Independent evaluation of the ability of spaceborne radar and lidar to retrieve the microphysical and radiative properties of ice clouds

The combination of radar and lidar in space offers the unique potential to retrieve vertical profles of ice water content and particle size globally, and two algorithms developed recently claim to have overcome the principal diffculty with this approach, that of correcting the lidar signal for extinction. In this paper "blind tests" of these algorithms are carried out, using realistic
94-GHz radar and 355-nm lidar backscatter profiles simulated from aircraft-measured size spectra, and including the effects of molecular scattering, multiple scattering and instrument noise. Radiation calculations are performed on the true and retrieved microphysical profiles to estimate the accuracy with which radiative flux profiles could be inferred remotely. It is found that the visible extinction profile can be retrieved independently of assumptions on the nature of the size distribution, the habit of the particles, the mean extinction-to-backscatter ratio or errors in instrument calibration. Local errors in retrieved extinction can occur in proportion to local fluctuations in extinction-to-backscatter ratio, but down to 400 m above the height of the lowest
lidar return, optical depth is typically retrieved to better than 0.2. Retrieval uncertainties are greater at the far end of the profile,
and errors in total optical depth can exceed 1, which changes the short-wave radiative effect of the cloud by around 20%. Longwave
uxes are much less sensitive to errors in total optical depth, and may generally be calculated to better than 2 W m2 throughout the profile. It is important for retrieval algorithms to account for the effects of lidar multiple scattering, as if this is neglected then optical depth is underestimated by approximately 35%, resulting in cloud radiative effects being underestimated
by around 30% in the short-wave and 15% in the long-wave. Unlike extinction coeficient, the inferred ice water content and
particle size can vary by 30% depending on the assumed mass-size relationship (a problem common to all remote retrieval algorithms). However, radiative fluxes are almost completely determined by the extinction profile, and if this is correct then errors in these other parameters have only a small effect in the short-wave (around 6% compared to clear-sky) and a negligible
effect in the long-wave.