Remote sensing of ice crystal asymmetry parameter using multi-directional polarization measurements – Part 2: Application to the Research Scanning Polarimeter

Abstract

Abstract. A new method to retrieve ice cloud asymmetry parameters from multi-directional polarized reflectance measurements is applied to measurements of the airborne Research Scanning Polarimeter (RSP) obtained during the CRYSTAL-FACE campaign in 2002. The method assumes individual hexagonal ice columns and plates serve as proxies for more complex shapes and aggregates. The closest fit is searched in a look-up table of simulated polarized reflectances computed for cloud layers that contain individual, randomly oriented hexagonal columns and plates with a virtually continuous selection of aspect ratios and distortion. The asymmetry parameter, aspect ratio and distortion of the hexagonal particle that leads to the best fit with the measurements are considered the retrieved values. Two cases of thick convective clouds and two cases of thinner anvil cloud layers are analyzed. Median asymmetry parameters retrieved by the RSP range from 0.76 to 0.78, and are generally smaller than those currently assumed in most climate models and satellite retrievals. In all cases the measurements indicate roughened or distorted ice crystals, which is consistent with previous findings. Retrieved aspect ratios in three of the cases range from 0.9 to 1.6, indicating compact particles dominate the cloud-top shortwave radiation. Retrievals for the remaining case indicate plate-like ice crystals with aspect ratios around 0.3. The RSP retrievals are qualitatively consistent with the CPI images obtained in the same cloud layers. Retrieved asymmetry parameters are compared to those determined in situ by the Cloud Integrating Nephelometer (CIN). For two cases, the median values of asymmetry parameter retrieved by CIN and RSP agree within 0.01, while for the two other cases RSP asymmetry parameters are about 0.03–0.05 greater than those obtained by the CIN. Part of this bias might be explained by vertical variation of the asymmetry parameter or ice shattering on the CIN probe, or both.

Resolving ice cloud optical thickness biases between CALIOP and MODIS using infrared retrievals

“…By conducting an infrared closure analysis, we have shown that the C5 parameterization is not representative of the globally averaged ice scattering properties. More recent investigations of the ice cloud asymmetry parameter suggest that most ice clouds have values around 0.75 in the visible spectrum van Diedenhoven et al, 2013). Additionally, use of the C5 ice cloud radiative model results in MODIS retrieval biases that are strongly dependent on the viewing angle, as demonstrated in Fig.…”

“…Other studies have also suggested that featureless (i.e., smooth) phase functions indicative of roughened or highly asymmetric aggregated habits with relatively small asymmetry parameters are needed to match aircraft and satellite observations (e.g., Baran et al, 2001;C.-Labonnote et al, 2000;van Diedenhoven et al, 2013). The Generalized Habit Model (GHM) ) was also tested but did not result in the same level of radiative closure with the IR IOT retrievals compared to the severely roughened aggregated columns (comparison shown in Fig.…”

“…It is important to note that this investigation focuses on ice clouds with optical depths less then 3.0 as this is the limit for CALIOP. Finally, it is recognized that using a fixed ice radiative model for global retrievals is only meaningful in a climatological sense and may be expected to breakdown in instantaneous and/or regional studies with variations in inferred ice models with region, cloud type, dynamics, and cloud-top height shown by Cole et al (2014) and van Diedenhoven et al (2014).…”

“…Evidently the habit sensitivity of the asymmetry parameter is also strong in both the 2.1 and 3.7 µm MODIS channels. While the asymmetry parameters of three severely roughened habits are not constant with effective size (though at 2.1 µm the aggregate plates and aggregate columns are nearly constant), the C5 model has much larger size sensitivity at both wavelengths van Diedenhoven et al, 2014;Yang et al, 2008). Aggregated columns, with smaller asymmetry parameters relative to C5, will result in a larger retrieved CER estimates.…”

“…To calculate the TOA cloudy radiances requires an accurate radiative transfer model, knowledge of the cloud boundaries, and well-characterized surface temperature/emissivity and atmospheric thermodynamic profiles. LBLDIS (Turner et al, 2003), a cloudy radiative transfer model, is used for this analysis. The model elegantly combines the clear-sky Line-By-Line Radiative Transfer Model (LBLRTM) (Clough and Moncet, 1992) with the Discrete Ordinates Radiative Transfer (DISORT) (Stamnes et al, 1988), a proven and accurate cloudy radiative transfer model.…”