The Atmospheric Moisture Budget over the Great Lakes: Comparing Reanalysis and CMIP5 Present-day Simulations

The Laurentian Great Lakes are crucial for the region's economy, recreation and water supply, yet the prediction of precipitation and its impact on the region's water balance is still poorly constrained. Here we use a suite of reanalysis products and CMIP5 global models to understand the factors that drive present-day precipitation in the region. We focus the key components of the atmospheric water balance - precipitation, evaporation and moisture flux divergence - to understand the dominant drivers of interannual variability of precipitation in the region and explain regional biases and their causes. We compare four different reanalysis products (ERA-Interim, NCEP CSFR, NASA MERRA and the NARR) with a subset of global AOGCMs to understand the role of the moisture budget on regional precipitation variability. Precipitation differences between three of the reanalysis products and observations range from 20-50%, with a shift towards earlier spring precipitation than observed. The NCEP CSFR product showing a bi-modal precipitation cycle with maxima in April and November, distinctly different from the unimodal annual cycle in the observations and other reanalysis products. Interestingly, this pattern is produced in several of the CMIP5 models as well. We evaluate the driver of this early spring bias in precipitation over the region by examining the moisture budget components, including local evapotranspiration and large-scale dynamics.