipitating boundary layers. Liquid water path and cloud droplet concentration decrease more rapidly for precipitating clouds and in deep boundary layers.

Reconciling and Validating the Cloud Thickness and Liquid Water Path Tendencies Proposed by R. Wood and J. J. van der Dussen et al.

2015 ◽  
Vol 72 (5) ◽  
pp. 2033-2040 ◽  
Author(s):  
Mohamed S. Ghonima ◽  
Joel R. Norris ◽  
Thijs Heus ◽  
Jan Kleissl
Keyword(s):  
Boundary Layer ◽  
Liquid Water ◽  
Mixed Boundary ◽  
Liquid Water Path ◽  
Water Path ◽  
Large Eddy ◽  
Heat And Moisture ◽  
Cloud Thickness ◽  
Detailed Derivation ◽  
Good Agreement

Abstract A detailed derivation of stratocumulus cloud thickness and liquid water path tendencies as a function of the well-mixed boundary layer mass, heat, and moisture budget equations is presented. The derivation corrects an error in the cloud thickness tendency equation derived by R. Wood to make it consistent with the liquid water path tendency equation derived by J. J. van der Dussen et al. The validity of the tendency equations is then tested against the output of large-eddy simulations of a typical stratocumulus-topped boundary layer case and is found to be in good agreement.

scholarly journals Cloud optical thickness and liquid water path. Does the <i>k</i> coefficient vary with droplet concentration?

2011 ◽  
Vol 11 (2) ◽  
pp. 5173-5215
Author(s):  
J.-L. Brenguier ◽  
F. Burnet ◽  
O. Geoffroy
Keyword(s):  
Correction Factor ◽  
Optical Thickness ◽  
Liquid Water ◽  
General Circulation ◽  
Field Experiments ◽  
General Circulation Models ◽  
Light Extinction ◽  
Liquid Water Path ◽  
Water Path ◽  
Droplet Concentration

Abstract. Cloud radiative transfer calculations in general circulation models involve a link between cloud microphysical and optical properties. Indeed, the liquid water content expresses as a function of the mean volume droplet radius, while the light extinction is a function of their mean surface radius. There is a small difference between these two parameters because of the droplet spectrum width. This issue has been addressed by introducing an empirical multiplying correction factor to the droplet concentration. Analysis of in situ sampled data, however, revealed that the correction factor decreases when the concentration increases, hence partially mitigating the aerosol indirect effect. Five field experiments are reanalyzed here, in which standard and upgraded versions of the droplet spectrometer were used to document shallow cumulus and stratocumulus topped boundary layers. They suggest that the standard probe noticeably underestimates the correction factor compared to the upgraded versions. The analysis is further refined to demonstrate that the value of the correction factor derived by averaging values calculated locally along the flight path overestimates the value derived from liquid water path and optical thickness of a cloudy column, and that there is no detectable correlation between the correction factor and the droplet concentration. It is also shown that the droplet concentration dilution by entrainment-mixing after CCN activation is significantly stronger in shallow cumuli than in stratocumulus layers. These various effects are finally combined to produce the best estimate of the correction factor to use in general circulation models.

scholarly journals Parameterizing cloud top effective radii from satellite retrieved values, accounting for vertical photon transport: quantification and correction of the resulting bias in droplet concentration and liquid water path retrievals

2018 ◽  
Vol 11 (7) ◽  
pp. 4273-4289 ◽  
Author(s):  
Daniel P. Grosvenor ◽  
Odran Sourdeval ◽  
Robert Wood
Keyword(s):  
Penetration Depth ◽  
Optical Depth ◽  
Liquid Water ◽  
Liquid Water Content ◽  
Cloud Base ◽  
Percentage Error ◽  
Time Variability ◽  
Liquid Water Path ◽  
Water Path ◽  
Droplet Concentration

Abstract. Droplet concentration (Nd) and liquid water path (LWP) retrievals from passive satellite retrievals of cloud optical depth (τ) and effective radius (re) usually assume the model of an idealized cloud in which the liquid water content (LWC) increases linearly between cloud base and cloud top (i.e. at a fixed fraction of the adiabatic LWC). Generally it is assumed that the retrieved re value is that at the top of the cloud. In reality, barring re retrieval biases due to cloud heterogeneity, the retrieved re is representative of smaller values that occur lower down in the cloud due to the vertical penetration of photons at the shortwave-infrared wavelengths used to retrieve re. This inconsistency will cause an overestimate of Nd and an underestimate of LWP (referred to here as the “penetration depth bias”), which this paper quantifies via a parameterization of the cloud top re as a function of the retrieved re and τ. Here we estimate the relative re underestimate for a range of idealized modelled adiabatic clouds using bispectral retrievals and plane-parallel radiative transfer. We find a tight relationship between gre=recloud top/reretrieved and τ and that a 1-D relationship approximates the modelled data well. Using this relationship we find that gre values and hence Nd and LWP biases are higher for the 2.1 µm channel re retrieval (re2.1) compared to the 3.7 µm one (re3.7). The theoretical bias in the retrieved Nd is very large for optically thin clouds, but rapidly reduces as cloud thickness increases. However, it remains above 20 % for τ<19.8 and τ<7.7 for re2.1 and re3.7, respectively. We also provide a parameterization of penetration depth in terms of the optical depth below cloud top (dτ) for which the retrieved re is likely to be representative. The magnitude of the Nd and LWP biases for climatological data sets is estimated globally using 1 year of daily MODIS (MODerate Imaging Spectroradiometer) data. Screening criteria are applied that are consistent with those required to help ensure accurate Nd and LWP retrievals. The results show that the SE Atlantic, SE Pacific and Californian stratocumulus regions produce fairly large overestimates due to the penetration depth bias with mean biases of 32–35 % for re2.1 and 15–17 % for re3.7. For the other stratocumulus regions examined the errors are smaller (24–28 % for re2.1 and 10–12 % for re3.7). Significant time variability in the percentage errors is also found with regional mean standard deviations of 19–37 % of the regional mean percentage error for re2.1 and 32–56 % for re3.7. This shows that it is important to apply a daily correction to Nd for the penetration depth error rather than a time–mean correction when examining daily data. We also examine the seasonal variation of the bias and find that the biases in the SE Atlantic, SE Pacific and Californian stratocumulus regions exhibit the most seasonality, with the largest errors occurring in the December, January and February (DJF) season. LWP biases are smaller in magnitude than those for Nd (−8 to −11 % for re2.1 and −3.6 to −6.1 % for re3.7). In reality, and especially for more heterogeneous clouds, the vertical penetration error will be combined with a number of other errors that affect both the re and τ, which are potentially larger and may compensate or enhance the bias due to vertical penetration depth. Therefore caution is required when applying the bias corrections; we suggest that they are only used for more homogeneous clouds.

Importance of Accurate Liquid Water Path for Estimation of Solar Radiation in Warm Boundary Layer Clouds: An Observational Study

2003 ◽  
Vol 16 (18) ◽  
pp. 2997-3009 ◽  
Author(s):  
Manajit Sengupta ◽  
Eugene E. Clothiaux ◽  
Thomas P. Ackerman ◽  
Seiji Kato ◽  
Qilong Min
Keyword(s):  
Boundary Layer ◽  
Solar Radiation ◽  
Observational Study ◽  
Liquid Water ◽  
Liquid Water Path ◽  
Water Path ◽  
Boundary Layer Clouds

scholarly journals The Role of Eddy Diffusivity Profiles on Stratocumulus Liquid Water Path Biases

2007 ◽  
Vol 135 (7) ◽  
pp. 2786-2793 ◽  
Author(s):  
Stephan R. de Roode
Keyword(s):  
Boundary Layer ◽  
Liquid Water ◽  
Sensible Heat Flux ◽  
Eddy Diffusivity ◽  
Specific Humidity ◽  
Entrainment Rate ◽  
Liquid Water Path ◽  
Water Path ◽  
Wide Range ◽  
Heat And Moisture

Abstract Results from simulations of the stratocumulus-topped boundary layer with one-dimensional versions of general simulation models typically exhibit a wide range of spread in the modeled liquid water path (LWP). These discrepancies are often attributed to differences in the modeled entrainment rate. Results from a large eddy simulation of the First International Satellite Cloud Climatology Project Regional Experiment I stratocumulus case are analyzed. The diagnosed eddy diffusivities for heat and moisture are found to differ by about a factor of 3. Moreover, both have a much larger magnitude than the ones typically applied in boundary layer parameterization schemes. Motivated by these results mean state solutions are analyzed for the specific case in which the vertical fluxes of heat and moisture are prescribed, whereas eddy diffusivity profiles are systematically varied by multiplication with a constant factor. The solutions demonstrate that any value, ranging from zero to a maximum adiabatic value, can be obtained for the LWP. In the subtropical parts over the ocean where horizontally extended stratocumulus fields persist, the surface sensible heat flux is typically small, whereas surface evaporation and entrainment of relatively dry air from above the surface can result in significant moisture fluxes. If the eddy diffusivity values are small, then the mean specific humidity will tend to decrease quite rapidly with height in order to support the humidity flux. This results in erroneous low humidity values in the upper part of the boundary layers causing low LWP values.

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