Care must be taken in interpreting the Rainfall or Liquid Precipitation data in the EPW files on the EnergyPlus Weather web site, assuming that's what you're referring to as "the weather files".
Please keep in mind that EPW is simply a data format, and that the weather files on the EnergyPlus Weather web site are not a unified set, but a collection of files from sources all over the world that have been translated into EPW format. Although liquid precipitation is a standard meteorological report element, its incorporation in processed weather files like the EPWs on the EnergyPlus Weather web site is a relatively new development. For example, neither the original TMY2 nor the IWEC weather files contain data on liquid precipitation. I haven't checked the other data sets, but it's also likely that many of them also do not contain data on liquid precipitation. This means that the Liquid Precipitation field in the EPW of those weather files will be blank.
The only sets of weather data that I'm confident have Liquid Precipitation are the newer TMY3 for the US and IWEC2 for international (not on the EP Weather web site but available from ASHRAE). Even for these two data sets, the Liquid Precipitation has been problematic for the following reasons: (1) some weather stations (27% of US and Canadian stations, 6% international) do not report it, (2) liquid precipitation is reported as amount and duration that can be overlapping, which does not fit well with file formats that expect single hourly values, (3) rainfall is very variable year-to-year and not used to date in selecting the months making up a "typical year" file, so the rainfall on such files cannot be expected to be average or typical.
As for units, in the raw data from NCDC it's in millimeters, as it is in the TMY3s, but in the IWEC2 it's in 0.1 millimeters for improved precision.
Therefore, my suggestion to anyone who's concerned about the rainfall/liquid precipitation in the EPW files to do ample QC (Quality Control), i.e.,
1. Make sure that the weather file does report it
2. If so, make sure that the units are correct by comparing them to what's on the original weather files
3. Compare the annual or monthly totals to other independent sources such as ASHRAE Standard 169 to see their representativeness.
Joe
Joe Huang White Box Technologies, Inc. 346 Rheem Blvd., Suite 108D Moraga CA 94556 yjhuang@xxxxxxxxxxxxxxxxxxxxxxxx http://weather.whiteboxtechnologies.com for simulation-ready weather data (o) (925)388-0265 (c) (510)928-2683 "building energy simulations at your fingertips"
On 25/09/2013 8:18 AM, Michela Buzzetti wrote:
Dear group
I need some clarification about the rainfall data present in the weather files.
The supplied data "Liquid Precipitation" is expressed in mm but the values are very small (example 0.03, 0.6 … mm). Moreover this value has the same order of magnitude in different Reports Frequency: hourly, monthly or yearly.
Best Regards
Michela
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