Shortwave cloud radiative properties using cloud radar and MISR.

Dr. Paquita Zuidema

NOAA Environmental Technology Laboratory

The radiative impacts of cloud spatial inhomogeneity are not realistically accounted for both in climate models and in the interpretation of satellite shortwave radiance measurements. The radiative effects can vary widely in importance depending on the underlying cloud structure. Studies employing high-resolution cloud observations are appealing, as such studies can realistically quantify the magnitude of a particular effect.

In this talk I will give an overview of ways in which two instruments contribute to understanding the radiative impacts of cloud spatial inhomogeneity. One instrument is a cloud radar, which can delineate and profile clouds at a high spatial resolution. The top-of-cloud two-dimensional shortwave radiation fields of marine stratocumulus clouds and tropical fair-weather cumuli are examined. The high-quality cloud depiction allows for a thorough evaluation of the independent pixel approximation (IPA), a popular radiative transfer approximation in which only horizontal variations in optical depth are taken into account. This works well at large scales (>~ 6km), e.g. for marine stratocumulus clouds, but not at smaller scales. The IPA performs spectacularly poorly for the tropical fair-weather cumuli. A major improvement to IPA for broken clouds is the Tilted Independent Pixel Approximation (TIPA), which takes the first-order contribution from the cloud sides into account.

A drawback of the cloud radar data is that it is two-dimensional, a limitation that becomes more important for broken clouds. The Multi-angle Imaging SpectroRadiometer (MISR), aboard the Terra satellite, allows the three-dimensional reconstruction of a cloud. MISR has the unique capability of measuring nine independent radiances for the same cloud scene.