Evaluation of the FLake Model in ERA5 for Lake Champlain

Abstract

Global model reanalyses of temperature and radiation are used for many purposes because of their spatial and temporal homogeneity. However, they use sub-models for lakes that are smaller than the model grid. This paper compares the simplified small-lake model, known as FLake, used in the European Centre global reanalysis known as ERA5, with observations made in and near Lake Champlain in northern Vermont. Lake Champlain is a challenging test for the ERA5 FLake model. The lake, which extends over several grid cells, is the lowest region at 30 m above sea level within complex mountain topography. The smoothing of the adjacent mountain topography means that the ERA5 grid cells containing the lake have higher mean elevations then 30 m, and this contributes to a small cool bias in FLake mid-summer temperatures. The seasonal cycle of FLake temperatures has a sharper peak than the observed lake temperatures. In winter, lake temperatures are close to 3°C, while the 30 m deep FLake mixed layer (ML) is near freezing. In May and June, FLake maintains a deep ML, while lake profiles are generally strongly stratified with peak temperatures near the surface several degrees above the model ML. One possible contributing reason is that inflowing river temperatures that are not considered by FLake are as much as 5°C above the lake surface temperature from April to June. The lake does develop a ML structure as it cools from the temperature peak in August, but the FLake ML cools faster and grows deeper in fall. We conclude that the vertical mixing in the FLake ML is stronger than the vertical mixing in Lake Champlain.

Daily surface temperatures for 185,549 lakes in the conterminous United States estimated using deep learning (1980–2020)

“…5d), but varied seasonally and across the range of temperatures (Fig. 5e,f) which is consistent with Betts et al (2020). Bias correction may be needed for most uses of the current ERA5 mixed layer temperature predictions.…”

“…For the ERA5 process‐based model used for comparison, we also bias‐corrected the output by adding 3.31°C to all predictions (referred to as ERA5*). This bias correction addressed a clear cold bias that currently exists in ERA5 in U.S. lakes (e.g., Betts et al 2020 found a 4°C cold bias of ERA5 on Lake Champlain in late spring; Muñoz‐Sabater et al 2021 reported a general cold bias across many lakes). The amount of bias correction was decided based on the intercept of a linear regression with slope 1 fit to observed vs. ERA5‐predicted temperatures.…”

“…Bachmann et al (2019), b T ¼ 16:14 þ 0:673Air À 0:0846Lat þ 0:0172Long À 0:00131Elev À 0:147Mon where the average over the folds of CV for each of the coefficients (air temperature [Air], latitude [Lat], longitude [Long], elevation [Elev], and month [Mon]) became the following equation: bT ¼ 20:368 þ 0:580Air À 0:159Lat þ 0:0347Long À 0:0015Elev þ 0:177MonFor the ERA5 process-based model used for comparison, we also bias-corrected the output by adding 3.31 C to all predictions (referred to as ERA5*). This bias correction addressed a clear cold bias that currently exists in ERA5 in U.S. lakes (e.g.,Betts et al 2020 found a 4 C cold bias of ERA5 on Lake Champlain in late spring; Muñoz-Sabater et al 2021 reported a general cold bias across many lakes). The amount of bias correction was decided based on the intercept of a linear regression with slope 1 fit to observed vs. ERA5-predicted temperatures.…”

Monitoring the Water Mass Balance Variability of Small Shallow Lakes by an ERA5-Land Reanalysis and Water Level Measurement-Based Model. An Application to the Trasimeno Lake, Italy

“…FLake is a one-dimensional model which represents fresh-water lakes [46][47][48], by parameterizing them as basins of fixed depth divided into a surface mixed layer, which is directly in contact with the atmosphere, and a deep layer, at the bottom. By means of the heat balance, it computes the behaviour over time of the temperature of the two layers: the surface mixed layer temperature, T ML , and the bottom layer temperature, T BL .…”