The Moderate Resolution Imaging Spectrometer (MODIS) is an earth-viewing sensor being developed as a facility instrument for the Earth Observing System (EOS) to be launched in the late 1990's. MODIS consists of two separate instruments that scan a swath width sufficient to provide nearly complete global coverage every 2 days from a polar-orbiting, sun-synchronous, platform at an altitude of 705 km. MODIS-N (nadir) will provide images in 36 spectral bands between 0.415 and 14.235-nu-m with spatial resolutions of 250 m (2 bands), 500 m (5 bands), and 1000 m (29 bands). These bands have been carefully selected to enable advanced studies of land, ocean, and atmospheric processes. In this paper we describe the status of MODIS-N and its companion instrument MODIS-T (tilt), a tiltable cross-track scanning spectrometer with 32 uniformly spaced channels between 0.410 and 0.875-nu-m. In addition, we review the various methods being developed for the remote sensing of atmospheric properties using MODIS, placing primary emphasis on the principal atmospheric applications of determining the optical, microphysical, and physical properties of clouds and aerosol particles from spectral reflection and thermal emission measurements. In addition to cloud and aerosol properties, MODIS-N will be used for determining the total precipitable water vapour and atmospheric stability. The physical principles behind the determination of each of these atmospheric products will be described herein, together with an example of their application to aircraft and/or satellite measurements. Extensions of these and related methods to MODIS observations pose an extraordinary challenge as well as a unique opportunity to enhance our understanding of the earth-atmosphere-ocean system.
