3D Radiative Transfer in the complex Topography of the Meteor Crater

Radiative transfer plays a key role in the energy exchange between the earth's surface and the atmosphere. Many models exist to simulate this exchange for the highly idealized case of a homogeneous atmosphere overlying a flat homogeneous surface. It is well known, however, that the radiative exchange of energy is further complicated by a more complex interface that is introduced by complex topography and varying surface properties: Shadows are cast by taller objects blocking direct solar radiation, diffuse radiation can be amplified due to reflecting surrounding terrain, solar radiation strongly varies on slopes with different exposure, and topography can block the emission of longwave radiation. All of these effects lead to the formation of different microclimates in mountainous terrain.

Only models like the Monte Carlo code for the physically correct tracing of photons in cloudy atmospheres, MYSTIC, can accurately calculate radiance and irradiance under arbitrarily 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 in MYSTIC 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.

As an example we show simulations of the radiation field in and around the topography of the Arizona Meteor Crater, a bowl-shaped, 165-m-deep basin with a diameter of 1200 m. The detailed observations from the 2006 Meteor Crater Experiment (METCRAX) allow the validation of the model simulations. 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 and the possibility to resolve the radiative effects that lead to the formation of radiation micro-climates in complex terrain.