[NARCCAP-discuss] extremely small precipitation value

[Jianhua Huang]

Hi Seth:

Thanks much for the information. In my research, I only care about whether
it is raining/snowing, not the volume. So the cutoff value is very
important. 

What does "per time period" mean here? I am using the 3-hourly pr data, and
the unit is kg m-2 s-1. If the density of rainwater is 1000kg per cubic
meter and "per time period" means an hour,  the unit is equal to mm m-2 s-1.
Then the cutoff number for raining /snowing is about 0.01*25.4/3600 =
7*10^(-5). Is that correct?

Thanks for any suggestion.

Jianhua


-----Original Message-----
From: narccap-discuss-bounces at mailman.ucar.edu
[mailto:narccap-discuss-bounces at mailman.ucar.edu] On Behalf Of Seth McGinnis
Sent: Tuesday, February 12, 2013 12:02 PM
To: Discussion of NARCCAP data - uses and questions
Subject: Re: [NARCCAP-discuss] extremely small precipitation value

Hi Jianhua,

All of the RCMs can put out very small precipitation values; those are the
values in the files, not a problem reading them into R.  The theoretical
lower limit is zero, so you should treat negative values as zero. (We have
plans to set them to zero in the published data at some point in the
future.)

Whether you should use a trace threshold and ignore values below that level
depends on how you are using the data.  The conventional trace value used in
meteorological observations is 0.01 inches per time period.

Cheers,

--Seth


On Mon, 11 Feb 2013 15:58:14 -0700
 "Jianhua Huang" <jh.eco.cas at gmail.com> wrote:
>Hi all:
>
> 
>
>I am using the precipitation (pr) data from rcm3-cgcm3 model. Some 
>values are very close to zero, for example 3*10^(-23),  and there are 
>even some negative values (like -3*10^(-21)). Should I treat these 
>values as zero? I don't know whether this problem is caused by the 
>software I used to process the data (I am using the ncdf4 package in 
>R). Anybody knows the theoretical smallest output from the rcm3-cgcm3 
>model, so that I can treat all values less than this value as zero?
>
> 
>
>Thanks
>
> 
>
>Jianhua
>
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