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Clear-Sky Window Channel Radiances: A Comparison between Observations and the ECMWF Model

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  • a Instituto de Meteorologia/Centro de Geofísica da Universidade de Lisboa, Lisbon, Portugal
  • | b European Centre for Medium-Range Weather Forecasts, Reading, United Kingdom
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Abstract

A comparison of clear-sky radiances of the Meteosat window channel with the European Centre for Medium-Range Weather Forecasts (ECMWF) model results is presented, aiming to assess both the model's performance and the quality of the observations. The comparison is made for four periods covering the seasonal cycle and for the Meteosat-7 (Africa and southern Europe) and Meteosat-5 (Middle East and the Indian subcontinent) disks. Results show 1) an underestimation of the diurnal cycle of model brightness temperature in clear-sky conditions with a cool bias during daytime of up to 7–8 K and 2) a much smaller and less widespread warm bias at night. Although such discrepancies are sensitive to surface characteristics (e.g., vegetation type, terrain elevation, and slope), problems in the model surface-to–boundary layer coupling are identified as the most likely model deficiency associated with the underestimation of diurnal amplitudes of skin temperature. A conditional error analysis also reveals strong error stratification with the model cloud cover and with the percent of clear-sky pixels associated with the Meteosat data, suggesting great care should be taken when using Meteosat clear-sky observations, particularly in the ITCZ and adjacent areas.

Corresponding author address: Dr. Isabel F. Trigo, Centro de Geofísica da Universidade de Lisboa, Faculdade de Ciências, Departamento de Física, Campo Grande, Ed. C8, Piso 6, 1749-016 Lisbon, Portugal. isabel.trigo@meteo.pt

Abstract

A comparison of clear-sky radiances of the Meteosat window channel with the European Centre for Medium-Range Weather Forecasts (ECMWF) model results is presented, aiming to assess both the model's performance and the quality of the observations. The comparison is made for four periods covering the seasonal cycle and for the Meteosat-7 (Africa and southern Europe) and Meteosat-5 (Middle East and the Indian subcontinent) disks. Results show 1) an underestimation of the diurnal cycle of model brightness temperature in clear-sky conditions with a cool bias during daytime of up to 7–8 K and 2) a much smaller and less widespread warm bias at night. Although such discrepancies are sensitive to surface characteristics (e.g., vegetation type, terrain elevation, and slope), problems in the model surface-to–boundary layer coupling are identified as the most likely model deficiency associated with the underestimation of diurnal amplitudes of skin temperature. A conditional error analysis also reveals strong error stratification with the model cloud cover and with the percent of clear-sky pixels associated with the Meteosat data, suggesting great care should be taken when using Meteosat clear-sky observations, particularly in the ITCZ and adjacent areas.

Corresponding author address: Dr. Isabel F. Trigo, Centro de Geofísica da Universidade de Lisboa, Faculdade de Ciências, Departamento de Física, Campo Grande, Ed. C8, Piso 6, 1749-016 Lisbon, Portugal. isabel.trigo@meteo.pt

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