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What methodology was used to generate Potential Evapotranspiration in the 11-member RCM?

The 11-member RCM uses the PET calculation from the Met Office Surface Exchange Scheme (MOSES).

In detail

The model calculation is as follows:-

PET = potential evapotranspiration = POT_EVAP/(1+rs/ra*Cp/(Cp+L*alpha))

POT_EVAP is the standard daily mean potential evaporation rate.

PET_FACTOR1 and PET_FACTOR2 are two factors which are used to calculatepotential evapotranspiration rates from POT_EVAP. They are defined as:

PET_FACTOR1 = ra(1+L*alpha/Cp)
PET_FACTOR2 = ra(1+L*alpha/Cp)*PE

It is done this way so that the user can prescribe stomatal resistance
rather than let the model derive it. You can use these outputs to infer
the effective daily mean stomatal resistance in the model.