Many useful methods for calibrating weather radars in the field and laboratory are summarized by Joe and Smith (2001) from presentations at the 2001 AMS Radar Calibration Workshop in Albuquerque, NM . Additional methods are described by Atlas (2002). Common external (full-system) calibration methods range from using metal spheres suspended from balloons to the use of solar radio emissions. Although the task is seemingly straightforward, it is challenging to devise methods that are accurate but not so difficult to conduct that they are impractical. The method used by NOAA’s Environmental Technology Laboratory (ETL) for calibrating its scanning X-band and Ka-band radars is described in this article. The radars are described by Martner et al. (2001, 2002).

NASA's Airborne Field Mill (ABFM) experiments at Kennedy Space Center (KSC) in 2000 and 2001 were designed to study cloud electrification processes and to assess various lightning launch commit criteria (LLCC) regulations (Merceret and Christian 2000) . In February 2001, the experiment included observations by a scanning 35-GHz cloud radar from NOAA's Environmental Technology Laboratory (ETL) to augment data from the project's cloud physics aircraft, ground-based electric field mill network, and operational weather surveillance radar. Its role in the ABFM was to provide a clearer context for interpreting the aircraft and field mill network data and to assess the cloud information content of the permanent weather surveillance radar that is routinely used as part of KSC launch commit decisions. This article encapsulates a much more thorough description by Martner et al. (2002a). Although Florida was suffering through a prolonged extreme drought in February 2001, two noteworthy cases presented themselves. On 03FEB01 coordinated aircraft, radar and electric field mill measurements were made of a mid-level stratiform cloud. The radar revealed that the cloud layer was approximately 2 km thick and extended upward to the -5 C altitude at 4.5 km MSL. As such, this cloud qualified as an LLCC "thick cloud" rule case and would have prevented a rocket launch at KSC. To paraphrase, this rule prohibits rocket launches, for fear of triggered lightning strikes, if a cloud is present that is more than 4500 ft thick and any part of it is within the 0 to -20 C temperature interval. Data from the 03FEB01 case and from another case of weaker clouds on 13FEB01are also examined to compare observations by the visiting high-------------------------Corresponding author: Brooks Martner, NOAA/ETL, 325 Broadway, Boulder, CO 80305; e-mail: brooks.martner@noaa.gov resolution cloud radar and those of the permanent, coarser resolution WSR-74C weather surveillance radar. The WSR-74C is a C-band radar that is routinely used by KSC meteorologists to assess cloud conditions over Cape Canaveral for LLCC decisions. Its ability to detect clouds over KSC is investigated using the cloud radar data as a baseline for comparison. ETL’s transportable NOAA/K cloud radar is a Ka-band Doppler, polarization-diversity system designed to provide detailed information about the structure, kinematics, and microphysical properties of nearby clouds (Martner et al. 2002b). During the ABFM, the NOAA/K radar was located at KSC about 1 km east of the Shuttle Landing Facility, well within the permanent KSC network of ground-based electric field mills and within 25 km of all launch pads on the Cape. An electric field mill from the University of Arizona was also temporarily installed adjacent to the radar. Scan images from NOAA/K were posted on the Worldwide Web in near realtime, and radio communications between the radar crew, the research aircraft, and the project command center were established to facilitate experiment operations.

NOAA/ETL originally built and operated its scanning Ka -band cloud radar in the early 1980's. This millimeter-wave radar, known as NOAA/K (Fig. 1), has been continually upgraded and refined since then. It has been used extensively in atmospheric field experiments across the United States and in other countries. In the 1990's it was particularly useful in cloud observation programs motivated by climate change, aircraft icing, and weather modification research. The radar characteristics, capabilities, and applications are described in this article.

A means to mark and track a moving and mixing volume within a cloud is useful for estimating the transport and dispersion of seeding material or other aerosols and thus for determining where and when the microphysical effects of the aerosols should occur. A proven approach has been to release and track chaff into a cloud and track it with circular polarization radar to estimate the rates of dispersion, loft, and dilution of ingested aerosol, as well as the filling of the cloud volume. The chaff, an effective tracer, is separable from the cloud because it strongly depolarizes the radar’s signal. A primary disadvantage of this cloud-volume-filling approach is that radar measurements of the microphysical evolution of the tracked cloud parcels may be masked by the chaff itself. To measure transport and dispersion in the cloud and simultaneously unambiguously examine the microphysical evolution in the same volume, a refined approach was devised to tag but not fill the cloud volume with chaff. Several experimental designs for chaff tagging are proposed for stratiform or gravity-wave clouds and for cumuliform clouds. The tagging technique will be more difficult to apply in convective clouds and is not yet tested. However, a “parallel lines” experiment designed for stratiform clouds was tested with hygroscopic seeding in a wave cloud with intrusions of convective cloud. The chaff lines were tracked for about one hour despite some ingestion by convective elements. This demonstrated that a seeded cloud volume could be bracketed and tracked effectively. A hygroscopic seeding signature, anticipated to appear as a line of enhanced reflectivity along a seeded flight path between the chaff lines, was not detected. A “tagged circle” experiment with silver iodide seeding was conducted in a precipitation wave cloud. A seeded circle was tagged with arcs of chaff, allowing it to be tracked efficiently. A seeding effect was indicated along the circular seeded path between the chaff arcs as signatures of enhanced reflectivity and values of depolarization indicative of pristine, new ice crystals. The results of these experiments demonstrate the potential of the chaff tagging technique.