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Radiation Dry Bias of the Vaisala RS92 Humidity Sensor

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  • 1 Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado
  • | 2 Bay Area Environmental Research Institute, Mountain View, California
  • | 3 National Center for Atmospheric Research, Boulder, Colorado
  • | 4 Laboratorio de Química de la Atmósfera, Universidad Nacional, Heredia, Costa Rica
  • | 5 Finnish Meteorological Institute, Sodankylä, Finland
  • | 6 Instituto Meteorológico Nacional, San José, Costa Rica
  • | 7 Aerospace Corporation, Los Angeles, California
  • | 8 Centro Nacional de Alta Tecnología, San José, Costa Rica
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Abstract

The comparison of simultaneous humidity measurements by the Vaisala RS92 radiosonde and by the Cryogenic Frostpoint Hygrometer (CFH) launched at Alajuela, Costa Rica, during July 2005 reveals a large solar radiation dry bias of the Vaisala RS92 humidity sensor and a minor temperature-dependent calibration error. For soundings launched at solar zenith angles between 10° and 30°, the average dry bias is on the order of 9% at the surface and increases to 50% at 15 km. A simple pressure- and temperature-dependent correction based on the comparison with the CFH can reduce this error to less than 7% at all altitudes up to 15.2 km, which is 700 m below the tropical tropopause. The correction does not depend on relative humidity, but is able to reproduce the relative humidity distribution observed by the CFH.

Corresponding author address: Holger Vömel, Cooperative Institute for Research in Environmental Sciences, University of Colorado, Campus Box 216, Boulder, CO 80309. Email: holger.voemel@colorado.edu

Abstract

The comparison of simultaneous humidity measurements by the Vaisala RS92 radiosonde and by the Cryogenic Frostpoint Hygrometer (CFH) launched at Alajuela, Costa Rica, during July 2005 reveals a large solar radiation dry bias of the Vaisala RS92 humidity sensor and a minor temperature-dependent calibration error. For soundings launched at solar zenith angles between 10° and 30°, the average dry bias is on the order of 9% at the surface and increases to 50% at 15 km. A simple pressure- and temperature-dependent correction based on the comparison with the CFH can reduce this error to less than 7% at all altitudes up to 15.2 km, which is 700 m below the tropical tropopause. The correction does not depend on relative humidity, but is able to reproduce the relative humidity distribution observed by the CFH.

Corresponding author address: Holger Vömel, Cooperative Institute for Research in Environmental Sciences, University of Colorado, Campus Box 216, Boulder, CO 80309. Email: holger.voemel@colorado.edu

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