On Tuesday 26 June 2007 01:18, José Antonio Ruiz Arias wrote: > > -----Mensaje original----- > > De: Dylan Beaudette [mailto:[EMAIL PROTECTED] > > Enviado el: lunes, 25 de junio de 2007 22:37 > > Para: José Antonio Ruiz Arias > > CC: [email protected]; [EMAIL PROTECTED] > > Asunto: Re: [GRASS-user] Use of coefbh and coefdh in r.sun > > > > On Monday 25 June 2007 12:27, José Antonio Ruiz Arias wrote: > > > Hi Dylan, > > > > Hi, thanks for the comments! > > > > > I will expose what I modestly think about. > > > > > > 1. does (1-coefbh)=coefdh? No. > > > I think you are confusing the clear-sky indices for the direct and > > > diffuse components with the direct and diffuse fractions. These > > > > latter > > > > > are both referred to the total global radiation coming into the > > > > ground > > > > > so that, considering the reflected component negligible, the addition > > > of the direct and diffuse fractions are always the unity. > > > > ok- this makes sense. > > > > > In turn, the clear-sky index > > > for the direct horizontal component (coefbh) is the fraction of > > > > direct > > > > > component attenuated by the clouds, that is, the ratio of direct beam > > > horizontal radiation under overcast-skies to the direct beam > > > horizontal radiation under clear-skies. The same is applied to the > > > > diffuse component. > > > > It sounds like in order to use 'real-sky' estimates I would need more > > than a > > pyranometer- as I would need to measure diffuse and beam components on > > both clear AND cloudy days... ?
Thanks for the reply, > Depending on the type of radiation you need (global, direct or diffuse), if > you want a 'real-sky' estimation with r.sun you will have to measure that > radiation for one (or a few sites - better) in your study area. That will > be your "ground-truth" to tune up the r.sun results and you can - hopefully > - expect to be representative for the whole study area (raster). Indeed. This makes sense now. I do not think that I will be able to estimate real-sky values for a couple reasons- mainly the short historical record, and only a single weather station with pyranometer. > > > 2. does coefdh correspond to the notion of the 'diffuse fraction' > > > which is commonly calculated via something called the 'clearness > > > index' ? I think I have answered above. > > > > Ok, it sounds like my proposed method was in error. > > > > > > In this case, Kt (see named attachment) and the derived Kd (see > > attachment) - are describing the diffuse fraction of total radiance? > > Values which cannot be used for the coefdh and coefbh inputs to r.sun. > > Yes. Maybe the definition help you to see the subtle difference among these > indices: > > Global_rad=Direct_rad + Diffuse_rad (If reflected rad is negligible) > Kt=Global_rad/Extraterrestrial_rad > Kb=Direct_rad/Global_rad and Kd=Diffuse_rad/Global_rad => Kb+Kd=1 Ok, this makes sense and I understand this now. > regardless the sky conditions. However, coefdh and coefbh are defined as: > > coefbh=horizontal direct radiation under any-sky (to be > measured)/horizontal direct radiation under clear-skies (r.sun) > coefdh=horizontal diffuse radiation under any-sky (to be > measured)/horizontal diffuse radiation under clear-skies (r.sun) > > which distinguishes among sky conditions. Thus, you multiply your clear-sky > estimation raster map (r.sun) for the corresponding coefficient coefbh or > coefdh calculated on your ground station (measured/r.sun). Got it. > > Would it be possible to disaggregate my pyranometer data in this way- > > giving my the real irradiance on the ground, compute the clear sky > > radiance at the same location with r.sun, and then compute the ratio of > > the two in order to get coefdh / coefbh ? > > Yes, you could use one of the regression equations to get Kd and Kb from > the global radiation provided by your pyranometer, then compute the > clear-sky raster maps with r.sun for your study area and compute the ratio > of the two. However, take always in account that these equations provide > you a statistical approach to your problem having a inherent statistical > error that depends on the regression chosen. In the bibliography you could > look for one in your site area. Indeed, this was my initial reason for not doing so. It appears as if I am back where I started estimating clear sky conditions from an estimated Linke value. > > > My personal opinion is that the best approach consists on using a > > > > more > > > > > or less approximated turbidity coefficient of Linke (a climatological > > > value could be the easiest choice) to get the clear-sky estimation > > > (r.sun) and then use the ground global radiation measurement > > > (supposing you have it, > > > obviously) to correct the clear-sky estimation provided by r.sun. > > > > This > > > > > approach can be also applied to both, the diffuse and direct > > > components of the radiation. The problem with the components is the > > > seldom availability of measurements. > > > > This was my initial goal, and seemed to work fairly well. I was able to > > compute T_L values accurately during the summer based on my pyranometer > > data, but there were not enough clear days in the 11yr history to > > compute realistic T_L values in winter months. Using a 15% diffuse > > fraction estimate for the entire year seemed to yield data which > > followed the 11yr mean values quite well.... However, I was hoping to > > avoid a static diffuse fraction- and make better use of the pyranometer > > data for the computation of 'real-sky' > > conditions. This may not be possible...? > > I think, rather than use a static 15% of diffuse fraction you should use a > regression equation that will give you slightly better estimations (remind > that if you have the global radiation, you have the clearness index, and > then, the diffuse fraction and the direct fraction through a regression > equation). The problem of estimating the sky turbidity is that you need > spectral measurements of the radiation and usually we do not have it. > However, there exist broadband methods to calculate it from direct beam > measurements. But this approach unavoidably needs of direct beam > measurements and, in the case of the TL, also needs the humidity (usually > precipitable water). I had originally tried this- but my 11 years of data did not have enough clear sky winter days, and therefore the computed T_L values were between 7 - 11 ; perhaps a little unrealistic. Thanks again, dylan > > > I hope to have cleared I little the question José A. > > > > Yes, thanks for the tips > > > > Cheers, > > > > Dylan > > > > > > -----Mensaje original----- > > > > De: [EMAIL PROTECTED] [mailto:grassuser- > > > > [EMAIL PROTECTED] En nombre de Dylan Beaudette Enviado el: > > > > lunes, 25 de junio de 2007 19:25 > > > > Para: [email protected] > > > > CC: [EMAIL PROTECTED] > > > > Asunto: [GRASS-user] Use of coefbh and coefdh in r.sun > > > > > > > > Hi, > > > > > > > > After consulting with an atmospheric scientist, I have decided that > > > > it might be simpler to estimate 'real-sky' radiance as opposed to > > > > trying to estimate the Linke turbidity for the estimation of > > > > 'clear-sky' conditions. > > > > > > > > It looks like r.sun can use input maps: coefbh and coefdh to > > > > compute > > > > > > real-sky radiance values. I would like to be sure that I am > > > > interpreting these inputs > > > > correctly: > > > > > > > > 1. does (1- coefbh) = coefdh ? > > > > 2. does coefdh correspond to the notion of the 'diffuse fraction' > > > > which is commonly calculated via something called the 'clearness > > > > index' ? > > > > > > > > > > > > If this is the case, then it is not all that difficult to compute > > > > coefdh from pyranometer data and the output from r.sun (mode 1) > > > > > > > > I will update the man page with my approach, if these assumptions > > > > are correct. > > > > > > > > cheers, > > > > > > > > PS: > > > > a relevent references is: > > > > > > > > Jacovides, C.; Tymvios, F.; Assimakopoulos, V. & Kaltsounides, N. > > > > Comparative > > > > study of various correlations in estimating hourly diffuse fraction > > > > of global solar radiation. Renewable Energy, 2006, 31, 2492 - 2504 > > > > > > > > > > > > -- > > > > Dylan Beaudette > > > > Soils and Biogeochemistry Graduate Group University of California > > > > at > > > > > > Davis > > > > 530.754.7341 > > > > > > > > _______________________________________________ > > > > grassuser mailing list > > > > [email protected] > > > > http://grass.itc.it/mailman/listinfo/grassuser > > > > > > > > __________ Informacisn de NOD32 2352 (20070625) __________ > > > > > > > > Este mensaje ha sido analizado con NOD32 antivirus system > > > > http://www.nod32.com > > > > -- > > Dylan Beaudette > > Soils and Biogeochemistry Graduate Group University of California at > > Davis > > 530.754.7341 > > > > > > __________ Información de NOD32 2354 (20070626) __________ > > > > Este mensaje ha sido analizado con NOD32 antivirus system > > http://www.nod32.com -- Dylan Beaudette Soils and Biogeochemistry Graduate Group University of California at Davis 530.754.7341 _______________________________________________ grassuser mailing list [email protected] http://grass.itc.it/mailman/listinfo/grassuser
