How to convert the units of specific cloud liquid water from ERA5 (kg/kg) to kg/m2?
I got the solution https://www.researchgate.net/post/How_to_convert_the_units_of_specific_cloud_liquid_water_from_ERA5_kg_kg_to_kg_m2

and 

https://www.nwpsaf.eu/site/download/documentation/rtm/docs_rttov12/rttov_gas_cloud_aerosol_units.pdf

In the above link the conversions between $\small\mathsf{kg/kg}$ , $\small\mathsf{g/m^3}$ , $\small\mathsf{kg/m^2}$ can be explicitly derived

EDIT
The summary of the first hyperlink is:


  $\small\mathsf{kg/kg}$ stands usually for kg liquid water per kg air.
  Multiplying the term with the calculated air density within the field will result into $\small\mathsf{kg/m^3}$, which means kg liquid water per $\small\mathsf{m^3}$ air. Then you have to multiply with the thickness of the liquid cloud, which will result into $\small\mathsf{kg/m^2}$."