3D radiative transfer simulations in complex terrain

(Abstract for the International Radiation Symposium 2008)

B. Mayer, S.W. Hoch, C.D. Whiteman

Monte Carlo codes can accurately calculate radiance and irradiance under arbitrarily complex conditions. As an example we show a simulation of the radiation in and around Meteor Crater, Arizona. Detailed observations during a meteorological experiment there in 2006 provided a rich data set with which to validate the simulations.

The Monte Carlo code for the physically correct tracing of photons in cloudy atmospheres, MYSTIC, makes it possible to simulate radiation in horizontally inhomogeneous atmospheres with complex boundary conditions such as scenes with broken clouds, complex topography and highly variable surface albedo. For the simulation of radiance and irradiance at specific sites, a backward Monte Carlo technique was implemented to allow faster calculations for the few locations where radiation measurements were made and calculations were desired. Solar and thermal radiation can be calculated with MYSTIC for arbitrarily oriented instruments, including slope-parallel orientations required for energy considerations.

Detailed measurements of the slope-parallel components of the surface radiation budget were made at Meteor Crater during the 2006 Meteor Crater Experiment (METCRAX). The Meteor Crater, a bowl-shaped, 1200-m-diameter basin that is 165 m deep and that is considered the best preserved meteor crater on earth, was formed by the impact of a meteorite 50,000 years ago. During the month of October 2006, incoming shortwave, outgoing shortwave, incoming longwave and outgoing longwave radiative fluxes were measured at 6 sites throughout the crater topography. At the crater floor and crater rim, these 4-component observations were made over quasi-horizontal surfaces, and diffuse radiation was measured using Licor pyranometers and shadow bands. The other 4 sites were on the lower and upper slopes of the western and eastern inner sidewalls of the crater basin. There, four-component instruments were mounted parallel to the underlying sloping surface.

MYSTIC was run to simulate the radiation field on the clear day of 21 October 2006, using a 10-m-horizontal-resolution digital elevation model for a 400 x 400 pixel grid centered on the crater. Simulations and observations show excellent agreement, demonstrating the high accuracy of the 3D radiative transfer simulations.

METCRAX simulations, October 21, 2006

Diurnal variations, forward calculation in low resolution:

These are some quick forward simulations - not very nice but they give a first idea what the diurnal variation of the irradiance may look like in and around the Meteor crater:

Global (horizontal) solar irradiance
Direct (horizontal) solar irradiance
Diffuse downward (horizontal) solar irradiance
Upward (horizontal) solar irradiance

High-resolution backward calculations:

Sky radiance at FLOOR

uvspec input file:
atmosphere_file ../data/atmmod/afglms.mc
solar_file ../data/solar_flux/atlas_plus_modtran

correlated_k kato2
output sum

sza 46.2652
phi0 350.21

mc_sample_grid 40 40 0.1 0.1
mc_photons 100000000

rte_solver montecarlo

mc_elevation_file ./mystic_dgm.txt
albedo 0.2