Jahresbericht 08 - PMOD/WRC
Jahresbericht 08 - PMOD/WRC
Jahresbericht 08 - PMOD/WRC
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26<br />
Scientific Research Activities<br />
Down-Welling Longwave Radiation Comparison Between Models<br />
and Measurements for Clear-Sky and Stratus Conditions<br />
Stefan Wacker and Julian Gröbner<br />
We performed two comprehensive radiative transfer<br />
studies considering clear-sky and stratus conditions<br />
for the Payerne BSRN site.<br />
In the first study, we compared modeled down-welling<br />
longwave radiation to measured down-welling longwavefluxes<br />
for nocturnal clear-sky conditions at Payerne by applying<br />
the Radiative Transfer Models MODTRAN and<br />
libRadtran. Absorption coefficients for libRadtran were generated<br />
using the line-by-line (i.e. highest possible resolution)<br />
ARTS model and the LOWTRAN band parameter<br />
model characterised by a much lower resolution of 20 cm -1 .<br />
The resolution of the MODTRAN band model is 2 cm -1 . We<br />
computed down-welling longwave radiation for the broadband<br />
wavelength range (2 to 300 µm) and for the wavelength<br />
range of the atmospheric window from 8 to 14 µm<br />
for 40 carefully selected nocturnal clear-sky cases at Payerne.<br />
Vertical profiles of pressure, temperature and humidity<br />
obtained from radiosondes launched at Payerne around<br />
23 UTC were incorporated into the models. The calculations<br />
were compared to the corresponding measurements,<br />
i.e. to a standard pyrgeometer and a modified CGR3-pyrgeometer<br />
which is only sensitive in the atmospheric window.<br />
Results revealed good agreement within the<br />
measurement uncertainty of ±2 Wm -2 between model and<br />
observations for both wavelength ranges using ARTS and<br />
MODTRAN (see Fig. 1). The calculations with LOWTRAN<br />
were outside the measurement uncertainty. The excellent<br />
results from this analysis can be also regarded as an independent<br />
validation of CGR3 measurements. Finally, the investigation<br />
revealed that pyrgeometers, sensitive only in<br />
the atmospheric window, are an attractive addition to support<br />
explicit validation of Radiative Transfer Models in the<br />
regime where water vapor spectra in the atmospheric window<br />
are not saturated.<br />
In the second study, we presented down-welling longwavefluxes<br />
and surface cloud radiative forcing results for singlelayered<br />
stratus cases at Payerne by using down-welling<br />
longwave radiation surface measurements and MODTRAN.<br />
Thirty cases with single-layered completely overcast stratus<br />
conditions were selected in fall and winter months between<br />
October and February in the period from 2000 to<br />
2005. Cloud base and cloud extent were determined for<br />
each case using synoptic observations, ceilometer data<br />
and humidity profiles obtained from radiosondes. These<br />
data were incorporated in MODTRAN together with pressure,<br />
temperature and humidity profiles of the radiosonde<br />
launched at 11 UTC at Payerne. The mean difference between<br />
model and measurements was 0.7 Wm -2 ±1.4 Wm -2<br />
for the 30 cases (see Fig. 2). We used these model results<br />
to deduce the surface longwave cloud radiative forcing<br />
which is a quantitative measure of the effect of clouds on<br />
the radiation budget relative to the clear-sky radiation<br />
budget. Cloud radiative forcing can be derived by subtracting<br />
the model results for clear-sky conditions from the<br />
corresponding cloudy computations. The obtained value<br />
of 81 Wm -2 for such single-layered, fully overcast stratus<br />
clouds can be regarded as corresponding to extreme conditions<br />
and will be less for all-sky cases which also include<br />
stratus situations that are not totally overcast as for example<br />
the value of 54 Wm -2 proposed by Dong et al. (2006).<br />
Figure 1. Errorbars indicating mean and standard deviation of observed<br />
minus modeled DLR of 40 clear-sky night situations at Payerne using<br />
various band parameter models. ARTS and MODTRAN calculations are<br />
within the measurement uncertainty of ±2 Wm -2 , whereas LOWTRAN is<br />
outside.<br />
Figure 2. Distribution of the differences between modeled and observed<br />
DLR for the 30 single-layered stratus cases at Payerne.<br />
References: Dong X., Xi B., Minnis P., 2006, A climatology of<br />
midlatitude continental clouds from the ARM SGP<br />
central facility. Part II: Cloud fraction and surface<br />
radiative forcing, J. Climate, 19, 1765-1783.
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