The Copernicus Arctic Regional reanalysis (CARRA) dataset is already in use for couple of years and hereafter we gathered some user questions, which were primarily collected from the registration forms of the forthcoming CARRA User Workshop (21 September, 2023, https://climate.copernicus.eu/copernicus-arctic-regional-reanalysis-carra-user-workshop) and also some user questions submitted to the Copernicus User Support. Hereafter we list these questions (some of the questions are edited for clarity) classified into some major categories and we try to give brief answers to them.
QUESTIONS RELATED TO SCIENTIFIC CONTENT OF CARRA
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Regarding the sea ice parametrisation (SICE): is that implemented with a constant sea ice thickness with number of layers or can the ice column actually evolve?
The SICE routine utilises thermodynamic relations to model the evolution of the sea ice thickness. The ice thickness is initialised once per production stream (different streams covering different time periods) with a one year spin up period each. Then the ice cover evolves freely as modelled by SICE without applying any external constraints. There is no consistency for perennial ice across the streams, and there is indeed a jump between the end of one stream and the beginning of a new one. This is only a problem in regions where there is ice all the time. For parts of the domain which are open some times of the year, the thickness will develop anew. Figure 1 below illustrates this. Here we see clear jumps between one stream and the next. These jumps would be more pronounced if only multi-year ice would be included.
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Some general description about the spin-up in CARRA can be found at Copernicus Arctic Regional Reanalysis (CARRA): Data User Guide#Singlelevelvariables
How do the heat flux calculations differ in CARRA from ERA5?
In both cases sensible and latent heat fluxes (H and LE, respectively) formulations are based on aerodynamic resistances. But there are some differences. We describe some of them below.
The expressions of H and LE in ERA5 and CARRA can be described as:
H=rho*Ch*U*Ft and LE=rho*Lv*Ch*U*Fq
where rho is the air density, Ch the turbulent exchange coefficient for heat and vapour, Lv is the latent heat of vaporization, U is the wind speed, Ft is a function of air and surface temperature, and Fq is a function of air and surface specific humidity, respectively. Ft and Fq represent the temperature and specific humidity gradients between surface and the environmental air, respectively.
In ERA5 and CARRA Ch, U, and Ft are different.
- In ERA5 Ch is calculated from the Obukhov length based on an iterative process to compute the Richardson number whereas CARRA uses directly the Richardson number (Louis 1979 formulation).
- In ERA5 U (wind modulus) accounts for free convection velocity w* but not in CARRA, which uses only the wind components u and v.
- In ERA5 Ft uses enthalpy where CARRA uses surface temperature.
Please note that the list of differences above might not be exhaustive.
What are major differences between the ERA5 reanalysis and CARRA?
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