Background & purpose: Overall, the top level objective of the ICE GENESIS project is to provide the European aeronautical industry with a validated new generation of 3D icing engineering tools (numerical simulation and test capabilities), for safe, efficient, and cost effective design and certification of future aircraft and rotorcraft. Besides Appendix C & O, this top level objective is also addressing snow conditions.

Today, with respect to snow, there are no validated engineering tools (test facility and numerical tools) available to support design of power plant systems by assessing the risk of snow accretion or accumulation. In addition, the characterisation of snow precipitation microphysics is not well defined in the available regulations (CS25/29) and guidance material (Advisory Circular AC29-2C, Acceptable Means of Compliance AMC25.1093) of the Federal Aviation Administration (FAA).

These regulations and the guidance material prescribe temperature conditions between -9 and +2°C, but solely provide rough recommendations for the types of snow conditions to be tested, if tests are required. Furthermore, the conditions are less detailed than for other icing conditions especially concerning number particle size distribution (PSD), snow particle density, knowledge about morphological shape and associated microphysical properties, distinction between wet and dry snowflake characteristics, others...

Context and limiting factors for quantifying snow microphysical properties / starting point: From literature we gathered information about snow microphysical properties at the beginning of the ICE GENESIS project, stating that snowflake diameters are mainly between 2 and 5 mm (Pruppacher & Klett; 2010), ranging up to 15 mm. Snowflake density varies, ranging from 0.005 to 0.2 g cm-3, being inversely proportional to snowflake diameter, i.e. the larger the flakes, the lower the density. This constant of proportionality between snowflake diameter and the density of the snowflake is almost four times larger for wet than for dry snowflakes. Overall, microphysical data from past measurement campaigns including precipitating ice during winter season, like OLYMPEX or GCPEX, are barely detailed in terms of statistics of solid particle / snow particle microphysical properties. As a further starting point for the work to be performed within WP5 and final deliverable D5.7, it has been also stated that ground based and some limited airborne in situ snow measurements in snow conditions performed in the past were rarely set up for pure and extremely detailed analysis of the microphysical properties of snow.

Often the microphysical measurements roughly served to validate precipitation related remote sensing retrievals within GPM for example, without presenting details on a series of microphysical parameters, as is needed within ICE GENESIS for WP10 snow accretion modelling.

This is why, within ICE GENESIS, proper ground and flight tests have been conducted in snow conditions, with the overall objective of a detailed characterization of all relevant snow and precipitation related microphysical parameters that were iteratively feeding the development of snow numerical tools and were used for comparison with artificial snow generated in test facilities during the ongoing project.
Main results / discoveries / findings: A close collaboration has been established between Technostream SNOW related workpackages of ICE GENESIS with WP5, WP7, and WP10, primarily in order to meet WP10 needs of 3D descriptors (size distributions of mass, volume, surface, area, sphericity, orthogonal or crosswise sphericity, ice effective density) for particle trajectory and particle melting models. This is why the measurements conducted during the ground based and airborne field-campaign in the Swiss Jura (GTRA documented in D5.5 and FTRA in D5.6) aimed to give momentum to quantitative and detailed snowfall microphysics including all above snow parameters at temperatures ranging from -10°C to +2°C. The combination of ground based remote sensing (multiple radar systems), in-situ (MASC snowflake camera) and aircraft-based measurement techniques (in situ and remote sensing) was designed to sample clouds and precipitation in the entire column and at different scales; from the large sampling volumes of radar data to the depiction of individual hydrometeors or imagers. The abundance of in-situ data offers the rare possibility to provide ground-truth for hydrometeor classification algorithms based on remotely sensed observations. The main scientific work within WP5 has then been the common quantification of 2D & 3D descriptors on the one hand from ATR-42 and MASC in-situ snow data and on the other hand from closure studies, including in situ validation of Doppler cloud radar TWC retrievals, of dual-frequency radar 2D and 3D descriptor retrievals, and of dual-polarization radar retrievals of crystal morphology and TWC.

Key observations from past data analysis (ID5.1) and mainly the ‘Les Eplatures’ campaign performed within the frame of ICE GENESIS are:
- 5-sec snow water content values reached 0.5-1.0 g/m³ rather frequently, with values peaking up to 1.5 g/m³ and few events up to 2.0 g/m³.
- Median mass diameters (MMD) varied significantly during single flight missions and from one flight to the other. Overall MMDs of 2-3 mm were rather often measured and maximum encountered snow MMDs were peaking up to 4 mm and sometimes even 5 mm, which are reasonably high values for snow MMDs.
- The crystal density and sphericity parameters as a function of particle size is strongly depending on the morphological shape. More or less rimed aggregates and graupel are comprising the main classes of snow particles.
- A final table is produced giving quantitative numbers within specific snow particle size ranges and next within three distinct temperature intervals (-10 to -4°C; -4 to 0°C; 0 to +2 °C) of number and mass fractions of different morphological classes, crystal density as a function of size and morphological classes, likewise for the sphericity parameters, others…).

Use of this deliverable / Future key issues: The 3D descriptor statistics are calculated and plotted in a series of figures as function of morphological class, of three distinct temperature ranges, and of the size of the snow particles. The scientific statistics are presented in the most important chapter 9 of this deliverable. Finally, all this information is condensed in quantitative tables, reading out values of ice density, sphericity, orthogonal sphericity, mass fractions, number fractions of morphological classes that will be still used within ICE GENESIS for snow accretion modelling, but can be also used in near future by regulation authorities, etc…

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