Comparison of in situ cirrus cloud water path and optical depth measurements with MODIS retrievals

With a global frequency of occurrence near 30%, cirrus clouds wield a strong influence over the radiation budget of the Earth’s climate system due to their location in the upper troposphere. Currently, global climate models (GCMs) are unable to accurately represent cirrus cloud feedbacks on the radiation and hydrological cycles due to a lack of understanding of how to parameterize the effects of cirrus. This inability to parameterize the microphysical properties of cirrus clouds can be attributed to a general lack of observations of these clouds and their dynamical environment in the upper troposphere. While aircraft provide direct measurements in this region, their use is limited due to expense, and ground-based remote sensors such as radars and lidars, while also quite useful, are limited to just a few locales. Satellite measurements, on the other hand, are global in nature but limited in the sense that the cloud properties must be derived through the use of complicated inversion algorithms. One of the newer satellite instruments currently on board the NASA Earth Observing System Terra and Aqua platforms, is the moderate resolution Imaging Spectroradiometer. MODIS observes upwelling reflectance and radiance from the Earth's atmosphere and surface in 36 narrow spectral intervals ranging from .62 μm to 14.385 μm. By combining measurement channels that are non absorbing and thus sensitive to total cross sectional area with other channels that are absorbing and include sensitivities to particle size, the observed radiances can provide estimates of optical depth (τ) and effective radius (re). Ice water path is calculated directly from these values. Validation of the retrievals is essential for eventual development of parameterizations that can be assimilated into GCMs.