Shading Calculations for the Radiation Instruments at Jabiru

Shading calculations for the radiation instruments at Jabiru D. M. O'Brien, R. M. Mitchell and Reinout Boers CSIRO Atmospheric Research PMB 1 Aspendale Victoria 3195 Australia 2000-05-24 CSIRO Atmospheric Research Internal Paper 18 Document reference: JAB 002A 1 Background The radiation instruments to be installed at Jabiru include shaded and unshaded pyranometers, a shaded pyrgeometer, a tracking sun-photometer and an all-sky camera (ASC). The pyranometers and pyrgeometer will be installed by the Australian Bureau of Mete- orology as part of a Baseline Surface Radiation Network (BSRN) station, whereas the sun-photometer and all-sky camera will be installed by CSIRO Atmospheric Research. In order to achieve the high quality of data that is demanded by the radiation community from BSRN stations, the instruments must be sited carefully to avoid shading and to min- imize obscuration of the horizon. This report describes the calculations that were carried out for the installation of the radiation instruments in the meteorological enclosure at Jabiru airport. The BSRN instruments are shown in figure 1, taken at the Australian Bureau of Meteorology site in Darwin. The white box in the foreground contains control, data acquisition and communications equipment. Immediately to the right of the box (with its support obscured by the box) is the shaded pyrgeometer mounted on a sun-tracker that positions the shading disc. To the left of the box is the shaded pyranometer, similarly mounted on a sun-tracker with a shading disc. The next post to the left supports the unshaded pyranometer that measures `global' solar radiation. The remaining instruments in the photograph are not part of the BSRN. Figure 1: Baseline Surface Radiation Network (BSRN) installation at Darwin airport. The sun-photometer is shown in figure 2, in this case a photograph of the CAR installation at Alice Springs airport. The optics module, to which the sun and sky collimators are attached, is the `shotgun' like object. It is mounted on a compact sun-tracker. The instrument to be deployed at Jabiru will use solar power, rather than mains power as in the instrument shown. Therefore, the Jabiru installation will have a solar panel mounted on the support post. The selection of the site at Jabiru airport was constrained by several factors: 1. access to power and telephone lines; 2 Figure 2: Sun-photometer installation at Alice Springs airport. 2. proximity to the standard meteorological instruments so that all instruments can be serviced daily by staff from Energy Resources Australia (ERA); 3. a row of trees with height varying between 5 m and 10 m about 30 m to the south of the meteorological enclosure limits the horizon; 4. a car park is located immediately to the west of the meteorological enclosure; 5. the unpaved area to the north-west is used by aircraft to taxi to freight containers near the runway, and therefore is subject to dust stirred up by the aircraft propellers; 6. the runway and surrounding apron to the north is susceptible to dust eddies in the dry season. These factors led to the decision to locate the radiation instruments to the east of the present meteorological enclosure, shown in the photo of figure 3 and the plan of figure 4. This report determines a configuration for the radiation instruments that either eliminates or minimizes the potential for shading. Particular attention is given to shading of the global radiation pyranometer and the sun-photometer by the masts of the automatic weather station (AWS) and the ERA weather station. In addition, the possibility of shading of the sun-photometer by the small shed in the meteorological enclosure and the BSRN station itself is examined carefully. Method Several candidate positions for the BSRN (points A{F) and sun-photometer (points G{I) were considered, as indicated on figure 5. Point X was the approximate position for the BSRN determined during a visit to Jabiru in February 2000 using nomographs of the sun's position. For each candidate position, the distance and bearing to the AWS mast were measured from the site diagram, and the solar elevation at the azimuth of the AWS was computed for every day of the year. To determine whether shading could occur, the solar 3 elevation was compared with the elevation of the top of the mast, based on the standard height of 10 m for the mast and an instrument height of 1.5 m. This process was repeated for the other (potentially) obscuring objects listed in table 1. The heights of the ERA mast and shed were assumed to be 10 m and 2.5 m respectively, while the BSRN shadow arms were assumed to be 1 m higher than the sun-photometer. The program to compute the solar elevation at a specified azimuth as a function of time was written especially for this task, but was based on the general purpose satellite navigation software developed by O'Brien. For each day of the year, the program uses a bisection strategy to determine the universal time when the sun transits the meridian (local solar noon), and then classifies the transit according to whether the sun lies to the north or the south. For northern transits, the solar azimuth decreases from its value at dawn, passes through zero at local solar noon, and becomes increasingly negative throughout the afternoon. For southern transits, the solar azimuth decreases from dawn to a minimum in mid-morning, then increases through 180◦ at local solar noon to a maximum in mid- afternoon, and finally decreases again until sunset. Thus, the sun may transit a given azimuth twice in a day. A bisection algorithm is used to find the times at which the sun transits the specified azimuth, but the update policy in the algorithm depends on whether the transit of the meridian is to the north or the south at local solar noon. Figure 3: Meteorological enclosure at the Jabiru airport in April 1999. The masts at the left and right of the photo are the AWS and ERA masts. The small shed referred to in the text is the white structure right of centre. Results BSRN The site proposed by the Australian Bureau of Meteorology for the global radiation pyranometer is point A on figure 5. Figure 7 shows the solar elevation in the direction of the AWS mast as a function of time throughout the year. The horizontal line in figure 7 is the elevation of the top of the mast. Shading of the pyranometer occurs for approximately six weeks in the middle of the year. Therefore, position A is unacceptable. Figure 8 shows similar calculations for the ERA mast. In this case the sun only transits the mast for approximately fifty days at the beginning and again at the end of the year. 4 In the remaining months, the sun transits to the north of the mast and shadowing is impossible. Even when the sun does pass behind the ERA mast, its elevation is so high that the shadow of the mast will not fall on the global pyranometer. Figures 9, 10 and 11 show that shading eliminates points B, C and E, whereas figures 12 and 13 show that points D and F are acceptable. Of the latter, point F is preferable, because it allows more room for the sun-photometer to the south, and because it has marginally better horizon to the south. As a final check, figure 14 demonstrates that the global pyranometer located at point F will not be shaded by the ERA mast. Sun-photometer The sun-photometer has three modes of operation: 1. staring directly at the sun; 2. scanning in the principal plane from 6◦ below the sun to the horizon in the back- scattering hemisphere; 3. scanning the almucantar at the zenith angle of the sun. Measurements made while staring at the sun determine the total optical thickness of the atmosphere, whereas the sky scans determine the size distribution and scattering properties of aerosol. It is clear that the sun-photometer should be sited so that it is not shaded and so that the sun maintains a 6◦ clearance above surrounding objects. The almucantar scans are performed for solar elevations down to a minimum of 10◦, so it is inevitable that some objects will fall into the field of view. However, measurements are not taken at equal angular increments on the sky scans, but rather are spaced tightly near the sun (where particle size information is most apparent) and are spaced more widely elsewhere. Thus, the probabilty of obscuration by objects not near the solar azimuth is low, and the impact of obscuration on interpretation of the data may be neglected. The possibility of displacing the sun-photometer to the east or west of the north-south line through the BSRN was considered. The argument in favour was that the principal plane scan conducted at local solar noon would not be obscured. However, this argument was rejected for four reasons: 1. on northern transits by the sun, the solar elevation is high and the sun comfortably clears the BSRN by the 6◦ margin; 2. on southern transits, the sun is high and the BSRN will obscure only measurements near the northern horizon, from which little useful information is expected; 3. if the sun-photometer is offset to either the east or the west, the angular width subtended by the BSRN at the sun-photometer is considerably larger than with the sun-photometer to the south, and therefore the BSRN could obscure more sky scans; 4. with the sun-photometer due south of the BSRN, only the noon principal plane scan when the sun is in the south will be affected, and so identification of these events is straightforward, thereby simplifying analysis of the data.

Load more