Demistify: an LES and SCM intercomparison of radiation fog

Abstract. An intercomparison between 10 single-column (SCM) and 5 large-eddy simulation (LES) models is presented for a radiation fog case study inspired by the LANFEX field campaign. 7 of the SCMs represent single-column equivalents of operational numerical weather prediction (NWP) models, whilst 3 are research-grade SCMs designed for fog simulation, and the LES are designed to reproduce in the best manner currently possible the underlying physical processes governing fog formation. The LES model results are of variable quality, and do not provide a consistent baseline against which to compare the NWP models, particularly under high aerosol or cloud droplet number (CDNC) conditions. The main SCM bias appears to be toward over-development of fog, i.e. fog which is too thick, although the inter-model variability is large. In reality there is a subtle balance between water lost to the surface and water condensed into fog, and the ability of a model to accurately simulate this process strongly determines the quality of its forecast. Some NWP-SCMs do not represent fundamental components of this process (e.g. cloud droplet sedimentation) and therefore are naturally hampered in their ability to deliver accurate simulations. Finally, we show that modelled fog development is as sensitive to the shape of the cloud droplet size distribution, a rarely studied or modified part of the microphysical parametrization, as it is to the underlying aerosol or CDNC.

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Demistify: a large-eddy simulation (LES) and single-column model (SCM) intercomparison of radiation fog

Atmospheric Chemistry and Physics ◽

10.5194/acp-22-319-2022 ◽

2022 ◽

Vol 22 (1) ◽

pp. 319-333

Author(s):

Ian Boutle ◽

Wayne Angevine ◽

Jian-Wen Bao ◽

Thierry Bergot ◽

Ritthik Bhattacharya ◽

...

Keyword(s):

Large Eddy Simulation ◽

Weather Prediction ◽

Cloud Droplet ◽

Droplet Size Distribution ◽

Radiation Fog ◽

Eddy Simulation ◽

Cloud Droplet Size Distribution ◽

Large Eddy ◽

Single Column ◽

Les Model

Abstract. An intercomparison between 10 single-column (SCM) and 5 large-eddy simulation (LES) models is presented for a radiation fog case study inspired by the Local and Non-local Fog Experiment (LANFEX) field campaign. Seven of the SCMs represent single-column equivalents of operational numerical weather prediction (NWP) models, whilst three are research-grade SCMs designed for fog simulation, and the LESs are designed to reproduce in the best manner currently possible the underlying physical processes governing fog formation. The LES model results are of variable quality and do not provide a consistent baseline against which to compare the NWP models, particularly under high aerosol or cloud droplet number concentration (CDNC) conditions. The main SCM bias appears to be toward the overdevelopment of fog, i.e. fog which is too thick, although the inter-model variability is large. In reality there is a subtle balance between water lost to the surface and water condensed into fog, and the ability of a model to accurately simulate this process strongly determines the quality of its forecast. Some NWP SCMs do not represent fundamental components of this process (e.g. cloud droplet sedimentation) and therefore are naturally hampered in their ability to deliver accurate simulations. Finally, we show that modelled fog development is as sensitive to the shape of the cloud droplet size distribution, a rarely studied or modified part of the microphysical parameterisation, as it is to the underlying aerosol or CDNC.

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Retrieval of Water Cloud Optical and Microphysical Properties from Combined Multiwavelength Lidar and Radar Data

Remote Sensing ◽

10.3390/rs13214396 ◽

2021 ◽

Vol 13 (21) ◽

pp. 4396

Author(s):

Yinchao Zhang ◽

Su Chen ◽

Wangshu Tan ◽

Siying Chen ◽

He Chen ◽

...

Keyword(s):

Size Distribution ◽

Droplet Size ◽

Weather Prediction ◽

Cloud Droplet ◽

Droplet Size Distribution ◽

Lookup Table ◽

Microphysical Properties ◽

Cloud Droplet Size Distribution ◽

Water Cloud ◽

532 Nm

The remote sensing of water clouds is useful for studying their spatial and temporal variations and constraining physical processes in climate and weather prediction models. However, radar-only detection provides inadequate information for the cloud droplet size distribution. Here, we propose a novel lookup-table method, which combines lidar (1064, 532 nm) and radar (8.6 mm) to retrieve profiles of cloud optical (backscatter coefficient and extinction coefficient) and microphysical properties (effective diameter and liquid water content). Through the iteration of the extinction-to-backscatter ratio, more continuous cloud optical characteristics can be obtained. Sensitivity analysis shows that a 10% error of the lidar constant will lead to a retrieval error of up to 30%. The algorithm performed precise capture of the ideal cloud signal at a specific height and at full height and the maximum relative error of the backscatter coefficients at 1064 nm and 532 nm were 6% and 4%, respectively. With the application of the algorithm in the two observation cases on single or multiple cloud layers, the results indicate that the microphysical properties mostly agree with the empirical radar measurements but are slightly different when larger particles cause signal changes of different extents. Consequently, the synergetic algorithm is capable of computing the cloud droplet size distribution. It provides continuous profiles of cloud optical properties and captures cloud microphysical properties well for water cloud studies.

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An Observational Study on Cloud Spectral Width in North China

Atmosphere ◽

10.3390/atmos10030109 ◽

2019 ◽

Vol 10 (3) ◽

pp. 109 ◽

Cited By ~ 3

Author(s):

Yuan Wang ◽

Shengjie Niu ◽

Chunsong Lu ◽

Yangang Liu ◽

Jingyi Chen ◽

...

Keyword(s):

Size Range ◽

Cloud Droplet ◽

Droplet Size Distribution ◽

Spectral Width ◽

Relative Dispersion ◽

Cloud Base ◽

Cloud Droplets ◽

Factors Affecting ◽

Cloud Droplet Size Distribution ◽

Stratiform Clouds

Cloud droplet size distribution (CDSD) is a critical characteristic for a number of processes related to clouds, considering that cloud droplets are formed in different sizes above the cloud-base. This paper analyzes the in-situ aircraft measurements of CDSDs and aerosol concentration ( N a ) performed in stratiform clouds in Hebei, China, in 2015 to reveal the characteristics of cloud spectral width, commonly known as relative dispersion ( ε , ratio of standard deviation (σ) to mean radius (r) of the CDSD). A new algorithm is developed to calculate the contributions of droplets of different sizes to ε . It is found that small droplets with the size range of 1 to 5.5 μm and medium droplets with the size range of 5.5 to 10 μm are the major contributors to ε, and the medium droplets generally dominate the change of ε. The variation of ε with N a can be well explained by comparing the normalized changes of σ and r ( k σ / σ and k r / r ), rather than k σ and k r only ( k σ is Δσ/Δ N a and k r is Δr/Δ N a ). From the perspective of external factors affecting ε change, the effects of N a and condensation are examined. It is found that ε increases initially and decreases afterward as N a increases, and “condensational broadening” occurs up to 1 km above cloud-base, potentially providing observational evidence for recent numerical simulations in the literature.

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Cloud heterogeneity on cloud and aerosol above cloud properties retrieved from simulated total and polarized reflectances

Atmospheric Measurement Techniques ◽

10.5194/amt-11-3627-2018 ◽

2018 ◽

Vol 11 (6) ◽

pp. 3627-3643 ◽

Cited By ~ 5

Author(s):

Céline Cornet ◽

Laurent C.-Labonnote ◽

Fabien Waquet ◽

Frédéric Szczap ◽

Lucia Deaconu ◽

...

Keyword(s):

Size Distribution ◽

Droplet Size ◽

Optical Thickness ◽

Incidence Angle ◽

Cloud Droplet ◽

Droplet Size Distribution ◽

Cloud Droplet Size Distribution ◽

Fractional Cloud ◽

Cloud Optical Thickness ◽

Heterogeneity Effects

Abstract. Simulations of total and polarized cloud reflectance angular signatures such as the ones measured by the multi-angular and polarized radiometer POLDER3/PARASOL are used to evaluate cloud heterogeneity effects on cloud parameter retrievals. Effects on optical thickness, albedo, effective radius and variance of the cloud droplet size distribution and aerosol parameters above cloud are analyzed. Three different clouds that have the same mean optical thicknesses were generated: the first with a flat top, the second with a bumpy top and the last with a fractional cloud cover. At small scale (50 m), for oblique solar incidence, the illumination effects lead to higher total but also polarized reflectances. The polarized reflectances even reach values that cannot be predicted by the 1-D homogeneous cloud assumption. At the POLDER scale (7 km × 7 km), the angular signature is modified by a combination of the plane–parallel bias and the shadowing and illumination effects. In order to quantify effects of cloud heterogeneity on operational products, we ran the POLDER operational algorithms on the simulated reflectances to retrieve the cloud optical thickness and albedo. Results show that the cloud optical thickness is greatly affected: biases can reach up to −70, −50 or +40 % for backward, nadir and forward viewing directions, respectively. Concerning the albedo of the cloudy scenes, the errors are smaller, between −4.7 % for solar incidence angle of 20∘ and up to about +8 % for solar incidence angle of 60∘. We also tested the heterogeneity effects on new algorithms that allow retrieving cloud droplet size distribution and cloud top pressures and also aerosol above clouds. Contrary to the bi-spectral method, the retrieved cloud droplet size parameters are not significantly affected by the cloud heterogeneity, which proves to be a great advantage of using polarized measurements. However, the cloud top pressure obtained from molecular scattering in the forward direction can be biased up to about 60 hPa (around 550 m). Concerning the aerosol optical thickness (AOT) above cloud, the results are different depending on the available angular information. Above the fractional cloud, when only side scattering angles between 100 and 130∘ are available, the AOT is underestimated because of the plane–parallel bias. However, for solar zenith angle of 60∘ it is overestimated because the polarized reflectances are increased in forward directions.

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Simultaneous retrieval of aerosol and cloud properties during the MILAGRO field campaign

Atmospheric Chemistry and Physics ◽

10.5194/acp-11-6245-2011 ◽

2011 ◽

Vol 11 (13) ◽

pp. 6245-6263 ◽

Cited By ~ 47

Author(s):

K. Knobelspiesse ◽

B. Cairns ◽

J. Redemann ◽

R. W. Bergstrom ◽

A. Stohl

Keyword(s):

Optical Properties ◽

Size Distribution ◽

Aerosol Optical Depth ◽

Optical Depth ◽

Droplet Size ◽

Cloud Droplet ◽

Droplet Size Distribution ◽

Cloud Droplet Size Distribution ◽

Distribution Parameters ◽

Aerosol Properties

Abstract. Estimation of Direct Climate Forcing (DCF) due to aerosols in cloudy areas has historically been a difficult task, mainly because of a lack of appropriate measurements. Recently, passive remote sensing instruments have been developed that have the potential to retrieve both cloud and aerosol properties using polarimetric, multiple view angle, and multi spectral observations, and therefore determine DCF from aerosols above clouds. One such instrument is the Research Scanning Polarimeter (RSP), an airborne prototype of a sensor on the NASA Glory satellite, which unfortunately failed to reach orbit during its launch in March of 2011. In the spring of 2006, the RSP was deployed on an aircraft based in Veracruz, Mexico, as part of the Megacity Initiative: Local and Global Research Observations (MILAGRO) field campaign. On 13 March, the RSP over flew an aerosol layer lofted above a low altitude marine stratocumulus cloud close to shore in the Gulf of Mexico. We investigate the feasibility of retrieving aerosol properties over clouds using these data. Our approach is to first determine cloud droplet size distribution using the angular location of the cloud bow and other features in the polarized reflectance. The selected cloud was then used in a multiple scattering radiative transfer model optimization to determine the aerosol optical properties and fine tune the cloud size distribution. In this scene, we were able to retrieve aerosol optical depth, the fine mode aerosol size distribution parameters and the cloud droplet size distribution parameters to a degree of accuracy required for climate modeling. This required assumptions about the aerosol vertical distribution and the optical properties of the coarse aerosol size mode. A sensitivity study was also performed to place this study in the context of future systematic scanning polarimeter observations, which found that the aerosol complex refractive index can also be observed accurately if the aerosol optical depth is larger than roughly 0.8 at a wavelength of (0.555 μm).

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Limitations of cloud droplet size distribution by Backus-Gilbert inversion of optical scattering data

Journal of the Optical Society of America ◽

10.1364/josa.66.000483 ◽

1976 ◽

Vol 66 (5) ◽

pp. 483 ◽

Cited By ~ 15

Author(s):

Madison J. Post

Keyword(s):

Size Distribution ◽

Droplet Size ◽

Cloud Droplet ◽

Droplet Size Distribution ◽

Optical Scattering ◽

Scattering Data ◽

Cloud Droplet Size Distribution

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Cloud heterogeneity effects on cloud and aerosol above cloud properties retrieved from simulated total and polarized reflectances

10.5194/amt-2017-413 ◽

2017 ◽

Cited By ~ 1

Author(s):

Céline Cornet ◽

Laurent C-Labonnote ◽

Frédéric Szczap ◽

Lucia Deaconu ◽

Fabien Waquet ◽

...

Keyword(s):

Size Distribution ◽

Droplet Size ◽

Optical Thickness ◽

Incidence Angle ◽

Cloud Droplet ◽

Droplet Size Distribution ◽

Cloud Droplet Size Distribution ◽

Fractional Cloud ◽

Cloud Optical Thickness ◽

Heterogeneity Effects

Abstract. Simulations of total and polarized cloud reflectance angular signatures such as the ones measured by the multi-angular and polarized radiometer POLDER3/PARASOL are used to evaluate cloud heterogeneity effects on cloud parameter retrievals. Effects on optical thickness, cloud albedo, effective radius and variance of the cloud droplet size distribution and aerosol above cloud optical thickness are analyzed. Three different clouds having the same mean optical thicknesses were generated: the first one with a flat top, the second one with a bumpy top and the last one with a fractional cloud cover. At small scale (50 m), for oblique solar incidence, the illumination effects lead to higher total but also polarized reflectances. The polarized reflectances even reach values that cannot be predicted by the 1D homogeneous cloud assumption. At the POLDER scale (7 km × 7 km), the angular signature is modified by a combination of the plane-parallel bias and the shadowing and illumination effects. In order to quantify effects of cloud heterogeneity on operational products, we ran the POLDER operational algorithms on the simulated reflectances to retrieve the cloud optical thickness and albedo. Results show that the cloud optical thickness is greatly affected: biases can reach up to −70 %, −50 % or +40 % for backward, nadir and forward viewing directions respectively. Concerning the cloud albedo, the errors are smaller, between −4.7 % for solar incidence angle of 20° and up to about 8 % for solar incidence angle of 60°. We also tested the heterogeneity effects on new algorithms that allow retrieving cloud droplet size distribution and cloud top pressures and also aerosol above clouds. Contrarily to the bi-spectral method, the retrieved cloud droplet size parameters are not significantly affected by the cloud heterogeneity, which proves to be a great advantage of using polarized measurements. However the cloud top pressure obtained from molecular scattering in the forward direction can be biased up to 120 hPa (around 1 km). Concerning the aerosol optical thickness (AOT) above cloud, the results are different depending on the available angular information. Above the fractional cloud, when only side scattering angles are available, the AOT can be underestimated because of the plane-parallel bias. For solar zenith angle of 60°, on contrary, it is overestimated because the polarized reflectances are increased in forward directions.

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Mid-level clouds are frequent above the southeast Atlantic stratocumulus clouds

Atmospheric Chemistry and Physics ◽

10.5194/acp-20-11025-2020 ◽

2020 ◽

Vol 20 (18) ◽

pp. 11025-11043

Author(s):

Adeyemi A. Adebiyi ◽

Paquita Zuidema ◽

Ian Chang ◽

Sharon P. Burton ◽

Brian Cairns

Keyword(s):

Cloud Droplet ◽

Droplet Size Distribution ◽

Aerosol Layer ◽

Cloud Optical Depth ◽

Low Level ◽

Cloud Properties ◽

Cloud Droplet Size Distribution ◽

Stratocumulus Clouds ◽

Absorbing Aerosols ◽

Cloud Tops

Abstract. Shortwave-absorbing aerosols seasonally overlay extensive low-level stratocumulus clouds over the southeast Atlantic. While much attention has focused on the interactions between the low-level clouds and the overlying aerosols, few studies have focused on the mid-level clouds that also occur over the region. The presence of mid-level clouds over the region complicates the space-based remote-sensing retrievals of cloud properties and the evaluation of cloud radiation budgets. Here we characterize the mid-level clouds over the southeast Atlantic using lidar- and radar-based satellite cloud retrievals and observations collected in September 2016 during the ORACLES (ObseRvations of Aerosols above CLouds and their intEractionS) field campaign. We find that mid-level clouds over the southeast Atlantic are relatively common, with the majority of the clouds occurring between altitudes of 5 and 7 km and at temperatures between 0 and −20 ∘C. The mid-level clouds occur at the top of a moist mid-tropospheric smoke-aerosol layer, most frequently between August and October, and closer to the southern African coast than farther offshore. They occur more frequently during the night than during the day. Between July and October, approximately 64 % of the mid-level clouds had a geometric cloud thickness less than 1 km, corresponding to a cloud optical depth of less than 4. A lidar-based depolarization–backscatter relationship for September 2016 indicates that the mid-level clouds are liquid-only clouds with no evidence of the existence of ice. In addition, a polarimeter-derived cloud droplet size distribution indicates that approximately 85 % of the September 2016 mid-level clouds had an effective radius less than 7 µm, which could further discourage the ability of the clouds to glaciate. These clouds are mostly associated with synoptically modulated mid-tropospheric moisture outflow that can be linked to the detrainment from the continental-based clouds. Overall, the supercooled mid-level clouds reduce the radiative cooling rates of the underlying low-altitude cloud tops by approximately 10 K d−1, thus influencing the regional cloud radiative budget.

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Simultaneous retrieval of aerosol and cloud properties during the MILAGRO field campaign

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-11-6363-2011 ◽

2011 ◽

Vol 11 (2) ◽

pp. 6363-6413 ◽

Cited By ~ 1

Author(s):

K. Knobelspiesse ◽

B. Cairns ◽

J. Redemann ◽

R. W. Bergstrom ◽

A. Stohl

Keyword(s):

Optical Properties ◽

Size Distribution ◽

Aerosol Optical Depth ◽

Optical Depth ◽

Droplet Size ◽

Cloud Droplet ◽

Droplet Size Distribution ◽

Field Campaign ◽

Cloud Droplet Size Distribution ◽

Aerosol Properties

Abstract. Estimation of Direct Climate Forcing (DCF) due to aerosols in cloudy areas has historically been a difficult task, mainly because of a lack of appropriate measurements. The Aerosol Polarimetry Sensor (APS), on the upcoming NASA Glory mission, has the potential to retrieve both cloud and aerosol properties because of its polarimetric, multiple view angle, and multi spectral observations. The APS airborne prototype is the Research Scanning Polarimeter (RSP), which has similar characteristics and can be used to demonstrate APS capabilities. In the spring of 2006, the RSP was deployed on an aircraft based in Veracruz, Mexico, as part of the Megacity Initiative: Local and Global Research Observations (MILAGRO) field campaign. On March 13th, the RSP over flew an aerosol layer lofted above a low altitude marine stratocumulus cloud close to shore in the Gulf of Mexico. We investigate the feasibility of retrieving aerosol properties over clouds using these data. Our approach is to first determine cloud droplet size distribution using the angular location of the cloud bow and other features in the polarized reflectance. The selected cloud was then used in a multiple scattering radiative transfer model optimization to determine the aerosol optical properties and fine tune the cloud size distribution. In this scene, we were able to retrieve aerosol optical depth, the fine mode aerosol size distribution and the cloud droplet size distribution to a degree of accuracy required for climate modeling. This required assumptions about the aerosol vertical distribution and the optical properties of the coarse aerosol size mode. A sensitivity study was also performed to place this case study in the context of the potential for future systematic APS observations of this kind, which found that the aerosol complex refractive index can also be observed accurately if the aerosol optical depth is larger than roughly 0.8 at a wavelength of 0.555 μm.

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