The applicability of physical optics in the millimetre and sub-millimetre spectral region. Part I: The ray tracing with diffraction on facets method

Abstract

Future satellite missions, from 2021 onwards, will obtain near-global measurements of cirrus at microwave and sub-millimetre frequencies. To realise the potential of these observations, fast and accurate light scattering methods are required to calculate scattered millimetre and sub-millimetre intensities from complex ice crystals. Here, the applicability of the ray tracing with diffraction on facets method (RTDF) in predicting the bulk scalar optical properties and phase functions of randomly oriented hexagonal ice columns and hexagonal ice aggregates at millimetre frequencies is investigated. The applicability of RTDF is shown to be acceptable down to size parameters of about 18, between the frequencies of 243 and 874 GHz. It is demonstrated that RTDF is generally well within about 10% of T-matrix solutions obtained for the scalar optical properties assuming hexagonal ice columns. Moreover, on replacing electromagnetic scalar optical property solutions obtained for the hexagonal ice aggregate with the RTDF counterparts at size parameter values of about 18 and greater, the bulk scalar optical properties can be calculated to generally well within ±5% of an electromagnetic-based database. The RTDF-derived bulk scalar optical properties result in brightness temperature errors to generally within about ±4 K at 874 GHz. Differing microphysics assumptions can easily exceed such errors. Similar findings are found for the bulk scattering phase functions. This finding is owing to the scattering solutions being dominated by the processes of diffraction and reflection, both being well described by RTDF. The impact of centimetre-sized complex ice crystals on interpreting cirrus polarisation measurements at sub-millimetre frequencies is discussed.

A general database of hydrometeor single scattering properties at microwave and sub-millimetre wavelengths

“…This potential inaccuracy must not be ignored, but should today be relatively small compared to all other uncertainties involved in full simulations and retrievals. For example, Baran et al (2017) reasoned in the same way and were also content with a 10 % accuracy. However, much higher discrepancies were found occasionally, particularly regarding backscattering.…”

Analysis of the internal electric fields of pristine ice crystals and aggregate snowflakes, and their effect on scattering

“…However, improvements to geometric optics methods have been shown to be accurate for x as small as 18, e.g. using methods such as ray tracing with diffraction on facets (RTDF; [46]). Hence it is possible that for the larger size parameters considered here, physical or geometric optics methods may be suitable approximations to apply.…”

The applicability of physical optics in the millimetre and sub-millimetre spectral region. Part II: Application to a three-component model of ice cloud and its evaluation against the bulk single-scattering properties of various other aggregate models

“…The bulk single-scattering properties of these two models are computed using the light-scattering methods described in Part I. In Part I, it was shown that the physical optics method was applicable in the microwave region down to size parameters of about 18, where the size parameter was defined as D/; this same definition of the size parameter is used in this paper [29]. The bridging between the electromagnetic and physical optics methods applied at size parameters smaller or greater than 18, respectively, was achieved using the ray tracing with diffraction on facets (RTDF) method developed by Ref.…”