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Integration of Global Positioning System and Scanning Water Vapor Radiometers for Precipitable Water Vapor and Cloud Liquid Path Estimates

In recent years the Global Positioning System (GPS) has proved to be a reliable instrument for measuring precipitable water vapor (PWV) (Bevis et al. 1992), offering an independent source of information on water vapor when compared with microwave radiometers (MWRs), and/or radiosonde observations (RAOBs) (Rocken et al. 1995, Basili et al. 2001). In our work we compared PWV from MWRs, GPS, and RAOBs examining in detail several assumptions in water vapor estimation. The objective was to combine GPS with MWR observations to get retrievals of cloud liquid path (CLP) that may be improvements over those obtained with MWR alone. We evaluated this method from data collected during the Cloudiness Inter-Comparison (CIC) Experiment intensive operational period (IOP) that was conducted at the Atmospheric Radiation Measurement (ARM) Program’s Southern Great Plains site (SGP) in North-Central Oklahoma. The SGP is a field measurement site consisting of a variety of in situ and remote-sensing instruments and is a powerful source for testing and analyzing concurrent data from different instruments. The instruments we considered in our work were three dual-channel scanning MWRs operated by ARM, the SuomiNet Ground-Based GPS operated by the National Oceanic and Atmospheric Administration/Forecast Systems Laboratory (NOAA/FSL) at the same site, and RAOBs, with the Vaisala RS90 humidity sensor, launched four times a day. Soundings from all of these sources were compared during the CIC-IOP. Clear and cloudy conditions were identified by use of ARM operational cloud-boundary products derived from lidar and cloud radar. Combined GPS and MWR measurements were considered for the retrieval of PWV and CLP for both clear and cloudy datasets. Good general agreement in the PWV measurement was found between the three different kinds of instruments, with root mean square (rms) differences of 1 to 2 mm. We focused our analysis on cloudy conditions to investigate a possible dependence of the PWV differences on meteorological conditions as well as on water vapor amount itself. This comparison offered the opportunity of testing the reliability of these instruments in the presence of clouds.