RADIATIVE-TRANSFER MODELING OF DISCONTINUOUS TREE CANOPIES AT MICROWAVE-FREQUENCIES

Development of a first-order radiative transfer model for predicting b ackscatter from tree canopies has been underway at the University of M ichigan Radiation Laboratory for some time. This model is known as the Michigan Microwave Canopy Scattering (MIMICS) model. The first-genera tion model, MIMICS I, was developed for canopies with continuous (clos ed) crown layers and its validity has been verified in several modelli ng analyses. This article presents the second-generation MIMICS model (MIMICS II) which accounts for canopies with discontinuous (open) crow n layer geometries. MIMICS II models open crown layers by treating the location, size and shape of the individual tree crowns as random vari ables. The backscattering coefficients for the canopy are then determi ned by introducing statistics derived from these parameters into the r adiative transfer solution. Application of the radiative transfer equa tions to the discontinuous canopy geometry is presented. The applicati on of random variables defining the crown geometry and the incorporati on of these variables into the radiative transfer solution is discusse d. The resulting model is a robust fully polarimetric solution that is applicable over a wide variety of canopy architectures. Model simulat ions are compared to results generated with the continuous canopy mode l. The effect of the open crown geometry is found to be most significa nt at shallow incidence angles and at high frequencies for trees with well-developed crowns. Under these conditions, the gaps in the crown l ayer give rise to a notable increase in crown layer transmissivity whi ch allows the radar to see through to the lower layers of the canopy m ore easily thereby directly affecting the backscatter contribution of the trunks and ground.