There are many different users of radar data (hydrologists, forecasters, ornithologists, general public, among others), and they can all benefit from polarimetry.
KNMI has installed two new dual-polarization weather radars in the 2nd half of 2016, and it is KNMI’s intention to fully utilize the polarimetric capabilities of these radars.
The focus of research on optimizing the use of dual-polarization radars at KNMI is on three aspects:
Polarimetric radars transmit and receive both horizontally and vertically polarized waves. Amplitude and phase differences, as well as temporal correlations between these two signals provide a wealth of information about the nature of the particles in the measurement volume. This in addition to the radar reflectivity and Doppler velocity that is also provided by single-polarization radars, which only transmit horizontally polarized waves. The additional information from dual-polarization radars can be related to the shape and orientation of the particles measured by the radar. This in turn can be related to the type of particles (rain drops, snowflakes, hail stones, but also birds, planes, buildings, etc), and in case of raindrops, their size.
The first and most visible benefit of polarimetric radars is the removal of clutter (or non-meteorological echoes). There are several existing methods to identify and remove non-meteorological echoes from polarimetric radar data. Several of these methods will be tested in this project, and the most suitable (combination of) method(s) will be operationally implemented. The fact that precipitation has very different characteristics with respect to shape and orientation than other echoes makes dual-polarization radar highly effective in this.
Information on what kind of precipitation has fallen on the ground (e.g., rain, hail, snow, wet snow) is very important for providing warnings for slippery road conditions (in case of snow or freezing rain), but also for e.g. producing hail probability maps. Dual-polarization radar can be used to determine the type of precipitation in the radar measurement volume. Because the radar measurement volume can be several km above the ground, numerical weather prediction model fields of temperature and humidity will be used to translate this to information on precipitation type on the ground. The precipitation type as determined aloft in the measurement volume is also directly relevant for e.g. aviation and warnings for hazardous weather (nowcasting).
Two major difficulties in accurate quantitative precipitation estimation using weather radar are the fact that the radar measures aloft (and the interest is in precipitation on the ground), and that the retrieval relation between radar reflectivity and rainfall intensity is highly uncertain due to variations in raindrop size distributions. Dual-polarization radar data will be used to estimate characteristics of the raindrop size distributions (the shape of a raindrop is strongly related to its size, which is measured by these radars), thereby reducing the uncertainty related to this. Uncertainties due to vertical variations of precipitation will be reduced by using precipitation type information from dual-polarization measurements in existing correction methods. The most notable effect is expected to be in stratiform precipitation that exhibits a clearly stratified vertical pattern, with an enhanced reflectivity in the melting layer (bright band), and greatly reduced reflectivities in the snow layer above.
These studies will be carried out in collaboration with the Royal Meteorological Institute of Belgium (RMI) and the Swedish Meteorological and Hydrological Institute (SMHI), and information will be exchanged with other European meteorological services through the EUMETNET project OPERA (http://www.eumetnet.eu/opera).
