Combining visible and infrared radiometry and lidar data to test simulations in clear and ice cloud conditions 1Dipartimento di Fisica, Università di Bologna, Italy
*now at: University of Edinburgh, School of GeoSciences, Edinburgh, UK
Received: 05 December 2009 – Published in Atmos. Chem. Phys. Discuss.: 18 March 2010 Abstract. Measurements taken during the 2003 Pacific THORPEX Observing System Test
(P-TOST) by the MODIS Airborne Simulator (MAS), the Scanning High-resolution
Interferometer Sounder (S-HIS) and the Cloud Physics Lidar (CPL) are compared
to simulations performed with a line-by-line and multiple scattering modeling
methodology (LBLMS). Formerly used for infrared hyper-spectral data analysis,
LBLMS has been extended to the visible and near infrared with the inclusion
of surface bi-directional reflectance properties. A number of scenes are
evaluated: two clear scenes, one with nadir geometry and one cross-track
encompassing sun glint, and three cloudy scenes, all with nadir geometry.
Revised: 29 June 2010 – Accepted: 30 July 2010 – Published: 09 August 2010
CPL data is used to estimate the particulate optical depth at 532 nm for the
clear and cloudy scenes and cloud upper and lower boundaries. Cloud optical
depth is retrieved from S-HIS infrared window radiances, and it agrees with
CPL values, to within natural variability. MAS data are simulated convolving
high resolution radiances. The paper discusses the results of the comparisons
for the clear and cloudy cases. LBLMS clear simulations agree with MAS data
to within 20% in the shortwave (SW) and near infrared (NIR) spectrum and
within 2 K in the infrared (IR) range. It is shown that cloudy sky
simulations using cloud parameters retrieved from IR radiances systematically
underestimate the measured radiance in the SW and NIR by nearly 50%,
although the IR retrieved optical thickness agree with same measured by CPL.
MODIS radiances measured from Terra are also compared to LBLMS simulations in
cloudy conditions, using retrieved cloud optical depth and effective radius
from MODIS, to understand the origin for the observed discrepancies. It is
shown that the simulations agree, to within natural variability, with
measurements in selected MODIS SW bands.
The impact of the assumed particles size distribution and vertical profile of
ice content on results is evaluated. Sensitivity is much smaller than
differences between measured and simulated radiances in the SW and NIR.
The paper dwells on a possible explanation of these contradictory results,
involving the phase function of ice particles in the shortwave.
Citation: Bozzo, A., Maestri, T., and Rizzi, R.: Combining visible and infrared radiometry and lidar data to test simulations in clear and ice cloud conditions, Atmos. Chem. Phys., 10, 7369-7387, doi:10.5194/acp-10-7369-2010, 2010.