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Calibration of Sun Radiometer–Based Atmospheric Water Vapor Retrievals Using GPS Meteorology

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  • 1 Centre d’Applications et de Recherches en Télédétection (CARTEL), Université de Sherbrooke, Québec, Canada
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Abstract

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 (V1) 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 (V1) 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.

* Current affiliation: Sciences Atmosphériques et Enjeux Environnementaux, Meteorological Service of Canada, Environment Canada, Montreal, Québec, Canada

Corresponding author address: Amadou Idrissa Bokoye, Sciences Atmosphériques et Enjeux Environnementaux, Meteorological Service of Canada, Environment Canada—Quebec Region, Place Bonaventure, 800, rue de la Gauchetière Ouest, Tour Nord-Est, bureau 7810, Montreal, QC H5A 1L9, Canada. Email: AmadouIdrissa.Bokoye@ec.gc.ca

Abstract

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 (V1) 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 (V1) 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.

* Current affiliation: Sciences Atmosphériques et Enjeux Environnementaux, Meteorological Service of Canada, Environment Canada, Montreal, Québec, Canada

Corresponding author address: Amadou Idrissa Bokoye, Sciences Atmosphériques et Enjeux Environnementaux, Meteorological Service of Canada, Environment Canada—Quebec Region, Place Bonaventure, 800, rue de la Gauchetière Ouest, Tour Nord-Est, bureau 7810, Montreal, QC H5A 1L9, Canada. Email: AmadouIdrissa.Bokoye@ec.gc.ca

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