Direct Numerical Simulation of Fog: The Sensitivity of a Dissipation Phase to Environmental Conditions

Abstract

The sensitivity of fog dissipation to the environmental changes in radiation, liquid-water lapse rate, free tropospheric temperature and relative humidity was studied through numerical experiments designed based on the 2007-Paris Fog observations. In particular, we examine how much of the stratocumulus-thinning mechanism can be extended to the near-surface clouds or fog. When the free troposphere is warmed relative to the reference case, fog-top descends and become denser. Reducing the longwave radiative cooling via a more emissive free troposphere favors thickening the physical depth of fog, unlike cloud-thinning in a stratocumulus cloud. Drying the free troposphere allows fog thinning and promotes fog dissipation while sustaining the entrainment rate. The numerical simulation results suggest that the contribution of entrainment drying is more effective than the contribution of entrainment warming yielding the reduction in liquid water path tendency and promoting the onset of fog depletion relative to the reference case studied here. These sensitivity experiments indicate that the fog lifting mechanism can enhance the effect of the inward mixing at the fog top. However, to promote fog dissipation, an inward mixing mechanism only cannot facilitate removing humidity in the fog layer unless a sufficient entrainment rate is simultaneously sustained.Atmosphere 2020, 11, 12 2 of 22 stage. From the modeling standpoint, this challenge is attributed to a lack of knowledge of the physical processes and limitations in the ability to represent these processes in models. Forecasting numerical models predominantly use single-column schemes and also require parameterization of turbulent processes [15][16][17]. The role of turbulence on fog evolution remains controversial. Some argue that turbulence hampers the development of fog [18,19], while others deduce that turbulence favors fog development [20]. Poor predictive capabilities of most fog simulation schemes are attributed to not accurately capturing the local characteristics of turbulent mixing and surface-atmosphere coupling [10,21].The role of entrainment in fog life cycle has been mostly highlighted in marine fog through observational campaigns [22][23][24] and numerical simulations [25][26][27][28]. High entrainment drying is suggested to be not conducive to fog dissipation unless the radiative cooling increases [25]. While enhancement of TKE and Deardroff's convective velocity during the dissipation phase of an advection-radiation fog event has been observed [24], it was shown that due to low entrainment rate, the vertical turbulent fluxes are not strong enough to provide a convective source to prompt fog dissipation [29]. The contribution of the entrainment of drier and warmer air from aloft to fog dissipation has been identified as crucial to the role of the high subsidence rate and the shortwave radiative flux [30]. Some investigators attribute marine fog dissipation to thermal turbulence and dry air entrainment in the fog-top region [23,24,31], and others to the li...

Observation, Simulation and Predictability of Fog: Review and Perspectives

“…By simulating several hundred million fog droplets as Lagrangian particles explicitly, this approach could resolve explicitly the diffusional growth of fog droplets, including Köhler theory and gravitational sedimentation representation [8]. Direct numerical simulations have also emerged as a valuable tool in resolving fog on scales in the order of a meter or less [17]. This type of simulation can provide insights into small scale radiation and turbulence interactions and the entrainment on fog evolution and dissipation.…”

Experimental study on the evolution of droplet size distribution during the fog life cycle

“…Droplets can conversely sediment by gravity (Bott, 1991 andDegefie et al, 2015) or turbulent motions (Tav et al, 2018), or evaporate if the supersaturation decreases due to heating or drying of the air mass, e.g., in the case of mixing with the residual dry air (Pilié et al, 1975;Choularton et al, 1981;Gerber, 1991). Additionally, Schwenkel and Maronga (2018) showed that different parametrizations of the activation and condensation processes impact the vertical extent and liquid water path of fog, which strongly affect the fog life cycle (Waersted et al, 2019 andKarimi, 2020).…”

A satellite‐based climatology of fog and low stratus formation and dissipation times in central Europe

“…The knowledge of formation and dissipation time of fog relies strongly on ground‐based observational data and localized process studies with numerical models such as large‐eddy simulations. These have been conducted for example in France (e.g., Haeffelin et al ., 2010; Dupont et al ., 2012; Wærsted et al ., 2019; Karimi, 2020) or in the Netherlands (Duynkerke, 1991; Steeneveld and de Bode, 2018) over time‐scales ranging from 6 days (Dupont et al ., 2012) up to 7 years (Wærsted et al ., 2019). According to these studies, radiation fog usually forms during the night through nocturnal cooling (Roach, 1995) and dissipates a few hours after sunrise (Haeffelin et al ., 2010; Bergot, 2016; Steeneveld and de Bode, 2018).…”