The determination of young sea ice thickness from space remains an elusive goal for those interested in the interaction of the oceans and the atmosphere, the thermal and chemical state of the ocean, and sea ice dynamics. Recent experiments and models have shown relationships between active and passive microwave signatures of new, growing ice and ice thickness. The two processes that dominate in determining the microwave signature are changes in dielectric properties and changes in surface roughness. In this paper we investigate the competition between these two processes in determining radar backscatter, the usefulness of surface roughness as an indicator of young ice thickness, and the optimum sensor parameters for observing changes in scattering linked to ice thickness. We present simulations that are based on radar observations made on laboratory-grown saline ice. These observations confirm that surface scattering dominates over volume scattering for 13.9 GHz radar backscatter from young, rough ice at most angles and for young, smooth ice below 30¿. Although rms roughness and backscatter (at 5.3 and 13.9 GHz, 23¿ incidence, and VV polarization) increase together after about 10 cm of ice growth under quiet conditions, it is unlikely that surface roughness and ice thickness are simply connected in real sea ice, where surface roughness can change rapidly due to the action of wind, waves, and snow. Simulations show, however, that formation of frost flowers is detectable by spaceborne radar and can serve to classify ice of roughly 5--20 cm thickness since it is a distinct, transient event that occurs under physical conditions that constrain the thickness of the ice. Our experimental data show that future sensors operating near 12¿ incidence may offer potential for probing the relationship between near-surface dielectric properties and ice thickness since the effects of variability in roughness and snowfall are minimized near this angle. ¿ American Geophysical Union 1996