The prediction of heat waves and drought conditions

During June, July and August 2003, an exceptional heat wave affected western and central Europe. The extreme drought and heat had heavy social, economic and environmental impacts, among them the death of thousands of the elderly. Other serious consequences were the destruction of large forest areas by fire, the depletion of aquatic ecosystems, the retreat of glaciers, power cuts and transport restrictions, and a decrease in agriculture production. In recent years, the number of such extremes was increasing, according to the Intergovernmental Panel on Climate Change (IPCC, 2001), and Meehl and Tebaldi (2004) also said that is possible that the number of heat waves similar to that of 2003 will increase in the future. As the long-range forecasts (greater than 1 month) for this phenomenon were not successful, it is thus important to try to understand which mechanisms can exacerbate or guide this kind of event.

In the specific case of the European heat wave in the summer of 2003, it has been shown that the synoptic circulation induced by the persistence of an anomalously strong anticyclone extended over Western Europe for most of the period May-August 2003 drove hot air from North Africa, producing strong subsidence motions on the right-hand side of the anticyclone (i.e. over central and Western Europe) during most of the time (see for instance Cassardo et al., 2007). The enhancement of the adiabatic compression related to these synoptic motions contributed to the observed temperature increments. A second enhancing factor that exacerbated the heat wave effects was the precipitation deficit recorded during the preceding spring, which caused low soil moisture values to be observed already at the beginning of the heat wave. This fact is not surprising as, according to Black and Sutton (2007), the soil water content in Europe's Mediterranean regions plays a critical role in the climate regulation across Europe and in the development of climate anomalies over Europe.

A thorough understanding of the physical mechanisms of climate is a baseline for their subsequent incorporation in the meteorological and climatological models. For instance, the ECMWF short-, medium- and long-range (up to 30 days) forecasts were successful in inferring the main features of the large-scale flow as well as the temperature anomalies, while the seasonal predictions were a failure, even considering the EPS (see Section 5). Grazzini et al. (2003) suggested that this problem could be due to a combination of (i) failure to predict the observed large Sea Surface Temperature (SST) anomalies in the Indian Ocean, and (ii) the above mentioned dependence on the soil moisture. The continuous refinement of the physical processes in the numerical atmospheric models could allow a more accurate prediction of such extreme phenomena.

Continue reading here: The importance of land surface conditions and of surface layer parameterizations

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