Understanding effective diameter and its application to terrestrial radiation in ice clouds 1Division of Atmospheric Sciences, Reno, NV 89512-1095, USA
11 Apr 2011
2SPEC, Inc., 5401 Western Ave., Boulder, CO 80301, USA
Received: 03 Oct 2010 – Published in Atmos. Chem. Phys. Discuss.: 02 Dec 2010 Abstract. The cloud property known as "effective diameter" or "effective radius",
which in essence is the cloud particle size distribution (PSD) volume at
bulk density divided by its projected area, is used extensively in
atmospheric radiation transfer, climate modeling and remote sensing. This
derives from the assumption that PSD optical properties can be uniquely
described in terms of their effective diameter, De, and their cloud
water content (CWC), henceforth referred to as the De-CWC assumption.
This study challenges this assumption, showing that while the De-CWC
assumption appears generally valid for liquid water clouds, it appears less
valid for ice clouds in regions where (1) absorption is not primarily a
function of either the PSD ice water content (IWC) or the PSD projected
area, and (2) where wave resonance (i.e. photon tunneling) contributes
significantly to absorption. These two regions often strongly coincide at
terrestrial wavelengths when De<~60 μm, which is where
this De-CWC assumption appears poorest. Treating optical properties
solely in terms of De and IWC may lead to errors up to 24%, 26%
and 20% for terrestrial radiation in the window region regarding the
absorption and extinction coefficients and the single scattering albedo,
respectively. Outside the window region, errors may reach 33% and 42%
regarding absorption and extinction. The magnitude and sign of these errors
can change rapidly with wavelength, which may produce significant errors in
climate modeling, remote sensing and other applications concerned with the
wavelength dependence of radiation.
Revised: 22 Mar 2011 – Accepted: 24 Mar 2011 – Published: 11 Apr 2011
Where the De-CWC assumption breaks down, ice cloud optical properties
appear to depend on De, IWC and the PSD shape. Optical property
parameterizations in climate models and remote sensing algorithms based on
historical PSD measurements may exhibit errors due to previously unknown PSD
errors (i.e. the presence of ice artifacts due to the shattering of larger
ice particles on the probe inlet tube during sampling). More recently
developed cloud probes are designed to mitigate this shattering problem.
Using realistic PSD shapes for a given temperature (and/or IWC) and cloud
type may minimize errors associated with PSD shape in ice optics
parameterizations and remote sensing algorithms.
While this topic was investigated using two ice optics schemes (the Yang et
al., 2005 database and the modified anomalous diffraction approximation, or
MADA), a physical understanding of the limitations of the De-IWC
assumption was made possible by using MADA. MADA allows one to approximate
the contribution of photon tunneling to absorption relative to other optical
processes, which reveals that part of the error regarding the De-IWC
assumption can be associated with tunneling. By relating the remaining error
to the radiation penetration depth in bulk ice (ΔL) due to
absorption, the domain where the De-IWC assumption is weakest was
described in terms of De and ΔL.
Citation: Mitchell, D. L., Lawson, R. P., and Baker, B.: Understanding effective diameter and its application to terrestrial radiation in ice clouds, Atmos. Chem. Phys., 11, 3417-3429, doi:10.5194/acp-11-3417-2011, 2011.