Category Modeling Solar Radiation at the Earth’s Surfac

Validation of the Model

As routine measurement of solar radiation in Lao PDR was not available prior to this project, five new pyranometer stations (Fig. 19.5) were established in different

Laotian cities at: Vientiane Capital, Laung Prabang, Xamnua, Thakhak and Pakxe. For each station, a pyranometer at Kipp & Zonen (model CM 11) is used to measure the global solar radiation. The voltage signal from the pyranometer is recorded by a data logger of Yokogawa (DC100) at a frequency of one second, which is averaged every 10 minutes. All pyranometers were newly calibrated from Kipp & Zonen. The global radiation data for the period of 6-8 months from these stations were collected during the project and used to test the model for the case of MTSAT data...

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Conversion of the Cloud-Atmospheric Albedo from Satellite Band (pA) to Broadband (pA)

The conversion of the satellite band (pA) to broadband (pA) cloud-atmospheric albedo needs solar radiation data from pyranometer stations and the satellite data of the same period. As such solar radiation data are not available in Lao PDR, the solar radiation measured at 4 pyranometer stations in Thailand, namely Chiang Mai (19.78°N, 98.980E), Ubon Ratchatani (15.25°N, 104.87°E), Songkhla (7.20°N, 100.60°E) and NakhonPathom (13.82°N, 100.04°E) is used.

The mathematical expression for the broadband cloud-atmospheric albedo (pA) is obtained from rearranging Eq. (19.4) to give:

Pa

(19.13)

InEq. (19.13), the atmospheric transmittance т was calculated by using Eq. (19.5) with the daily global radiation (H) measured at the four stations. The other parame­ters of Eq. (19...

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Absorption and Scattering of Aerosols

Southeast Asia is well-known for the its high atmospheric aerosols loads due to in­tensive biomass burning both from agricultural activities and forest fire (Von Hoyningen-Huene et al. 1999). These atmospheric aerosols play a very im­portant role in the depletion of the incoming solar radiation. In the calculation of solar radiation from satellite data, it is necessary to know the amount of solar radia­tion depleted by aerosols. Ideally, this depletion should be obtained from a network of sunphotometer stations. Unfortunately, this is not possible in developing coun­tries. As atmospheric aerosol loads are closely related to visibility data which are generally observed in most meteorological stations...

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Surface Albedo

The surface albedo is estimated using the p’EA from the images collected at 12:30 h local time. These images are examined for a given month and pixels are selected with the lowest value to create the cloud-free composite image for that month. The effect of cloud shadows are assumed to be negligible because the shadows of the
clouds are almost underneath the clouds at 12:30 h in Lao PDR throughout the year. The composite image is converted to the surface albedo using parameterization de­veloped from the 5S radiative transfer model (Tanre’ et al. 1986; Janjai et al. 2006).

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Absorption Coefficients of Water Vapour, Ozone and Gases

These absorption coefficients are all determined from ground-based measurements. In its general form, the absorption coefficients ai for the atmospheric constituent і is calculated from:

where a’ and ai are the absorption coefficients of constituent i in satellite band (X1; X2) and broadband, respectively. I0x is the spectral extraterrestrial radiation, Tjx is the spectral transmittance for constituent i. the calculation of spectral trans­mittance for each atmospheric constituent is explained as follows.

Water Vapour

The spectral transmittance of water vapour was computed from the relationship given in Iqbal (1983):

TWX = exp[-0.238kwXwmr/(1 + 20.07kwXwmr)°A5] (19.8)

where kWX is the spectral extinction coefficient for water vapour, w is the monthly average pricipitable water and mr is ...

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Earth-Atmospheric Albedo (pEa)

Each pixel of all rectified satellite images is converted into earth-atmospheric albedo using a calibration tables provided by the satellite data agencies. As the calcu­lation of global solar radiation is on the daily average basis, the values of earth – atmospheric albedo are averaged over a day to obtain the daily mean values. These mean values are again averaged over a month to get a monthly mean of daily earth – atmospheric albedo pA).

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Satellite Model

The model used in this work is modified from our previous work (Janjai et al. 2005) by accounting for the aerosols absorption of the upwelling path. According to the modified model, the incident solar radiation which enters the earth’s atmosphere is scattered back to the outer space by air molecules and clouds with the cloud – atmospheric albedo of pA and by atmospheric aerosols with the albedo of p’aer. The rest of the radiation continues to travel downwards and is absorbed by ozone, gases, water vapour and aerosols with the absorption coefficients of a’o, a’g, aW and a’aer, respectively. Upon reaching the surface, the radiation flux is reflected back by the ground with the albedo of p’G. As it travels upwards through the atmosphere, it is

Fig. 19...

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Preparation of Satellite Data

The satellite data used in this work are the digital image data from 4 geostationary satellites: GMS 4, GMS 5, GOES 9 and MTSAT. They were recorded from the visible channel of these satellites. The data periods of GMS 4, GMS 5, GOES 9 and MTSAT are: January-September, 1995, October, 1995-May, 2003, June, 2003-July, 2005 and August, 2005-December, 2006, respectively.

The nine hourly images per day (8:30 am-4.30 pm) for the total period of 12-year (1995-2006) with approximately 35,000 images from these satellites were used in this work. Each image consists of a matrix of pixels which records solar radiation reflected from the earth-atmospheric system in the form of gray levels.

A program computer written in IDL (Interactive Data Language) was developed to read and display these digital image...

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