| Popis: |
The effects of aircraft icing comprise of (i) simple inconveniences, like flight delays, (ii) increased fuel consumption and (iii) severe performance degradations, potentially resulting in the loss of control over the aircraft. The accretion of ice onto the aircraft is considered the reason for several documented fatal crashes. Because of this safety hazard for aviation, weather services around the world use aircraft icing prediction schemes to forecast the location and intensity of icing environments, based on the output of numerical weather prediction models. Although aircraft icing occurs by the freezing of supercooled liquid droplets impinging onto the aircraft’s surface, current icing prediction schemes do not consider microphysical quantities, like the droplets’ size or their spectrum, in their design. Analytical icing related algorithms tend to respect the droplets’ size, but in a simplified, heuristic way. The present work develops a new physically based aircraft icing parameterization employing the two main physical processes involved in aircraft icing, namely the droplet impingement and the freezing of the impinged droplets. The former is represented by the droplets’ impingement efficiency, which is computed by evaluating droplet trajectories in the vicinity of the object. The freezing model used in this new parameterization allows for the differentiation of different icing phases, a dry icing phase solely accreting rime ice and a wet icing phase accreting glaze ice and a liquid water film due to the release of latent heat whilst freezing, allowing for a more detailed forecast and analysis. This new aircraft icing parameterization is implemented in a numerical weather forecast model to simulate icing conditions for two former case studies and an additional icing incident. The simulations reproduce the results of the case studies properly and enhance them with additional icing related quantities. Contrary to currently used heuristic approaches to the impingement efficiency, this new physically based approach gives a greater differentiation of high impingement efficiency values. Using this new parameterization, the present work further investigates the role of microphysical quantities by conducting sensitivity studies focused on the explicit consideration of the droplet spectrum and the aerosol load, which is represented by the cloud droplet number density for one experiment and by the number density of aerosols acting as cloud condensation nuclei for the droplet nucleation parameterization for the other. Since the new aircraft icing parameterization introduces further influencing quantities, a further sensitivity investigation focusing on the relative airspeed is conducted. The present work shows that the explicit consideration of the cloud droplet spectrum affects the impingement and icing related quantities only minorly. The alteration of the cloud droplet number density, as a proxy for the effect of the aerosol load, has large implications for the impingement, accretion rates and amounts as well as composition of the accreted material. Altering the number density of cloud condensation nuclei instead affects the impingement mainly in low impinging environments and the composition of the accreted material due to impacts on the accretion of rime ice. Glaze ice and liquid water accretion are largely unaffected. The relative airspeed also affects the droplet impingement, but its largest effect is on the accretion rates caused by the impinging water and the composition of the accreted material. The newly developed physically based parameterization of aircraft icing also gives the possibility to design a new online icing prediction scheme, as it is implemented into an operatively used numerical weather forecast model. This requires additional efforts in designing an appropriate icing intensity scale as well as in testing, validating and tuning. |
