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Comparison of modeled and observed cloud-free longwave downward radiation over the Alps

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Differences between computed and observed cloud-free longwave downward radiation () were examined both for Payerne and six further radiation sites of the Alpine Surface Radiation Budget (ASRB) network. was computed by the complex radiative transfer model (RTM) MODTRAN v4.0 and by a simplified RTM used in the numerical weather prediction model "alpine model" (aLMo). MODTRAN computed in Payerne using sounding data and ASRB observed show an average difference of 1.5 W m (± 3.5 W m), and −3.2 W m ± 3.6 W m) for night- and daytime respectively from 1996 to 2001. Nighttime is underestimated by the aLMo RTM in the order of −20 W m compared to ASRB measurements both at Payerne and at six further ASRB sites. However bias is strongly reduced in Payerne during daytime. For comparison MODTRAN computed using aLMo forecasted atmospheric profiles shows mean differences in the order of −3 W m both for night- and daytime. Hence the comprehensive radiation scheme used in aLMo is mainly responsible for the substantial underestimation of cloud-free .

Differenzen zwischen berechneter und gemessener wolkenfreier langwelliger Gegenstrahlung (LDR) wurden an der Station Payerne und an sechs weiteren Stationen des alpinen Strahlungsmessnetzes ASRB untersucht. Die LDR-Berechnungen wurden mit dem komplexen Strahlungstransfermodell MODTRAN v4.0 und mit einem vereinfachten Strahlungstransfermodell durchgeführt, welches im Wettervorhersagemodell aLMo an der MeteoSchweiz verwendet wird. MODTRAN-Berechnungen mit Hilfe von Sondierungsdaten in Payerne zeigen eine mittlere LDR-Differenz zu ASRB Messungen von 1.5 W m−2 (± 3.5 W m−2) um Mitternacht, und −3.2 W m−2 (± 3.6 W m−2) am Mittag von 1996 bis 2001. LDR-Prognosen des aLMo Strahlungstransfermodells für Payerne und sechs weitere ASRB-Stationen unterschätzen nachts die langwellige Gegenstrahlung in der Größenordnung von −20 W m−2. Die LDR-Differenz in Payerne ist jedoch während des Tages deutlich reduziert. Im Gegensatz dazu liegt die mittlere LDR-Differenz zwischen MODTRAN-Berechnungen mit Hilfe von aLMo Atmosphärenprofilen und ASRB-Messungen bei rund −3 W m−2 tags über. Daraus läasst sich schließen, dass das in aLMo verwendete Strahlungstransfermodell eine substantielle Unterschätzung der wolkenfreien langwelligen Gegenstrahlung verursacht.

Document Type: Research Article


Publication date: 2005-02-01

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  • Meteorologische Zeitschrift (originally founded in 1866) is the joint periodical of the meteorological societies of Austria, Germany and Switzerland. It accepts high-quality peer-reviewed manuscripts on all aspects of observational, theoretical and computational research out of the entire field of meteorology, including climatology. Meteorologische Zeitschrift represents a natural forum for the meteorological community of Central Europe and worldwide.
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