An Improved Retrieval Of Cloud Liquid Water Path For ARM Microwave Radiometers
Liljegren, J.C., Ames Laboratory
Eighth Atmospheric Radiation Measurement (ARM) Science Team Meeting
The Atmospheric Radiation Measurement (ARM) Program has deployed dual-frequency microwave water radiometers (MWRs) at its Cloud and Radiation Testbed (CART) sites in the U. S. Southern Great Plains (SGP), the Tropical Western Pacific (TWP), and the North Slope of Alaska/Adjacent Arctic Ocean (NSA/AAO). Although the integrated water vapor amount provided by these instruments has enjoyed increasing application, the primary purpose of these instruments has been to provide measurements of the integrated liquid water path in clouds. The liquid water path measurements have been widely used by ARM investigators to test cloud life cycle predictions in cloud resolving models and single column models, to investigate and develop multifractal representations of cloud structure, and to estimate cloud radiative and microphysical properties in conjunction with other ARM instrumentation, such as the Multi-Filter Rotating Shadowband Radiometer (MFRSR) and the Millimeter Cloud Radar (MMCR). In the course of assisting with these investigations, shortcomings in the present statistical retrieval, with which the cloud liquid water path is derived from the microwave measurements, have become evident. Chief among these are the reliance on climatology rather than ancillary measurements in the estimation of the liquid water path. Not only does this require a separate retrieval (and a priori dataset) for each instrument location, but neither the synoptic and diurnal variations in the "dry air" contribution to the microwave signal due to molecular oxygen nor the strong dependence of microwave emission by clouds on the temperature of the cloud water (and therefore on cloud height) are accounted for. The objective of this research has been to develop a retrieval for cloud liquid water path that is 1) independent of local climatology and may thus be used at all ARM CART sites, and 2) accounts for the variations in oxygen and liquid water contributions. The new retrieval uses measurements of surface pressure, temperature and humidity to directly account for the oxygen contribution. In this way, the wandering (positive and negative) clear sky baseline that has often been (mis)interpreted as a lack of instrument sensitivity to thin clouds is addressed, and the same retrieval may be used at all instrument locations. The new retrieval uses the IR thermometer to estimate cloud temperature and account for the temperature-dependent liquid emission; whereas the statistical retrieval uses a single climatological average cloud temperature and thus underestimates the liquid water path for clouds that are warmer than the climatological average cloud temperature and over-estimates the liquid water path for colder-than-average clouds. The temperature reported by the IR thermometer is adjusted to account for 1) the contribution of water vapor to the IR signal which causes the IR temperature to exceed the cloud base temperature in high-humidity conditions, and 2) the effect of cloud thickness which causes the cloud base temperature to exceed the liquid-weighted mean cloud temperature. Comparisons of the liquid water paths and precipitable water vapor amounts from the statistical and new retrievals for the SGP and TWP CART sites has been very encouraging.
Note: This is the poster abstract presented at the meeting; an extended version was not provided by the author(s).
