Three Dimensional Radiative Effects in Passive Millimeter/Sub-Millimeter All-sky Observations

Abstract

This study was conducted to quantify the errors prompted by neglecting three-dimensional (3D) effects, i.e., beam-filling and horizontal photon transport effects, at millimeter/sub-millimeter wavelengths. This paper gives an overview of the 3D effects that impact ice cloud retrievals of both current and proposed (Ice Cloud Imager) satellite instruments operating at frequencies of ≈186.3 and ≈668 GHz. The 3D synthetic scenes were generated from two-dimensional (2D) CloudSat (Cloud Satellite) observations over the tropics and mid-latitudes using a stochastic approach. By means of the Atmospheric Radiative Transfer Simulator (ARTS), three radiative transfer simulations were carried out: one 3D, one independent beam approximation (IBA), and one-dimensional (1D). The comparison between the 3D and IBA simulations revealed a small horizontal photon transport effect, with IBA simulations introducing mostly random errors and a slight overestimation (below 1 K). However, performing 1D radiative transfer simulations results in a significant beam-filling effect that increases primarily with frequency, and secondly, with footprint size. For a sensor footprint size of 15 km, the errors induced by neglecting domain heterogeneities yield root mean square errors of up to ≈4 K and ≈13 K at 186.3 GHz and 668 GHz, respectively. However, an instrument operating at the same frequencies, but with a much smaller footprint size, i.e., 6 km, is subject to smaller uncertainties, with a root mean square error of ≈2 K at 186.3 GHz and ≈7.1 K at 668 GHz. When designing future satellite instruments, this effect of footprint size on modeling uncertainties should be considered in the overall error budget. The smallest possible footprint size should be a priority for future sub-millimeter observations in light of these results.

Physical characteristics of frozen hydrometeors inferred with parameter estimation

“…This neglects the polarising effects of scattering from particles, which can transfer energy from one polarisation to another, and causes optical properties to vary as a function of polarisation and viewing angle. RT-TOV v13.0 will have a simplified treatment of polarised scattering from preferentially oriented frozen hydrometeors; this reduces errors in the simulated vertical-horizontal polarisation difference by 10-15 K (Barlakas et al, 2021). This is not used in the main study; however, the parameter estimation is updated at the end, taking account of the new polarisation scheme, to provide a self-consistent final configuration for RTTOV v13.0 (Sect.…”

“…This configuration was important for testing the other developments towards v13.0. In particular, it was used by Barlakas et al (2021) to develop a representation of polarised scattering and to tune the required polarisation ratio parameter. In areas affected by strong scattering, the new approach to polarisation increases brightness temperatures by up to around 7 K in vertically polarised channels and decreases them by the same amount in horizontally polarised channels.…”

Evaluation of the impacts of ice cloud vertical inhomogeneity on spaceborne passive submillimeter‐wave simulations

“…Then we can get outputted IWC and D eff profiles in a 3D coordinate system with a 1 km × 1 km horizontal resolution and a 50-m vertical resolution. Horizontal heterogeneity effects have been demonstrated in Barlakas and Eriksson (2020), and this effect is not considered in this study, which means that each profile was considered as an independent sample or pixel. Thus, the IWP of the simulated ice cloud can be obtained by vertically integrating IWCs with height, and the vertically averaged D eff can be also obtained according to the actual…”

Synergistic radar and sub-millimeter radiometer retrievals of ice hydrometeors in mid-latitude frontal cloud systems

“…Systematic vertical orientation of particles of a given shape in the cloud would effectively alter their scattering properties. For observations at nadir, particle orientation can increase the extinction of the large plate aggregate of up to 20 % ( Barlakas et al, 2021). To first order, the increase in extinction can be expected to cause a similar overestimation of the retrieved IWC.…”