RAIN

Quantitative precipitation estimation in a mountainous region is of high practical use. Think of monitoring and nowcasting precipitation, river flow and sea level forecasting, hydrogeological risk management, water management for hydroelectric power, road maintenance, and tourism. Yet, neither gauge networks nor weather radars give a perfect answer. Gauge observations in a mountainous region such as the European Alps suffer from severe errors because of wind, snow-drift, and high spatial variability. Radar scientists and engineers, on the other hand, are faced with severe beam shielding, strong ground clutter, and difficult operating conditions. A sophisticated elimination of ground clutter, and automatic robust algorithms to correct for shielding and profile effects are a must if one needs quantitative estimates of precipitation amounts in a routinely manner. A gauge directly measures the water flux at the ground through a surface of typically 200 cm2, whereas the radar makes use of backscattering of radar waves by hydrometeors in the atmosphere. The radar thus only provides an indirect measurement of precipitation rates at the surface of the earth. The advantage of the radar is its capability of real-time high-resolution monitoring over a large area. The combination of the two types of precipitation observations is not straightforward. Real-time adjustment of radar-derived precipitation fields with the water flux measured over 200 cm2 of a gauge may make things worse. The reason is a difference in the sampling area of about eight orders of magnitude, and the fact that precipitation often exhibits high spatial variability.

The meteorologist wants to have observations from the first signs of precipitation of 0.1 mm/h to heavy hailstorms with over 100 mm/h, within the full range of radar coverage up to about 200 km. Including the range of signal fluctuations this corresponds to 20 orders of magnitude in terms of signal power between the transmitted power and the weakest signal to be detected. This is more than we can cope with using today's receiver technology. The hydrologist, on the other hand, may need high precision because 10% more rain may break the dam. A really challenging task.

Quantitative estimation of precipitation is much more demanding than qualitative monitoring of location and size of storms. Several factors, both instrumental and meteorological, affect the accuracy of radar estimates. These include instability of the hardware, ground clutter, beam shielding by horizon and obstacles, enhancement of signal by melting snow, vertical profile of reflectivity, overshooting in shallow precipitation, signal attenuation in heavy rain, signal attenuation by water on the radome, variations in rain-drop size distributions, and enhancement of signal by presence of hail. Which of these factors are most relevant in a given context depends on the climate, the type of application and radar design.

MeteoSwiss has more than forty years of experience with radar operation in a mountainous region. Many efforts went into the optimization of hardware stability and data processing of the radar network in order to obtain quantitative precipitation estimates that meet both the meteorologist's and the hydrologist's requirements.

Recent modifications in the algorithms of the operational MeteoSwiss radar product RAIN resulted in a significant reduction of bias, scatter and false alarms. The critical success index (CSI), which is a combination of the false alarm rate (FAR) and the probability of detection (POD), shows a continuous increase over the last 8 years. This was demonstrated by an objective large-scale verification of RAIN using measurements of a dense raingauge network over all of Switzerland. The above map shows the 3 operational MeteoSwiss C-band radars and the raingauge network used for verification.

Operational applications
Nowcasting precipitation, automatic pre-alert for heavy rainfall, hydro-geological risk management, sea-level prediction, water management for hydro power, road maintenance, tourism.


Related links
OPERA http://www.chmi.cz/OPERA
VOLTAIRE http://www.voltaireproject.org

Further reading
Germann and Joss, 2004: Operational Measurement of Precipitation in Mountainous Terrain. In: Weather Radar - Principles and Advanced Applications, edited by P. Meischner, Springer Verlag, 52-77.

Contact
MeteoSwiss, Radar and Satellites, CH-6605 Locarno-Monti, Switzerland