RESULTS FROM A SIMPLE INFRARED CLOUD DETECTOR

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1 RESULTS FROM A SIMPLE INFRARED CLOUD DETECTOR A. Maghrabi 1 and R. Clay 2 1 Institute of Astronomical and Geophysical Research, King Abdulaziz City For Science and Technology, P.O. Box 6086 Riyadh 11442, Saudi Arabia amaghrabi@kacst.edu.sa 2 School of Chemistry and Physics, University of Adelaide, 5005, South Australia, Australia rclay@physics.adelaide.edu.au ABSTRACT Cloud cover must be continuously observed by some method to fully determine its nature for various applications e.g. solar energy applications and energy budget studies. In the past, several methods and techniques have been employed to detect the presence of clouds. The University of Adelaide has developed simple Infrared (IR) cloud monitors for cloud detection. The monitors were developed with 3 o and 90 o fields of view (FOV) and have a spectral response which extends from 5.5 µm to above 20 µm, centered on a wavelength of 10 µm. They were operated in vertical and scanning modes. Some of the results obtained from the 3 o FOV detector are presented, both at the zenith as well as at different zenith angles in the City of Adelaide, an urban site having a mid-latitude Mediterranean climate. INTRODUCTION Clouds are highly variable and exhibit a wide range of spatial extents and lifetime length scales. They are characterized by their type, coverage, liquid water content, droplet concentration and droplet size. They vary by height, temperature, density and their constituents. Clouds scatter considerable visible radiation and therefore reflect some of the incoming solar radiation. They also emit infrared and microwave radiation. Clouds are fairly opaque at IR wavelengths, which makes them good candidates for detection using IR methods. Knowledge of cloud cover and properties must be obtained through observation for solar energy research, radiation budget studies, for astronomical and atmospheric applications. Cloud observations have usually been made using the naked eye at semiregular intervals, but may be made using a number of techniques. Radar, laser reflection, weather satellite photographs, and attenuation of starlight are examples of methods that have been used to detect the presence of clouds (Bird 1996). Recently, infrared techniques have been used for cloud detection above astronomical observatories (e.g. Ashley and Jurcevic 1991 and Buckley et al. (1999). This method is the focus of this paper. 1

2 INFRARED TECHNIQUE FOR CLOUD DETECTION Sloan, Shaw and Williams (1955) showed that infrared measurements of the sky at wavelengths between 8 µm and 14 µm are sensitive to the presence of clouds. Clouds are detectable through the blackbody radiation that they emit. Assuming that a cloud represents a grey body at terrestrial temperatures ~ 300 K, Wien s Law implies that it will radiate with maximum intensity at a wavelength of ~10 µm, in the IR part of the spectrum. It follows then that an IR cloud monitor can be used to detect clouds. Clouds in thermal equilibrium with the air at a particular height appear warm against the cold background clear sky. The amount of contrast between the clear and cloudy sky temperatures depends on the wavelength band selected, the optical depth (or emissivity) of the cloud and the intensity of emission from the clear sky. The clear sky temperature typically depends on the screen temperature and the amount of water vapour in the atmosphere. The expected relationship between clear sky temperature and air temperature is due to the fact that the temperature in the part of the atmosphere where the radiation originates is correlated by some combination of radiation, turbulence, and advective process to the surface temperature. The dependence of the atmospheric temperature on the amount of water vapour is generally needed, since atmospheric water vapour strongly absorbs the infrared radiation as a result of both infrared continuum and selective absorption by its vibro-rotational and rotational absorption bands centred at infrared wavelengths longer than 5 µm. DESCRIPTION AND CHARACTERISTICS OF THE DETECTOR The University of Adelaide has developed simple single pixel thermopile IR cloud detectors as an inexpensive tool for cloud detection over astronomical and astrophysical observatories. The design concepts, calibrations and the constructions of these detectors are given in Maghrabi (2007) and Maghrabi and Clay (2008). A brief summary of these detectors will be given here. The detectors are based on commercial IR thermopile sensors. The sensor elements are hermetically sealed into a housing and an additional optical filter is used to better define the optical pass band. We use an optical filter having a 5.5 µm spectral short wave cutoff. We designed the detectors to have 90 o and 3 o fields of view (FOVs). The detectors have been operating on the roof of the Physics building at the University of Adelaide, in four sites in Saudi Arabia, and in one site at the Pierre Auger Observatory in Argentina. Here, we discuss the Adelaide site, at which both detectors were placed on the roof of the physics building. They were installed in two different modes. One detector with the 90 o FOV was fixed to view vertically. The other with the 3 o FOV was used as a scanning detector. The latter detector was placed on a rotator that allowed us to scan the sky from East to the West (horizon to horizon) through the zenith in 3 o steps, every 5 minutes, figure (1). The electronics provides three output temperatures from the detector (for more details about the technical issues of these detectors see Maghrabi and Clay 2008). These are: the uncompensated temperature, the ambient temperature of the detector, and the 2

3 compensated temperature. The three outputs are related to each other by the following formula: Tsky = Tsky + T compensated uncompensated ambient Where Tsky uncompensated is the temperature difference between the detector reference temperature (T ambient )and the target compensated temperature (Tsky compensated ). The detectors were calibrated in terms of output using a black body of known temperature to fill their fields of view. The experimental error in the sky temperature measurements from the detector is about 2 o C (Maghrabi 2007). The results presented in this paper illustrate the scanner detectors ability to detect clouds at the zenith and at different zenith angles. RESULTS AND DISCUSSION Results from the Scanner at the Zenith The detectors were tested for their sensitivity to cloud coverage under different sky conditions. These conditions varied according to the cloud cover from clear, through partly cloudy, to totally overcast skies. Figure (2) shows hourly data taken between April 27 th and May 1 st 2005 by the detector with the 3 o FOV at the zenith. The three temperatures in the figure correspond to the three detector output temperatures (Tsky compensated, Tsky uncompensated and Tair ambient). Low compensated sky temperatures represent a clear sky, and high temperatures (from -5 o C to above 0 o C) indicate low, thick clouds. The sky experienced both clear and cloudy times over this period. The sky stayed clear for the first day, from the afternoon of the 27 th of April until 1 PM the next day. The sharp increase on the morning of March 28 th was due to the presence of mid-level clouds where the sky becomes clear again after only an hour. After this point (from midday of April 28 th ) there was a build up of clouds observed. During this cloudy period (1 PM April 28 th to 1 AM April 29 th ) the sky experienced overcast periods where the compensated sky temperature remained between the 5 to 10 o C mark. The sky then experienced cloudy and clear sky conditions represented by increases and decreases in the sky temperatures. At 10 AM on the morning of the 29 th of April the sky remained clear for more than 12 hours, after which clouds started to develop in the field of view. It is evident from the above results that the distinction between the clear sky and the clouds in the FOV is straightforward at most times. A cloud discrimination criterion can be readily determined from the data set. Also, the cloud detector s data generally agrees with cloud information found in Weather Bureau records, but are more detailed. Figure (3) shows histograms summarizing data from the cloud monitor at the zenith over 1300 hours of night-time operation. The uncompensated sky temperature is shown for the three types of sky. The first histogram (1 st on the left) is of 220 data points taken from totally overcast skies. It shows the expected range of temperatures from the detector, which is between 3

4 0 o C to around 15 o C. The variation in the temperature depends mainly on the cloud properties, e.g. cloud height; a mean of -6 o C is found in such cases. The second histogram (the middle figure) is of 930 data points taken from totally clear skies. The temperature ranges between 39 o C and 25 o C, with a mean of 27 o C. This variation in temperatures is attributed to changing atmospheric conditions, e.g. air temperature and the amount of water vapor. Partly overcast skies are those that showed clouds and clear sky periods at the same time. The temperature ranges for this case vary according to the sky clearness or the amount of cloud cover in the FOV. The temperature ranges are intermediate between the clear sky and overcast cases and are shown in the last histogram. These histograms show that there is clear, measurable separation between the cloudy and clear skies and confirm the ability of the detector in various atmospheric conditions. Measurements from the Scanning at Different zenith Angles Figure (4) shows an example of two full scans from the scanner detector showing totally overcast and totally clear skies. The former scan was taken on the 25 th of April 2005 at 9 AM for a totally overcast sky with uniformly distributed low-level clouds. The clear sky scan was taken on July 22 nd 2001 at 19:30. The temperature plotted is the uncompensated sky temperature. The two curves are characterized by the absence of any structure through all the zenith angles. However, the cloudy sky appears warmer than the clear sky by about 20 o C at zenith angles between 30 o to 150 o. This difference decreases until it reaches of about 0 to 5 degrees at the horizon, where for both situations the sky temperature is similar or very close to the screen (air) temperatures. This is a consequence of the increasing optical depth of the radiating molecules (particularly water) within the band pass of the detector. These two curves represent the ideal situation for the two most extreme sky conditions: totally clear and totally overcast. The curve of the overcast conditions may differ according to the type and distribution of the clouds. On the other hand, the clear sky curve is standard and the range of sky temperatures may depend on the prevailing atmospheric conditions. The presence of clouds at any zenith angle may cause structure to appear in the curve as seen in Figure 5. Figure (5) shows another example of three full scans from the scanner detector during the night of 26 th November 2002 from 6:30 to 6:40 in the morning with changing cloud coverage. The sky temperatures range from 35 o C at the zenith to -3 o C near the horizon. The three curves are characterized by the presence of different structures. An increase in the sky temperature at any zenith angle indicates the existence of clouds in the FOV of the detector. At 6:30 am the scanner indicates the presence of clouds at different zenith angles mainly between 15 o 20 o, 39 o 70 o, 109 o and 115 o. These are represented by an increase in the sky temperature from a smooth expected ideal clear sky, which may be similar to that of the clear sky found in figure (4). At 6:35 the clouds start to move toward the west. For example the cloud found at the 50 o angle in the previous scan is now located at 55 o angle. At 6:40 the sky started to clear toward the eastern side, particularly at those zenith angles previously detecting clouds, but some detectable clouds were still observed at the zenith angle 140 o. 4

5 It is clear that the scanner provides us with a useful tool for detecting clouds over the entire sky dome. CONCLUSION We have designed and developed simple single pixel thermopile IR detectors for monitoring clouds for the purpose of atmospheric research and for use in astronomical and astrophysical observatories. The performance of these detectors was tested over a long period of time and in different extreme conditions. They have shown a stable and accurate performance as well as superior cloud detection ability. Data from these detectors are reliable and can be used in future atmospheric studies. This includes the capability of using cloudy skies data to determine, for example, the height of the cloud. ACKNOWLEDGEMENT We appreciate the Australian Bureau of Meteorology and particularly, the assistance of Mr. Bruce Brooks for his help in providing the weather data. Diagrams: 5

6 Fig.1: Shows the scanner detector at the roof of the physics department at the University of Adelaide, Adelaide. The figure shows the scanner at different zenith angles. Clockwise; the scanner at the west toward the hills, at the zenith, towards the sea and finally at 10 o from the zenith toward the sea. The picture also shows the configuration of the detectors on the scanner. The three detectors with larger sizes are those with 3 o FOV while those with smaller sizes are those with 90 o FOV Temperature Tsky (uncompensated) Tair Tsky(Compensated) -60 4/27/ /28/ /28/ /29/ /29/ /30/ /30/ /1/05 01 Time mm:dd/yy hh Fig.2: Shows a sample of data taken between May 27 th to June 5 th 2005 from the STD detector with 3 o FOV at the zenith. The data is presented in one-hour resolution. 6

7 Fig. 3: Histograms (frequency distributions) of the uncompensated sky temperatures for 3 o FOV at the zenith. The first (1st on the left) is for 222 hours of data for completely overcast skies. The second histogram shows 930 data points for completely clear skies. Partly overcast skies are shown in the last histogram with a total of 230 data points. 0 Overcast Clear ky Ts Angle Fig.4: East-West scans through the zenith for clear and overcast compensated sky temperatures, in o C. The zenith angle is reported in degrees. 7

8 0-5 06:30PM 06:35PM 6:40PM Tsky Angle Fig. 5: Shows a sample from scanner with different cloud covers between 6:30 am to 6:40 am. 8

9 References Bird, D.J., 1996, Proceeding of the Auger meeting on Nitrogen Fluorescence Cosmic Ray detectors (Salt Lake City). Maghrabi A. H. (2007); Ground based measurements of IR atmospheric radiation from Clear and Cloudy Skies, PhD Thesis, University of Adelaide, Adelaide, Australia. Maghrabi A. H. and Clay R. W. (2008), Design and Development of a Simple Infrared Monitor for Cloud Detection, Energy Cons. & Mang., in press. Sloan, R. Shaw,J.H and Williams, S.D., J. Opt. Soc. Am., BRIEF BIOGRAPHY OF PRESENTER DR: Abdullrahman H Maghrabi Assistance Professor of Research Astronomy Department, Astronomy and Geophysics Research Institute, King Abdulaziz City for Science and Technology, Fields of Interest Solar energy applications, Atmospheric studies - particularly long wave radiation, low energy cosmic rays and observational Astrophysics PhD The University of Adelaide (Infrared Atmospheric monitoring) 2000 MSc The University of Adelaide (High Energy Astrophysics) 1996 BSc King AbdulAziz University, Jeddah,Saudi Arabia ( Physics-Astronomy) 9