Remote sensing of total precipitable water vapor by microwave radiometers and GPS during the 1997 Water Vapor Intensive Operating Period

During September 15 to October 5, 1997, an intensive experiment to measure atmospheric water vapor was conducted at the Department of Energy´s Atmospheric Radiation Measurement Program´s Cloud and Radiation Testbed (CART) site in north central Oklahoma. This experiment was called the 1997 Water Vapor Intensive Operating Period (WVIOP´97). Among the goals of WVIOP´97 was the comparison of different methods of measuring precipitable water vapor (PWV). This paper focuses on the inter-comparison of PWV measurements by ground-based microwave radiometers, Global Positioning System-PWV instruments (GPS-PWV), and by radiosondes. For this experiment 3-hourly radiosondes for in situ measurements were deployed. Microwave radiometric measurements of water vapor in the zenith direction were made by two independent radiometers that produce PWV and cloud liquid path from brightness temperature T_b measurements. These radiometers were operated by the Environmental Technology Laboratory and operate at the frequency pairs of (20.6, 31.65 GHz) and (23.87, 31.65 GHz). The calibration of the instruments was achieved by the so-called “tip cal” method and their data, in terms of derived PWV are compared. Two independent GPS-PWV instruments were also operated 9 km apart and, in addition, two different processing algorithms were applied to the data from each of these instruments