Alpert & al. 2002 - A

Référence bibliographique complète
ALPERT P., BEN-GAI T., BAHARAD A., et al. The paradoxical increase of Mediterranean extreme daily rainfall in spite of decrease in total values. Geophys. Res. Lett., 2002, vol. 29, n° 11.

Abstract: Earlier reports indicated some specific isolated regions exhibiting a paradoxical increase of extreme rainfall in spite of decrease in the totals. Here, we conduct a coherent study of the full-scale of daily rainfall categories over a relatively large subtropical region - the Mediterranean - in order to assess whether this paradoxical behavior is real and its extent. We show that the torrential rainfall in Italy exceeding 128 mm/d has increased percentage-wise by a factor of 4 during 1951-1995 with strong peaks in EI-Nino years. ln Spain, extreme categories at bath tails of the distribution (light: 0-4 mm/d and heavy/torrential: 64 mm/d and up) increased significantly. No signifiçant trends were found in Israel and Cyprus. The consequent redistribution of the daily ramfall categories -torrential/heavy against the moderate/light intensities - is of utmost interest particularly in the semi-arid sub-tropical regions for - purposes of water management, soil erosion and flash floods impacts.

Hydrology, precipitation, floods, global change, climate dynamics

Organismes / Contact

(1) - Paramètre(s) atmosphérique(s) modifié(s)
(2) - Elément(s) du milieu impacté(s)
(3) - Type(s) d'aléa impacté(s)
(3) - Sous-type(s) d'aléa

Pays / Zone
Massif / Secteur
Site(s) d'étude
Période(s) d'observation
Mediterranean area (Spain, Italy Cyprus, Israel)  

265 stations in Mediterranean-Spain, ltaly, Cyprus and Israel


(1) - Modifications des paramètres atmosphériques

Class B (Light-Moderate) is the number one contributor with about 34-39% immediately followed by the Heavy categories C1(27-3l%) and C2 (16-20%).

The Light category A (7 -11 %) is next, and closing-tlle-list are the Torrential categories, D1(4-8%) and D2(l-4%).

In Spain and Italy, the D-categories increased, while the central C-categories decreased. ln Spain, moreover, the lower extremes increased as well, indicating a very strong increase of the daily rainfall variability. No significant trends were evident in Israel and Cyprus.

There is a clear dividing line between the Heavy-Torrential categories (C2,D1I and D2) which show an increasing trend, and the weaker categories (A,B,C1) which show a highly significant decreasing trend. The Heavy to Torrential contribution (C2 + Dl + D2), increases from 23% in the 1950s to about 31% from the total annual rainfall in the 1990s.

In Italy, the Torrential contributions Dl and D2 exhibit the largest inter-annual fluctuations, compared to the other categories.

Extreme daily rainfall increases in spite of the fact that total rainfall general1y decreases. For instance, torrential rainfall exceeding 128 mm/d contributes 4-5% of the total Italy rainfall in the 1990s compared to only 1% in the 1950s ; an increase by a factor of 4. Furthermore, heavy to torrential categories above 32 mm/d contributed in the 1950s only 23% of Italy rainfall and this share increased to 32% in the 1990s.

Italy stations during 1880-1996 have shawn similar trends for heavy rainfall contributed mostly by the rainfall in summer and in the transition seasons [Brunetti et al., 1999].

Relation to EI-Nino:
It is interesting to note that Torrential rainfall categories Dl and D2 tend to peak in El-Nino years, particularly noticed for Italy, for the EI-Nino years 1953, 1965, 1982/3, 1986/7. In 1983 and 1986, for instance, the D2 category alone contributed over 15% compared to an anriual average of only 1-4%. The link between Torrential rainfall and EI-Nino seems to strengthen in recent last decades.


Such a scenario illustrates the existence of a substantial change in the rainfall distribution over a relatively large subtropical region, the Mediterranean, in which the "increase in variance" overcomes the "reduction in the mean" [Meehl et al., 2000]. These results strongly illustrate the trends suggested due to global green-house gag warming. Both modeling [Hennessy et al., 1997], and observational studies [Brunetti et al., 1999], show that this reduction is associated with fewer rainy days. The latter is explained by the increase in the frequency and persistence of sub-tropical anticyclones, particular1y over the Mediterranean.


Informations complémentaires (données utilisées, méthode, scénarios, etc.)

Construction of 6 daily rainfall categories as powers of 2 in analyzing the 1951-1995 trends for 265 Mediterranean stations : in Mediterranean-Spain (182 stations), Italy (42), Cyprus (3) and Israel (38). Analysis in each location following the same exact methodology.

Selected daily rainfall categories in mm/day :
A : Light : 0-4 mm/d,
B : Light-Moderate : 4-16 mm/d;
C1 : Moderate-Heavy : 16-32 mm/d;
C2 : Heavy : 32-64 mm/d;
Dl : Heavy-Torrential : 64-128 mm/d; and
D2 : Torrential : 128-up mm/d.

Daily rain values in powers of 2, i.e., [2 to the power of n] as indicated above, allow the analysis of the contributions of several rainfall categories to the rainfall totals and how these contributions change with time. Earlier studies have generally adopted a number of different but single thresholds in order to define heavy rain which made it difficult to compare among these studies. In a recent review [Easterling et al., 2000], for instance, five different thresholds were listed for heavy rain, i.e. 20, 25.4, 50, 50.8 and 100 rnm/d. These threshold values and others found in the literature, can be well fitted to the suggested [2 to the power of n] definitions of heavy rainfall, i.e. 20 to 16, 50 to 64 and 100 to 128.

The monotonous non-linear time trends were tested for significance using Spearman's rank correlation.

(2) - Effets du changement climatique sur le milieu naturel

Sensibilité du milieu à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)

(3) - Effets du changement climatique sur l'aléa

Paramètre de l'aléa
Sensibilité du paramètres de l'aléa à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)

(4) - Remarques générales


(5) - Syntèses et préconisations

Références citées :

- Brunetti, M., L. Buffoni, M. Maugeri, and T. Nanni, Precipitation intensity in northern Italy, Int. J. Climatol., 20,1017-1031, 1999.

- Easterling, D. R., et al., Observed variability and trends in extreme climate events: A briefreView, Bull. Amer. Meteor. Soc., 81, 417-425, 2000.

- Hennessy, K. J., J. M. Gregory, and J. F. B. Mitchell, Changes in daily precipitation under enhanced greenhouse conditions, Climate Dynamics, 13,667-680, 1997.

- Meehl, G. A., et al., An introduction to trends in extreme weather and climate events: Observations, socioeconomic impacts, terrestrial ecological impacts, and model projections, Bull. Amer. Meteor. Soc., 81, 413-416, 2000.