Impact of Cloud Cover on Global UV Irradiance over Baghdad, Iraq

Abstract

Six months of continuous of hourly records of Ultraviolet irradiance recorded at Baghdad, (33.34 o N, 44.45 o E, 34 m a.m.s.l.), Iraq, were combined with concurrent synoptic cloud observations to investigate the relative influence of clouds on UV irradiance. The ratio of UV irradiance to the total solar irradiance increases with cloud cover, especially for cloud cover greater than 4 octas, In order to determine the effect of cloud on UV irradiance, a cloud modification factor was considered, it is defined as the ratio between the UV measurements and the corresponding clear sky UV irradiance that would be expected for the same time period and atmospheric conditions. It was shown that the effect of cloud with 4 octas increases demonstrating the spectral dependence of the cloud radiative extinction, while the impact of cloud cover less than 4 octas is relatively constant and ranged between (3.6 - 4.3%). Broadband solar irradiance (0.3-3 µm), global, ultraviolet global irradiance, photo synthetically active irradiance (0.4-0.7 µm) and other meteorological variables were continuously recorded for the period between first of October 1994 and 30 March 1995 in the Radiometric Station of the Center of Energy and Environment Researches (33.34 0 N, 44.45 0 E, 34 m a.m.s.l.), in Baghdad, Iraq. The measurements were taken every 10 minutes. Solar global irradiance was measured using Eppely pyranometer, An Eppley TUVR radiometer was used to measure the ultraviolet irradiance (0.29-0.385 µm) on a horizontal surface. Air temperature and relative humidity were also recorded for the same period. From this database, hourly values have been generated for the mentioned period to guarantee that a wide range of seasonal conditions and solar elevation angles are covered. It is important to point out that there are significant differences in temporal and cosine response between the radiometers used for the measurements of UV and broadband solar irradiance. Consequently hourly values have been computed, in order to reduce the temporal effects that could be introduced in obtaining the ratio of UV radiation to broadband solar radiation. These undesirable effects can reach quite a significant level, especially under highly changing conditions, which are usually associated with broken cloud fields. III. RESULTS AND DISCUSSION