Create → RTS File for Qnet Shortwave FluxThe input variables for modeling the net flux of shortwave
radiation are defined as follows:
For each variable, you may choose from the droplist of data types. For the "Scalar" data type, enter a numeric value with the units indicated in the dialog. For the other data types, enter a filename. Values in files must also use the indicated units. Single grids and grid sequences are assumed to be stored as RTG and RTS files, respectively. Time series are assumed to be stored as text files, with one value per line. For a time series or grid sequence, the time between values must coincide with the timestep provided. For DEMs with pixel geometry and bounding box given in terms of Geographic coordinates, the latitude and longitude of each pixel is used in the calculations. For DEMs with a "fixed-length" pixel geometry (e.g. UTM coordinates), which tend to span smaller areas, the dialog prompts for a single lat/lon pair to be used in the calculations. timestep: The timestep between frames in the new grid sequence (RTS file) should typically be about one hour and should match the timestep that will be used to model the Snowmelt or Evaporation processes. The number of frames in the RTS file will depend on the start and stop times, as well as the timestep. The start time, stop time and timestep should match those used to create the new longwave radiation file with extension "*.Qn-LW". time zone: The boundaries of time zones can be very irregular and a time zone map should be consulted if you are unsure. The time zone is not simply a function of the longitude. You can select an adjacent time zone to include the effect of Daylight Savings Time. Time zones with non-integer offsets from GMT are not yet supported. factor: This number can be used to account for cloud or canopy cover. It is simply multiplied by the "clear sky" value of QSW. There are many different methods of computing this factor from the fraction of the sky covered by clouds, C, and the fraction covered by forest canopy, F. slope: Topographic slopes (not slope angles) are specified as dimensionless numbers [m/m]. A RiverTools grid (RTG file) with extension "_slope.rtg", "_mf-slope.rtg" or "_dinf-slope.rtg" can be used. aspect: Aspect is specified as an angle measured in radians counter-clockwise from due east (the standard convention). A RiverTools grid (RTG file) with extension "_mf-angle.rtg" or "_dinf-angle.rtg" can be used for the (continuous-angle) aspect grid. Equations Used to Compute Shortwave RadiationSee References below. Notes on the EquationsThis routine creates a grid sequence (RTS file format) of clear-sky solar radiation flux, QSW, (shortwave radiation) that is used for the energy-balance methods of Snowmelt and Evaporation. Direct, diffuse and back- scattered radiation fluxes are all modeled. Properties of the atmosphere such as air temperature, relative humidity and dust attenuation are used as well as surface/topographic properties such as slope angle, aspect angle and surface albedo. The approach followed here closely follows the one outlined in Appendix E of Dingman (2002). However, instantaneous vs. day-integrated radiation fluxes are used and the optical air mass is modeled using the widely used method of Kasten and Young (1989). QSW is set to zero between the times of local sunset and local sunrise, so frames in the RTS file that correspond to nighttime hours will contain only zeros. For additional information, see the help page for the longwave radiation calculator. ReferencesDingman, S.L (2002) Physical Hydrology, 2nd ed., Prentice Hall, New Jersey. (see Appendix E) Kasten and Young (1989) (for the optical air mass equation) Liston, G. ******* Marks and Dozier (1992) ******* Water Resources Research. Whitman, A.M. (2003) A simple expression for the equation of time, online document, http://www.sunspot.noao.edu/sunspot/pr/answerbook/expl-5.html |