Marc Stéfanon

Heatwave classification over Europe and the Mediterranean region

A method for defining and classifying heatwave events in the Euro-Mediterranean region is presented. The definition is based on the 95th centile of the local temperature probability density function, with additional criteria for spatial and temporal extension. The heatwave events are then classified into six classes by cluster analysis. The six heatwave patterns of Europe are described and compared to the existing literature. The most catastrophic extreme heatwaves (e.g. 2003 in Europe, 2010 in Russia) are shown to belong to one of these classes. It is then shown that the different classes are associated with different physical mechanisms. The effect of synoptic circulation and hydrological conditions are notably investigated. In particular, a drought appears to be a pre-requisite to heatwave occurrence in some clusters but not all.

Soil moisture-temperature feedbacks at meso-scale during summer heat waves over Western Europe

This paper investigates the impact of soil moisture-temperature feedback during heatwaves occurring over France between 1989 and 2008. Two simulations of the weather research and forecasting regional model have been analysed, with two different land-surface models. One resolves the hydrology and is able to simulate summer dryness, while the other prescribes constant and high soil moisture and hence no soil moisture deficit. The sensitivity analysis conducted for all heatwave episodes highlights different soil moisture-temperature responses (1) over low-elevation plains, (2) over mountains and (3) over coastal regions. In the plains, soil moisture deficit induces less evapotranspiration and higher sensible heat flux. This has the effect of heating the planetary boundary layer and at the same time of creating a general condition of higher convective instability and a slight increase of shallow cloud cover. A positive feedback is created which increases the temperature anomaly during the heatwaves. In mountainous regions, enhanced heat fluxes over dry soil reinforce upslope winds producing strong vertical motion over the mountain slope, first triggered by thermal convection. This, jointly to the instability conditions, favors convection triggering and produces clouds and precipitation over the mountains, reducing the temperature anomaly. In coastal regions, dry soil enhances land/sea thermal contrast, strengthening sea-breeze circulation and moist cold marine air advection. This damps the magnitude of the heatwave temperature anomaly in coastal areas, expecially near the Mediterranean coast. Hence, along with heating in the plains, soil dryness can also have a significant cooling effect over mountains and coastal regions due to meso-scale circulations.

Effects of interactive vegetation phenology on the 2003 summer heat waves

This paper investigates the impact of accounting for interactive plant phenology on the simulation of the June and August 2003 European heat waves. A sensitivity analysis is conducted here by using the WRF atmospheric model and the ORCHIDEE land-surface model over France with (1) a prescribed vegetation corresponding to year 2002 and (2) a dynamical vegetation model that leaves the vegetation freely evolving. It has been found that, accounting for the phenology dynamics has opposite effects on both events, it damps the temperature anomaly in June, while it amplifies the temperature anomaly in August. The evolution of leaf area index in the two simulations reveals the early and fast development of agricultural vegetation in the simulation with freely evolving vegetation. The vegetation also decays earlier in 2003 than during normal years. This behavior has two consequences. In June, the larger foliage development, caused by higher springtime insolation, contributes to enhanced evapotranspiration and therefore land surface cooling which limit the temperature anomaly during the heat wave. This effect is not as visible in mountainous regions where the presence of forest and the absence of agriculture do not lead to the same modulation of the local water cycle. In August, the early leave fall and the critical soil moisture stress contribute to largely suppress evapotranspiration and to enhance sensible heat flux thus amplifying the temperature anomaly. The modulation of the temperature anomaly caused by the effect of interactive vegetation phenology can reach 1.5C for an average total anomaly of about 8C (i.e. 20%).

European heatwave in July 2006: Observations and modeling showing how local processes amplify conducive large-scale conditions

July 2006 was particularly warm in Europe. The consistency of this kind of anomaly with large-scale circulation conditions or local processes is a key issue for regional climate evolution. Using observations from space and ground-based observatory, together with simulations from regional model, shows that two concomitant but disconnected drivers explain this heatwave. The first driver corresponds to large-scale conditions (specific atmospheric condition with advection of continental air favoring clear sky). The second condition relates to local processes (dry soil, amplifying surface temperature in heatwave for first 5 days, and making this event warm enough to induce a monthly mean anomaly). This large-scale event is studied at a site in northern France, where comprehensive observation data carefully reanalyzed are available. A regional model is able to produce the amplitude of the event, for both temperature and cloud large-scale anomalies. Coupling model and observations allow discriminating the surface contribution to the temperature anomaly.

Modification of land‐atmosphere interactions by CO2 effects: Implications for summer dryness and heat wave amplitude

Plant stomata couple the energy, water and carbon cycles. We use the framework of Regional Climate Modeling to simulate the 2003 European heat wave and assess how higher levels of surface CO2 may affect such an extreme event through land-atmosphere interactions. Increased CO2 modifies the seasonality of the water cycle through stomatal regulation and increased leaf area. As a result, the water saved during the growing season through higher water use efficiency mitigates summer dryness and the heat wave impact. Land-atmosphere interactions and CO2 fertilization together synergistically contribute to increased summer transpiration. This, in turn, alters the surface energy budget and decreases sensible heat flux, mitigating air temperature rise. Accurate representation of the response to higher CO2 levels, and of the coupling between the carbon and water cycles are therefore critical to forecasting seasonal climate, water cycle dynamics and to enhance the accuracy of extreme event prediction under future climate.