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

[Joe Huang]

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


On Aug 27, 2014, at 3:59 PM, "Collinge, William Overton" <woc6 at pitt.edu> wrote:

> 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/
> 
> 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"
> 
> 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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