New Digital Technical Document on improving the calibration of the water vapour channels of the radiometers.

A new Digital Technical Document entitled “Application of the Campanelli’s Technique to calibrate the water vapour channels of the CIMELs  sun photometers at the Izaña Observatory”, by Pedro Miguel Romero Campos et al., was recently published in the Publications online AEMET’s website section.
This work is a practical application of the technique described by Campanelli et al., in his recent paper published in 2010 ⁽¹⁾. In our case, the technique is applied to 7 CIMEL sun photometers of the AERONET network were operating at Izaña Observatory during 2009 and first quarter of 2010. In the Technical Document an introduction to the classical approach, classically used to characterize the filters and obtain precipitable water, was performed. Then the innovations introduced by Campanelli, as an alternative treatment, which produce better results than the classical methodology, were introduced. These developments are basically two:

1.    To characterize the water vapour filters with constants obtained from in situ measurements of the instruments. These constants would be valid for one-month period approximately. This methodology does not use radiative transfer models, as it is done in the classical treatment.
2.    Channels are calibrated using measurements of precipitable water from external and high accuracy sources, and using a technique named type II langley. In this work, we use as external sources the Vaisala RS-92 radiosondes launched from Güímar, the GPS receiver measuring at Izaña Observatory and the Fourier Transform Infrared Radiometer (FTIR). The type II langley technique introduces into the independent variable the amount of precipitable water measured by the external source and then it fits to a straightforward line of slope -1 through a least-squares method using the langley-modified equation in its logarithmic version. The calibration values obtained are the points of intersection of the fits with the ordinate axis. These calibration values will also be valid for one month.

The good results obtained indicate that calibration errors are considerably reduced with respect to the classical methodology. We think the methodology applied to CIMEL photometers can serve as a validation methodology, because Campanelli applied the new methodology to PREDE sunphotometers. We recommend this technique as a good alternative to be used from now on AERONET and other networks where it is necessary to perform calibrations of water vapour sunphotometers/radiometeres filters

Technical Document Reference: P.M. Romero Campos, E. Cuevas Agulló, O. García Rodríguez, A.J. Berjón Arroyo, V.E. Cachorro Revilla. Aplicación de la técnica de Campanelli para la calibración de los canales de vapor de agua de fotómetros CIMEL en el Observatorio Atmosférico de Izaña. Nota Técnica nº 2 de AEMET. Centro de Investigación Atmosférica de Izaña. Agencia Estatal de Meteorología, Septiembre de 2011.  NIPO: 784-11-010-6.

PDF Document here:
http://www.aemet.es/es/divulgacion/publicaciones/detalles/Campanelli

(1) CAMPANELLI, M., A. LUPI, T. NAKAJIMA, V. MALVESTUTO, C. TOMASI and V. ESTELLÉS (2010). Summertime columnar content of atmospheric water vapor from ground-based Sun-sky radiometer measurements through a new in situ procedure. J. Geophys. Res., 115, D19304, doi: 10.1029/2009JD013211.