Improved ground-based liquid water path retrievals using a combined infrared and microwave approach

Abstract

Radiative transfer modeling through cloudy atmospheres requires accurate measurement of the cloud properties. In liquid water clouds, accurate measurements of the liquid water path (LWP) are required, especially when the LWP is less than 100 g/m 2. A new ground-based retrieval algorithm was developed that retrieves LWP using infrared (8-13 μm and 3-4 μm) and microwave (23.8 and 31.4 GHz) radiance observations, as microwave radiance is sensitive to a large range of LWP (less than 5 to over 1000 g/m 2) and infrared radiance is extremely sensitive to LWP less than approximately 60 g/m 2. The random error in the retrieved LWP is estimated to be less than 4% when the LWP is less than 50 g/m 2 but, for larger LWPs, it increases to at least 12 g/m 2. The algorithm is also able to retrieve effective radius (r e) when the LWP is less than approximately 60 g/m 2. This new retrieval algorithm was applied to data collected by the Atmospheric Radiation Measurement (ARM) program's Mobile Facility at Pt. Reyes, California, from July to August 2005. Daytime retrieved r e values agree well to airborne in situ observations, with a mean bias of 0.11 μm. A radiative transfer model utilizing the retrieved cloud properties was used to compute the surface and top of the atmosphere fluxes. The flux residuals (observed minus calculated) demonstrate a significant reduction in scatter (approximately a factor of 2 for LWP < 40 g/m 2) relative to flux calculations where the cloud properties were determined by the microwave-only retrieval algorithm MWRRET. Copyright 2007 by the American Geophysical Union.