From: ScienceDaily
Published November 30, 2013 08:28 AM

Forecasting heavy rain events

It is difficult to forecast heavy precipitation events accurately and reliably. The quality of these forecasts is affected by two processes whose relative importance has now been quantified by a team at the Laboratoire d'Aérologie (CNRS / Université Toulouse III-Paul Sabatier). The researchers have shown that these processes should be taken into account in low wind speed events. Their findings should help forecast these events, which repeatedly cause significant damage, especially in south-eastern France. They are first published online the November 28, 2013 in the Quarterly Journal of the Royal Meteorological Society.

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Nearly every autumn, the countries of the Mediterranean Basin are stricken by heavy rainfall and flash floods, which endanger populations and can cause significant property damage. South-eastern France is frequently affected by these events, caused by the interaction between topography and the still warm and moisture-laden air masses from the Mediterranean Sea. Weather forecasters are able to predict these events and issue weather warnings. However, simulating their evolution on different time scales remains difficult, just like forecasting the strength and location of precipitation, the two parameters that broadly determine the extent of flooding.

Researchers from CNRS and Université Toulouse III-Paul Sabatier at the Laboratoire d'Aérologie investigated two phenomena that play a key role in meteorology: the microphysics of hydrometeors (rain, snow and ice pellets)[1] and atmospheric turbulence. The aim was to determine the relative effect of these two processes on forecast sensitivity. To do this, the scientists looked at five heavy rainfall episodes that took place between September 2010 and November 2011 in south-eastern France and for which measured data was available. For each event, ensemble simulations were carried out using the Meso-NH[2] atmospheric research model, giving more or less importance to each of the two processes.

Man in rainstorm image via Shutterstock.

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