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Nowcasting the risk of snowfall and freezing rain with radar and ground data Schmid, Willi ; Mathis, Albert ; Keller, Urs

By: Contributor(s): Language: English Language: French Series: ; topic V-181Publication details: XIth international winter road congress 2002, Sapporo [Japan] / XIe congres international de la viabilite hivernale 2002, Sapporo [Japon]. Paper, 2002Description: 7 sSubject(s): Bibl.nr: VTI 2002.0071Location: Abstract: A reliable forecast of the danger of snowfall or freezing rain is required for an optimal planning of snow or ice removal on the roads. Risk forecasts in the time range 0-2 hours would be especially useful. Salt could be distributed, or special actions to prevent accidents or traffic breakdowns could be initiated in time. Up to now, forecast procedures are mainly based on local ground measurements of temperature, humidity and precipitation. Forecasts on the onset and evolution of precipitation are normally delivered by weather services or by monitoring systems presenting weather maps, model forecasts, satellite or radar data. Few attempts have been made up to now to combine local data and short-term forecasts of precipitation based on satellite or radar data in a proper nowcasting system. In this contribution, we present a new concept to combine various risk factors to the desired risk forecasts of snowfall or freezing rain. The risk factors are: (1) The probability that precipitation stronger than a given level of intensity will occur in the subsequent 1-2 hours. This risk factor is obtained with RainCast, a fully automated radar nowcasting procedure developed at ETH; (2) The probability that this precipitation will be snow or supercooled rain. This probability is determined using ground measurements of temperature, humidity and precipitation. In a hilly or mountainous region one has the possibility to use pseudo-profiles of temperature and humidity for calculation of this risk factor if the measuring stations are distributed over various layers in altitude; and (3) The probability that liquid precipitation will fall an a frozen surface. The resulting nowcasting procedure is called RainCast+. Various forecast products can be distributed to the users: forecast images, local risk forecasts and warning messages. The procedure allows selection of the desired warning regions and warning criteria in a highly flexible manner. The skill of the forecasts, based on a statistical analysis of large data samples, is promising. The system is now being installed for operational use in Switzerland north of the Alps.
Item type: Reports, conferences, monographs
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A reliable forecast of the danger of snowfall or freezing rain is required for an optimal planning of snow or ice removal on the roads. Risk forecasts in the time range 0-2 hours would be especially useful. Salt could be distributed, or special actions to prevent accidents or traffic breakdowns could be initiated in time. Up to now, forecast procedures are mainly based on local ground measurements of temperature, humidity and precipitation. Forecasts on the onset and evolution of precipitation are normally delivered by weather services or by monitoring systems presenting weather maps, model forecasts, satellite or radar data. Few attempts have been made up to now to combine local data and short-term forecasts of precipitation based on satellite or radar data in a proper nowcasting system. In this contribution, we present a new concept to combine various risk factors to the desired risk forecasts of snowfall or freezing rain. The risk factors are: (1) The probability that precipitation stronger than a given level of intensity will occur in the subsequent 1-2 hours. This risk factor is obtained with RainCast, a fully automated radar nowcasting procedure developed at ETH; (2) The probability that this precipitation will be snow or supercooled rain. This probability is determined using ground measurements of temperature, humidity and precipitation. In a hilly or mountainous region one has the possibility to use pseudo-profiles of temperature and humidity for calculation of this risk factor if the measuring stations are distributed over various layers in altitude; and (3) The probability that liquid precipitation will fall an a frozen surface. The resulting nowcasting procedure is called RainCast+. Various forecast products can be distributed to the users: forecast images, local risk forecasts and warning messages. The procedure allows selection of the desired warning regions and warning criteria in a highly flexible manner. The skill of the forecasts, based on a statistical analysis of large data samples, is promising. The system is now being installed for operational use in Switzerland north of the Alps.

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