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.\" RCSid $Id$ |
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.TH "GENSSKY" "1" "8/31/24" "RADIANCE" "" |
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.SH "NAME" |
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genssky - generates a RADIANCE description of the spectral daylight sources |
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.SH "SYNOPSIS" |
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\fBgenssky month day hour [-y year] [-d AOD] [-c cloud_cover] [-l file] [-g grefl] [-n nproc] [-r res] [-f out]\fR |
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.SH "DESCRIPTION" |
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\fIGenssky\fR produces a RADIANCE scene description of spectral sky |
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and ground using the precomputed atmospheric scattering method |
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(Bruneton et al). For a given atmospheric condition, a set of |
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Radiance data files are pre-computed and reused for other solar |
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positions. Precomputation can be sped up using multithreading with |
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the \fI-n\fR flag. Extraterestrial solar radiation are spectrally |
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resolved from 380-780nm at 20nm interval and constant regardless |
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of location and time. The resulting sky source is in the same spectra |
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range. The output are three files defaults to {prefix}.rad, |
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{prefix}_sky.hsr, {prefix}_ground.hsr. The default prefix is "out" |
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and can be changed using \fI-f\fR flag. The two .hsr files stores |
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the fisheye spectral image of the sky and ground, with a default |
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resolution of 128x128, which can be changed using \fI-r\fR flag. |
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.PP |
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Non-clear conditions are modeled by linearly interpolating between |
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clear sky and CIE overcast sky. The resulting sky is the weight |
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(cloud cover) average of clear and overcast sky. The overcast sky |
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has a CCT of 6415K, from which the daylight spectra was computed. |
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.PP |
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Genssky models the Mie scattering differently from the original |
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implementation. Instead of assuming exponential decay uniformly |
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across the spectral range, the Mie scattering coefficients, as a |
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functinon of the altitude, are computed into a lookup table using |
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libradtran radiative transfer solver and interpolated directly |
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during precomputation. The default Mie scattering profile is OPAC |
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continental average, and can be overridden using the \fI-l\fR flag. |
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The overall aerosol profile can also be scaled linearly by applying |
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a broadband aerosol depth value using the \fI-d\fR flag. |
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.PP |
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Rayleigh scattering as a function of altitude is assumed to be |
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exponential decay. The Rayleigh scattering coefficients at sea level |
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are taken from Anderson et al (1986). There are a total of five |
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sets of Rayleigh scattering data: mid-latitude summer, mid-latitude |
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winter, sub-arctic summer, sub-arctic winter, and tropical. Latitudes |
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are divided at 23.5 and 62.5 deg and summer is from month [4-9]. |
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The scale-height of each Rayleigh scattering profile are tuned to |
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fit the corresponding results from a libradtran calculation. |
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.PP |
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\fIgenssky\fR can be used with the following input parameters. |
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.PP |
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\fB-a Latitude in degree, north positive. Used along with datetime to determine solar angle and Rayleigh scattering profile. |
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.br |
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\fB-c Total cloud cover, value ranging from 0(clear)-1(overcast), default=0. |
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.br |
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\fB-d Broadband aerosol optical depth, default: 0.115. This parameter linearlly scales the overall aerosol scattering, default=0.115. |
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.br |
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\fB-f Output file name, default="out" |
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.br |
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\fB-g Average ground reflectance, default=0.2 |
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.br |
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\fB-l Custom Mie scattering profile file |
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.br |
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\fB-m Standard meridian, west positive. For example, UTC-8:00 = 120. Used to calculated solar angle. |
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.br |
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\fB-n Number of threads for precomputation, no benefits beyond 16 threads |
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.br |
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\fB-o Longitude in degree, west positive. Used to calculated solar angle. |
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.br |
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\fB-r Sky and ground hyperspectral image resolution. default=128. Increase this value to reduce artifacts at horizon. |
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.br |
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.SH "EXAMPLES" |
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To generate a clear sky for March 2th at 3:15pm standard time at a site latitude of 42 degrees, 108 degrees west longitude, and a 110 degrees standard meridian, using 8 threads: |
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.br |
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genssky 3 2 15.25 -a 42 -o 108 -m 110 -n 8 |
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For the same sky but with 100 percent total cloud cover: |
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.br |
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genssky 3 2 15.25 -a 42 -o 108 -m 110 -n 8 -c 1 |
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.br |
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.SH "FILES" |
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/usr/local/lib/ray/mie_ca.dat |
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.br |
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.SH "AUTHOR" |
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Taoning Wang |
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.br |
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.SH "SEE ALSO" |
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gensky(1), gensky(1), rcomb(1), rcontrib(1), rpict(1), rtpict(1), |
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rtrace(1), xform(1) |
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.br |
