Seeking Help: Understanding WACCM Data's Time and time_bnds Variables

[YY XIA]

Hi, everyone.
I downloaded the WACCM data with a 6 - hour output frequency for a specific month (in 2024) and region from the website (WACCM Download Subset).

After reading the downloaded NetCDF (NC) file, the "time" variable shows values like 0, 0.25, 0.5, 0.75, 1, 1.25…30.75, with the unit being days from the start time. I'm puzzled about whether the four sets of data per day represent the average values of the four 6 - hour time periods, or the instantaneous values at 00:00, 06:00, 12:00, and 18:00.

If each data profile represents a 6 - hour average, does it correspond to the time periods of UT 00:00 - 06:00, 06:00 - 12:00, 12:00 - 18:00, and 18:00 - 24:00?

Moreover, there is another variable named “time_bnds” in the file. Its long_name is ‘time interval endpoints (Size: 2×124, Dimensions: nbnd, time; 31 [days] * 4[data profiles per day] = 124). For example, the first four columns of “time_bnds” variable in the first row are [0, 0, 0.25, 0.5, 1], and in the second row are [0, 0.25, 0.5, 0.75]. If the first and second rows represent the start and end times of the time interval corresponding to each data profile respectively, why are the two value in the first row and the second row of the first column both 0? In other words, why are the first set of time interval endpoints of each day the same in "time_bnds"?

Thank you again for your patience and help!

 

[strandwg]

"time" represents the current model time, and also the time at which instantaneous fields are saved, and "time_bnds" represents the end points of the averaging interval for which averaged fields are saved.

If an output field has the attribute "cell_methods" then it's an averaged field; if that attribute is missing, the output field is an instantaneous value.

Does this answer your questions?

 

[YY XIA]

Thank you so much for your prompt and clear explanation! It really helps me a lot to understand the definitions of "time" and "time_bnds" variables.

Just to confirm with you: if the output field read from the NC file doesn't have the attribute "cell_methods", even though the file contains the "time_bnds" variable, does that mean the output field represents an instantaneous value rather than an averaged one?

Your clarification is extremely valuable to my research. Thank you again for taking the time to help me!

 

[strandwg]

Thank you so much for your prompt and clear explanation! It really helps me a lot to understand the definitions of "time" and "time_bnds" variables.

Just to confirm with you: if the output field read from the NC file doesn't have the attribute "cell_methods", even though the file contains the "time_bnds" variable, does that mean the output field represents an instantaneous value rather than an averaged one?

Your clarification is extremely valuable to my research. Thank you again for taking the time to help me!

You are correct. The presence of "cell_methods" as a variable's attribute indicates it's time-averaged.

 

[YY XIA]

Thank you so much for taking the time to respond! I truly appreciate your assistance with this question.