UV-model intercomparison - COST 713.
Results for the Swiss Model

Anne Renaud(1), Pierre Eckert(2) and Johannes Staehelin(1)

(1) Institute for Atmospheric Science, ETH-Zurich, Switzerland
(2) Swiss Meteorological Institute, Geneve-Cointrin, Switzerland
 

1    Calculation procedure

The results presented here are calculated with a two-steps procedure:

• calculation of the value for the station Davos (1610 m a.s.l.) for the given atmospheric conditions. Two statistical models based on the UV-Biometer measurements (Solar Light Co. model 501) from may 95 to april 96 at Davos are used: one model for the no-snow conditions and one for the snow conditions (see 5. Information on the statistical models)
• adaptation to obtain value for the given altitude. This adaptation is based on the comparison between UV-Biometer measurements at Davos and Payerne (490 m a.s.l.) from may 95 to april 96 (see 6. Information on the altitude adaptation).

2    Proposed jobs for the UV-model comparison

Results are given for the jobs 1-9, 28-36 and 55-72. The reasons for this choice are:

• the aerosol optical depth is not part of the model input data. Therefore the results for the jobs 10-18 and 19-27 are the same as for the jobs 1-9. And the results for the jobs 37-45 and 46-54 are the same as for the jobs 28-36.
• the surface albedo is not an input parameter of the model. For this comparison the no-snow model was used for albedo 0.03 and the snow model for albedo 0.5. We don't have any intermediate case for the 0.1 albedo (jobs 73-81).
• the models can also not be used with confidence for a very low total ozone amount combined with a sea-level surface, since the situation is too far from the conditions at Davos (jobs 82-87)
• because of the restriction of the measurements used for the statistical estimation (zenith angle < 70 degrees), the models cannot be used for very large zenith angles (jobs 88-90).

3    Remarks on the calculation time and results

• the computation time is negligible since the basic models are very simple.
• since our analysis is based on UV-Biometer measurements we are not able to provide reliable data for the CIE weighted spectrally integrated irradiance. The difference is expected to vary with varying total ozone amount and zenith angle.
• the parameters in the snow and no-snow models were estimated to fit the UV-Biometer data in the range of total ozone and of zenith angle we have at Davos, for snow and no snow conditions, respectively. The results given for zenith angle of 15 degrees and for total ozone smaller then 240 DU are obtained from the extrapolation of the model outside of the domain of the analysis.
• an extrapolation was also used to obtain a value at sea-level (altitude effect).

4    Results

(1) jobnr

(2) zenith angle

(3) total ozone

(4) surface albedo

(5) altitude

(6) UV-Biometer [W/m2]

(1)(2)(3) (4)   (5) (6) 
 1 15 190 0.03    0 0.308 
 2 30 190 0.03    0 0.245 
 3 60 190 0.03    0 0.069 
 4 15 285 0.03    0 0.226 
 5 30 285 0.03    0 0.174 
 6 60 285 0.03    0 0.041 
 7 15 380 0.03    0 0.175 
 8 30 380 0.03    0 0.133 
 9 60 380 0.03    0 0.031 
28 15 190 0.5     0 0.335 
29 30 190 0.5     0 0.272 
30 60 190 0.5     0 0.083 
31 15 285 0.5     0 0.248 
32 30 285 0.5     0 0.196 
33 60 285 0.5     0 0.052 
34 15 380 0.5     0 0.186 
35 30 380 0.5     0 0.144 
36 60 380 0.5     0 0.035 
55 15 183 0.03 2000 0.476 
56 30 183 0.03 2000 0.379 
57 60 183 0.03 2000 0.109 
58 15 278 0.03 2000 0.347 
59 30 278 0.03 2000 0.268 
60 60 278 0.03 2000 0.064 
61 15 373 0.03 2000 0.269 
62 30 373 0.03 2000 0.204 
63 60 373 0.03 2000 0.047 
64 15 183 0.5  2000 0.517 
65 30 183 0.5  2000 0.42 
66 60 183 0.5  2000 0.13 
67 15 278 0.5  2000 0.382 
68 30 278 0.5  2000 0.302 
69 60 278 0.5  2000 0.081 
70 15 373 0.5  2000 0.287 
71 30 373 0.5  2000 0.222 
72 60 373 0.5  2000 0.054 

5    Information on the statistical models

5.1    Basis model

5.2    Basis data set

• two-minutes global UV-Biometer measurements (Solar Light Co., model 501) at Davos restricted to zenith angles smaller than 70 degrees and clear-sky (free-sun) conditions. The free-sun criterion is based on sunshine duration per 10 minutes, completed with a minimal accepted value of the ratio direct/global dependent on the zenith angle.
• total ozone daily averages at Arosa (1820 m a.s.l.; 12 Km from Davos).

5.3    Estimation of the parameters a0-a5

5.3.1 No-snow model

• estimation of the parameters a0-a5 with the data restricted to days without snow at Davos and without snow at the station Weissfluhjoch (2690 m a.s.l., ca 6 Km from Davos) to avoid the influence of the large albedo in the neighbourhood.
• Range of total ozone: 240 - 330 DU. Range of zenith angles: 23 - 70 degrees.

5.3.2 Snow model

• estimation of the parameters a0-a5 with data restricted to days with snow at Davos and at the station Weissfluhjoch.
• range of total ozone: 250 - 420 DU. Range of zenith angles: 28 - 70 degrees.

6    Information on the altitude adaptation

The altitude adaptation is based on the comparison between all weather global UV-Biometer measurements at Davos (1610 m a.s.l.) and Payerne (490 m a.s.l.) during the period may 95 - april 96.

Envelopes of the data were calculated for the two stations and the altitude effect was estimated as a function of the julian day (increase of global UV-Biometer for 1000m in percent with reference station at Davos). The estimated typical summer value (mean july-august) of the altitude effect is -18%/1000m (reference at Davos).

Do you have remarks?, questions? Please contact:

Anne Renaud
Institute for Atmospheric Science
ETH-Hoenggerberg
CH-8093 Zurich
Switzerland

e-mail: anne@atmos.umnw.ethz.ch

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