Objective Determination of Cloud Heights and Radar Reflectivities Using a Combination of Active Remote Sensors at the ARM CART Sites

TLDR

The ARM Program deploys millimeter‑wave cloud radars at CART sites, complementing optical ceilometers and lidar to provide comprehensive vertical hydrometeor distributions essential for cloud microphysics and radiative effect studies. The authors present an algorithm that fuses data from these active sensors to objectively determine hydrometeor height distributions and estimate radar reflectivities, vertical velocities, and Doppler spectral widths with optimized accuracy. Applied to nine months of Oklahoma CART data, the algorithm merges and optimizes measurements from the radar’s four operating modes—using advanced phase‑coded pulse compression for 10–15 dB higher sensitivity—while addressing range sidelobes, aliasing, and coherent averaging. The merged radar data agree with ceilometer and lidar cloud‑base heights within 20–30 m, miss only 5.9 % of laser‑detected clouds (vs.

Abstract

The U.S. Department of Energy's Atmospheric Radiation Measurement (ARM) Program is deploying sensitive, millimeter-wave cloud radars at its Cloud and Radiation Test Bed (CART) sites in Oklahoma, Alaska, and the tropical western Pacific Ocean. The radars complement optical devices, including a Belfort or Vaisala laser ceilometer and a micropulse lidar, in providing a comprehensive source of information on the vertical distribution of hydrometeors overhead at the sites. An algorithm is described that combines data from these active remote sensors to produce an objective determination of hydrometeor height distributions and estimates of their radar reflectivities, vertical velocities, and Doppler spectral widths, which are optimized for accuracy. These data provide fundamental information for retrieving cloud microphysical properties and assessing the radiative effects of clouds on climate. The algorithm is applied to nine months of data from the CART site in Oklahoma for initial evaluation. Much of the algorithm's calculations deal with merging and optimizing data from the radar's four sequential operating modes, which have differing advantages and limitations, including problems resulting from range sidelobes, range aliasing, and coherent averaging. Two of the 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 approximately 5.9% of the clouds detected by the laser systems. Using data from only the radar's two less-sensitive conventional pulse modes would increase the missed detections to 22%–34%. A significant remaining problem is that the radar's lower-altitude data are often contaminated with echoes from nonhydrometeor targets, such as insects.