Using passive and active microwave observations to constrain ice particle models

Abstract

Abstract. Satellite microwave remote sensing is an important tool for determining the distribution of atmospheric ice globally. The upcoming Ice Cloud Imager (ICI) sensor will provide unprecedented measurements at sub-millimetre frequencies, employing channels up to 664 GHz. However, the utilization of such measurements requires detailed data on how individual ice particles scatter and absorb radiation, i.e., single scattering data. Several single scattering databases are currently available, with the one by Eriksson et al. (2018) specifically tailored to ICI. This study attempts to validate and constrain the large set of particle models available in this database, to a smaller and more manageable set. A combined active and passive model framework is developed and employed, which converts CloudSat observations to simulated brightness temperatures (TBs) measured by the GPM Microwave Imager (GMI) and ICI. Simulations covering about one month in the tropic pacific ocean are performed, assuming different microphysical settings realized as combinations of particle model and particle size distribution (PSD). Firstly, it is found that when the CloudSat inversions and passive forward model are considered separately, assumed particle model and PSD have a considerable impact on both radar retrieved ice water content (IWC) and simulated TBs. Conversely, when the combined active and passive framework is employed instead, the uncertainty due to assumed particle model is significantly reduced, essentially due to a compensation effect between bulk extinction at passive frequencies and radar reflectivity. Furthermore, simulated TBs for almost all the tested microphysical combinations, from a statistical point of view, agree well with GMI measurements (186.31 and 190.31 GHz), indicating the robustness of the simulations. However, it is difficult to identify a particle model that outperforms any other. One aggregate particle model, composed of columns, yields marginally better agreement to GMI compared to the other particles, mainly for the most severe cases of deep convection. Of tested PSDs, the one by McFarquhar and Heymsfield (1997) is found to give the best overall agreement to GMI and also yields radar dBZ-IWC relationships closely matching measurements by Protat et al. (2016). Only one particle, modelled as an air-ice mixture spheroid, performs poorly overall. On the other hand, simulations at the higher ICI frequencies (328.65, 334.65, and 668.2 GHz) show significantly higher sensitivity to the assumed particle model. This study thus points to the potential use of combined ICI and 94 GHz radar measurements to constrain ice hydrometeor properties in radiative transfer (RT), using the method demonstrated in this paper.

Towards an operational Ice Cloud Imager (ICI) retrieval product

“…Ice hydrometeors presently constitute one of the components in Earth's atmosphere that are least constrained by observation and modelling systems. There is even a persistent large spread among zonal means of IWP (Waliser et al, 2009;Eliasson et al, 2011;Duncan and Eriksson, 2018). ICI will provide observations that could be used to decrease these uncertainties inside both weather forecasting and stand-alone retrievals, as well as by model verification through "satellite simulators".…”

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

“…Accurate radiative transfer simulations of brightness temperatures necessitate realistic representations of the hydrometeor's size, density, shape, and size distribution. Although several studies focused on the impacts of the microphysical properties on brightness temperatures simulated at low frequencies (up to ≈200 GHz) [66][67][68][69][70], limited research has been conducted at sub-mm wavelengths [71]. Even though such studies are of great importance on their own, they are beyond the scope of this paper.…”

“…In the framework of this study, different ice hydrometeors' shapes have been tested, but their sensitivity to the 3D radiative effects was found to be of low importance. Consequently, a comprehensive assessment of the latter effects have been conducted only for large plate aggregates that are found to be one of the realistic scatterers at sub-mm wavelengths [71]. For liquid particles (cloud water and rain), the optical properties have been derived from Mie theory in the case of spherical particles.…”

“…Note here that IWC and RWC are estimated separately, with a temperature of 273.15 K being the decisive indicator. For details, the reader is referred to [71]. Figure 2 illustrates the IWC field corresponding to the example latitudinal slice of the synthetic surrogate radar reflectivity field depicted in Figure 1b.…”

Synthesizing the Vertical Structure of Tropical Cirrus by Combining CloudSat Radar Reflectivity With In Situ Microphysical Measurements Using Bayesian Monte Carlo Integration

“…All data are shown in SI unit. The ARTS particle habits are roughly classified into three categories based on their density (Ekelund et al, 2019). The low‐density habits, the habits for a m < 0.08 and b m < 2, have rapid growth in diameter if the particle mass increases slightly.…”