Cloud Optical Property Retrieval at the ARM SGP Site Using the Non-Local Independent Pixel Approximation
Oreopoulos, L., Cahalan, R.F., Marshak, A., and Wen, G., Climate and Radiation Branch, NASA GSFC/University of Maryland, Baltimore County
Eighth Atmospheric Radiation Measurement (ARM) Science Team Meeting
While the Independent Pixel Approximation (IPA) has been the standard for cloud optical property retrieval from space, it has been shown to be inadequate on a pixel per pixel scale due to horizontal photon transport and other 3-D effects. In this poster, we evaluate the ability of the Non-local Independent Pixel Approximation (NIPA) to improve retrieved optical depth and effective radius statistics as a function of spatial scale. The basic idea is to use the inverse NIPA to "roughen" radiance fields that have been smoothed (compared to their IPA counterparts) by horizontal photon transport. Radiance data from NH mid-latitude storm track clouds are provided by Landsat at the ARM Southern Great Plains (SGP) site. We extend NIPA by introducing a 3-D direct beam source function term for oblique illumination and broken cloud conditions, and by using the concept of the "local radiative smoothing scale". IPA and NIPA retrievals of optical depth and effective radius (hence, liquid water path) from the Landsat radiances are compared to each other and to ground measurements (e.g., from microwave radiometry). We examine the scale dependence of flux differences between "standard" and "improved" retrievals by feeding back the retrieved properties to plane-parallel and 3-D radiative transfer codes. Implications for remote sensing and large-scale modeling are also discussed.
