Moisture recycling and the maximum of precipitation in spring in the Iberian Peninsula

Abstract

In the semiarid interior of the Iberian Peninsula, the topographic insulation from the surrounding seas promotes the role of internal sources of moisture and water recycling in the rainfall regime. In inland Iberia, the annual cycle of precipitation often has a distinctive peak in the springtime, when evapotranspiration (ET) is the highest, in contrast to the coastal areas, where it is more closely related to the external moisture availability and synoptic forcing, with a maximum in winter-autumn and a pronounced minimum in the summer. In this work we investigate the role of land surface water fluxes in the intensification of the hydrological cycle in the Iberian spring. We used data from 5 km resolution WRF-ARW model simulations over the Iberian Peninsula for eleven months of May (2000–2010). To bring out the effect of ET fluxes, we conducted experiments where ET water over land was removed from the system. Our findings indicate that the impact of ET on precipitation is on average very large (37 % increase). The impact is particularly strong in the interior north and northeast areas where the observed annual rainfall cycle has a peak in May, suggesting that the role of surface water fluxes is very important there. To investigate whether this role is as a water source or to amplify precipitation dynamics, we computed the recycling ratio analytically from the model data. In addition, we developed a procedure to quantify the amplification impact by comparing the recycling ratio and the relative change in precipitation between control and experiments with ET removed. Results show that the role of surface water fluxes on precipitation depends on large-scale forcing and moisture advection. When the synoptic forcing and moisture advection are strong, such as in fronts associated to Atlantic storms, the impact of ET fluxes is small. When there is potential for convection, as is commonly the case of late spring in the Iberian Peninsula, ET fluxes have a significant impact. Surface moisture fluxes moisten the boundary layer and increase moist static energy, strengthening convective processes, and their role goes from being a primary water source for precipitation (recycling) to have mostly an amplification effect as external moisture availability increases. Our findings show that for the eleven simulated May cases over the Iberian Peninsula, the role of ET fluxes in activating recycling is important and explains 27–58 % of their total impact on precipitation, depending on the method used to calculate the recycling ratio. This indicates that the complementary effect of ET on amplifying rainfall from external sources of moisture is comparable in magnitude to the recycling mechanism and important as well.