Inter-comparison of two land-surface models applied at different scales and their feedbacks while coupled with a regional climate model

摘要

Downstream models are often used in order to study regional impacts ofclimate and climate change on the land surface. For this purpose, they areusually driven offline (i.e., 1-way) with results from regional climatemodels (RCMs). However, the offline approach does not allow for feedbacksbetween these models. Thereby, the land surface of the downstream model isusually completely different to the land surface which is used within theRCM. Thus, this study aims at investigating the inconsistencies that arisewhen driving a downstream model offline instead of interactively coupledwith the RCM, due to different feedbacks from the use of different landsurface models (LSM). Therefore, two physically based LSMs which developedfrom different disciplinary backgrounds are compared in our study: while theNOAH-LSM was developed for the use within RCMs, PROMET was originallydeveloped to answer hydrological questions on the local to regional scale.Thereby, the models use different physical formulations on different spatialscales and different parameterizations of the same land surface processesthat lead to inconsistencies when driving PROMET offline with RCM output.Processes that contribute to these inconsistencies are, as described in thisstudy, net radiation due to land use related albedo and emissivity differences, theredistribution of this net radiation over sensible and latent heat, for example, dueto different assumptions about land use impermeability or soil hydraulicreasons caused by different plant and soil parameterizations. As a result,simulated evapotranspiration, e.g., shows considerable differences of max. 280 mm yr?1. For a full interactive coupling (i.e., 2-way) between PROMET and theatmospheric part of the RCM, PROMET returns the land surface energy fluxesto the RCM and, thus, provides the lower boundary conditions for the RCMsubsequently. Accordingly, the RCM responses to the replacement of the LSMwith overall increased annual mean near surface air temperature (+1 K) andless annual precipitation (?56 mm) with different spatial and temporalbehaviour. Finally, feedbacks can set up positive and negative effectson simulated evapotranspiration, resulting in a decrease of evapotranspiration South of the Alps amoderate increase North of the Alps. The inconsistencies are quantified andaccount for up to 30% from July to Semptember when focused to an area around Milan, Italy.