Comparison of ground based, satellite estimated and modeled solar UV Index in three major cities of Nepal

This paper presents the comparison of solar UV Index (UVI) at three major cities of Nepal (Kathmandu, Pokhara and Biratnagar). Kathmandu (27.72° N, 85.32⁰ E), Pokhara (28.22° N, 83.22° E) and Biratnagar (26.45° N, 87.27° E) are located at an elevation of 1350m, 800m and 72m respectively from the sea level. The NILUUV irradiance meter is used to record UV radiation on these stations. The Ozone Monitoring Instrument (OMI) on board, NASA EOS Aura space craft, is a nadir viewing spectrometer that measures solar reflected and back scattered light in ultraviolet and visible spectrum. The UVSPEC model from the Libradtran package version 1.01 is also used to reconstruct the global spectral irradiance from 290 to 450nm at 0.5nm resolution. The study is based on one year (2009) data for Kathmandu (KTM), Pokhara (PKR) and Biratnagar (BRT). OMI overestimates the ground based data (June-September) by 50.7–93.9% in KTM, 48.8–70.4% in PKR and 33.3–62.0% in BRT. Radiative transfer (RT) clear sky calculation with ground based (GB) measurement was used to determine the influence of clouds and aerosols on the surface UV radiation. On common clear day in March 1, 2009 the relative difference between calculated clear sky UVI and ground based UVI was found to be 44.7% at KTM, 41.8% at PKR and 42.3% at BRT at 7 UT. The correlation coefficient (r) between GB and OMI noon time ozone showed a good agreement and were found to be 0.92 at KTM, 0.91 at PKR and 0.91 at BRT respectively from January to April, 2009.