Thanks Miguel. 1. I won't be able to re-generate the roadcast until over the weekend; sorry to be a burden, but do you have time to change the air temperature and dewpoint temperatures so that they match the observed and then re-run the model ?
2. Yes I understand about site exposure, and yes we could adjust the cloud cover values to control the amount of solar flux. Would prefer to have some other parameter to control this other than cloud cover though..I'll think about that... 3. So the definition of the melting snow condition is when snow is falling on a road that is already above freezing ? Regards Iain -----Original Message----- From: Miguel Tremblay [mailto:[EMAIL PROTECTED] Sent: March 20, 2008 1:38 PM To: Iain Russell Cc: '[email protected]' Subject: Re: [Metro-developers] High pavement forecast temperature with melting snow road condition Iain Russell wrote: > Thanks for the analysis Miguel; I will re-run the model as soon as possible > as you suggested. > > In the meantime I have some other questions and comments; > > 1. One hypothesis which we have formed is that there could be a high > sensitivity dependence on the forecast cloud cover (cc parameter) within the > model. For example in the case provided here, the cc forecast values (in > octas) from 17 UTC to 23 UTC on 14th March are : > > 17Z : 8 > 18z : 8 > 19z : 7 > 20z : 7 > 21z : 6 > 22z : 6 > 23z : 7 > > The forecast road temperature shoots upwards after 18z as your chart shows, > just when the cloud cover values become less than overcast. i.e. the solar > flux values at the road surface may become very high due to broken cloud > cover. Do you have an opinion on this ? > > Yes. I did an experiment with cloud cover set to 0. You can see the results in both zip files in attachment. Colors at the top of vix_st.png represent the cloud cover (different shades of grey for 1-8 and blue when 0). When CC is at 0, the ST goes up to 22°C instead of 16. I dont think the difference between 6,7 and 8 in values in the cloud cover would explain the difference. But we will be in better position with the results using the true AT values. > 2. For some locations where we run the model, the roads are in deep canyons > which even now are not receiving direct sunlight (even when skies are clear > or partly cloudy). Does the model take into account any terrain blockages to > direct sunlight ? > > No. Since there is no parameter in the station configuration file, there is no way to METRo to know that. Every site is different. I suggest that you modify the CC based on your experience about where the sun is (visible or not) from every location for every hour for every day of year (sun altitude, and position, is not the same though the season). As you probably guessed, this question is a difficult one and every answer will be specific to each site. > 3. It seems odd that the model would predict "melting snow" with road > temperatures as high as 17 C; wouldn't the snow have to melt first and > perhaps the residual water mostly evaporate before the road temperature could > get that high ? Perhaps there's some error in the model physics > formulation...or perhaps we are interpreting the "melting snow" condition > incorrectly ? > > I had a look at the input and I understand why this is the result for road condition. If you look at the documentation for "road condition" (http://documentation.wikia.com/wiki/Road_condition_%28METRo%29), melting snow is defined like "Solid precipitation on the road when the temperature of the surface of the road is above the freezing point." If you look at the atmospheric input forecast you have given, there is precipitation of snow from the beginning of the road forecast until March 15th at 3 AM (represented by the thin blue line on vix_ac.png image in original.zip). During this period, the road temperature goes over the freezing point, starting at 17Z on March 14th. Since, for this period and until 3 AM the following day, the road temperature is above zero and there is snow falling, the snow is melting and the condition is "melting snow". Is this answer your question? Regards, Miguel Tremblay _______________________________________________ METRo-developers mailing list [email protected] https://mail.gna.org/listinfo/metro-developers
