Bulk hydrometeor optical properties for microwave and sub-mm radiative transfer in RTTOV-SCATT v13.0

Abstract

Abstract. Satellite observations of radiation in the microwave and sub-mm spectral regions (broadly from 1 to 1000 GHz) can have strong sensitivity to cloud and precipitation particles in the atmosphere. These particles (known as hydrometeors) scatter, absorb and emit radiation according to their mass, composition, shape, internal structure, and orientation. Hence, microwave and sub-mm observations have applications including weather forecasting, geophysical retrievals and model validation. To simulate these observations requires a scattering-capable radiative transfer model and an estimate of the bulk optical properties of the hydrometeors. This article describes the module used to integrate single-particle optical properties over a particle size distribution (PSD) to provide bulk optical properties for the Radiative Transfer for TOVS microwave and sub-mm scattering code, RTTOV-SCATT, a widely-used fast model. Bulk optical properties can be derived from a range of particle models including Mie spheres (liquid and frozen) and non-spherical ice habits from the Liu and Atmospheric Radiative Transfer Simulator (ARTS) databases, which include pristine crystals, aggregates and hail. The effects of different PSD and particle options on simulated brightness temperatures are explored, based on an analytical two-stream solution for a homogeneous cloud slab. The hydrometeor scattering "spectrum" below 1000 GHz is described, along with its sensitivities to particle composition (liquid or ice), size and shape. The optical behaviour of frozen particles changes in the frequencies above 200 GHz, moving towards an optically thick and emission-dominated regime more familiar from the infrared. This region is previously little explored but will soon be covered by the Ice Cloud Imager (ICI).

Implementation of a Discrete Dipole Approximation Scattering Database Into Community Radiative Transfer Model

“…The maximum renormalization orders found by Geer et al. (2021) were also mostly less than 0.1 but there were orders reaching nearly 0.5 for the Field et al. (2007) tropical PSD and up to 9.1 for the Field et al.…”

“…It should be noted that some RT solvers may require the scattering parameters in a different format such as the single scattering albedo and the asymmetry parameter, but as discussed in Section 5, these parameters can be derived from what has been provided in the ARTS database. The ARTS dataset has also been implemented in RTTOV (Radiative Transfer for TOVS) and it has been evaluated in the ECMWF data assimilation system (Geer, 2021; Geer et al., 2021). We limited the rain droplet size to 10 mm while calculating the bulk scattering properties because larger drop sizes may not be realistic.…”

“…(2019), McFarquhar and Heymsfield (1997), and modified gamma distribution recommended by Geer et al. (2021). In most cases, the water content calculated using the number density (Equation ) was a few order of magnitude different from the water content used as input to estimate the PSD parameters.…”

“…Geer et al. (2021) suggest using the ratio of input water content ( Ψ ) to implied or computed water content ( Ψ c ), r = Ψ/Ψ c , to scale the calculated n (D) as n ′( D ) = n ( D ) × r . The implied water content ( Ψ c ) can be calculated using the mass of the particles and number density as follows: $$${{\Psi}}_{c}=\int \nolimits_{{D}_{\mathrm{min}}}^{{D}_{\mathrm{max}}}m(D)n(D)dD=\int \nolimits_{{D}_{\mathrm{min}}}^{{D}_{\mathrm{max}}}\alpha {D}^{\beta }n(D)dD$$$ …”

“…Unlike infrared observations which are normally saturated in the presence of optically thick clouds, MW instruments continue to provide useful information even in the presence of deep convective clouds. Data assimilation systems generally use a fast radiative transfer (RT) model as the forward operator to assimilate satellite observations into the first guess provided by the NWP models (Baordo & Geer, 2016; Geer et al., 2021; Moradi, Evans, et al., 2020).…”