Progress in Understanding Water Vapor's Role in Models

Ackerman, T. P., University of Washington

General Circulation and Single Column Models/Parameterizations

Cloud Modeling

N/A


Time-height cross sections of water vapor mixing ratio, which is observed directly by the ARM Raman lidar at 10-min and approximately 100 m resolution, and relative humidity for 29 November through 2 December 2002. The bottom panel shows the comparison of the precipitable water vapor observed by the Raman lidar and the collocated microwave radiometer. The time-height cross sections, as well as the integrated field, show the large variability in water vapor that exists over the ARM Southern Great Plains site.


Time-height cross sections of water vapor mixing ratio, which is observed directly by the ARM Raman lidar at 10-min and approximately 100 m resolution, and relative humidity for 29 November through 2 December 2002. The bottom panel shows the comparison of the precipitable water vapor observed by the Raman lidar and the collocated microwave radiometer. The time-height cross sections, as well as the integrated field, show the large variability in water vapor that exists over the ARM Southern Great Plains site.

After years of sustained research efforts into the accuracy of atmospheric water vapor measurements, researchers from the U.S. Department of Energy's ARM Program have succeeded in reducing measurement uncertainties from greater than 25% to less than 3%. This improvement is critically important for understanding trends in water vapor concentrations over the past several decades, which are intimately linked to our understanding of climate change over that same period. In addition, these measurements are important constraints on the current generation of climate models, which, as they are fine-turned, will allow scientists to better predict climate change. A large part of the recent success stemmed from a series of water vapor intensive observation periods (WVIOPs) conducted at the ARM site in Oklahoma between 1996 and 2000. The goals of these WVIOPs were to characterize the accuracy of the new ARM operational water vapor observations from the microwave radiometer and Raman lidar and to use these new measurements to assess the accuracy of routine radiosonde measurements. ARM scientists have made great progress in addressing both goals.

The ARM Program has pioneered the use of operational microwave radiometers and Raman lidar to measure water vapor column concentrations and water vapor vertical profiles, respectively. These operational instruments were paired with additional research instruments during the 1996, 1997, 1999, and 2000 WVIOPs to verify their accuracy and determine the accuracy of water vapor profile measurements from routine radiosondes. The unique datasets obtained during these WVIOPs have led to many results, including the discovery and characterization of a large (> 25%) variability in the water vapor profiles from Vaisala RS-80H radiosondes, which are used world-wide. The RS-80 error is essentially height-independent and can be treated as a calibration factor error. The microwave radiometer, on the other hand, measures microwave energy emitted by water vapor in the atmosphere and is self-calibrating. Consequently, the ARM microwave observations provide a stable reference that can be used to remove most of the sonde-to-sonde calibration variability.

Water vapor profiles retrieved from two Raman lidars, which have both been calibrated to the ARM microwave radiometer, showed agreement to within 5% for all altitudes below 8 km during two WVIOPs. The mean agreement of the total precipitable water vapor from different techniques has converged significantly from early analysis that originally showed differences up to 15%. Retrievals of total precipitable water vapor (PWV) from the ARM microwave radiometer are now found to be only 3% moister than PWV derived from new GPS results, and about 2% drier than the mean of radiosonde data after a recently defined sonde dry-bias correction is applied (the goal is an absolute accuracy of better than 2% in total column water vapor). The scientists also found that observations from different collocated microwave radiometers show larger differences than expected and are attempting to resolve the remaining inconsistencies.