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Thanks to Paul Hirose, who provided me wih the necessary data, I had the
possibility to compare the refraction values already posted here, and which
were calculated with the temperature profiles from U.S. Standard Atmosphere
Supplements 1966 (USStAS-66), with those using the COSPAR International
Reference Atmosphere (CIRA) from 1986 (CIRA-86).

The CIRA-86 data sets allow to calculate for a given day of the year the
temperature distribution in the atmosphere for heights from 01.km to almost
120km and for latitudes from 80deg south to 80 deg north. With the data of
this model, a temperature is calculated by summation of temperature values
from annual, semi-annual and a ter-annual temperatur cycles to the annual
average temperature. A day is defined as a fraction of the year, ranging
from 0.0 to 12.0. In order to receive comparable results to the January and
July data calculated with USStAS-66 the middle of each of the two months,
i.e 0.5 and 6.5 was selected.

Differences between the two models:
The profiles in USStAS-66 seem to represent average values for the
corresponding month. The CIRA-86 are representative for a given day. (Note:
Paul could actually find lateron also montlhly averaged data calculated with
the CIRA-86 model; they reched me however after the calculations were
already done.) A more important difference between the two models,
especially when calculating refrction at the horizon, is the different
resolution of temperature values at low heights. The CIRA-86 model provides
there only values for fixed points at heights of 0.1km, 2.2km and 4.2km. The
graphs in USStAS-66 are at lower heights somewhat more detailed; their
profiles for January show clearly for latitudes of  60deg and 75 deg
temperature inversions between heights of 0km and approx. 2km. The CIRA-86
temperature profiles for mid January show there no inversion for latitude of
60deg and an indication of an inversion for latitude 75deg.
Note: The CIRA-86 data do not provide a temperature at 0km height. For
calculating the refractions the the lapse rate value for the range between
2.2km and 0.1km was just continued for the heights below.

Results:
The refractions calculated with the two models compare well. At an altitude
of 2 deg the differences between the two models of the values averaged over
the latitudes are for January and July less than one tenth of an arc min. At
an altitude of 1 deg this difference is still less than half of an arc min.
At lower altitudes the differences between the two models increase somewhat
due to the above mentioned differences in the models.

Conclusion:
There seems to exist a noticable tendency for temperature inversions during
the winter season at higher latitudes which has a remarkable influence on
the refraction at the horizon. In order to make better refraction values
available for such situations, like e.g. in the North Sea during the winter
months, the "systematics" of these inversions have to be analysed more in
detail.

As a geste of saying thankyou to all the help I received here in the Nav-L
list, I make the results, which are fully documented in two Excel-files
(1.4MB and 0.3MB), available to all those who are interested in the subject.
The files can be found under the follwing links:
http://rapidshare.de/files/4903523/Refraction_comparing_results_US_Atm_1966.xls.html
http://rapidshare.de/files/4903630/Refraction_comparing_results_CIRA_1986.xls.html
To obtain the files click on the corresponding link, go to the bottom of the
page and click on "free", wait for about one minute until the file shows up
for downloading.

Have a nice week-end.

Marcel

   

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