Enhancement of Atmospheric Radiation Measurement Surface Meteorological Observations During the Fall 1996 Water Vapor Intensive Observation Period

This work describes in situ moisture sensor comparisons performed in conjunction with the first Water Vapor (WV) Intensive Observation Period (IOP) conducted at the Atmospheric Radiation Measurement (ARM) Program Southern Great Plains (SGP) Cloud and Radiation Testbed (CART) site during September of 1996. Numerous remote sensing instruments (e.g., two Raman Lidar, two Atmospheric Emitted Radiance Interferometers [AERI], and a suite of 13 microwave radiometers) were assembled at the CART site This paper describes the temperature calibrations and the RH during the IOP. The in situ measurements were used for intercomparisons performed at the Oklahoma Mesonet using calibration and verification. In addition, this work was meant the temperature and RH chambers developed there. This to help assess the current observing strategy in an effort to work was done in conjunction with the first WV IOP (Amer. make improvements to the routine continuous measurements. Meteor. Soc. 1997) that took place at the ARM Program SGP To accomplish these goals, verification of the in situ 1994) measurements was required. Therefore, a laboratory intercomparison of the in situ moisture sensors (nine capacitive The WV IOP was designed to reduce the uncertainty in the chip relative humidity sensors and four chilled mirror sensors) specification of the vertical water vapor profile derived from was performed at the Oklahoma Mesonet temperature (T) and various state-of-the-science moisture-measuring devices, relative humidity (RH) testing and calibration facility. Tests including both in situ and remote sensing instrumentation. were conducted both before and after the instruments were Reducing observational errors of in situ sensors is integral to used in the IOP, making it possible to detect instrument the characterization of the spectral radiative state of the problems prior to deployment of sensors and to determine if atmosphere and the subsequent use in radiative transfer instrument failure or drift occurred during the IOP. studies. Although measurements were made throughout the Results from the laboratory comparisons indicate that most of was to characterize the lowest 1 km of the atmosphere (which the RH sensors tested were within manufacturer specifications contains a significant fraction of the columnar water vapor) by and were capable of measuring RH with an accuracy of 2% to concentrating the majority of the observations in this region. 3% RH; one instrument was not within manufacturer specifications prior to the IOP and apparently drifted during Measurements of water vapor were made using numerous the IOP. The chilled mirror sensors proved to be accurate in instruments including balloon soundings, the CART Raman the laboratory, with agreement generally for Lidar (Goldsmith et al. 1997) (CART instruments refer to dewpoints above . those sensors or instruments that have been installed at the Preliminary results comparing in situ moisture measurements equipment), the NASA Goddard Raman Lidar, two AERIs with remotely sensed atmospheric moisture will be presented (Smith et al. 1997; Feltz et al. 1997), a Twin Otter aircraft and additional applications will be discussed. As a consequence of this work, modifications were made to the ARM CART calibration procedures, and there are now redundant temperature and RH measurements so that sensor drift or calibration errors can be detected. These modifications to the observation and calibration strategy led to improvements in the continuous routine measurements at the ARM CART Site.