WV

OVERVIEW There is a recognized need both within and outside the ARM community to improve the state-of-the-art in measuring the water vapor in a vertical column, both range-resolved and column-integrated. A series of IOPs that have moisture characterization as their focus will be conducted to provide a means to address this issue.

With the recent delivery of the CART Raman lidar to the SGP CART site, the suite of instruments that measure water vapor at the site is now complete; hence the time has come to implement the first of these IOPs. The independent intent to operate the NASA/GSFC Raman lidar at the SGP CART site during September 1996 provides strong incentive to hold the IOP at that time to take advantage of the presence of this second Raman lidar system.

A three-week campaign, operating from September 5-25, 1996 (coincident with the Fall ARM-UAV IOP), that focuses on optimizing the capabilities of CART instrumentation for characterizing atmospheric water vapor, will hence be conducted. This first IOP will attempt to emphasize measurement of water vapor in the lowest kilometer (especially the lower 200 m) of the atmosphere, and also provide an overall calibration for the CART Raman lidar.

GOALS The main goal of the series of Water Vapor IOPs is to obtain the best possible water vapor measurements, both range-resolved and column-integrated, under all conditions (clear/cloudy, day/night, etc.), from a fusion of a variety of measurements. The primary goal of the first IOP will be to improve (or at least to characterize the limits) of the ability of CART instrumentation to characterize the lowest kilometer (and especially the lower 200 m) of the atmosphere.

A secondary goal of the first IOP, which meshes well with the primary goal, is a careful calibration of the CART Raman lidar independently of the BBSS.

Additional wide-ranging intercomparisons of water vapor measurements will also be carried out to the extent possible (in particular, intercomparisons of column-integrated measurements); this should be a natural and very fruitful outcome of the IOP.

The first Water Vapor IOP should provide valuable insight about how to proceed with the subsequent IOPs in the series.

DISCUSSION An important feature of the first IOP (and perhaps others) is represented by the addition of chilled mirror hygrometers and a tethersonde to the instrumentation already available or planned at the CART site. The chilled mirrors hygrometers are crucial, since they offer the lowest practical absolute calibration uncertainties. By placing the chilled mirror hygrometers on a tethersonde, and potentially on the ground and/or the tower, accurate point measurements can be made to transfer calibration to the high spatial resolution profiling instruments, the Raman lidar, and the BBSS sondes.

Both the Raman lidar and the sondes are important, because together they span the altitude range of interest. The full altitude range can be spanned with accurate measurements, assuming that the Raman lidar calibration is highly stable and altitude independent (for which there is good evidence) and that the sondes can be corrected, especially in the key region below the Raman lidar lower limit, using ground, tower, and tethersonde point observations. A tethersonde with minimal overlap of the lowest Raman lidar levels would seem to be adequate.

The resulting profiles will allow testing the other hypotheses important to improving absolute water vapor accuracies when a lesser complement of instrumentation is available, namely that radiation observations (which don't tightly constrain the detailed high spatial resolution profiles) can be used to constrain the overall absolute calibration of both sondes and the Raman lidar. Specifically, we will test the important hypothesis that highly accurate knowledge of the 22 GHz water line spectroscopy (rigorously constrained by accurate laboratory Stark Effect observations) and microwave calibration yield a tight constraint on total precipitable water vapor. Initial results also show promise that selected lines of the AERI infrared spectrum can serve as important transfer standards for absolute water vapor calibration (because selected lines can provide information independent of the important continuum and aerosol contributions that vary slowly with wavelength).

Activity Summary

SCHEDULE This IOP will be conducted from September 10 - 30, 1996 (coincident with the Fall ARM-UAV IOP). Instruments that do not require supervision will be operated continuously during this period. Instruments that do require supervision are presently planned to be operated for 8-hour periods each day. Because it is necessary to cover as broad a range of environmental conditions as possible, the daily 8-hour period will be shifted across the diurnal cycle as deemed appropriate during the IOP (but will be maintained as a contiguous 8-hour block).

Responses to Site Operations Questionnaires for Water Vapor IOP

Timeline

Campaign Data Sets