A comparative study on SPCTRAL2, SPCTR-1881 and SMARTS2 models using direct normal solar irradiance in different bands for Cairo and Aswan, Egypt

The estimated values of Direct Normal Solar Irradiance (DNSI) in different bands, using the three spectral models SPCTRAL2, SPCTR-1881 (modified SPCTRAL2) and SMARTS2, are compared to corresponding values measured in a polluted (urban/industrial) area of Cairo and an unpolluted area in Aswan. The measured data of DNSI are used as input to all models, while some of these data have been selected (56 measurements for Aswan and 94 for Cairo) to cover different atmospheric conditions in the period 1991-1996. The statistical indicators, t-statistic and absolute percentage error ε%, show that SPCTRAL2 model is not suitable for estimation of DNSI in the visible band b2 (530-630 nm), but it is more suitable in the infrared band b4 (695-2900 nm). SPCTR-1881 model is not suitable for estimation of DNSI in b4, but it is efficient for b2. SMARTS2 model is the most suitable for most bands with the lowest percentage deviation, so that the ε% range for Cairo is 0.93-5.2% with t-values in the range 0.002-0.006 with Ångström turbidity parameters α=0.4 and β=0.21-0.28. For Aswan the ε% range is 0.74-4.29% with t-values in the range 0.019-0.181 (α=0.8 and β=0.12-0.14).