Project Summary and Governing Hypotheses
Fog consists of suspended water droplets or ice crystals in the lower atmosphere formed by favorable collusion of dynamic, microphysical, physicochemical, thermodynamic and surface processes, and is identified by conditions with near-ground visibility less than 1 km. It negatively affects societal functions such as transportation, communications and ecosystem wellbeing. Characterization of the fog environment is also imperative for directed high energy laser, free-space optics and remote detection applications. The predictability of fog and its electro-optic propagation characteristics, however, is amongst the poorest in meteorology.
This project (Fatima) seeks leaps of fundamental knowledge and prediction and detection capabilities pertinent to fog in marine environments, in particular, Marine Sea Fog (MSF) that forms over shallow seas and shelves as well as on coastal Ice Fog (IF).
The approach includes theoretical analysis, detailed process studies, novel high-resolution numerical simulations, extensive field measurements and predictive modeling in collaboration with practitioners.
The project is multidisciplinary, and draws expertise from a group of researchers covering the scale continuum from large regional weather systems (synoptic) to fog droplets of micrometer scales that grow on aerosols of nanoscales (microphysical).
The team has experience in field measurements and analyses, theoretical and numerical modeling, satellite remote sensing and forecasting with numerical weather prediction (NWP) models.
The objectives are to understand and quantify the intricate interplay between processes underlying the lifecycles of MSF and IF, and represent them in forecasting models via:
- Deploying leading-edge instrumentation, including novel measurement technologies, to probe from synoptic to fog-spawning smallest scale of turbulent motions (Kolmogorov K scales);
- Theoretical/numerical analyses that push frontiers of two-phase turbulence, especially non-linear dynamics of turbulence-droplets interactions;
- Delving into droplet microphysics, thermodynamics, ocean-surface processes and large-scale forcing;
- Developing microphysical parameterizations for NWP models to help improve the prediction of visibility (Vis); and
- Understanding and modeling the impacts of fog and turbulence on electro-optical (EO) propagation.
The project is underpinned by eight hypothesis:
Hypothesis 1 (On fog climatology) Warmer humid airflow along negative SST gradients in collusion with a cooling sensible air-sea heat flux provides conditions for fog formation. This is favored by pronounced air-sea interactions (e.g., elevated wind speeds), intense oceanic turbulent mixing responsible for SST gradients (e.g., shelf mixing) and specific synoptic forcing types.
Hypothesis 2: (On the sea-surface influence) Unlike low-level layered clouds, sea-surface processes control the details of fog lifecycle through air-sea momentum, heat and water-vapor fluxes, local surface gravity-wave field and active wave breaking that produces sea surface aerosols SSAs, while sea-surface properties such as air-sea temperature differential and low-level wind shear playing a significant role.
Hypothesis 3: (On the hydrometeor effects) Precipitation from clouds above the fog layer has profound impacts on the fog lifecycle by moistening the sub-cloud layer, scavenging fog droplets, suppressing surface SSA production and modifying surface waves and turbulent fluxes
Hypothesis 4: (On the dynamical origins of fog) While synoptic to microscales influence fog genesis, the critical (rate determining) step is the outer (integral) scale eddies feeding turbulent kinetic energy (TKE) to the dissipating Kolmogorov K scales via a nonlinear energy cascade across the inertial subrange. It is within these K eddies that temperature/moisture homogenizes and spawns fog droplets around embedded nuclei by vapor condensation, the rate of which is determined by aerosol physicochemical properties and dynamical conditions within K eddies.
Hypothesis 5: (On thermodynamics) Radiative cooling and heating are crucial for the lifecycle of MSF through their link to microphysical and turbulence processes. In concert they determine fog characteristics, type (stratus lowering versus advective) and heterogeneity.
Hypothesis 6: (On electro-optical EO propagation) Optical attenuation in MSF can be parameterized using appropriate fog integrated microphysical and MABL turbulence parameters.
Hypothesis 7: (On fog detection based on EM propagation characteristics) A two-wavelength microwave MW and near infra-red NIR scintillometer can be used to infer microphysical properties, forms of precipitation, and evolutionary stages of different fog types at scales of 1 km, which are most relevant to NWP models.
Hypothesis 8: (On elucidating the dynamics of Ice Fog) Low-level inversion layer(s) may play an important role in IF production through the interaction of supercooled droplets and ice nuclei in mixed phase. Inversions trap aerosol particles via ‘stratification drag’ facilities nucleation. The longevity of the inversion, and hence the IF formation, depends on (destabilizing) buoyant convection induced by inversion-top LWR cooling (which also provides moisture for IF via entrainment) and the strength of the (stabilizing) inversion.