Articles | Volume 11, issue 6
Geosci. Model Dev., 11, 2139–2152, 2018
Geosci. Model Dev., 11, 2139–2152, 2018

Model evaluation paper 12 Jun 2018

Model evaluation paper | 12 Jun 2018

Comparison of observed and modeled cloud-free longwave downward radiation (2010–2016) at the high mountain BSRN Izaña station

Rosa Delia García1,2,3, Africa Barreto4,2,3, Emilio Cuevas2, Julian Gröbner5, Omaira Elena García2, Angel Gómez-Peláez2,a, Pedro Miguel Romero-Campos2, Alberto Redondas2, Victoria Eugenia Cachorro3, and Ramon Ramos2 Rosa Delia García et al.
  • 1Air Liquide España, Delegación Canarias, Candelaria, 38509, Spain
  • 2Izaña Atmospheric Research Center (IARC), State Meteorological Agency (AEMET), Santa Cruz de Tenerife, Spain
  • 3Atmospheric Optics Group, Valladolid University, Valladolid, Spain
  • 4Cimel Electronique, Paris, France
  • 5Physikalisch-Meteorologisches Observatorium Davos, World Radiation Center (PMOD/WRC), Davos, Switzerland
  • anow at: Meteorological State Agency of Spain (AEMET), Delegation in Asturias, Oviedo, Spain

Abstract. A 7-year (2010–2016) comparison study between measured and simulated longwave downward radiation (LDR) under cloud-free conditions was performed at the Izaña Atmospheric Observatory (IZO, Spain). This analysis encompasses a total of 2062 cases distributed approximately evenly between day and night. Results show an excellent agreement between Baseline Surface Radiation Network (BSRN) measurements and simulations with libRadtran V2.0.1 and MODerate resolution atmospheric TRANsmission model (MODTRAN) V6 radiative transfer models (RTMs). Mean bias (simulated − measured) of  <  1.1 % and root mean square of the bias (RMS) of  <  1 % are within the instrumental error (2 %). These results highlight the good agreement between the two RTMs, proving to be useful tools for the quality control of LDR observations and for detecting temporal drifts in field instruments. The standard deviations of the residuals, associated with the RTM input parameters uncertainties are rather small, 0.47 and 0.49 % for libRadtran and MODTRAN, respectively, at daytime, and 0.49 to 0.51 % at night-time. For precipitable water vapor (PWV)  >  10 mm, the observed night-time difference between models and measurements is +5 W m−2 indicating a scale change of the World Infrared Standard Group of Pyrgeometers (WISG), which serves as reference for atmospheric longwave radiation measurements. Preliminary results suggest a possible impact of dust aerosol on infrared radiation during daytime that might not be correctly parametrized by the models, resulting in a slight underestimation of the modeled LDR, of about −3 W m−2, for relatively high aerosol optical depth (AOD  >  0.20).

Short summary
A 7-year comparison study between measured and simulated longwave downward radiation under cloud-free conditions has been performed at BSRN Izaña. Results show an excellent agreement with a mean bias (simulated–measured) less than 1.1 % and RMSE less than 1 %, which are within the instrumental error (2 %).