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Abstract
A ground-based optical array instrument for the measurement of shapes, sizes, and fall velocities of freely falling hydrometeors is presented. The instrument, the Hydrometeor Velocity and Shape Detector (HVSD), is designed to accurately measure hydrometeors greater than 1 mm in diameter that yield the main contribution to radar backscatter and rain rate in moderate to heavy precipitation.
The optical system of the HVSD consists of two horizontal and parallel light beams with a small vertical offset, directed toward two arrays of photodiodes. Each hydrometeor falling through the measuring area is recorded twice with a slight time difference. The two corresponding images of each particle are matched automatically, based on shape and fall pattern characteristics. After two images are matched, the fall velocity of the original hydrometeor is calculated and its actual cross section is reconstructed.
The HVSD was calibrated using simulated raindrops and ice particles in the laboratory. It has an inherent undersizing problem of small raindrops, for which an empirical correction is derived. Size measurements of submillimeter hydrometeors are generally of significant uncertainty. The quality of the matching algorithm was analyzed by comparing rainfall data recorded with the HVSD, a Joss–Waldvogel disdrometer, and a rain gauge. The results demonstrate the HVSD's capability to measure properties of single hydrometeors and integral precipitation parameters. Snowfall measurements can also be used to investigate characteristics of aggregates and rimed particles. Median fall velocities and natural fall velocity variability can be studied.
Abstract
A ground-based optical array instrument for the measurement of shapes, sizes, and fall velocities of freely falling hydrometeors is presented. The instrument, the Hydrometeor Velocity and Shape Detector (HVSD), is designed to accurately measure hydrometeors greater than 1 mm in diameter that yield the main contribution to radar backscatter and rain rate in moderate to heavy precipitation.
The optical system of the HVSD consists of two horizontal and parallel light beams with a small vertical offset, directed toward two arrays of photodiodes. Each hydrometeor falling through the measuring area is recorded twice with a slight time difference. The two corresponding images of each particle are matched automatically, based on shape and fall pattern characteristics. After two images are matched, the fall velocity of the original hydrometeor is calculated and its actual cross section is reconstructed.
The HVSD was calibrated using simulated raindrops and ice particles in the laboratory. It has an inherent undersizing problem of small raindrops, for which an empirical correction is derived. Size measurements of submillimeter hydrometeors are generally of significant uncertainty. The quality of the matching algorithm was analyzed by comparing rainfall data recorded with the HVSD, a Joss–Waldvogel disdrometer, and a rain gauge. The results demonstrate the HVSD's capability to measure properties of single hydrometeors and integral precipitation parameters. Snowfall measurements can also be used to investigate characteristics of aggregates and rimed particles. Median fall velocities and natural fall velocity variability can be studied.
Shallow, maritime cumuli are ubiquitous over much of the tropical oceans, and characterizing their properties is important to understanding weather and climate. The Rain in Cumulus over the Ocean (RICO) field campaign, which took place during November 2004–January 2005 in the trades over the western Atlantic, emphasized measurements of processes related to the formation of rain in shallow cumuli, and how rain subsequently modifies the structure and ensemble statistics of trade wind clouds. Eight weeks of nearly continuous S-band polarimetric radar sampling, 57 flights from three heavily instrumented research aircraft, and a suite of ground- and ship-based instrumentation provided data on trade wind clouds with unprecedented resolution. Observational strategies employed during RICO capitalized on the advances in remote sensing and other instrumentation to provide insight into processes that span a range of scales and that lie at the heart of questions relating to the cause and effects of rain from shallow maritime cumuli.
Shallow, maritime cumuli are ubiquitous over much of the tropical oceans, and characterizing their properties is important to understanding weather and climate. The Rain in Cumulus over the Ocean (RICO) field campaign, which took place during November 2004–January 2005 in the trades over the western Atlantic, emphasized measurements of processes related to the formation of rain in shallow cumuli, and how rain subsequently modifies the structure and ensemble statistics of trade wind clouds. Eight weeks of nearly continuous S-band polarimetric radar sampling, 57 flights from three heavily instrumented research aircraft, and a suite of ground- and ship-based instrumentation provided data on trade wind clouds with unprecedented resolution. Observational strategies employed during RICO capitalized on the advances in remote sensing and other instrumentation to provide insight into processes that span a range of scales and that lie at the heart of questions relating to the cause and effects of rain from shallow maritime cumuli.
