The effects of lake representation on the regional hydroclimate in the ECMWF Reanalyses

Abstract

Lakes are an integral part of the geosphere, but they are challenging to represent in Earth system models, which either exclude lakes or prescribe properties without simulating lake dynamics. In the ECMWF interim reanalysis (ERA-Interim), lakes are represented by prescribing lake surface water temperatures (LSWT) from external data sources, while the newer-generation ERA5 introduces the Freshwater Lake (FLake) parameterization scheme to the modeling system with different LSWT assimilation data sources. This study assesses the performance of these two reanalyses over three regions with the largest lakes in the world (Laurentian Great Lakes, African Great Lakes, and Lake Baikal) to understand the effects of their simulation differences on hydrometeorological variables. We find that differences in lake representation alter the associated hydrological and atmospheric processes and can affect regional hydroclimatic assessments. There are prominent differences in LSWT between the two datasets that influence the simulation of lake-effect snowstorms in the Laurentian winters and lake land-breeze circulation patterns in the African region. Generally, ERA5has warmer LSWT in all three regions for most months (by 2 12 K) and its evaporation rates are up to twice the magnitudes in ERA-Interim. In the Laurentian lakes, ERA5 has strong biases in LSWT and evaporation magnitudes. Over Lake Baikal and the African Great Lakes, ERA5 LSWT magnitudes are closer to satellite-based datasets, albeit with a warm bias (1 4 K), while ERA-Interim underestimates the magnitudes. ERA5 also simulates intense precipitation hot spots in lake proximity that are not visible in ERA-Interim and other observation-based datasets. Despite these limitations, ERA5 improves the simulation of lake land circulation patterns across the African Great Lakes.