PILPS 2e Experimental Design, Response to Comments
Thank-you to everyone who took the time to review this document and submit your questions. A summary of the questions and comments we received is provided below. The revised experimental design will be posted on the PILPS 2e web site (http://www.hydro.washington.edu/~Lettenmaier/CurrentResearch/PILPS-2E/index.html) , as soon as a few outstanding issues are resolved.
Another figure has been added to show the location of precipitation, temperature and gauging stations.
A separate file with hourly cloud cover fraction (tor_cloud_1hr.nc) will be provided as an optional variable.
Yes. We have reconsidered this decision, this snow/rain temperature threshold will remain fixed and should not be altered by modelers.
Figure 2 has been revised to show this connection.
A file of 30 arc second resolution elevations (GTOPO30) will be provided instead of the proposed 5 minute DEM. Since this represents a much finer resolution than the remainder of the land surface characteristics, it will be provided in a separate file: tor_ele_30sec.nc.
See above, elevation data at 30 arc second resolution will be supplied for each grid cell, in file tor_ele_30sec.nc.
This experiment is designed so that soil parameters for the entire basin can be adjusted following calibration of the two 'calibration' sub-basins. Therefore, everyone will not be using identical soil parameters, even if only one parameter set is specified. We will therefore offer a choice of initial parameters sets, as originally indicated, rather then trying to justify a choice of one over another.
We are not planning to specify values for thermal conductivity and volumetric heat capacity since they are so strongly dependent on soil moisture and ice content. If a constant value is required, we suggest that you could use the equations of Johnasen [as reported in Farouki, O.T., Thermal Properties of Soils, in Series on Rock and Soil Mechanics, vol 11, Trans. Tech., Clausthal-Zellerfeld, Germany, 1986] and Flerchinger and Saxton, Simultaneous heat and water model of a freezing snow-residue-soil system, II Field verification, Trans. ASAE, 32, 573-578, 1989]. We will provide a suggested thermal damping depth and lower temperature boundary condition based on the soil temperature data (5, 20, 50 and 100 cm depth) available from the Abisko Research Station in the headwaters of the Torne River.
Unfortunately, the vegetation data that will be used do not indicate a specific threshold. The best interpretation we can offer is that closed shrubland contains more shrubland than grassland, and open shrubland contains more grass than shrubs. We suggest that you assume open shrubland contains less than 50% shrubs, while closed shrubland contains greater than 50% shrubs.
Once again we are trying to avoid specifying too many parameters since many models use them differently (e.g. transmittance of solar radiation is dependent on solar zenith angle, etc.). We will provide suggested values of the following parameters for each vegetation type: root distribution, radiation transmissivity, architectural resistance, stomatal resistance, displacement height and roughness height. It is not required that these values be used, however any deviations from the suggested values should be documented. A questionaire will be provided to clarify what sort of description is needed.
The value for landcover height of woodland should be 10 m, which is corrected in the revised document.
The reference height for wind, temperature and humidity measurements is 2 m, which is corrected in the revised document.
This is acceptable. The albedo of fresh snow can also be considered a 'suggested' variable, with any variations to be documented.
The calibration and validation pairs have been switched as suggested, and the revised document so indicates.
Yes, saturated hydraulic conductivity can be changed for the calibration basins. However, since this value can reasonably vary over orders of magnitude, we are not restricting the range to within 20% of the values in Table 3. Values for saturated hydraulic conductivity should be within two orders of magnitude of the values in Table 3.
We have attempted to calculate a representative precipitation lapse rate to apply to the mountainous areas, through the evaluation of annual precipitation at three station pairs. Two of these pairs are in the mountainous region to the northwest (Katterjakk (elev. 560 m)/Abisko (elev. 386 m) and Katterjakk/Kiruna (elev. 450 m) and one is in the north (Keinovuopio (elev. 450 m)/Karesuando (elev. 329 m). There was some concern that the Abisko station lies in a rain shadow, and in fact the lapse rate calculated for this station was much higher than the others and was discarded. Following interpolation, the daily precipitation values were lapsed to the mean grid cell elevation. We are still discussing the validity of this technique with SMHI, but the final data set will include some sort of orographic enhancement of precipitation.
In addition, our original proposal to disaggregate daily precipitation to hourly is not possible. Therefore, the hourly precipitation will be calculated as 1/24th of the daily value. Since the annual hydrograph in this area is dominated by snow melt, the error associated with this assumption should not be large. Andy Pitman, (Macquarie University) has proposed an experiment (possibly run on some sub-set of the models) to test the sensitivity of simulated results to the method of interpolation of meteorological data. We will explore the possibility of testing model sensitivity to the disaggregated precipitation at the same time.
The average surface temperature for the grid cell should be a weighted average for each surface category over which surface temperature is calculated.
Exactly. Other differences between both temperatures may arise from the numerical implementation of the energy balance. We cannot assume that the same temperature is used to compute the turbulent flux and the long-wave radiation. Thus the distinction between both temperatures is needed but very simple models may not show any difference between the two values.
Averaging the T4 values (weighted for surface area) for all radiative surfaces and taking the fourth root results in the effective radiative temperature of all surfaces in the model grid cell. This temperature to the fourth power results in the average outgoing longwave flux for the entire grid cell, whereas a weighted average of surface temperature will not necessarily conserve this longwave flux. So yes, the surface radiative temperature should be the same as that calculated from the weighted grid mean upward longwave radiation.
Yes, but that is what we are looking for. The other soil moisture parameters in Table 4 should be adequate for evaluation of absolute quantities.
The schedule will be relaxed by popular request!
A final version of the experimental design and forcing data, which incorporates all of these comments, will be released in one to two weeks, once we finalize decisions regarding the choice of calibration basins and complete our quality checks on the revised precipitation data set (see Comments 14 and 16).
We will aim for simulated results to be returned to the University of Washington by October 15, 2000.