Stratus–Fog Formation and Dissipation: A 6-Day Case Study

Dupont, Jean-Charles; Haeffelin, Martial; Protat, Alain; Bouniol, Dominique; Boyouk, Neda; Morille, Yohann

doi:10.1007/s10546-012-9699-4

Cited by 60 publications

(48 citation statements)

References 33 publications

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“…However, these events are not classified as stratus lowering as they were followed rapidly by formation of fog at the surface. A delay of 30 min between the formation at 150 m height and at the ground seems too short to be a stratus lowering, which is mainly driven by the evaporation of slowly falling droplets that cool the sub-cloud layer (Dupont et al, 2012). This suggests that this type of radiation fog could be linked with, and specific to, the configuration of the SIRTA site.…”

Section: Presentation Of the Observed Casementioning

confidence: 99%

Large eddy simulation of radiation fog: impact of dynamics on the fog life cycle

et al. 2017

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Abstract. Large eddy simulations (LESs) of a radiation fog event occurring during the ParisFog experiment are studied with a view to analyse the impact of the dynamics of the boundary layer on the fog life cycle. The LES, performed with the Meso-NH model at 5 m resolution horizontally and 1 m vertically, and with a 2-moment microphysical scheme, includes the drag effect of a tree barrier and the deposition of droplets on vegetation. The model shows good agreement with measurements of near-surface dynamic and thermodynamic parameters and liquid water path. The blocking effect of the trees induces elevated fog formation, as actually observed, and horizontal heterogeneities during the formation. It also limits cooling and cloud water production. Deposition is found to exert the most significant impact on fog prediction as it not only erodes the fog near the surface but also modifies the fog life cycle and induces vertical heterogeneities. A comparison with the 2 m horizontal resolution simulation reveals small differences, meaning that grid convergence is achieved. Conversely, increasing numerical diffusion through a wind advection operator of lower order leads to an increase in the liquid water path and has a very similar effect to removing the tree barrier. This study allows us to establish the major dynamical ingredients needed to accurately represent the fog life cycle at very high-resolution.

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Section: Presentation Of the Observed Casementioning

confidence: 99%

Large eddy simulation of radiation fog: impact of dynamics on the fog life cycle

et al. 2017

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“…An advantage of SIRTA is the continuous measurements by several ground-based remote sensing instruments. Such instruments have been proven useful for the study of the fog life cycle: the attenuated backscatter from a ceilometer can detect the growth of aerosols preceding fog formation , while a cloud radar can provide information about the fog vertical development and properties once it has formed (Teshiba et al, 2004;Boers et al, 2012;Dupont et al, 2012). In this study, we use the observations from several instruments of SIRTA (Table 1) to analyse periods when fog occurred.…”

Section: Observational Site and Instrumentationmentioning

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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“…Doppler cloud radars are amazing tools to characterize cloud and fog properties and to improve their representation in models (Illingworth et al 2007;Bouniol et al 2010;Haeffelin et al 2010;Maier et al 2012;Dupont et al 2012). Depending on the scientific application, they can be deployed from ground or ship (Moran et al 1998;Kollias et al 2007a), aircraft (Horie et al 2000;Li et al 2001;Wolde and Pazmany 2005;Delanoë et al 2013;Hagen et al 2014), or satellite (Stephens et al 2002;Illingworth et al 2015).…”

Section: Introductionmentioning

confidence: 99%

“…This very high-vertical resolution mode is ideal for fog and low-stratus studies (Maier et al 2012;Dupont et al 2012). Figure 4 shows two examples of low-cloud/fog measurements.…”

mentioning

confidence: 99%

BASTA: A 95-GHz FMCW Doppler Radar for Cloud and Fog Studies

Delanoë

Protat

Vinson

et al. 2016

Journal of Atmospheric and Oceanic Technology

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Doppler cloud radars are amazing tools to characterize cloud and fog properties and to improve their representation in models. However, commercially available cloud radars (35 and 95 GHz) are still very expensive, which hinders their widespread deployment. This study presents the development of a lower-cost semioperational 95-GHz Doppler cloud radar called the Bistatic Radar System for Atmospheric Studies (BASTA). To drastically reduce the cost of the instrument, a different approach is used compared to traditional pulsed radars: instead of transmitting a large amount of energy for a very short time period (as a pulse), a lower amount of energy is transmitted continuously. By using a specific signal processing technique, the radar can challenge expensive radars and provide high-quality measurements of cloud and fog. The latest version of the instrument has a sensitivity of about −50 dBZ at 1 km for 3-s integration and a vertical resolution of 25 m. The BASTA radar currently uses four successive modes for specific applications: the 12.5-m vertical resolution mode is dedicated to fog and low clouds, the 25-m mode is for liquid and ice midtropospheric clouds, and the 100- and 200-m modes are ideal for optically thin high-level ice clouds. The advantages of such a radar for calibration procedures and field operations are also highlighted. The radar comes with a set of products dedicated to cloud and fog studies. For instance, cloud mask, corrected Doppler velocity, and multimode products combining the high-sensitivity mode and high-resolution modes are provided.

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Stratus–Fog Formation and Dissipation: A 6-Day Case Study

Cited by 60 publications

References 33 publications

Large eddy simulation of radiation fog: impact of dynamics on the fog life cycle

Large eddy simulation of radiation fog: impact of dynamics on the fog life cycle

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

BASTA: A 95-GHz FMCW Doppler Radar for Cloud and Fog Studies

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