Question on UTC offset in SUEWS prepare and forcing data in local time

General
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I have a query regarding the handling of forcing time and the UTC offset setting in SUEWS Prepare.

Following the SUEWS documentation on forcing data requirements ( 4.2. Meteorological Forcing Data — SUEWS 2026.1.28 documentation ), I converted my ERA5 meteorological forcing data from UTC to local time (IST, UTC+5.5) prior to preparing the meteorological input files using UMEP / SUEWS Prepare.

During the SUEWS Prepare workflow, there is an option to specify the UTC offset. I would like to clarify the purpose of this setting and how it is intended to be used in this context.

Specifically:

  1. If the forcing data have already been converted to local time before ingestion into SUEWS Prepare, should the UTC offset still be set to +5.5, or should it be set to 0?

  2. What role does the UTC offset play internally (e.g. for solar geometry, or internal time conversions)?

  3. I noticed that changing the UTC offset value does not appear to modify the prepared forcing file timestamps, even when UTC time is used. Is this expected behavior?

I am using SUEWS Prepare to generate the input files and then running the simulations using SuPy. I want to ensure that the forcing time conventions are handled correctly and consistently across the workflow.

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Hi @Divya, welcome to the forum — and great questions! The UTC offset setting can be confusing, so let me walk through what’s happening under the hood.

Short answer: since your ERA5 data have already been converted to IST (local time), you should set the UTC offset to +5.5. This tells SUEWS the relationship between your timestamps and UTC, which the model needs internally.

Now to your specific questions:

1. Should the offset be +5.5 or 0 when data are already in local time?

Set it to +5.5. The offset is not a conversion instruction — it’s a declaration. You’re telling SUEWS: “these timestamps are in a timezone that is 5.5 hours ahead of UTC.” The model then uses that information for its internal physics calculations.

2. What does the UTC offset actually do internally?

It’s used for solar geometry — specifically, calculating the sun’s position (zenith and azimuth angles). Internally, SUEWS converts your local timestamps back to Universal Time for the solar position algorithm via:

ut_time = (hour - UTC_offset) / 24 + min/(60*24) + sec/(3600*24)

This UT time feeds into the Julian day calculation, which determines the Earth-Sun geometry. Getting this wrong would shift your solar radiation timing, affecting the entire surface energy balance.

3. Why don’t the timestamps change when I adjust the UTC offset?

That’s expected behaviour. The UTC offset is a site property, not a forcing data transformation. It lives alongside latitude, longitude, and altitude as metadata about your study area. SUEWS reads your forcing timestamps as-is and uses the offset only when it needs to compute solar angles internally.

In summary: provide forcing data in local time (as the documentation specifies), and set the UTC offset to match your local timezone (+5.5 for IST). The two work together — local timestamps plus offset gives SUEWS everything it needs.

Hope that clears things up — feel free to follow up if anything is still unclear!

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Thank you so much! This is very helpful.