Evaluation of NASA POWER Reanalysis Data for Daily Weather Variables and Observed Long-Term Climate Trends in Nuwara Eliya, Sri Lanka

Accurate climate data is essential for planning climate resilience and understanding regional climate variability, particularly in high-altitude, data-scarce regions such as Nuwara Eliya, Sri Lanka. This study evaluates the performance of NASA POWER reanalysis data (0.5° latitude × 0.625° longitude resolution) by comparing it with ground-based meteorological observations from 2009 to 2021, addressing a gap that remains largely unexplored in the existing literature. Key climate variables, including precipitation, temperature, wind speed, and relative humidity (daily data), along with cloud cover (monthly data, 2017–2020), were validated using the correlation coefficient (R), mean error (ME), root mean square error (RMSE), and percent bias (PBIAS). The results showed strong agreement for temperature (bias-corrected R = 0.66), wind speed (R = 0.72), cloud cover (R = 0.92), and relative humidity (R = 0.63). Precipitation analysis revealed that dry-day overestimation drove positive biases in FIM, SIM, and NEM, whereas SWM biases resulted from underestimation of monsoon rainfall intensity. The Mann-Kendall test and Sen’s slope estimator were applied to detect long-term trends (1990–2020), revealing a significant increase in minimum temperature (0.01 °C/year) and shortwave radiation (0.2 W/m²/year), and a decline in wind speed (−0.03 m/s/year), indicating significant climate shifts in the region. This study provides a comprehensive validation of NASA POWER data in Sri Lanka’s central highlands, underscores the need for localised bias correction, and demonstrates its potential for broader application in similar high-elevation, data-constrained regions for climate impact assessment and resource planning.