Retrieving microphysical properties of liquid water clouds from infrared and microwave observations using an optimal estimation framework

From a radiation budget perspective, the most important microphysical property of a liquid water cloud is its liquid water path (LWP), or the vertical integral of cloud liquid water content, in the cloud. The distribution of LWP depends greatly upon the location of interest; however, results from the Department of Energy's Atmospheric Radiation Measurement (ARM) program suggests that over 50% of clouds that contain liquid water at the Southern Great Plains, North Slope of Alaska, and Tropical Western Pacific sites have LWP < 100 g/m2. However, the primary tool used by the ARM program to retrieve the LWP is the 2-channel microwave radiometer (MWR), which observes downwelling energy at 23.8 and 31.4 GHz, and the uncertainty in the LWP retrieved from these observations is 20-30 g/m2. This translates into a very large uncertainty in the LWP for clouds with small LWP.

Infrared radiance is very sensitive to the LWP when the LWP is less than approximately 50 g/m2. We have developed an algorithm to retrieve LWP (and effective radius) from radiance observations made by the Atmospheric Emitted Radiance Interferometer (AERI) using the optimal estimation framework. Here, we have extended this algorithm to include observations from both the MWR and AERI, thereby providing sensitivity to LWP over a large range of LWP. This new algorithm is evaluated using data collected by the ARM mobile facility at Pt. Reyes, California, during the Marine Stratus Radiation Aerosol and Drizzle (MASRAD) IOP in 2005. To quantify the accuracy of the algorithm, the microphysical properties retrieved by this algorithm are used in a radiative transfer model to compute the shortwave diffuse flux, which are then compared against observations.