Development of monthly average hourly maps of vitamin D weighted solar ultraviolet radiation for Thailand using an empirical model from ground- and satellite-based data

Abstract

This thesis presents the development of maps of monthly average hourly vitamin D weighted solar ultraviolet radiation (DUV) for Thailand using an empirical model from ground- and satellite-based data. Spectral solar ultraviolet radiation measured by a spectrophotometer model DMc150 of Bentham Instruments installed at a roof top of Science building at Silpakorn University, Nakhon Pathom (13.82 oN, 100.04 oE) in years 2014 and 2016 were used to find a relationship between DUV and erythemal solar ultraviolet radiation (EUV). Broadband radiometers model 501A of Solar Light company installed at four stations namely, Chiang Mai (18.78 oN, 98.98 oE), Ubon Ratchathani (15.25 oN, 104.87 oE), Nakhon Pathom (13.82 oN, 100.04 oE), and Songkhla (7.20 oN, 100.60 oE) were used to measure EUV and then these values were converted to DUV using the relationship between DUV and EUV producted from the data at Nakhon Pathom station. The DUV data obtained from the radiometers at the four stations during 2014-2018 were used for modelling and validation processes. Another ground-based input data is visibility obtained from observation at 125 meteorological stations in Thailand. The visibility data was converted to aerosol optical depth. Satellite-based input data used in this thesis consist of a satellite-derived cloud index from MTSAT-2 and Himawari-8 satellites, and total ozone column from OMI/AURA satellite. In addition, solar zenith angle, air mass, and extraterrestrial DUV irradiance were also calculated. These input data were collected during 2012-2021, and were interpolated to obtain the data as grids covering the region of Thailand. An empirical model for estimating monthly average hourly DUV irradiance using the data from the four UV monitoring stations during 2014-2016 was developed as a function of aerosol, cloud index, ozone, solar zenith angle, air mass, and extraterrestrial DUV irradiance. The model was validated by calculating DUV irradiance using the proposed model and compared the values obtained from this model with those obtained from the ground-based measurement during 2017-2018. The difference between DUV irradiances estimated from the proposed model and those obtained from the measurement was presented in terms of root mean square difference (RMSD) and mean bias difference (MBD). The RMSD and MBD were found to be 12.7% and -0.2%, respectively. Afterward, this model was used to estimate monthly average hourly DUV irradiance covering the region of Thailand during 2012-2021. The result was presented as maps and these maps show diurnal, seasonal and geographical variations of DUV irradiance.

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