Nowcasting involves high spatial and temporal resolution forecasts of weather developments for the next few minutes and up to a maximum of six hours ahead. MeteoSwiss uses these short-term forecasts to, among other things, predict thunderstorms, hail and heavy rainfall. This means that nowcasting is an indispensable tool for civil protection.
Short-term forecasts can be generated for any number of meteorological parameters, such as temperature, sunshine or precipitation. The main focus of nowcasting, however, is on forecasting and issuing warnings for phenomena which have particular practical implications, such as severe thunderstorms, heavy rain, hail and snow, as well as wind and fog. The systems developed for this purpose combine all available information in real time in order to predict something as precisely as possible, such as the amount of rain in a river basin in the coming few hours. Measurements and model data form the basis of these short-term forecasts. The measurement data from the weather radar network and meteorological satellites play a particularly important role here.
The many applications of nowcasting
MeteoSwiss uses nowcasting for a wide variety of applications. Many of these help in the early detection of hazards such as thunderstorms or heavy rainfall.
|Name||Application||Source(s) of data|
|TRT||Thunderstorm warning, prediction of the centre of a thunderstorm, hail forecasting||Radar, lightning measurements, COSMO|
|CombiPrecip||High resolution precipitation measurement with extensive geographical coverage||Amalgamation of radar and rain measurement data|
|NASS||Heavy rainfall warnings||Radar, CombiPrecip|
|COALITION||Thunderstorm development||Satellites, radar, lightning measurements, COSMO, orography (vertical structure of the earth's surface)|
|INCA||Temperature, wind, rain etc.||COSMO, SwissMetNet, Radar, CombiPrecip|
|NWC/SAF||Clouds and thunderstorms||Satellites, COSMO|
|REAL||Probabilistic precipitation measurement using Ensemble||Radar, rain gauges|
|NORA||Heavy rain in the Alps||Radar, SwissMetNet, radiosonde|
|POH, MESHS||Likelihood of hail; hailstone size||Radar, COSMO|
|Energie||Nowcasting for renewable energies||Radiation measurements at ground level; satellites
|Lagrange||Extrapolation of precipitation fronts and cells||CombiPrecip, NORA, REAL, orography
|Hydrometeor||Identification of hail, graupel, snow, ice, drizzle and raindrops||Polarimetric radar measurements, COSMO
Nowcasting in civil protection: the example of the thunderstorm
Thunderstorms occur on a regular basis during the six months around summer. This can lead to local flash flooding and landslides, particularly in the Alps and pre-Alpine regions. In addition, strong gusts of wind and large hailstones up to several centimetres in diameter can occur in the vicinity of a thunderstorm. Thunderstorms endanger humans and animals, as well as causing considerable damage every year to vehicles, buildings and agriculture, and disrupting road, rail and air traffic. It is therefore imperative that up-to-date weather information and forecasts for the coming few hours are available in order to allow the relevant authorities, event organisers and individuals to take the necessary precautions in good time. Nowcasting is therefore one of the cornerstones of civil protection.
The weather warnings generated using nowcasting techniques are issued by MeteoSwiss, available i.a. by email or smartphone app. The Swiss government's natural hazards portal provides information on current hazards.
The limitations of numerical weather models
Numerical weather models such as COSMO, used by MeteoSwiss, calculate regional weather forecasts for a few days ahead. From these forecasts, the probability of thunderstorms can also be estimated. However, there are limitations as to how accurately the time and location can be predicted in which heavy precipitation, hail and thunderstorm cells will occur. This is primarily because there is not sufficiently precise information available to the weather models on the temperature and humidity distribution or the state of the clouds at the start of the forecast, in order to be able to generate a forecast that is accurate to within a few kilometres and minutes.
Combination of numerous weather data in real time
With the nowcasting systems developed by MeteoSwiss, factors such as the position and strength of thunderstorms, heavy rain and hail cells can be forecast for up to six hours ahead. For this purpose, radar, satellite and lightning measurements in real time are combined with model data to produce as detailed and precise predictions as possible on the development and path of thunderstorm cells. Initial nowcasting systems were restricted to the use of weather radar data, however, the new generation includes additional monitoring data in order to further improve the forecasting. Together with those from radar, measurements from satellites, lightning and meteorological stations, and the forecasts from the COSMO weather model are also used. Algorithms that amalgamate numerous information sources are known as expert systems. These allow the characteristics and the path of thunderstorm cells to be tracked over a period of time. On the basis of changes in the storm cell, estimates can be made as to the potential for hazards within the coming hours.
The expert systems TRT and COALITION developed by MeteoSwiss have now been in operation for several years. The forecasters obtain information from these systems about the current and future developments of thunderstorms, and can thus convey warnings promptly to all the relevant official institutions and the media in the case of a prevailing or impending hazard.
The role of measurement data
A key factor in the success of nowcasting is the rapid communication and processing of measurement data from different monitoring systems. The most important of these are data from weather satellites and weather radar, as well as signals from lightning localisation antennae. The combination of these systems allows optimal observation of clouds, thunderstorms, heavy rain cells and hail cells in their various stages of development. The satellite data are particularly useful for observing the initial cloud formation phase when there is no precipitation. Radar data then provide further information on clouds as soon as precipitation has begun. During the most intensive phase of a thunderstorm, all three systems can be used to ascertain the characteristics of the storm.
The geostationary satellite "Meteosat Second Generation" (MSG) is used for the observation of clouds. This satellite takes measurements in the solar and thermal wavelength range. Solar radiation is the radiation which comes from the sun. From this measurement, the MSG satellite can identify how quickly clouds are thickening, whether they consist of ice or water, and how large the droplets or ice crystals are. In the thermal wavelength range, the temperature of the upper cloud layer can be determined. This cools very rapidly when a thunderstorm develops. For Europe, these data are available every five minutes and have a spatial resolution of a few kilometres.
A weather radar is used to detect areas of precipitation. RADAR is an abbreviation, and stands for RAdio Detection And Ranging. The radar apparatus emits an electromagnetic signal that is bounced back as an echo from surrounding objects. The length of time it takes for the signal to travel indicates the distance between the radar and the target, while the type of echo reveals the characteristics of the object detected. The weather radar thus provides information on the speed and size of the precipitation particles, and whether they are in the form of water droplets, graupel, snow or hail.
Lightning localisation functions with the help of antennae that detect electromagnetic waves from the lightning. The determination of the exact location of a lightning strike involves a combined process. The lightning flash location is ascertained by triangulation, which is based on measurements taken of the same lightning flash at various antennae. Each antenna also ascertains the precise time at which the electromagnetic wave reaches it. By comparing these times, the coordinates of the lightning flash can be calculated. Under normal circumstances, the strike can be located to an accuracy of around one kilometre.