[Bldg-sim] Precipitation value in stat extension file.

[Collinge, William Overton]

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+?

From: Bldg-sim [mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Michael J Witte
Sent: Wednesday, August 27, 2014 3:52 PM
To: bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] Precipitation value in stat extension file.

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



On 8/27/2014 12:50 PM, Joe Huang wrote:
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/<http://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<mailto:yjhuang at whiteboxtechnologies.com>

http://weather.whiteboxtechnologies.com for simulation-ready weather data

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On 8/27/2014 8:59 AM, Mayank Bhatnagar wrote:
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



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