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.
Highlights
One of the largest sources of uncertainties in numerical weather prediction (NWP) and climate models concerns clouds containing ice [1]
By means of the Atmospheric Radiative Transfer Simulator (ARTS), a study has been carried out in order to explore the errors prompted by omitting three-dimensional (3D) radiative transfer at millimeter/sub-millimeter wavelengths
This study gives an insight of the 3D effects, i.e., beam-filling and horizontal photon transport effects, which contaminate cloud ice retrievals from both current and forthcoming satellite missions, with a focus on the Ice Cloud Imager (ICI)
Summary
One of the largest sources of uncertainties in numerical weather prediction (NWP) and climate models concerns clouds containing ice [1]. The properties of such clouds are only poorly constrained in models [2] and an unmet need for trustworthy global observations of ice cloud properties exists [3]. Frequencies below ≈90 GHz can penetrate nearly the entire atmosphere, and provide information about water vapor over ocean in clear sky conditions In cloudy conditions, these frequencies are sensitive to hydrometeors, with the lower frequencies (up to ≈30 GHz) giving an insight to heavy precipitation, while frequencies between 30 and 90 GHz render information about liquid water clouds.
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