Radiation Fog. Part I: Observations of Stability and Drop Size Distributions

Price, J. D.

doi:10.1007/s10546-010-9580-2

Cited by 88 publications

(115 citation statements)

References 44 publications

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“…droplet activation and a rise the observed and LES cloud droplet numbers . Fig 9(b) shows a small secondary peak in the size spectra around 10 µm developing due to the increasing turbulence, and the evolution of the spectra and cloud droplet numbers after this point is very similar to that already reported in Price (2011) and Tonttila et al (2017), hence we do not focus on it here. The well-mixed layer continues to grow throughout the night, until the entire fog layer is well-mixed and turbulent by 08 UTC.…”

Section: Les Analysis 10supporting

confidence: 61%

“…for the droplet concentration at the lowest model level, tapering up to a value determined by the concentration of aerosol (Osborne et al, 2014) at 150 m. This tapering process is a pragmatic attempt at representing the fact that cloud drop numbers in fog tend to be lower than those 15 observed higher up in the atmosphere (Gultepe et al, 2009;Price, 2011), however it appears that it is failing to represent some key features. The drop numbers in fog are believed to be low because, despite the abundance of aerosol from which to form cloud droplets near the surface, the lack of any appreciable updrafts in the near surface stable boundary layer results in weak supersaturations.…”

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confidence: 99%

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Aerosol-fog interaction and the transition to well-mixed radiation fog

Boutle¹,

Price²,

Kudzotsa³

et al. 2017

Preprint

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Abstract. We analyse the development of a radiation fog event and its gradual transition from optically thin fog in a stable boundary layer to well-mixed optically-thick fog. Comparison of observations and a detailed large-eddy simulation demonstrate that aerosol growth and activation is the key process in determining the onset of adiabatic fog. Weak turbulence and low supersaturations lead to the growth of aerosol particles which can significantly affect the visibility, but do not significantly interact with the long-wave radiation, allowing the atmosphere to remain stable. Only when a substantial fraction of the aerosol 5 become activated into cloud droplets can the fog interact with the radiation, becoming optically thick and well-mixed. Modifications to the parametrization of cloud droplet numbers in fog, resulting in lower and more realistic concentrations, are shown to give significant improvements to an NWP model, which initially struggled to accurately simulate the transition. Finally, consequences of this work for common aerosol activation parametrizations used in climate models are discussed, demonstrating that many schemes are reliant on an artificial minimum value when activating aerosol in fog, and adjustment of this minimum 10 can significantly affect the sensitivity of the climate system to aerosol radiative forcing.

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Section: Les Analysis 10supporting

confidence: 61%

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confidence: 99%

Aerosol-fog interaction and the transition to well-mixed radiation fog

Boutle¹,

Price²,

Kudzotsa³

et al. 2017

Preprint

Self Cite

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“…However, accurate and general relationships cannot be found from Z alone, since Z is most sensitive to the largest droplets, which may only weakly impact LWC and r eff . As the shape of the DSD varies significantly during and between fog events (Boers et al, 2012;Gultepe et al, 2007;Price, 2011), retrievals of LWC and r eff using Z alone will only be rough estimates, even in the absence of drizzle. A synergy with the more reliable LWP from MWR is therefore used in several methods in the literature, with varying approaches for vertically distributing this liquid water inside the cloud.…”

Section: Appendix B: Estimation Of Vertical Profiles Of Microphysicalmentioning

confidence: 99%

Radiation in fog: quantification of the impact on fog liquid water based on ground-based remote sensing

Wærsted¹,

Haeffelin

Dupont

et al. 2017

Atmos. Chem. Phys.

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Abstract. Radiative cooling and heating impact the liquid water balance of fog and therefore play an important role in determining their persistence or dissipation. We demonstrate that a quantitative analysis of the radiation-driven condensation and evaporation is possible in real time using groundbased remote sensing observations (cloud radar, ceilometer, microwave radiometer). Seven continental fog events in midlatitude winter are studied, and the radiative processes are further explored through sensitivity studies. The longwave (LW) radiative cooling of the fog is able to produce 40-70 g m −2 h −1 of liquid water by condensation when the fog liquid water path exceeds 30 g m −2 and there are no clouds above the fog, which corresponds to renewing the fog water in 0.5-2 h. The variability is related to fog temperature and atmospheric humidity, with warmer fog below a drier atmosphere producing more liquid water. The appearance of a cloud layer above the fog strongly reduces the LW cooling relative to a situation with no cloud above; the effect is strongest for a low cloud, when the reduction can reach 100 %. Consequently, the appearance of clouds above will perturb the liquid water balance in the fog and may therefore induce fog dissipation. Shortwave (SW) radiative heating by absorption by fog droplets is smaller than the LW cooling, but it can contribute significantly, inducing 10-15 g m −2 h −1 of evaporation in thick fog at (winter) midday. The absorption of SW radiation by unactivated aerosols inside the fog is likely less than 30 % of the SW absorption by the water droplets, in most cases. However, the aerosols may contribute more significantly if the air mass contains a high concentration of absorbing aerosols. The absorbed radiation at the surface can reach 40-120 W m −2 during the daytime depending on the fog thickness. As in situ measurements indicate that 20-40 % of this energy is transferred to the fog as sensible heat, this surface absorption can contribute significantly to heating and evaporation of the fog, up to 30 g m −2 h −1 for thin fog, even without correcting for the typical underestimation of turbulent heat fluxes by the eddy covariance method. Since the radiative processes depend mainly on the profiles of temperature, humidity and clouds, the results of this paper are not site specific and can be generalised to fog under different dynamic conditions and formation mechanisms, and the methodology should be applicable to warmer and moister climates as well. The retrieval of approximate emissivity of clouds above fog from cloud radar should be further developed.

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“…In particular, the model introduced in this work accurately preserves the characteristics of the aerosol size distribution both inside and outside of clouds, making it ideal for studying the impact of removal processes, cloud processing and evaporation on the particle size distribution, as well as the associated feedbacks on cloud properties, precipitation formation and boundary layer dynamics. The model is evaluated by experimenting on two very different cases: one comprising marine stratocumulus clouds based on the DYCOMS-II dataset (Stevens et al, 2003), and another focusing on a radiation fog event based on the findings of (Porson et al, 2011;Price, 2011). The results are compared with earlier studies and models with a simple bulk microphysics scheme, and similarities and differences are analysed and explained in detail.…”

Section: Introductionmentioning

confidence: 99%

UCLALES–SALSA v1.0: a large-eddy model with interactive sectional microphysics for aerosol, clouds and precipitation

et al. 2017

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Abstract. Challenges in understanding the aerosol-cloud interactions and their impacts on global climate highlight the need for improved knowledge of the underlying physical processes and feedbacks as well as their interactions with cloud and boundary layer dynamics. To pursue this goal, increasingly sophisticated cloud-scale models are needed to complement the limited supply of observations of the interactions between aerosols and clouds. For this purpose, a new large-eddy simulation (LES) model, coupled with an interactive sectional description for aerosols and clouds, is introduced. The new model builds and extends upon the well-characterized UCLA Large-Eddy Simulation Code (UCLALES) and the Sectional Aerosol module for Large-Scale Applications (SALSA), hereafter denoted as UCLALES-SALSA. Novel strategies for the aerosol, cloud and precipitation bin discretisation are presented. These enable tracking the effects of cloud processing and wet scavenging on the aerosol size distribution as accurately as possible, while keeping the computational cost of the model as low as possible. The model is tested with two different simulation set-ups: a marine stratocumulus case in the DYCOMS-II campaign and another case focusing on the formation and evolution of a nocturnal radiation fog. It is shown that, in both cases, the size-resolved interactions between aerosols and clouds have a critical influence on the dynamics of the boundary layer. The results demonstrate the importance of accurately representing the wet scavenging of aerosol in the model. Specifically, in a case with marine stratocumulus, precipitation and the subsequent removal of cloud activating particles lead to thinning of the cloud deck and the formation of a decoupled boundary layer structure. In radiation fog, the growth and sedimentation of droplets strongly affect their radiative properties, which in turn drive new droplet formation. The size-resolved diagnostics provided by the model enable investigations of these issues with high detail. It is also shown that the results remain consistent with UCLALES (without SALSA) in cases where the dominating physical processes remain well represented by both models.

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Radiation Fog. Part I: Observations of Stability and Drop Size Distributions

Cited by 88 publications

References 44 publications

Aerosol-fog interaction and the transition to well-mixed radiation fog

Aerosol-fog interaction and the transition to well-mixed radiation fog

Radiation in fog: quantification of the impact on fog liquid water based on ground-based remote sensing

UCLALES–SALSA v1.0: a large-eddy model with interactive sectional microphysics for aerosol, clouds and precipitation

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