Calibration of sun radiometer-based atmospheric water vapor retrievals using GPS meteorology

A study of the validation and calibration process for integrated water vapor (IWV) measurements derived from sun radiometry at the 940-nm solar absorption channel employed in the Aerosol Robotic Network (AERONET) Aerosol Canada (AEROCAN) is presented. The sun radiometer data are compared with GPS meteorology records used as a reference. Three Canadian sites from different climatic regimes covering the period 2000-04 are considered. The observations from five different sun radiometers (IWV-SUN) were processed using the initial AERONET IWV retrieval procedure (VI) whereas GPS-derived IWV (IWV-GPS) was retrieved using "GPSpace" software developed by the Geodetic Survey division of Natural Resources Canada. A sensitivity study is carried out to highlight the influence of both central wavelength and signal amplitude on the 940-nm filter characteristics, which are instrument dependent and can drift due to aging. The comparison between IWV-SUN (VI) and IWV-GPS shows an average rmse of 0.23 +/- 0.11 g cm(-2) (22%) and a mean bias of -0.09 +/- 0.16 g cm(-2) (9%). Furthermore, it is shown that the use of GPS for determining the 940-nm channel calibration constants for the solar radiometers improves IWV retrievals (rmse reduced by about 35% and bias by a factor of 3-10) without any knowledge of the 940-nm filter characteristics. These results are discussed within the context of the new AERONET IWV processing procedure (V2), which accounts for solar 940-nm region filter characteristics. The GPS receiver technique appears to be a powerful calibration tool because of its continuous observation capability, its robustness, and its operational simplicity.