There are a number of issues with the cloud fraction profile and the low, medium, high and total cloud fraction diagnostics.
- For ERA-Interim and other ECMWF products from IFS cycles before CY33R1, High/Medium/Low/Total Cloud Cover (HCC/MCC/LCC/TCC) diagnostics are calculated using the Geleyn and Hollingsworth (1979) computation method. This is a computationally efficient implementation and is reported as maximum-random, but it is not strictly maximum-random overlap as there is also some randomness if there is a minimum in the cloud fraction profile (which results in slightly higher cloud cover than with strict maximum-random). [Geleyn, J-F., and A. Hollingsworth, (1979). An economical analytical method for the computation of the interaction between scattering and line absorption of radiation. Beitr. Phys. Atmos., 52, 1-16.]
- HCC/MCC/LCC/TCC are calculated from the start of timestep profile of cloud fraction, whereas the output profile of cloud fraction is from the end of timestep. This will explain a large part of the inconsistencies between the prognostic cloud fraction profile and diagnostic HCC/MCC/LCC/TCC.
- In IFS cycles before CY33R1, there is a bug which means the LCC is underestimated if there is cloud in the profile either side of the the MCC/LCC boundary and cloud fraction is less in the lower layer than in the higher layer. This also applies to MCC for the HCC/MCC interface.
Given the above, in order to have consistent data, we suggest users calculate their own HCC/MCC/LCC/TCC from the cloud fraction profile.