Miguel Tremblay wrote: > Julie Prestopnik wrote: >> Miguel Tremblay wrote: >> >>> Julie Prestopnik wrote: >>> >>>> My supervisor asked me to pass along this question: >>>> >>>> The attached files pertain to the Denver snowstorm on 12/20/06. In the >>>> input to METRo, 28cm of snow fall during the first 33 hours. The cloud >>>> cover is complete throughout that time period. The road temperature is >>>> below 0C through the entire period. If the road is below freezing, I >>>> would not expect any runoff since the precipitation does not turn to >>>> liquid. Yet, the snow depth never gets above 1.7cm. I understand that >>>> some compaction may be taking place, but standard snow-to-liquid >>>> ratios are near 10-1. This compaction ratio is well beyond that. Where >>>> does all the precip go? >>>> >>>> Is there an implicit traffic model within METRo? Or, is the snow >>>> carried away by the wind? I'm just grasping for possible >>>> explanations... >>>> >>>> >>>> -Julie >>>> >>> Hi Julie, >>> >>> Excellent question and the timing of it is very surprising. >>> >>> I did a commit of file lib_therm.f this morning: I have translated >>> the comments of the subroutine RODCON (for road condition). >>> Everything about the water/snow runoff is in this routine. >>> >>> In simple words, there is a value (MEA in lib_therm.f) that is used >>> to parametrize this runoff. >>> >>> From the publication of METRo in JAM, page 2029, it is written: >>> « Snow removal, either by traffic or maintenance operations, >>> is also parameterized with the same exponential >>> relaxation function, with WS replacing Wl in (10). >>> This parameterization stems from the observation that >>> the passage of cars tends effectively to remove all but >>> a thin layer of snow at the road surface. » >>> >>> You can play with the different variables controlling the runoff. >>> Mainly MEA and the factors "3e-3" and "3e-4" used in the computation >>> of ER1 and ER2. >>> >>> If you have some remarks/comments/questions about those parameters, >>> do not hesitate since these are assumptions. >>> >>> Miguel >>> >> >> >> I have a follow-up question regarding the factor 3e-4 in the ER2 >> computation. We changed the 3e-4 value to 0 and re-ran the case we were >> looking at. As was noted in a previous email, this case was a major >> snow event in the Denver, Colorado area. With that last term in the ER2 >> equation being set to 0, we did see an increase in the snow depth on the >> road. However, at the time the snow depth was at its highest value in >> the output file (7.77cm - 2006-12-21T15:20Z), 27cm had already fallen >> during that period. In addition, the air temperature and the road >> temperature remained below freezing during the entire period. >> >> We're trying to determine why there is such a difference between the >> amount of total snowfall and the total snow on the road. Can you tell us >> what processes are suppressing the snow buildup on the roadway? >> >> I am re-attaching the input files for this case, as well as attaching >> the output files from the original run (roadcast.000.99.xml.orig) and >> also from the run where we set the 3e-4 value to 0 >> (roadcast.000.99.xml.metro3.0.4.snow_depth_ER2_0). >> >> Thanks for your help, >> >> Julie >> > Hi Julie, > > I had a look at your case and I created the images using ViX with the > 0.0 factor instead of 3e-4. > > The explanation for the fact that the amount in the ice/snow reservoir > do not exceed 7.77cm is that the vapor flux (evaporation) compensate for > the precipitation rate (look at the light sky blue line on vix_fl.png > image, the values are pretty high. Color legend here: > http://documentation.wikia.com/wiki/Graphical_display_%28METRo%29#Flux). > > The question is: is it normal that all the precipitation that fell > evaporate? I think not but, if you look at your input, you'll see that > your dew point is pretty low compare to the air temperature (green and > red line, respectively, on vix_st.png image). I don't think that it is > possible to have 27 cm of snow during 24h and a dew point 5°C below the > air temperature. > > In order to investigate further, could you check if the dew point and > air temperature are consistent with what happened? Maybe you could > check what would be the output if the dew point where less cold (warmer > :)). > > Just tell me what your results are, and what you think, and I'll have a > look at it. > > Miguel
Thank you, Miguel! We hadn't noticed the difference between the dew point values and the air temperature values. It turns out that our dew point values were not consistent with the obs. We going to investigate our forecast a bit more. Thanks again for your help. Julie _______________________________________________ METRo-developers mailing list [email protected] https://mail.gna.org/listinfo/metro-developers
