High-resolution retrieval of cloud liquid water profiles using the collocated ARM ka- and W-band radars
Authors
Warren J. Wiscombe — Brookhaven National Laboratory
Karen Lee Johnson — Brookhaven National Laboratory
Yangang Liu — Brookhaven National Laboratory
Dong Huang — NASA - Goddard Space Flight Center
Category
Cloud Properties
Description
Dual-frequency radar observations at the Southern Great Plains Central Facility site on May 6, 2006: (a) radar reflectivity factor at 95 GHz by the WACR with the ARSCL lidar cloud base shown as red line; (b) the dual-frequency radar cloud LWC retrieval using the constrained approach present in this letter; (c) the retrieval using the direct approach (Hogan et al. 2005). For better visualization, negative values are shown in black, and values larger than 2.0 gm-3 are shown in green. Note that the direct approach is designed only for high signal-to-noise ratio cases, or equivalently, low resolution. The comparison is only to show the benefits of the constrained approach for high-resolution retrievals. (d) Time series of the radar LWPs in comparison with the reference LWP from the microwave radiometer; (e) the histograms of the corresponding difference in LWP (radar LWP-MWR LWP).
Most of the existing radar algorithms for retrieving cloud liquid water content (LWC) make use of empirical Z-LWC relationships that are based on various questionable assumptions. They work poorly under precipitating conditions, and the uncertainty in the retrievals is difficult to quantify. We have not seen much progress on these approaches for decades. On the other hand, the dual-frequency radar attenuation approach makes no assumptions about the cloud drop size distribution and is based on simple physics. Thus, this approach can provide accurate (unbiased) retrieval of cloud LWC. Previous studies showed that, however, the precision of the dual-frequency retrieval is very poor; either a long radar dwell time or averaging over many range gates is needed to improve the retrieval precision. This poster shows that, by virtue of advanced mathematical inversion techniques like total variation regularization, accurate retrieval of vertically resolved cloud LWC at high temporal and spatial resolution is achievable using operational cloud radars. The validity of this dual-frequency approach is demonstrated using the co-located Ka-band and W-band cloud radars operated by the Atmospheric Radiation Measurement (ARM) Climate Research Facility. The liquid water path calculated from the radars agrees closely with that from a microwave radiometer, with mean difference of 70 gm-2 for precipitating clouds and 30 gm-2 for non-precipitating clouds. Comparison with lidar measurements reveals that the dual-frequency retrieval also reasonably captures the cloud base height of drizzling clouds—something that is very difficult to determine from radar reflectivity alone. We have applied the dual-frequency approach to the ARM radar observations from 2006 to 2008 and have produced a three-year cloud LWC vertical profile data set.
