Thursday, 26 January 2012: 2:45 PM
Optimizing Weather Radar Through Ground-Based Disdrometer Data
Room 239 (New Orleans Convention Center )
To ensure a timely warning of impending flooding, it is necessary to measure the amount and spatial distribution of precipitation quickly and accurately. This goal is achieved with a combination of weather radar measurements (spatial information with reduced accuracy) and ground-based disdrometer measurements. Normally, precipitation radar measures the radar reflectivity (Z) on the basis of ground shadowing only from a height of 1,000 to 2,000 m above the ground, thus providing information to localize precipitation areas. In order to accurately determine the precipitation input, a quantitative recording of the precipitation intensity (R) on the ground is required. This requires both extrapolation of the radar data to the ground and derivation of the reflectivity factor Z to the precipitation rate R. Usually, the spatial distribution of the precipitation rate from the radar measurement is adjusted with data from rain gauges on the ground with the shortest time delay possible. A potential problem is the so-called 'bright band', i.e. the snow/rain melting layer which is usually about 200 to 500 meters below the zero degree boundary. The bright band can lead to overestimates of the precipitation rate, since melting snow is highly reflective. However, the height of the bright band is not always accurately recorded by weather radars. Conventional rain collectors can also fail in this respect; although they supply values to adjust the precipitation intensity R, they do not take account of either the type of precipitation or the height of the bright band. As a present weather sensor with a modern optical disdrometer, the OTT Parsivel2 supplies the type and distribution of precipitation at the reference height with the ground temperature. Based on this information it is possible to adjust weather radar data without delay. Precipitation forecasts that are derived from such data become considerably more precise and thereby improve the quality of flood forecasts.
Despite the many advantages of the modern optical disdrometer, human weather observers are still indispensable for many locations. However, modern measuring devices such as the OTT Parsivel2 optical disdrometer offer a variety of functions that exceed human recording capacity and are able to deliver very accurate data in a timely manner. They are extremely robust and reliable; they never go on holiday or suffer from the frailties of human health and they can withstand extreme weather conditions.