Large-eddy simulation of radiation fog with comprehensive two-moment bulk microphysics: impact of different aerosol activation and condensation parameterizations

Abstract. In this paper we study the influence of the cloud microphysical parameterization, namely the effect of different methods for calculating the supersaturation and aerosol activation, on the structure and life cycle of radiation fog in large-eddy simulations. For this purpose we investigate a well-documented deep fog case as observed at Cabauw (the Netherlands) using high-resolution large-eddy simulations with a comprehensive bulk cloud microphysics scheme. By comparing saturation adjustment with a diagnostic and a prognostic method for calculating supersaturation (while neglecting the activation process), we find that, even though assumptions for saturation adjustment are violated, the expected overestimation of the liquid water mixing ratio is negligible. By additionally considering activation, however, our results indicate that saturation adjustment, due to approximating the underlying supersaturation, leads to a higher droplet concentration and hence significantly higher liquid water content in the fog layer, while diagnostic and prognostic methods yield comparable results. Furthermore, the effect of different droplet number concentrations is investigated, induced by using different common activation schemes. We find, in line with previous studies, a positive feedback between the droplet number concentration (as a consequence of the applied activation schemes) and strength of the fog layer (defined by its vertical extent and amount of liquid water). Furthermore, we perform an explicit analysis of the budgets of condensation, evaporation, sedimentation and advection in order to assess the height-dependent contribution of the individual processes on the development phases.

Download Full-text

Large-eddy simulation of radiation fog with comprehensive two-moment bulk microphysics: Impact of different aerosol activation and condensation parameterizations

10.5194/acp-2018-1139 ◽

2018 ◽

Author(s):

Johannes Schwenkel ◽

Björn Maronga

Keyword(s):

Cloud Droplet ◽

Large Eddy Simulations ◽

Diffusional Growth ◽

Radiation Fog ◽

Eddy Simulation ◽

Large Eddy ◽

Growth Method ◽

Explicit Analysis ◽

Temporal Influence ◽

Comparison Of The Results

Abstract. In this paper we study the influence of the cloud microphysical parameterization on large-eddy simulations of radiation fog. A deep fog case as observed at Cabauw (Netherlands) is investigated using high-resolution large-eddy simulations with different microphysics treatments for activation and diffusional growth. A comparison of the results indicates that the commonly applied assumption of saturation adjustment produces at maximum 6.9 % higher liquid water paths compared to the explicit diffusional growth method but has no significant influence on the general life cycle of the fog layer. Differences are found to be the most pronounced at the top of the fog layer where the highest supersaturations occurs. Furthermore, the effect of different cloud droplet number concentrations is investigated by using a selection of common activation schemes. We find, in line with previous studies, a positive feedback between the cloud droplet number concentration and both the optical thickness and the strength of the fog layer. Furthermore, we perform an explicit analysis of the budgets of microphysical quantities in order to assess which processes have the largest spatial and temporal influence on the development of the fog layer.

Download Full-text

Modelling study on the formation of pockets of open cells in marine stratocumulus cloud during CLARIFY

10.5194/egusphere-egu2020-16043 ◽

2020 ◽

Author(s):

Emma Simpson ◽

Tom Choularton

Keyword(s):

Cloud Model ◽

Direct Interaction ◽

Cloud Microphysics ◽

Large Eddy Simulations ◽

Radiation Budget ◽

Microphysics Scheme ◽

Marine Stratocumulus ◽

Open Cell ◽

The Earth ◽

Large Eddy

Due to the wide spread nature of marine stratocumulus cloud they have a significant impact on the Earth&#8217;s radiation budget. Such clouds are sensitive to the presence of aerosol, which can promote the break-up of a cloud deck into pockets of open cell convection (POC). The transition from a cloud deck to pockets of open cells changes the overall cloud albedo thus affecting the Earth&#8217;s radiation budget. The representation of stratocumulus cloud and the transition to POCs is poorly represented in current climate and weather models. This study aims to improve understanding of this process using extensive in-situ measurements made during the CLARIFY campaign of stratocumulus cloud decks, transition areas between overcast and open cell cloud structures as well as areas of POCs, to inform and compare to large-eddy simulations.A variety of different aerosol situations occurred during CLARIFY, combinations of polluted/clean boundary layer and polluted/clean conditions above the cloud layer. Large-eddy simulations are conducted to investigate the sensitivity of clouds to changes in the observed aerosol conditions with a particular focus on whether or not the change in aerosol initiates cloud breakup.The MetOffice NERC Cloud model (MONC) is used to preform the large-eddy simulations and employs the CASIM cloud microphysics scheme which includes activation of aerosol particles to cloud drops. Such a model set-up allows direct interaction between aerosols and clouds. Observations from CLARIFY are used to initialise and evaluate model simulations.

Download Full-text

An intercomparison of radar-based liquid cloud microphysics retrievals and implication for model evaluation studies

Atmospheric Measurement Techniques Discussions ◽

10.5194/amtd-4-7109-2011 ◽

2011 ◽

Vol 4 (6) ◽

pp. 7109-7158 ◽

Cited By ~ 1

Author(s):

D. Huang ◽

C. Zhao ◽

M. Dunn ◽

X. Dong ◽

G. G. Mace ◽

...

Keyword(s):

Great Plains ◽

Liquid Water ◽

Model Evaluation ◽

Evaluation Studies ◽

Cloud Droplet ◽

Liquid Water Content ◽

Cloud Microphysics ◽

Effective Radius ◽

High Altitudes ◽

Liquid Water Path

Abstract. To assess if current radar-based liquid cloud microphysical retrievals of the Atmospheric Radiation Measurement (ARM) program can provide useful constraints for modeling studies, this paper presents intercomparison results of three cloud products at the Southern Great Plains (SGP) site: the ARM MICROBASE, University of Utah (UU), and University of North Dakota (UND) products over the nine-year period from 1998 to 2006. The probability density and spatial autocorrelation functions of the three cloud Liquid Water Content (LWC) retrievals appear to be consistent with each other, while large differences are found in the droplet effective radius retrievals. The differences in the vertical distribution of both cloud LWC and droplet effective radius retrievals are found to be alarmingly large, with the relative difference between nine-year mean cloud LWC retrievals ranging from 20% at low altitudes to 100% at high altitudes. Nevertheless, the spread in LWC retrievals is much smaller than that in cloud simulations by climate and cloud resolving models. The MICROBASE effective radius ranges from 2.0 at high altitudes to 6.0 μm at low altitudes and the UU and UND droplet effective radius is 6 μm larger. Further analysis through a suite of retrieval experiments shows that the difference between MICROBASE and UU LWC retrievals stems primarily from the partition total Liquid Water path (LWP) into supercooled and warm liquid, and from the input cloud boundaries and LWP. The large differences between MICROBASE and UU droplet effective radius retrievals are mainly due to rain/drizzle contamination and the assumptions of cloud droplet concentration used in the retrieval algorithms. The large discrepancy between different products suggests caution in model evaluation with these observational products, and calls for improved retrievals in general.

Download Full-text

The Spiderweb Structure of Stratocumulus Clouds

Atmosphere ◽

10.3390/atmos11070730 ◽

2020 ◽

Vol 11 (7) ◽

pp. 730

Author(s):

Georgios Matheou ◽

Anthony B. Davis ◽

João Teixeira

Keyword(s):

Liquid Water ◽

Evaporative Cooling ◽

Liquid Water Content ◽

Entrainment Rate ◽

Liquid Water Path ◽

Integral Boundary ◽

Water Path ◽

Large Eddy ◽

Stratocumulus Clouds ◽

Distinctive Structure

Stratocumulus clouds have a distinctive structure composed of a combination of lumpy cellular structures and thin elongated regions, resembling canyons or slits. The elongated slits are referred to as “spiderweb” structure to emphasize their interconnected nature. Using very high resolution large-eddy simulations (LES), it is shown that the spiderweb structure is generated by cloud-top evaporative cooling. Analysis of liquid water path (LWP) and cloud liquid water content shows that cloud-top evaporative cooling generates relatively shallow slits near the cloud top. Most of liquid water mass is concentrated near the cloud top, thus cloud-top slits of clear air have a large impact on the entire-column LWP. When evaporative cooling is suppressed in the LES, LWP exhibits cellular lumpy structure without the elongated low-LWP regions. Even though the spiderweb signature on the LWP distribution is negligible, the cloud-top evaporative cooling process significantly affects integral boundary layer quantities, such as the vertically integrated turbulent kinetic energy, mean liquid water path, and entrainment rate. In a pair of simulations driven only by cloud-top radiative cooling, evaporative cooling nearly doubles the entrainment rate.

Download Full-text

Gamma Distribution Parameters for Cloud Drop Distributions from Multicylinder Measurements

Journal of Applied Meteorology and Climatology ◽

10.1175/jamc-d-13-0306.1 ◽

2014 ◽

Vol 53 (6) ◽

pp. 1606-1617 ◽

Cited By ~ 3

Author(s):

Kathleen F. Jones ◽

Gregory Thompson ◽

Keran J. Claffey ◽

Eric P. Kelsey

Keyword(s):

Water Content ◽

Gamma Distribution ◽

Liquid Water ◽

Liquid Water Content ◽

Cloud Microphysics ◽

Drop Diameter ◽

Cloud Models ◽

Median Volume ◽

Drop Number ◽

Mount Washington

AbstractThe liquid water content and drop diameters in supercooled clouds have been measured since the 1940s at the summit of Mount Washington in New Hampshire using a rotating multicylinder. Many of the cloud microphysics models in the Weather Research and Forecasting Model (WRF) assume a gamma distribution for cloud drops. In this paper, years of multicylinder data are reanalyzed to determine the best-fitting gamma or monodisperse distribution to compare with parameters in the WRF cloud models. The single-moment cloud schemes specify a predetermined and constant drop number density in clouds, which leads to a fixed relationship between the median volume drop diameter and the liquid water content. The Mount Washington drop number densities are generally larger and best-fit distributions are generally narrower than is typically assumed in WRF.

Download Full-text

The ability of the ICE-T microphysics scheme in HARMONIE-AROME to predict aircraft icing

Weather and Forecasting ◽

10.1175/waf-d-21-0104.1 ◽

2021 ◽

Author(s):

Tim Carlsen ◽

Morten Køltzow ◽

Trude Storelvmo

Keyword(s):

Water Content ◽

Numerical Weather Prediction ◽

Liquid Water ◽

Prediction Models ◽

Weather Prediction ◽

Supercooled Liquid ◽

Liquid Water Content ◽

Aircraft Icing ◽

Microphysics Scheme ◽

Numerical Weather

Abstract In-cloud icing is a major hazard for aviation traffic and forecasting of these events is an important task for weather agencies worldwide. A common tool utilised by aviation forecasters is an icing intensity index based on supercooled liquid water from numerical weather prediction models. We seek to validate the modified microphysics scheme, ICE-T, in the HARMONIE-AROME numerical weather prediction model with respect to aircraft icing. Icing intensities and supercooled liquid water derived from two 3-month winter season simulations with the original microphysics code, CTRL, and ICE-T are compared with pilot reports of icing and satellite retrieved values of liquid and ice water content from CloudSat-CALIPSO and liquid water path from AMSR-2. The results show increased supercooled liquid water and higher icing indices in ICE-T. Several different thresholds and sizes of neighbourhood areas for icing forecasts were tested out, and ICE-T captures more of the reported icing events for all thresholds and nearly all neighbourhood areas. With a higher frequency of forecasted icing, a higher false-alarm ratio cannot be ruled out, but is not possible to quantify due to the lack of no-icing observations. The increased liquid water content in ICE-T shows a better match with the retrieved satellite observations, yet the values are still greatly underestimated at lower levels. Future studies should investigate this issue further, as liquid water content also has implications for downstream processes such as the cloud radiative effect, latent heat release, and precipitation.

Download Full-text

An evaluation of size-resolved cloud microphysics scheme numerics for use with radar observations Part II: Condensation and evaporation

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-20-0213.1 ◽

2021 ◽

Author(s):

Hyunho Lee ◽

Ann M. Fridlind ◽

Andrew S. Ackerman

Keyword(s):

Weather Forecasting ◽

Cloud Microphysics ◽

Model Framework ◽

Microphysics Scheme ◽

Climate Change Research ◽

Numerical Diffusion ◽

Vertical Advection ◽

Large Eddy ◽

Fundamental Process ◽

Turbulence Effects

AbstractAccurate numerical modeling of clouds and precipitation is essential for weather forecasting and climate change research. While size-resolved (bin) cloud microphysics models predict particle size distributions without imposing shapes, results are subject to artificial size distribution broadening owing to numerical diffusion associated with various processes. Whereas Part 1 addressed collision-coalescence, here we investigate numerical diffusion that occurs in solving condensation and evaporation. In a parcel model framework, all of the numerical schemes examined converge to one solution of condensation and evaporation as the mass grid is refined, and the advection-based schemes are recommended over the reassigning schemes. Including Eulerian vertical advection in a column limits the convergence to some extent, but that limitation occurs at a sufficiently fine mass grid, and the number of iterations in solving vertical advection should be minimized to reduce numerical diffusion. Insubstantial numerical diffusion in solving condensation can be amplified if collision-coalescence is also active, which in turn can be substantially diminished if turbulence effects on collision are incorporated. Large-eddy simulations of a drizzling stratocumulus field reveal that changes in moments of Doppler spectra obtained using different mass grids are consistent with those obtained from the simplified framework, and that spectral moments obtained using a mass grid designed to effectively reduce numerical diffusion are generally closer to observations. Notable differences between the simulations and observations still exist, and our results suggest a need to consider whether factors other than numerical diffusion in the fundamental process schemes employed can cause such differences.

Download Full-text

Evaluation of Microphysics Schemes in Tropical Cyclones using Polarimetric Radar Observations: Convective Precipitation in an Outer Rainband

Monthly Weather Review ◽

10.1175/mwr-d-19-0378.1 ◽

2021 ◽

Author(s):

Dan Wu ◽

Fuqing Zhang ◽

Xiaomin Chen ◽

Alexander Ryzhkov ◽

Kun Zhao ◽

...

Keyword(s):

Water Content ◽

Liquid Water ◽

Liquid Water Content ◽

Cloud Microphysics ◽

Convective Precipitation ◽

Polarimetric Radar ◽

Error Source ◽

Ice Water Content ◽

Ice Water ◽

Ice Phase

AbstractCloud microphysics significantly impact tropical cyclone precipitation. A prior polarimetric radar observational study by Wu et al. (2018) revealed the ice-phase microphysical processes as the dominant microphysics mechanisms responsible for the heavy precipitation in the outer rainband of Typhoon Nida (2016). To assess the model performance regarding microphysics, three double-moment microphysics schemes (i.e., Thompson, Morrison, and WDM6) are evaluated by performing a set of simulations of the same case. While these simulations capture the outer rainband’s general structure, microphysics in the outer rainbands are strikingly different from the observations. This discrepancy is primarily attributed to different microphysics parameterizations in these schemes, rather than the differences in large-scale environments due to cloud-environment interactions. An interesting finding in this study is that the surface rain rate or liquid water content is inversely proportional to the simulated mean raindrop sizes. The mass-weighted raindrop diameters are overestimated in the Morrison and Thompson schemes and underestimated in the WDM6 scheme, while the former two schemes produce lower liquid water content than WDM6. Compared with the observed ice water content based on a new polarimetric radar retrieval method, the ice water content above the environmental 0 °C level in all simulations is highly underestimated, especially at heights above 12 km MSL where large concentrations of small ice particles are typically prevalent. This finding suggests that the improper treatment of ice-phase processes is potentially an important error source in these microphysics schemes. Another error source identified in the WDM6 scheme is overactive warm-rain processes that produce excessive concentrations of smaller raindrops.

Download Full-text

Comparison of Observed and Simulated Drop Size Distributions from Large-Eddy Simulations with Bin Microphysics

Monthly Weather Review ◽

10.1175/mwr-d-18-0242.1 ◽

2019 ◽

Vol 147 (2) ◽

pp. 477-493

Author(s):

Mikael K. Witte ◽

Patrick Y. Chuang ◽

Orlando Ayala ◽

Lian-Ping Wang ◽

Graham Feingold

Keyword(s):

Liquid Water ◽

Drop Size ◽

Potential Temperature ◽

Liquid Water Content ◽

Liquid Water Path ◽

Cloud Drop ◽

Large Eddy Simulation Model ◽

Surface Precipitation ◽

Large Eddy ◽

Bin Microphysics

Abstract Two case studies of marine stratocumulus (one nocturnal and drizzling, the other daytime and nonprecipitating) are simulated by the UCLA large-eddy simulation model with bin microphysics for comparison with aircraft in situ observations. A high-bin-resolution variant of the microphysics is implemented for closer comparison with cloud drop size distribution (DSD) observations and a turbulent collision–coalescence kernel to evaluate the role of turbulence on drizzle formation. Simulations agree well with observational constraints, reproducing observed thermodynamic profiles (i.e., liquid water potential temperature and total moisture mixing ratio) as well as liquid water path. Cloud drop number concentration and liquid water content profiles also agree well insofar as the thermodynamic profiles match observations, but there are significant differences in DSD shape among simulations that cause discrepancies in higher-order moments such as sedimentation flux, especially as a function of bin resolution. Counterintuitively, high-bin-resolution simulations produce broader DSDs than standard resolution for both cases. Examination of several metrics of DSD width and percentile drop sizes shows that various discrepancies of model output with respect to the observations can be attributed to specific microphysical processes: condensation spuriously creates DSDs that are too wide as measured by standard deviation, which leads to collisional production of too many large drops. The turbulent kernel has the greatest impact on the low-bin-resolution simulation of the drizzling case, which exhibits greater surface precipitation accumulation and broader DSDs than the control (quiescent kernel) simulations. Turbulence effects on precipitation formation cannot be definitively evaluated using bin microphysics until the artificial condensation broadening issue has been addressed.

Download Full-text

Understanding aerosol–cloud interactions through modeling the development of orographic cumulus congestus during IPHEx

Atmospheric Chemistry and Physics ◽

10.5194/acp-19-1413-2019 ◽

2019 ◽

Vol 19 (3) ◽

pp. 1413-1437 ◽

Cited By ~ 2

Author(s):

Yajuan Duan ◽

Markus D. Petters ◽

Ana P. Barros

Keyword(s):

Water Content ◽

Liquid Water ◽

Cloud Droplet ◽

Liquid Water Content ◽

Cloud Microphysics ◽

Turbulent Dispersion ◽

Cloud Base ◽

Droplet Growth ◽

Aerosol Cloud ◽

Aerosol Cloud Interactions

Abstract. A new cloud parcel model (CPM) including activation, condensation, collision–coalescence, and lateral entrainment processes is used to investigate aerosol–cloud interactions (ACIs) in cumulus development prior to rainfall onset. The CPM was applied with surface aerosol measurements to predict the vertical structure of cloud development at early stages, and the model results were evaluated against airborne observations of cloud microphysics and thermodynamic conditions collected during the Integrated Precipitation and Hydrology Experiment (IPHEx) in the inner region of the southern Appalachian Mountains (SAM). Sensitivity analysis was conducted to examine the model response to variations in key ACI physiochemical parameters and initial conditions. The CPM sensitivities mirror those found in parcel models without entrainment and collision–coalescence, except for the evolution of the droplet spectrum and liquid water content with height. Simulated cloud droplet number concentrations (CDNCs) exhibit high sensitivity to variations in the initial aerosol concentration at cloud base, but weak sensitivity to bulk aerosol hygroscopicity. The condensation coefficient ac plays a governing role in determining the evolution of CDNC, liquid water content (LWC), and cloud droplet spectra (CDS) in time and with height. Lower values of ac lead to higher CDNCs and broader CDS above cloud base, and higher maximum supersaturation near cloud base. Analysis of model simulations reveals that competitive interference among turbulent dispersion, activation, and droplet growth processes modulates spectral width and explains the emergence of bimodal CDS and CDNC heterogeneity in aircraft measurements from different cloud regions and at different heights. Parameterization of nonlinear interactions among entrainment, condensational growth, and collision–coalescence processes is therefore necessary to simulate the vertical structures of CDNCs and CDSs in convective clouds. Comparisons of model predictions with data suggest that the representation of lateral entrainment remains challenging due to the spatial heterogeneity of the convective boundary layer and the intricate 3-D circulations in mountainous regions.

Download Full-text