Clarification regarding forcing height (z) when using ERA5-derived EPW forcing in SUEWS Simple

[reeveugm]

Hi,

I would like to ask for clarification regarding the correct use of forcing height (z) in SUEWS Simple. This question is not about a software bug, but about the correct scientific methodology for using reanalysis forcing.

Background
I am using SUEWS Simple (UMEP) for neighbourhood-scale urban energy balance simulations.
My forcing originates from ERA5, downloaded through the ShinyWeatherData service.
The forcing contains the standard ERA5 near-surface variables:

• Air temperature at 2 m
• Relative humidity at 2 m
• Wind speed at 10 m
• Pressure
• Radiation
• Precipitation
• etc..

Originally, the forcing was provided as an EPW file, but because I experienced issues reading EPW directly, I converted it into the standard SUEWS forcing TXT format. The meteorological variables themselves were not modified during the conversion.
Therefore, the forcing still represents:

• Wind speed at 10 m
• Temperature at 2 m
• Relative humidity at 2 m

Initially, I set
z= 10
because I assumed the forcing height should match the ERA5/EPW reference height.
However, SUEWS terminated and faced error resolved in https://github.com/UMEP-dev/UMEP/issues/804#issuecomment-3655780837 and in https://github.com/UMEP-dev/SUEWS/issues/1035

At that time, one of the developers reproduced the error and replied:

"What you can do is, either to set a height that is acceptable or find a new forcing dataset. SUEWS is a local model that calculate fluxes in the inertial sub-layer i.e. two-three times the mean building height."

Since my study area has an average building height of approximately 8 m, I changed
z= 30
which allowed the simulations to run successfully.

Current concern
While preparing my manuscript, I read the latest documentation and found several statements.

1. Forcing documentation :

The forcing height (z) tells SUEWS where your data originate.

and

For reanalysis data (e.g., ERA5): check the reference height of the dataset.

2. EPW documentation:
It also states

Using EPW data with a different forcing height (e.g., z = 50) will cause incorrect wind profile calculations, as SUEWS assumes all forcing data originate from the specified height.

This seems to suggest that using

• wind = 10 m
• temperature = 2 m
• RH = 2 m

while specifyingz = 30 m is physically inconsistent.

However...
The earlier recommendation to increase z above the roughness sublayer appears to suggest the opposite. Therefore, I am unsure how these two recommendations should be interpreted together.

My questions:

1. Is it scientifically acceptable to use ERA5-derived forcing (10 m wind, 2 m temperature/RH) while specifying
   z= 30
   simply because the original forcing height results in (z-zd) < 0?

2. Does SUEWS internally extrapolate forcing variables from their original measurement heights to the specified z, or does it simply assume that all forcing variables already originate from the specified forcing height?

3. Is the recommendation to increase z above the roughness sublayer intended as
   a physical modelling recommendation,
   or
   merely a workaround to satisfy the model requirements when (z-zd) < 0?

4. For ERA5 forcing, what would be the recommended workflow?
   Keep z = 10 m?
   Apply an external profile correction before running SUEWS?
   Another approach?

5. If simulations have already been completed using
   z = 30
   while the forcing still represents ERA5 10 m / 2 m variables, would this generally be considered an invalid forcing configuration, or simply a less ideal approximation?

Any clarification would be greatly appreciated, as this issue directly affects the methodological validity of my simulations.

Thank you very much for your time.