CALIPSO UTC and Profile time in L2 data

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ses2271
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CALIPSO UTC and Profile time in L2 data

by ses2271 » Wed Dec 21, 2022 6:21 pm America/New_York

Hello there,

I'm trying to collocate CALIPSO retrievals with those from another instrument. I've downloaded the L2 dataset that I need (CAL_LID_L2_05kmAPro-Standard-V4-20), but when I look at the two time variables available ("Profile_UTC_Time" and "Profile_Time") these are in formats that I don't recognize and don't know how to convert to more standard datetime formats.

For example, the first time values for the Profile_UTC_Time in one of the data files is 80101.36959844. From both the metadata and the name of this file (''CAL_LID_L2_05kmAPro-Standard-V4-20.2008-01-01T08-52-14ZD.hdf'), I can see that the scan starts at "2008-01-01T08:52:14Z," which makes me think that the 80101 corresponds to 2008-01-01. However, I'm not sure about the numbers following the decimal point; I checked and they don't correspond to the number of seconds past midnight (8:52:14 would be 31934 seconds). Do you know how these values should be interpreted?

As for the Profile_Time variable, I'm getting values like 473294144.8958215 for the first observation in the above file, and I'm not sure how to interpret that. I do understand the difference between the local profile time and the UTC time, just not how to interpret the values that exist in the dataset. Any insight you can provide, or documentation you can point me towards, would be very appreciated!

Best,
Sarah
Last edited by ses2271 on Thu Dec 22, 2022 11:39 pm America/New_York, edited 1 time in total.

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ASDC - ingridgs
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Re: CALIPSO UTC and Profile time in L2 data

by ASDC - ingridgs » Thu Dec 22, 2022 10:14 am America/New_York

Hello @ses2271 ,

Thank you for your question. A Subject Matter Expert has been notified and will answer your question shortly. Please stand by!

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Re: CALIPSO UTC and Profile time in L2 data

by ASDC - ingridgs » Wed Jan 04, 2023 10:36 am America/New_York

Hello @ses2271,
The science team response is pasted below:

A description of each of these time parameters contained in the file can be found here:

https://www-calipso.larc.nasa.gov/resources/calipso_users_guide/data_summaries/profile_data_v420.php#profile_time

Profile_Time
Time, expressed in International Atomic Time (TAI). Units are in seconds, starting from January 1, 1993. For the 5 km profile products, three values are reported: the time for the first pulse included in the 15 shot average; the time at the temporal midpoint (i.e., at the 8th of 15 consecutive laser shots); and the time for the final pulse.

Profile_UTC
Similar to Profile Time, but for time expressed in Coordinated Universal Time (UTC), and formatted as 'yymmdd.ffffffff', where 'yy' represents the last two digits of year, 'mm' and 'dd' represent month and day, respectively, and 'ffffffff' is the fractional part of the day.


So UTC time 80101.36959844 would be Jan. 1st, 2008 8:48:45


Thank you,

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