Parameterizing Size Distribution in Ice Clouds Page: 3 of 89
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monomodal spectra), De and Vf. This assumes p > 1.035 (P < 1.035 indicates broad
PSD associated with relatively large ice particles with small ice crystals contributing little
to PSD projected area). To test the physics of this retrieval algorithm, MODIS radiances
from the TERRA satellite overpass on 2 February 2006, corresponding to cirrus in situ
sampling during the TWP-ICE experiment, were converted to p values. Retrieved cirrus
pixels were matched with in situ measurements spatially and within 4 minutes
temporally. These in situ PSD measurements were unique in that the probes were
designed to minimize the problem of shattering. Thus a comparison between retrieved
and measured PSD should be meaningful. The comparison of the retrieved PSD from
MODIS with PSD measurements from the Proteus aircraft are shown in Appendix B,
Fig. 3, left panel. It is seen that using the monomodal PSD assumption results in a
retrieved PSD slope very similar to the observed slope. Naturally the retrieved and
observed De and mean size agreed as well. Comparisons between satellite retrievals
and in situ measurements are hard to come by and this ARM case study has been very
useful in validating this retrieval algorithm.
c. Arctic cirrus
Provided that p for tropical anvil and in situ cirrus is essentially the same for
midlatitude in situ cirrus (as the journal literature indicates), the above work
characterizes the cirrus PSD for both tropical anvil and midlatitude synoptic (in situ)
cirrus. Note that some of the PSD schemes considered in Section 2a were derived from
midlatitude synoptic cirrus while others were derived from tropical anvil cirrus
Cirrus PSD for the Arctic were characterized by (1) developing a PSD
parameterization based on cirrus in situ PSD measurements from the ARM M-PACE
project and (2) using AERI radiance measurements during M-PACE to estimate the
concentrations of small ice crystals. This work was undertaken primarily by the PhD
student on this project and is described in Appendix C. The methodology for developing
the Arctic PSD parameterization is described in the Appendix of Mitchell et al. (2009;
see Appendix A). Prior to the completion of the Arctic PSD scheme, the Ivanova PSD
scheme for midlatitude cirrus was used as a surrogate for Arctic PSD so that the M-
PACE AERI measurements could be used to do a preliminary evaluation of the
concentrations of small ice crystals in Arctic cirrus. The results of that study are
described in Appendix C. No evidence was found that Arctic PSDs are bimodal or that
elevated or anomalously high concentrations of small ice crystals exist. For typical
cirrus IWC of 10 mg liter-, total ice particle concentrations were -3-8 per liter.
3. Retrieval of the ice fraction in Arctic mixed phase clouds
a. AERI and radar retrievals
An interesting finding from this research shared by other published studies is that
the mean value of p is roughly constant for all temperatures corresponding to all-ice
conditions (T < -36 C). For thin to moderately thick cirrus, p ~ 1.06 0.04. For warmer
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DeSlover, Daniel & Mitchell, David L. Parameterizing Size Distribution in Ice Clouds, report, September 25, 2009; Madison, Wisconsin. (digital.library.unt.edu/ark:/67531/metadc925707/m1/3/: accessed September 24, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.