Wednesday, 23 January 2008
Long-term synthesis of ARM millimeter cloud radar and micropulse lidar observations
Exhibit Hall B (Ernest N. Morial Convention Center)
The Department of Energy's Atmospheric Radiation Measurement (ARM) program continuously operates zenith-pointing Ka-band (35 GHz, 8.6 mm) Millimeter Cloud Radars (MMCRs) in conjunction with micropulse lidars (MPLs) and a large suite of additional meteorological instrumentation at five fixed global sites. The ARM Climate Research Facility (ACRF) is comprised of sites in the Southern Great Plains (north-central Oklahoma), North Slope of Alaska (Barrow) and three Tropical Western Pacific locations (Darwin, Australia; Manus, Papua New Guinea; and Nauru Island). Multi-year data sets are available for each site. In addition, ACRF has a highly instrumented mobile site, which includes a W-band (95 GHz, 3.2 mm) ARM Cloud Radar (WACR) and MPL among its instrumentation. A WACR is also in operation at the fixed Southern Great Plains site. The ARM Mobile Facility has had ten-month deployments in Niamey, Niger and the Black Forest, Germany, and in 2008 will be deployed in Shanghai, China. At each ACRF site, data from the cloud radars (MMCR or WACR), and MPL, as well as ceilometer and surface precipitation measurements have been synthesized to produce best-estimate time-height profiles of hydrometeor locations, radar reflectivities, mean Doppler velocities and Doppler spectral widths using the Active Remote Sensing of CLouds (ARSCL) value-added products. None of the instruments alone can see the entire vertical cloud profile at all times. Cloud radars can miss thin clouds, particularly cirrus clouds, and cannot clearly distinguish cloud boundaries from precipitation and drizzle. Lidars cannot penetrate thick low-level cloud to see higher cloud layers that may lie above. The MMCR has several distinct operating modes, each optimized for specific types and locations of clouds and precipitation, while the WACR alternates between co-polar and cross-polar measurements. The ARSCL software incorporates the different radar observing modes while correcting them for possible artifacts, such as velocity aliasing or pulse-coding effects. The resulting best-estimate reflectivity observations are merged with MPL and ceilometer-determined cloud bases to separate cloud from precipitation returns and to help in the identification and removal of insect and other non-hydrometeor “clutter” radar returns. In addition, lidar observations of thin cirrus clouds, which the radar alone can miss due to incomplete radar beam filling or insufficient sensitivity, are incorporated into the product's results. Examples of the benefits of combining radar and lidar observations using the ARSCL products are discussed. Results are shown from a study of upper level cloud observations made using MMCR or WACR observations alone, and in conjunction with the MPL.