Microwave remote sensing is important for observing\nthe mass of ice hydrometeors. One of the main error\nsources of microwave ice mass retrievals is that approximations\naround the shape of the particles are unavoidable. One\ncommon approach to represent particles of irregular shape\nis the soft particle approximation (SPA). We show that it is\npossible to define a SPA that mimics mean optical particles of\navailable reference data over narrow frequency ranges, considering\na single observation technique at the time, but that\nSPA does not work in a broader context. Most critically, the\nrequired air fraction varies with frequency and application, as\nwell as with particle size. In addition, the air fraction matching\nestablished density parameterisations results in far too\nsoft particles, at least for frequencies above 90 GHz. That is,\nalternatives to SPA must be found.\nOne alternative was recently presented by Geer and\nBaordo (2014). They used a subset of the same reference data\nand simply selected as ââ?¬Å?shape modelââ?¬Â the particle type giving\nthe best overall agreement with observations. We present\na way to perform the same selection of a representative particle\nshape but without involving assumptions on particle size\ndistribution and actual ice mass contents. Only an assumption\non the occurrence frequency of different particle shapes\nis still required. Our analysis leads to the same selection of\nrepresentative shape as found by Geer and Baordo (2014).\nIn addition, we show that the selected particle shape has the\ndesired properties at higher frequencies as well as for radar\napplications.\nFinally, we demonstrate that in this context the assumption\non particle shape is likely less critical when using mass\nequivalent diameter to characterise particle size compared to\nusing maximum dimension, but a better understanding of the\nvariability of size distributions is required to fully characterise\nthe advantage.\nFurther advancements on these subjects are presently difficult\nto achieve due to a lack of reference data. One main\nproblem is that most available databases of pre calculated optical\nproperties assume completely random particle orientation,\nwhile for certain conditions a horizontal alignment is\nexpected. In addition, the only database covering frequencies\nabove 340 GHz has a poor representation of absorption\nas it is based on outdated refractive index data as well as\nonly covering particles having a maximum dimension below\n2mm and a single temperature.
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