Radiation relevant cloud information derived from atmospheric circulation model results
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Radiation relevant cloud information derived from atmospheric circulation model results Mathias Schreier, Peter Koepke, Joachim Reuder, Harry Schwander, Jan Schween. Introduction. Circulation Model.

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Radiation relevant cloud information derived from atmospheric circulation model results

Mathias Schreier, Peter Koepke, Joachim Reuder, Harry Schwander, Jan Schween


Circulation Model

Beside the solar zenith angle, clouds are the most important factor on the solar radiation reaching the Earth‘s surface. Even in the UVB-spectral range with its distinct total ozone dependence, it dominates the temporal and spatial variability of the incoming radiation.

The general problem to get cloud properties relevant for radiation is the calculation from available model output quantities (relative humidity, liquid water content and ice water content). An algorithm is presented that allows the determination of cloud cover from humidity profiles resulting from atmospheric circulation model calculations. On the basis of regional climate simulations by the MM5/MCCM model cloud cover has been derived for potential future climatic conditions.

Regional climate simulations have been performed for 5 years around 1990 and 5 years under the assumption of 2xCO2 conditions for Southern Germany (BayFORKLIM, 1999, Grell et al., 2000). For every 3 hours profiles of humidity (relative humidity (rh), liquid water content (LWC), ice water content (IWC)) are available with an horizontal resolution of 15km15km in 21 vertical layers. The figure shows the selected gridpoints used for the determination of cloud information (open circles) for different Bavarian sites. The corresponding stations of the German Weather Service with routine synoptic observations are denoted by the purple circles.






Cloud Cover



Determination of cloud cover from MM5/MCCM humidity profiles

The model layers have been separated for water clouds into low level (0km-2km above ground) and mid-level (2km-7km above ground) clouds. For each of the two cloud layers and every daytime model time step an averaged hypothetical relative humidity (rh*) has been calculated over the layers and 3X3 gridpoints. In case of the presence of liquid water, it has been converted to watervapour, therefore rh* can reach supersaturation.

The cumulative relative frequency of humidity rh* has been determined from the MM5/MCCM output for each month, representing climatic conditions around 1990 for the 7 investigated sites.

Both data sets have been combined to define a functional dependency between rh* and NL, that varies with month and site, which can be used for 2xCO2 conditions.

The observed cloud cover cumulative frequency of low clouds (NL) has been evaluated from synoptic network data for the time period 1985 to 1995 .

Derivation of CMFs for UV


BayFORKLIM, Klimaänderungen in Bayern und ihre Auswirkungen, Final Report, 1999.

Dameris, M., V. Grewe, R. Hein, C. Schnadt, C. Brühl, and B. Steil, Assessment of the future development of the ozone layer., Geophys. Res. Letters, 25, 3579-3582, 1998.

Grell, G., L. Schade, R. Knoche, A. Pfeiffer, and J. Egger, Nonhydrostatic climate simulations of precipitation over complex terrain, J. Geophys. Res., 105, D24, 29595-29608, 2000.

Reuder, J., P. Koepke, and M. Dameris, Future UV radiation in Central Europe modelled from ozone scenarios, J. Photochem. Photobiol., B., 61, 3, 94-105, 2001.

Schwander, H., A. Kaifel, A. Ruggaber and P. Koepke, Spectral radiative transfer modeling with minimized computation time by use of neural-network technique, Appl. Opt., 40, 3, 331-335, 2001.

Schwander, H., P. Koepke, A. Kaifel and G. Seckmeyer, Modification of spectral UV irradiance by clouds, J. Geophys. Res., 107 (D16), AAC7-1 to AAC7-12, 2002

Schween, J., Koepke, P., Reuder, J., Modellierung zeitlich und räumlich variabler UV-Strahlung unter dem Aspekt ihrer biologischen und photochemischen Wirkung, Report, 2003.

Schwander, H., P. Koepke, M. Mech, A. Oppenrieder, J. Reuder, and J. Schween, STAR – System for Transfer of Atmospheric Radiation – Version 2002, freely available via

http://www.meteo.physik.uni-muenchen.de/strahlung/uvrad/Star/STARinfo.htm, 2002

The cloud effect on the irridiance is usually described by the so-called cloud modification factor (CMF) that represents the ratio between the radiation in the presence of clouds and the radiation under cloudfree but otherwise identical atmospheric conditions.

The conversion of cloud cover into a combined CMFL,M for UV spectral range has been performed by an algorithm on the basis of an artifical neural network (Schwander et al., 2002). Necessary input parameters are NL, NM, the solar zenith angle and the surface albedo.

In contrast to CMFL,M for water clouds, the CMFH for high cirrus cloud cover is derived directly from the ice water path

The final overall CMF for all clouds has been calculated under the consideration of multiple scattering effects.


An algorithm has been developed, which transforms humidity, liquid water and ice water profiles from circulation models into cloud cover at different cloud levels. These data can be converted to CMFs, which is possible for the total solar wavelength region.

The results are applied to the UV-spectral range. Here, due to the predicted recovery of the stratospheric ozone for future 2CO2-conditions, a reduction of UV-exposition of 5-10% could be expected for conditions neglecting changes in cloudiness (Reuder et al., 2001).

The consideration of cloud effects, however, indicates that the predicted reduction in cloudiness will overcompensate the ozone effect especially during the summer months with anyway high UV impact. The result is an predicted increase in UV-exposition of 5-20% during summertime, compared to ratios of 1990 (Schween et al., 2003).

Future UV scenarios

The CMFs have been used to calculate erythemally weighted UV radiation for 1CO2 and 2CO2 conditions. The radiation transfer calculations have been carried out with STAR (Schwander et al., 2002).

Total ozone data have been taken from average values around 1990 for recent conditions and a scenario assuming the most probable for conditions at the estimated time of doubled CO2 conditions (Reuder et al., 2001).

Aerosol has been considered with its annual variability, but without changes between recent and future climatic conditions.

In case of snow the albedo has been calculated by an algorithm of Schwander at al. (2001), in other cases the albedo for grass was used.

The erythemally effective UV radiation has been calculated every 3 hours for 5 years of 1CO2 and 2CO2 conditions. From that the averaged daily UV-exposition has been determined on a monthly basis.

Abteilung für Strahlung und Satellitenmeteorologie

Arbeitsgruppe UV-Strahlung

Theresienstrasse 37, 80333 München, Germany

E-mail: [email protected]

[email protected]