Land - atmosphere coupling in climate models over North America; understanding inter-model differences

García García, Almudena (2020) Land - atmosphere coupling in climate models over North America; understanding inter-model differences. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The interactions between the lower atmosphere and the land surface are associated with weather and climate phenomena such as the duration, frequency and intensity of extreme temperature and precipitation events. Thus, the representation of land- atmosphere interactions in climate model simulations is crucial for projecting future changes in the statistics of extreme events as realistically as possible. Given the importance of the land-atmosphere interaction, the purpose of the thesis is to evaluate climate simulations performed by General Circulation Models (GCMs) and Regional Climate Models (RCMs) and examine the role of the Land Surface Model (LSM) component and the horizontal resolution over North America. For this purpose, I analyze a large set of simulations from GCMs and RCMs used by the fifth assessment report of the Intergovernmental Panel on Climate Change (IPCC) as well as my own simulations performed by the Weather Research and Forecasting (WRF) model. Results show that GCM simulations present large uncertainties in the representation of land-atmosphere interactions in comparison with observations. This work also reveals a dependence of the simulated land-atmosphere interactions on the LSM components used in regional and global simulations. Additionally, the LSM component is identified as an important source of uncertainty in the simulation of extreme temperature and precipitation events. Increasing the horizontal resolution also affects the simulation of land-atmosphere interactions, which lead to the intensification of precipitation, evapotranspiration and soil moisture at low latitudes; that is increased latent heat flux, soil moisture, and precipitation. The impact of both factors, horizontal resolution and the LSM, is larger in summer in agreement with the summer intensification of land-atmosphere interactions reported in the literature. The comparison of model simulations and observations indicates that the use of the most comprehensive LSM component available in WRF, the Community Land Model version 4 (CLM4), leads to a better representation of temperature climatologies. In contrast, finer horizontal resolutions are associated with larger biases in the WRF simulation of precipitation climatology, due to the overestimation of precipitation in the WRF model. Due to the large effect of the LSM component on the simulation of near-surface conditions shown in this dissertation, the use of simple version of LSM component in GCMs, RCMs or reanalyses can be an important limitation in climate simulations and reanalysis products.

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