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How to calculate water content in atmosphere...


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Hello

I would like to calculate the precipitable water content from the known relative humidity. I tried to do this as proposed on the first page of this document

http://www.alma.nrao.edu/memos/html-memos/...238/memo238.pdf

However, for locations with very high water content I obtain a difference of about a factor 2. One of those locations is Manaus (Brazil). Looking at their sounding data of the first days in May (2006):

http://weather.uwyo.edu/cgi-bin/sounding?r...&STNM=82332

indicate a water content of about 60mm (see last line of a sounding report). Using the above suggested equation provides only values of about 30mm. I checked my calculations several times and they seem to be ok. Also the equations seem to be ok (or I made the same mistake as they did).

Where is here the problem, or, how could the precipitable water content be better calculated from relative humidity?

Thank you for your help!

Regards MET

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Are you using the two values "of the Lifted Condensation Level" for temp and pressure?

No. It looks like I have to explain a bit more on what I intend to do.

If I understand you correct you refer with the "Lifted Condensation Level" to the data provided from the soundings. For this analysis I actually can not use the sounding data. Too many locations considered in the analysis are not sounding locations. The only data available are those which can be obtained from e.g. Wunderground, i.e. Temperature, Dew Point Temperature, sea level Pressure, Relative Humidity, Wind etc. The data for the Water Content is not provided and has therefore to be estimated from the available data. It's therefore those data (Wunderground) which I used for estimating the Water Content with the suggested formula. Could it be that the assumed height of the column of 1.5km may vary so much that the result for Manaus differs by a factor of 2?

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With the help of Google I found here http://en.wikipedia.org/wiki/Lifted_condensation_level how the height of the lifted condensation level can be estimated with the difference between surface temperature and dew point temperature. I thought that this height would give a better estimate than 1.5 km scale height. Using the sounding data of Manaus (see above link) the LCL height would be about half of the water vapour scale height (1.5km) whereas the precipitable water content from the sounding suggests that the scale height should be twice the 1.5 km.

At this point I'm a bit lost. I would therefore very much appreciate if someone could indicate how the water vapour content can be estimated from surface data such as, temperature, dew point temperature, pressure (sea level or surface) and relative humidity.

Thanks for your help.

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Posted
  • Location: Coventry,Warwickshire
  • Location: Coventry,Warwickshire

    I think there is an assumption that the water content above the surface is uniform in those calculatiosn and you need to look at a number of levels.

    Try the equations Here

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    Thank you BrickFielder for indicating this paper. Unfortunately it doesn't help to solve my problem of estimating the water content from *surface* data. It's only by exception that also data at higher levels are available, like e.g. the above mentioned soundings from Manaus which I used for checking the estimation procedure.

    As you indicated, estimating the water content from surface data requires that some assumptions are made, like e.g. the scale height of 1.5 km. Any ideas on how this scale height could be estimated (instead of assuming a fixed value) with the available surface data (temperature, dew point temperature, pressure and relative humidity)? Is there e.g. a relationship/approximation between the height of the LCL (which can be estimated from surface and dew point temperatures) and the scale height?

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    • 3 months later...
    Posted
  • Location: City of Gales, New Zealand, 150m ASL
  • Location: City of Gales, New Zealand, 150m ASL

    I'm coming into this very late but it has been perplexing me though I don't know if the OP is still around. Even if he is not, the continued discussion would be of interest.

    When it comes to the water vapour partial pressure that is obviously not given in soundings so it has to be inferred. And it tells you in that paper how to do it. You need theta (sounding), RH (sounding) and e (2.718...blah). So no problems with that.

    I am fairly tired so not much time to think about this but with all that data, just plot it on a t-phi gram for the first few significant and standard levels. Blank tephis are on the web. easy to find the LCL from this data, just look at the surface plots of temp and dewpoint. Follow a constant mixing ratio line up from the dewpoint, follow a dry abiabat up from the temp and where they intersect, that's at the pressure value of the LCL.

    Only thing is, surely the precipitable water content is going to depend on the means of ascent. LCL => Forced ascent, i.e. over mountains. I would think that in Manaus, convective ascent is more important. For this you need to know the Level of Free Convection which gives the convective cloud base whereas LCL would give you the cloud base for forced ascent. Convective condensation level also can be inferred.

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