The model yields Tj or T}(t) and the respective f parameters corresponding to each of 11 total sky cover values. To find the frequency /that the transmission efficiency T is equalled or exceeded, for example, a linear interpolation is performed using these tabular data. A similar procedure is used for T(t), while Pean be calculated straightforwardly using Eq. 4. Statistical parameters were calculated on a seasonal and annual basis for each site using the computer code pa vail, written expressly for manipulating the sky-cover data base. In addition, the code is set up to statistically reduce data for multiple sites within a geographical region. Cloud Transmission Model I. The first cloud transmission model is the most conservative and is utilized to establish a lower bound on the calculated power availability. We assume that only thin cirriform, middle, and stratiform clouds are partially transmissive when they are observed at total sky covers less than or equal to 6 tenths. Furthermore, we are implicitly assuming that the thickness of these cloud forms increases in a manner directly proportional to sky cover so that the cloud-form transmission efficiency decreases with increasing sky cover. Numerically, model 1 specifies [1] zero transmission efficiency for cumuliform or mixed-form clouds, [2] zero transmission efficiency for all cloud forms observed at total sky covers greater than 6 tenths, [3] the cirriform transmission efficiency decreases from 90%* to 30% as the total sky cover increases from 0 to 6 tenths, in correlation with the lower range of observations in Fig. 7 of Ref. (1), and [4] the transmission efficiency of middle and stratiform clouds decreases from 40% to 10% and 60% to 20%, respectively, in the same sky cover range. Also, to account for statistical variations in the persistence probabilities, Ej values used in model 1 were reduced by 8%, consistent with the observational results of Lund (9) and the conservative nature of this model. Cloud Transmission Model 2. The second model represents the best estimate for cloud transmission under conditions not involving hole boring. The model is based largely on subjective judgments and some comments and observations on the sensitivity of the power availability model to the estimated cloud-form transmission efficiencies will be presented in a later section. Specifically, model 2 assumes [1] zero transmission efficiency through cumuliform clouds, [2] the cirriform transmission efficiency decreases from 90% to 35% as the total sky cover increases from 0 to 10 tenths, in correlation with the Kuhn-Weickmann curve in Fig. 7 of Ref. (1), |3] the transmission efficiency of middle and stratiform clouds decreases from 40% to 10% and 60% to 20%, respectively, over the same sky cover limits, and [4] the transmission efficiency for mixed cloud forms decreases from 30% to 0% as the total sky cover increases from 0 to 10 tenths. Therefore, compared with the first model, model 2 allows partial transmission during certain overcast conditions and through mixed cloud forms if they are observed during periods of lesser sky cover. Cumuliform clouds are again assumed to be completely opaque. Cloud Transmission Model 3. To complete the power-availability model, a third cloud transmission model is proposed which assumes substantial penetration of certain cloud types by hole boring. We have limited the laser-beam power density by safety and environmental constraints so that not all cloud types are penetrable. A description of the hole-boring calculations is given in Ref. (1). *Thin cirriform clouds are often present but unobservable when ground-based total sky cover data are taken.
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