Weather types and spatial variability of precipitation in the Iberian Peninsula

The relationship between circulation weather types (WTs) and the spatial variability of precipitation across the Iberian Peninsula were studied using a high density, quality controlled, homogenized monthly precipitation database with approximately 3000 stations and interpolated to a 10km grid. The circulation WTs were computed using an objective version of the Lamb classification centered on the Iberian Peninsula. A total of 26 WTs were selected for the period 1948-2003. Daily WTs were grouped to obtain their monthly frequencies, and used as potential independent variables in a linear least-square non-negative regression model with a forward stepwise selection. Results show the impact of each WT on precipitation in the Iberian Peninsula with a spatial resolution which had never been achieved before and additionally were obtained on a monthly scale highlighting the large seasonal cycle observed in each class, and including significantly different patterns in winter and summer. Nevertheless, results confirm that most of the precipitation in the Iberian Peninsula is produced by just a few WTs, with W, SW and C being the most influential. The association between WTs and precipitation is more robust in winter months and for the western IP areas, while it is lower during summer months and for the eastern IP areas. Spatial analysis revealed that precipitation on the Mediterranean coastland is mostly related with easterly flows (NE, E, SE and their hybrid counterparts), while on the Cantabrian coastland. N and NW flows are the most influential WTs. In general, cyclone-related types are the least frequent ones and also the most efficient in generating precipitation; while anticyclone-related types have the highest frequencies, but also the lowest contribution to total monthly rainfall in the Iberian Peninsula.