Rain Gauge and Disdrometer Measurements during the Keys Area Microphysics Project (KAMP)

Abstract

Four impact disdrometers and 27 tipping bucket rain gauges were operated at 11 different sites during August and September 2001, as part of the Keys Area Microphysics Project. The rain gauge and disdrometer network was designed to study the range dependency of radar calibration and rainfall verification in tropical storms. The gauges were collocated at eight sites, while three to five gauge clusters were deployed at three sites. Four disdrometers were also collocated with rain gauges. Overall the experiment was quite successful, although some problems did occur including flooding of gauge loggers, vandalism, and excessive noise at disdrometer sites. Both a south-to-north and east-to-west rainfall gradient was observed, whereby the gauges on the western and northern sides of the Lower Keys recorded more rainfall. Considering the campaign-long rain accumulations, collocated gauges agreed well, with differences generally less than 2%, except for one gauge cluster where the rain accumulation difference was attributed to individual gauge calibration error. The duration of a rain event was sensitive to the definition of a rain event, while this was not a factor in rain intensity. Only 7% of the rain events had significant storm total differences in excess of 2.5 mm. All of these events occurred at storm conditional mean and maximum rain rates higher than 5 and 50 mm h?1, respectively. Nevertheless, there were many other rain events for which the storm total differences were not significant in heavy rainfall. Combining most of the rain events from all collocated gauge sites, the correlation coefficient and mean percent absolute difference between the gauge storm totals were 0.99 and about 9%, respectively, on average. A rain gauge was typically able to measure rainfall within ±1.2 mm. As the storm total increased, the standard deviation of the rain total difference and correlation coefficient increased, while mean percent absolute difference decreased. Considering the gauge that recorded higher overall accumulation as the reference, and ignoring the natural variability of rainfall between collocated gauges, the gauge rainfall error was about 9%. Two disdrometers that were placed away from noise sources performed well and recorded higher rainfall accumulation than their collocated rain gauges.