Measurements of net radiation and its components in semi-arid climate of Baghdad

Abstract

Regular measurements of incoming global radiation, net short wave radiation and total net radiation were taken above a natural prairie grass during a one year period. Air temperature and relative humidity measurements were also taken simultaneously. Variations in the seasonal patterns of these quantities have been presented and their distributions discussed. Two linear relationships between the monthly means, both for 24 h period and daytime, of net radiation and global radiation were developed. Analysis of the hourly regressions of net radiation and global radiation for clear skies yields four values of heating coefficients: winter, spring, summer and autumn. Heating coefficients were consistently positive, and the values generally found are small compared with unity. For the climate of Baghdad, good agreement is found between the hourly observed total net radiation and estimates made with the simple empirical scheme presented by Holtslag and Van Ulden. For all clear skies data, it appears that the root mean square error is σ=28 W m −2 for the net radiation, which is 7% of the observed average hourly values, and that the correlation coefficient is 0.992. Two expressions for the temperature difference between the air and the sky were obtained. In summer, the temperature difference does not exceed 10 °C, while during winter months, the difference is ≈20 °C. This, in turn, is related to the amount of precipitable water vapour.