Earthdata Forum

Hello.

I'm modelling heat transfer in a domain that's being heated by many sources, one of them being solar radiation. I did the computations on many solar geometry angles every 10 minutes and got to the point where the measured daily insolation (provided by POWER) is nearly equal to the one I get from the mathematical model. This is, mostly, because I'm using the All Sky Insolation Clearness Index (ALLSKY_KT) alongsinde the solar angles and the solar constant acoording to the time and place I'm studying.

The problem arises when I try to compute the energy received at near 90°zenith angle (sunrise and sunset) becuase KT is given on a hourly basis and the values around these times should be smaller than the neighbouring hourly average. I used some correlations in order to compute the air mass and then KT but the values (at least for the sunrise) seem to be way smaller and I get an abrupt change between the computed values and the ones I got from POWER.

So my question is, how the hourly average is computed? Is the value of HR = 6 the average between 6 AM and 7 AM? This is a very simple question but I'd prefer to be certain about the numbers I'm using.

Thanks!

EDIT: I'm evaluating solar irradiance at 29.03° North for September 24th up until the 26th.

Hi jahirbgoode,

A subject matter expert has been contacted relating to your question on how the hourly averaged parameter is computed.

Thank you,

David W.
NASA Langley ASDC DAAC Lifecycle

Dear Jahirbgoode,

To get started to answer this question, it would help you could specify the latitude and time period of the data that you are evaluating? This will impact the final answer.

Thanks.

Paul Stackhouse

Hello, Paul.

Right now, I'm evaluating solar irradiance at 29.03° North for September 24th up until the 26th.

Thanks.

Hi, Whitney.

I specified it in my last comment. I edited the original post too and just in case it's for 29.03° North for September the 24th to 26th.

These data are based on satellite measurements, and the measurements are intermittent, not continuous, and time interpolation and space averaging are used to get data on regular time stamps and regular grid boxes.

If an hour is relatively far from sunrise and sunset, the value for that hour is based on more than one observations and the value can thus be more reliable. If an hour is around sunrise or sunset, there may not be adequate observations for the interpolation and averaging and, therefore, the value is less reliable.

In addition, around sunrise or sunset, the cloud and aerosol information as well as the airmass are less reliable, plus, the topography may complicate the issue. All these make the values around sunrise and sunset less reliable, and the errors, instead of the normal small percentage, could be hundreds percent.