Explaining Rainfall Accumulations over Several Days in the French Alps Using Low-Dimensional Atmospheric Predictors Based on Analogy

Abstract

We propose a new approach to explain multiday rainfall accumulation over a French Alpine watershed using large-scale atmospheric predictors based on analogy. The classical analogy framework associates a rainfall cumulative distribution function (CDF) with a given atmospheric situation from the precipitation accumulations yielded by the closest situations. The analogy may apply to single-day or multiday sequences of pressure fields. The proposed approach represents a paradigm shift in analogy. It relies on the similarity of the local topology mapping the pressure field sequences, somehow forgetting the pressure fields per se. This topology is summarized by the way the sequences of pressure fields resemble their neighbors (dimensional predictors) and how fast they evolve in time (dynamical predictors). Although some information—and hence predictability—is expected to be lost when compared with classical analogy, this approach provides new insight on the atmospheric features generating rainfall CDFs. We apply both approaches to geopotential heights over western Europe in view of assessing 3-day rainfall accumulations over the Isère River catchment at Grenoble, France. Results show that dimensional predictors are the most skillful features for predicting 3-day rainfall—bringing alone 60% of the predictability of the classical analogy approach—whereas the dynamical predictors are less explicative. These results open new directions of research that the classical analogy approach cannot handle. They show, for instance, that both dry sequences and strong rainfall sequences are associated with singular 500-hPa geopotential shapes acting as local attractors—a way of explaining the change in rainfall CDFs in a changing climate.

Characterizing large‐scale circulations driving extreme precipitation in the Northern French Alps

“…Among 138 EPS in the Isère River catchment, respectively 113 and 21 were associated with Atlantic and Mediterranean circulations, pointing out the Atlantic influence as the main influence driving EPS in the Isère River catchment, as also shown for extreme daily precipitation in Figure 2 of Blanchet et al (2020). Among 122 EPS in the Drac River catchment, respectively 78 and 42 were associated with Atlantic and Mediterranean circulations, pointing out a more balanced contribution of either influence in producing EPS in the Drac River catchment, again in coherence with Figure 2 of Blanchet et al (2020). Due to concomitance in EPS, there are 142 distinct EPS associated with Atlantic circulations and 49 distinct EPS associated with Mediterranean circulations (Figure 9).…”

“…Four atmospheric descriptors are used, among which three are based on analogy, that is, they account for the relative position of the atmospheric states with respect to each Faranda et al (2017aFaranda et al ( , 2017b and Messori et al (2017). They proved to be skilful for predicting precipitation in the region in Blanchet and Creutin (2020). A fourth nonanalogy descriptor is added.…”

Past Evolution and Recent Changes in Western Europe Large-scale Circulation

“…Changes in LSC characteristics are investigated using four atmospheric descriptors introduced in previous works Blanchet et al, 2018;Blanchet and Creutin, 2020). These descriptors are based on daily 500 hPa geopotential height fields over Western Europe (rectangle in Fig.…”

“…The singularity cannot be directly related to the frequency of occurrence of a given flow direction since a geopotential shape is never perfectly reproduced (T W S k,k > 0). Very low singularities even appear to be rare in the climatology, which means that the atmosphere spends much time exploring quite unseen patterns than very closely coming back to an already seen pattern Blanchet and Creutin, 2020).…”

“…In this paper, we propose a contribution to fill this gap by employing four atmospheric descriptors that characterize the 500 hPa geopotential height field and that allow the consideration of dynamical trends within the main atmospheric influences. These descriptors were introduced in previous works, and they were shown to explain precipitation variability and extremes in the Northern French Alps (Blanc et al, 2021b;Blanchet and Creutin, 2020). They characterize i) the stationarity of a flow direction (celerity), ii) how well a flow direction is reproduced in the climatology (singularity, relative singularity), and iii) the intensity of the low and the high pressure systems (Maximum Pressure Difference).…”