Comparison of columnar water vapor measurement during the fall 1997 ARM Intensive Observation Period: optical methods

Optical methods can provide water vapor data from ground-based, airborne or spaceborne measurements of direct or reflected sunlight in spectral channels in and adjacent to water vapor absorption bands. The water-vapor transmittance T_w derived from these measurements has to be translated into water vapor amounts. Although this relationship is well known qualitatively, it has proven difficult to quantify. Attempts to do so for water-vapor absorption bands in the near-infrared date back to 1912. Recent findings that the H₂O line intensities in the visible and near infrared portion of the widely used HITRAN-96 database were in error and that H₂O lines might be missing from the current databases have sparked renewed discussion of the accurate conversion of measured water-vapor transmittance into amounts of water vapor. In the fall of 1997 the Atmospheric Radiation Measurement (ARM) program conducted an Intensive Observation Period (IOP) to study water vapor at its Southern Great Plains (SGP) site. Among a large number of systems such as radiosondes, microwave radiometers, Raman lidars, Global Positioning System, and an infrared spectrometer, four optical instruments were present to measure water vapor. The authors focus on the four optical instruments: the NASA Ames Airborne Tracking Sunphotometer (AATS-6), a CIMEL CE-318 sun/sky photometer, a multi-filter rotating shadowband radiometer (MFRSR), and a rotating shadowband spectroradiometer (RSS)