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Accumulations are described in section ERA5-Land data documentation. The accumulations in monthly means (stream=moda/mnth) are described in section monthly means
How to use lake-related fields
Independently whether a model grid point is over a lake or not, the IFS computes lake variables all over the globe, at each grid-box. This is to ease output field aggregation at diverse model resolutions and to have a warm start of the model with shorter spin-up time if lake cover is upgraded, i.e., it is still a decent lake initial condition if lake location are updated or a new lake is added operationally. Lake depths (input parameter for our lake parametrization) are specified for each grid-box either with in-situ values or with a default 25 m value; over ocean we use ocean bathymetry. Worth to mention that the later default values will be changed soon (extra information in this HESS reference). The computed lake variable values are not taken into account in the total grid-box flux calculations if lake is not present in the grid-box.
Lake location in the model is determined by lake cover field (parameter name CL, in fraction: 0 - grid-box has no lakes, 1 - grid-box is fully covered with lake/s). Lake depths are presented in the field DL (in meters).
The ECMWF model also contains an ice module, a snow module and a bottom sediments module. The present setup of the IFS is running with no bottom sediment and snow modules (snow accumulation over ice is not allowed and snow parameters are used only for albedo purposes). In the implementation in the IFS lake ice can be fractional within a grid-box with inland water (10 cm of ice means 100 % of a grid-box or tile is covered with ice; 0 cm of ice means 100 % of the grid-box is covered by water; in between a linear interpolation is applied) (Manrique-Sunen et al., 2013). At present, the water balance equation is not included for lakes and the lake depth and surface area are kept constant in time (IFS Documentation, 2017, chapter 8 and 11). Lake parametrization also requires the lake fraction CL, lake depth DL (preferably bathymetry), and lake initial conditions. DL is the most important external parameter that uses the lake parametrization.
New HESS lake paper (https://www.researchgate.net/publication/333362092_Upgraded_global_mapping_information_for_earth_system_modelling_an_application_to_surface_water_depth_at_ECMWF)
IFS documentation (https://www.ecmwf.int/en/elibrary/18714-part-iv-physical-processes) Chapter 8 and 11.
Known issues
At the time of writing (May 2019) we are aware of these issues with ERA5-Land:
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