Precipitation value in stat extension file.

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Dear All,

I am doing some analysis on weather file in which I need precipitation
value in mm or inches. However, when I look on stat extension file, there
is monthly precipitation value in kPa. As I know 'kPa' is a unit of
pressure. Please let me know if anyone has conversion factor for monthly
precipitation value from kPa to mm(inches).

The epw extension file has hourly precipitation value in mm. However, all
the weather files do not have hourly precipitation values.

I'll be grateful if anyone can guide me on this. :)

Regards,

Mayank

Mayank Bhatnagar's picture
Joined: 2011-10-02
Reputation: 0

Mayank,

You're right. kPa is a unit of pressure, so somehow the header units you're using have
been misaligned. I won't comment on what you see in the EPW, but if you look at the
source weather file, which probably uses a TMY3 or TMY2/IWEC format (the only ones I know
that contain precipitation data), there are two columns for liquid precipitation, one for
depth in mm followed by one for duration in hr.

Having said that, from my investigations in creating the IWEC2 weather files I've
concluded that this way of reporting the liquid precipitation can lead to missing or
double counting the amount of precipitation. The reason is that in the actual weather
station reports, there may be multiple reports in one record, e.g.,
100mm over the past 1 hour and 500mm over the past 24 hours
as well as reports that overlap in time, e.g.,
200mm over the past 4 hours
followed several hours later by
400mm over the past 24 hours

I've attached the brief-up about liquid precipitation from the IWEC2 final report with
more details.

The other thing to consider is that if you're using a "typical year" weather file, the
liquid precipitation on that file are a random selection and may not represent long-range
averages. Depending on your level of interest, it's best to go back to the source
historical weather files such as those available at NCDC: www.ncdc.noaa/oa/climate/isd/

Joe

Joe Huang
White Box Technologies, Inc.
346 Rheem Blvd., Suite 205A
Moraga CA 94556
yjhuang at whiteboxtechnologies.com
http://weather.whiteboxtechnologies.com for simulation-ready weather data
(o) (925)388-0265
(c) (510)928-2683
"building energy simulations at your fingertips"

Joe Huang's picture
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Reputation: 406

Sorry, there was a typo in the link to the ISD. It should be
www.ncdc.noaa.gov/oa/climate/isd/ (I tested it this time :-) )

Joe Huang
White Box Technologies, Inc.
346 Rheem Blvd., Suite 205A
Moraga CA 94556
yjhuang at whiteboxtechnologies.com
http://weather.whiteboxtechnologies.com for simulation-ready weather data
(o) (925)388-0265
(c) (510)928-2683
"building energy simulations at your fingertips"

Joe Huang's picture
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Joined: 2011-09-30
Reputation: 406

Not sure what this section of the EnergyPlus stat file is reporting
(will get this fixed or clarified)

- Monthly Indicators for Precipitation/Moisture (kPa)
Jan Feb Mar Apr May Jun Jul
Aug Sep Oct Nov Dec
1.4 1.6 1.8 1.8 2.3 2.6
2.9 2.8 2.7 2.2 1.8 1.3

But if you keep searching for "precipitation" you'll find what I think
you're looking for

- Monthly Statistics for Liquid Precipitation mm
Jan Feb Mar Apr May Jun Jul
Aug Sep Oct Nov Dec
Total 117 43 97 0 0 83
0 0 0 0 0 31
Max Hourly 36 13 14 0 0 17
0 0 0 0 0 6

That said, I would echo Joe's comments about using this information with
caution. Some data sources do not contain actual precip amounts but
only a flag for raining or not raining. The weather processor will fill
in approximate precip amounts in that case.

Mike

Michael J. Witte's picture
Joined: 2011-10-01
Reputation: 0

The variable in kPa is probably showing the total column water vapor. kPa water vapor divided by atmospheric pressure (101.3 kPa or thereabouts at sea level) would be an aggregate measure of absolute humidity i.e. an average over the vertical profile at some specific location.

Not sure what it is subsequently used for in E+?

WOC6 at pitt.edu's picture
Joined: 2014-07-16
Reputation: 0

Sounds liked you're referring to Precipitable Water, which has no relationship to the actual liquid precipitation falling on the ground. PW is reported in the TMY2/3 weather files, mainly because it's used along with a few other arcane variables (aerosol optical depth, albedo) in modeling solar radiation. These are all echoed in the EPW, which content-wise is 99% identical to the TMY2/3s, but I can't think of any reason why any of these variables would be used in EnergyPlus.

Another reason I'm doubtful of your explanation is that PW is typically reported in depth (0.1 cm in the TMYs) and not pressure.

Sent from my IPad

Joe Huang

Joe Huang's picture
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Joined: 2011-09-30
Reputation: 406

Thank you for your valuable suggestions.

Thank you joe, I am looking on source files for precipitation/ rainfall
data.

I hope, I will get your suggestions in future also.

Regards,
Mayank
On Aug 28, 2014 6:26 AM, "Joe Huang"
wrote:

Mayank Bhatnagar's picture
Joined: 2011-10-02
Reputation: 0

Mayank,

It would help if I knew the extent and purpose of your investigation into
precipitation/rainfall data. For example, what are the location(s)? Is it related to
assessing building energy performance, like for a green roof, etc. Is it a hypothetical
study for average conditions or a specific study of an actual building, time, and place ?

Joe

Joe Huang
White Box Technologies, Inc.
346 Rheem Blvd., Suite 205A
Moraga CA 94556
yjhuang at whiteboxtechnologies.com
http://weather.whiteboxtechnologies.com for simulation-ready weather data
(o) (925)388-0265
(c) (510)928-2683
"building energy simulations at your fingertips"

Joe Huang's picture
Offline
Joined: 2011-09-30
Reputation: 406