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Evaluation of Satellite-Derived Erythemal and Broadband UV-A Irradiance Under the Impact of Tropopause Height and Cloud Fraction over Iraq, 2015–2025

Mustapha A. A Jebar, Mustafa Abbass

Solar ultraviolet (UV) radiation is a public-health concern in Iraq, where UV levels in summer are high and ground-based UV instrumentation does not sufficiently cover this region. This study employs eleven years of daily satellite data (1 January 2015 to 31 December 2025) to investigate the influence of tropopause height and cloud fraction on the local-noon erythemal UV irradiance and broadband UV-A (320–400 nm) at three regions in Iraq: Erbil (north), Baghdad (centre) and Nasiriyah (south). Broadband UV-A was reconstructed from Ozone Monitoring Instrument (OMI) spectral irradiances at 310, 324 and 380 nm; tropopause height was taken from the AIRS/Aqua Level-3 daily product and cloud fraction from the OMI cloud retrieval, both through the NASA GES DISC Giovanni system. Pearson correlation and multivariable linear regression were conducted on raw daily data and on de-seasonalised anomalies, and correlations were ranked by cloud coverage. Nasiriyah had the highest mean UV irradiance and tropopause height (15,591 m) and the lowest mean cloud fraction (0.141); Erbil showed the reverse (13,426m; 0.343, respectively), with Baghdad being an intermediate between the two (14,345 m; 0.188, respectively). The pooled regression model explained 34.8% of the variance in erythemal UV irradiance, but the anomaly model explained only 6.2%, showing that most of the raw association between tropopause height and UV irradiance reflects the shared seasonal cycle. Once seasonality was removed, cloud fraction was the dominant predictor of broadband UV-A anomalies (β=−0.241, against 0.064 for tropopause height) and matched tropopause height for erythemal UV anomalies (β = −0.162 and 0.165, respectively). The correlation between tropopause height and UV irradiation also showed weakness from clear-sky (pooled r = 0.573) through partly cloudy (0.412) to cloudy (0.219) conditions. Day-to-day UV variability over Iraq is therefore governed largely by cloud cover, and the relationship between tropopause height and UV irradiance is best assessed on de-seasonalised and cloud-free days data.

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