Evaluation of precipitation variability over complex terrain based on observations and high resolution WRF v4.2.2 modelling

Abstract. Heterogeneity in the occurrence, amount and distribution of precipitation in mountainous areas is relevant for water resources and stresses the need for high-altitude observations and high-resolution modeling over complex terrain. This work focuses on the analysis of precipitation in the Sierra de Guadarrama, a mountain range at the center of the Iberian Plateau that provides water resources for several million citizens living within it and in nearby cities, mostly Madrid. The precipitation climatology is assessed from available observations and high resolution climate model simulations during the period 1991–2020. For this purpose, data from 37 stations located in the Sierra de Guadarrama and surrounding lowlands, with altitudes ranging from 600 to 2200 m a.s.l have been analyzed and compared to model simulations. A regional simulation with the WRF 4.2.2 model, driven with boundary conditions provided by the ERA5 reanalysis, was performed. The simulation has three nested domains with increasing resolutions of 9, 3 and 1 km, with the smallest domain covering the target region, including the Sierra de Guadarrama and surroundings. The comparison of the different data sources aims at characterizing the precipitation distribution over the area, assessing the performance of ERA5 and the potential added value of the increasing resolution of the WRF simulation in reproducing the observations. Results show that the increase in WRF resolution from 9 to 3 km produces a better representation of the overall climatology of precipitation and the altitudinal gradients. The 1 km resolution simulation represents better the shape of the probability distribution of precipitation, including extremes, but overestimates total accumulations, mostly due to an overestimation of the occurrence of wet days. Orography plays a very relevant role in activating precipitation and precipitation gradients with altitude are best captured at 1 km resolution, as well as extreme values. However, some seasonal gradients are best matched by the 3 km resolution. Furthermore, summer convective precipitation is slightly improved, but still poorly simulated, likely because the simulation does not allow for convection-permitting at any resolution. The results are discussed with the support of some sensitivity tests changing convection parameterizations and allowing for convection-permitting schemes (CPS). The results are relevant to understand the behaviour of precipitation in complex terrain, to better interpret mountain climates and also the hydrology of the center of the Iberian Peninsula.