Background

The new generation of ice cloud bulk scattering models is a significant improvement over previous models. The first generations of models, used for MODIS Collection 4 and earlier products, were based on a set of 12 particle size distributions based on only five size bins for a fixed habit prescription. The largest particle diameter in the single scattering library was approximately 750 microns. The Version 2 models were based on 1117 particle size distributions, and added complexity by including new habits (such as the droxtal), inclusion of more size bins and larger particles, and adopted smooth particles (except for the aggregate of solid columns). These models were adopted for MODIS Collection 5 ice cloud products.

The limitations in the Version 2 models became evident over time, including gaps in spectral coverage, discontinuities in the extinction/absorption efficiencies, differences caused by adoption of different aspect ratios in the solar and IR single-scattering libraries, and more. These new models address these limitations and include many research advancements since the Version 2 models were developed. For example, the single-scattering calculations are now calculated using the recently updated ice index of refraction published by Warren and Brandt (JGR, 2008).

One of the larger issues is that the assumption of smooth particles is inconsistent with studies using polarization data from POLDER.

Some Frequently Asked Questions

1. What is the primary difference between the Version 2 used for MODIS Collection 5 and the new models?

To get to the point, the primary differences are twofold:

(a) use of particle roughening primarily affects the solar bands that are used in the inference of optical thickness and effective particle size. The adoption of severely roughened particles will act to decrease the inferred optical thickness and increase the effective particle size.

(b) the new models are based on much improved microphysical data, especially with regards to the data used for low values of effective diameter (Deff < 50 microns). In the Version 2 models, the numbers of small particles were artificially increased as part of a sensitivity study with small particles being increased by a factor of 100 or 1000. In the new models, microphysical data are now available from very cold and optically thin ice clouds that have almost no particles larger than 250 microns, so the small particle models are now based solely on the microphysical data. These data come primarily from the SCOUT field campaign. The small particle models are especially important in IR-based retrievals.

2. For those of us who want to explore the polarization aspects of ice clouds, will the full phase matrix now be provided?

Yes - the scattering models for PARASOL will soon be provided in the netCDF format and contain phase functions for P₁₁, -P₁₂/P₁₁, P₂₂/P₁₁, P₃₃/P₁₁, P₄₃/P₁₁, and P₄₄/P₁₁. We will also be providing the phase matrix for a range of solar wavelengths from 0.4 to 3 µm. We are still testing the full phase matrix properties.

3. The previous models included something called the delta-transmitted energy term - will this term be included in the new models?

In a word: no. The fraction of delta-transmitted energy is essentially an artifact of the geometrical ray-tracing code. One way to think about this term is that it is the fraction of energy associated with photons that enter one facet of a hexagonal particle that leave the opposing facet without scattering. Treatment of this term in radiative transfer calculations caused confusion for a number of users. Because the new single-scattering calculations adopt a new treatment of forward scattering, there is no longer such a term in the ice cloud bulk scattering models.

4. What new parameters will be provided in the models?

At the request of a colleague, the models now include a parameter that defines the extinction coefficient divided by the ice water content (beta/IWC). This parameter is a function of the effective diameter.

5. Will the microphysical data be available to the community?

Yes - the Microphysical Data link provides access to the data for each of the field campaigns used in this work. Dr. Heymsfield and colleagues submitted a manuscript to the Journal of Atmospheric Sciences (April 2012) that provides a wealth of background.