Solar Radiation Modeling:
TOPORAD and TOPQUAD
Don Cline
Background
Purpose
The models TOPORAD and TOPQUAD are for computing the spatial distribution
of solar radiation throughout an area of interest. TOPORAD computes the
instantaneous solar radiation distribution, i.e. computes the
distribution for one instant in time. TOPQUAD integrates the solar
radiation inputs over the course of a day, to yield the total solar
radiation loading. TOPQUAD calls TOPORAD several times for one day, then
performs the integration.
To run TOPORAD and TOPQUAD, you must use IPW: the Image Processing Workbench,
written by Jim Frew and Jeff Dozier at UC Santa Barbara. IPW utilizes UNIX
primitives to perform a variety of image processing functions. This tutorial
and the subsequent exercises will get you started in IPW, but only far enough to run the models and perform some basic IPW functions.
Digital Elevation Models
The basic data required to run TOPORAD and TOPQUAD is a digital elevation model (DEM).
A DEM is a matrix, or grid of elevations, with one elevation for each element
of the matrix. The spatial resolution of the DEM refers to the dimension, in
ground units, of each side of the grid element (elements are usually square,
but don't have to be). For example, USGS standard product DEMs come in
different spatial resolutions, depending on their source: DEMS produced from
data related to 7.5 min quadrangles is nominally 30 m x 30 m, while other
USGS DEM products have resolutions of ~90 m x 90 m, etc.
To do much with DEMs, we have to know the "address" of each element in the
grid. Usually, we do this with row/column addresses. If the origin of the
DEM is the upper left corner, then the top row is row one, and the left-most
column is column one. Then the address of the upper-left cell is (1,1).
The terms ith row, jth column are used to refer to generic cell addresses, especially in coding. In remote sensing, it is more common to
refer to rows as "lines" and columns as "samples".
We also have to be aware of the range of elevations in a DEM, and how that
range can be represented in digital values. 256 values from 0-255 can be coded
as 8-bit data, but that is not enough for most places. In Colorado, where
elevations may range from 2000-4000 m in the same DEM, we need at least 2000
values. 12-bit data can represent 4096 values, which is sufficient for most
places.
For this exercise in using TOPQUAD and TOPORAD, we'll be using a DEM for
Niwot Ridge in the Colorado Front Range. It has a spatial resolution of
20 m x 20 m, and has 175 lines (rows) and 423 samples (columns). Elevations
in the DEM range from 3107 m to 4095 m. The data comes to you as an ASCII
file with 1 field (elevations) and 74025 lines (175 x 423 = 74025). The origin
of the DEM is the upper left corner, and the ASCII file lists each element
starting with 1,1, 1,2, 1,3... 2,1, 2,2, 2,3... etc.
Atmospheric Parameters
To run either TOPORAD or TOPQUAD, you have to know (or guess) something
about the atmosphere. Obviously, whether it's clear or overcast significantly
affects the radiation regime. But, perhaps less obviously, the gaseous
constituents, particulates, etc. also affect the radiation regime, and
must be accounted for even under clear skies.
The three atmospheric parameters needed for these models are called tau,
omega, and gamma (t,w,g).
Modeling Exercise
There are four components to this lab. The first two are tutorials to
demonstrate how to import data into IPW, and how to run TOPORAD and TOPQUAD.
The third component is a set of exercises to perform. Follow the links below
to each of these components.
1. Tutorial: Converting ASCII elevation data to IPW format.
2. Tutorial: Viewing IPW images using xv2.
3. Tutorial: Running TOPORAD and TOPQUAD
4. Exercises