Program Science Objectives
The following objectives are to be achieved during the Fatima-IF:
- Investigate ice crystal formation and nucleation characteristics during IF events in the presence of low boundary-layer clouds
- Investigate ice fog and low-cloud particle phase conditions (e.g., mixed phase) during ice crystal formation, associated turbulence processes, and vertical drift of ice crystals
- Investigate physical and chemical properties of Arctic aerosols akin to IF life cycle as well as study the diffusion of aerosols and moisture during IF formation
- Evaluate air-sea turbulent fluxes under freezing fog conditions and their evolution/influence on the life cycle of IF
- Investigate how ambient microphysical and flow/turbulence properties affect ice crystal spectra, in particular, the characterization function of Ta and relative humidity with respect to relative humidity of ice (RHi)
- Understand the impacts of turbulence on cold fog microphysical processes and IF life cycle
- Develop microphysical parameterization for IF size spectra and ice crystal number concentration Ni for ABL ice clouds with focus on numerical modeling applications
- Evaluate ice-crystal optical properties such as scattering, absorption, and extinction coefficients for IF conditions
- Evaluate vertical profiles of IF microphysical characteristics based on NSA-site remote sensing platforms (to help improve WRF/LES NWP regional simulations over the Arctic environments)
Observations will be used to test the components of the following hypothesis proposed in the original Fatima MURI proposal:
H1: “Under some conditions, low-level inversion layer(s) play(s) an important role in IF production through the interaction of supercooled droplets and ice nuclei in mixed phase. Inversions trap aerosol particles via the ‘stratification drag’ (Srdić-Mitrović et al. 1999) and facilitates nucleation. The longevity of the inversion, and hence the IF formation, depends on (destabilizing) buoyant convection in the inversion area induced by inversion-top long-wave radiative cooling (which also provides moisture for IF via entrainment) and the strength of the (stabilizing) inversion. These opposing effects can be expressed by a Richardson number criterion [here is the buoyancy jump at the inversion, the inversion-layer thickness, and the buoyant convection velocity]. The inversion stability requires , where is a critical value, but given the multi-phase complexity other parameters such as , and are also expected to play a role (here is the integral length scale of turbulence induced by cloud-top instability, is the mixing ratio above the inversion, the net surface buoyancy flux, and the net radiation flux at the cloud top; see Fig. 2)”
Additional Hypotheses to be tested as part of the proposed work are:
H2: If surface turbulence in ABL is strong enough (for example, the height of the inversion is smaller than the Monin-Obukhov length scale up to which the mechanical turbulence propagates under all stability conditions), the role of the inversion is effaced by surface friction velocity ), and hence IF formation and dissipation are dominated by the surface energetics, moisture and aerosols.
H3: Even if the inversion is not destroyed by surface turbulence, there exists a criterion for decoupling of surface from the low-level clouds. If coupled, the lower ABL is more or less mixed in , and if decoupled the lower ABL tends to be stable , thus affecting IF formation and its dynamics. The relevant dynamic parameters can be determined by a thoughtful scaling analysis.
H4: IN parameterization for IF is significantly different from that of high-level clouds e.g., cirrus, and abundance of aerosols in ABL due to anthropogenic sources leads to a large Ni, which augments extinction of radiation and lower .