Tuesday, 11 January 2005
A lidar-radar observing system for monitoring optical and microphysical properties of cirrus
Through complicated and nonlinear feedbacks, cirrus clouds play an important role determining the current and future states of the climate system. Depending on their optical properties, cirrus have the potential to either heat or cool the lower atmosphere—a radiative forcing that ties back in to atmospheric circulation and cirrus formation itself. To better understand this role, both large- and small-scale cloud systems must be observed to gain a more detailed knowledge of cloud extrinsic (spatial and macrophysical) and intrinsic (optical and microphysical) properties. The application of a lidar+radar observing system for this purpose is of prime interest due to the inherently complementary measurements provided by these sensors, and their anticipated simultaneous availability upon the EOS CALIPSO and CloudSat missions scheduled for launch in 2005. Although the lidar as a stand-alone observing system proves useful for resolving cirrus cloud optical properties,the addition of the radar system is essential in resolving two key cloud microphysical parameters: particle number concentration and characteristic diameter of the crystal size distribution. The ability of these two active instruments to probe the internal structure of clouds adds a new dimension to our current understanding of clouds—forming a seminal dataset of tomorrow’s Earth Information System. The projected two years of data collection is also essential for understanding seasonal clouds structure and their role in global weather patters.
In preparation for this important scientific mission, significant resources have been dedicated to field programs (e.g., the Cirrus Regional Study of Tropical Anvils and Cirrus Layers (CRYSTAL)-Florida Area Cirrus Experiment (FACE)) seeking to address some of the challenges of a remote sensing system. In particular, an adequate treatment of non-spherical particles that dominates cirrus clouds in the forward radiative models and issues involving lidar multiple scattering effects and radar Mie effects.
This presentation outlines a multisensor retrieval of cirrus optical properties, cast in the optimal estimation theory framework. Tests on both synthetic and CRYSTAL-FACE data demonstrate both the validity, robustness, and limitations of the current method, with considerations given for the analogous CloudSat/CALIPSO observing system.