Objective determination of cloud heights and radar reflectivities using a combination of active remote sensors at the ARM CART sites

The U.S. Department of Energy's Atmospheric Radiation Measurement (ARM) Pro gram is deploying sensitive, millimeter-wave cloud radars at its Cloud and Radiation Test Bed (CART) sites in Oklahoma, Alaska, and the tropical weste rn pacific Ocean. The radars complement optical devices, including a Belfor t or Vaisala Laser ceilometer and a micropulse lidar, in providing a compre hensive source of information on the vertical distribution of hydrometeors overhead at the sites. An algorithm is described that combines data from th ese active remote sensors to produce an objective determination of hydromet eor height distributions and estimates of their radar reflectivities, verti cal velocities, and Doppler spectral widths, which are optimized for accura cy. These data provide fundamental information for retrieving cloud microph ysical properties and assessing the radiative effects of clouds on climate. The algorithm is applied to nine months of data from the CART site in Okla homa for initial evaluation. Much of the algorithm's calculations deal with merging and optimizing data from the radar's four sequential operating mod es, which have differing advantages and limitations, including problems res ulting from range sidelobes, range aliasing, and coherent averaging. Two of rile modes use advanced phase-coded pulse compression techniques to yield approximately 10 and 15 dB more sensitivity than is available from the two conventional pulse modes. Comparison of cloud-base heights from the Belfort ceilometer and the micropulse lidar confirms small biases found in earlier studies, but recent information about the ceilometer brings the agreement to within 20-30 m. Merged data of the radar's modes were found to miss appr oximately 5.9% of the clouds detected by the laser systems. Using data from only the radar's two less-sensitive conventional pulse modes would increas e the missed detections to 22%-34%. A significant remaining problem is that the radar's lower-altitude data are often contaminated with echoes from no nhydrometeor targets, such as insects.