Experimental and numerical studies of small-scale rainfall measurements and variability

A high-resolution rainfall observatory was established at the Iowa Institute of Hydraulic Research to support studies of small scale rainfall variability. It includes a vertically pointing X-band radar, a two-dimensional video disdrometer, an optical rain gauge, a standard tipping bucket raingauge, and high temporal resolution sensors for measurements of wind velocity and direction, temperature, humidity, and pressure. All the instruments are collocated. The observatory is being upgraded to include a Doppler processor for the radar, a mobile platform to enable participation in community-organized hydrometeorological experiments, and a network of about 15 high-resolution raingauges to be installed at a nearby airport. The airport network design includes innovative concepts of dual sensors constituting a single observational point, connected to the same data logger. The observational points are separated with distances ranging from 10-1000 metres. We present comparisons of data collected by the various sensors and discuss implications for radar-rainfall estimation. The capabilities of the experimental setup at IIHR will facilitate numerical studies of rainfall measurements. In particular, we present results of computational fluid dynamics calculations of the measurement error of the video disdrometer based on the high resolution observations of drop size distribution and wind velocity.