Low-level wind variability at Jaisalmer and its representation in ERA5 Reanalysis

Bela Amol Lodh, Vishal Dixit, G. S. Bhat, Pramit Kumar Deb Burman

Jaisalmer has grown to be one of the important wind energy hubs in India following the development of the Jaisalmer Wind Park. In this study we assess the variability of wind speed at various scales, and the occurrence of wind ramps (defined as the change in wind speed over a certain period) at Jaisalmer using high-quality in-situ measurements. A distinct diurnal pattern in wind ramps is seen in winter as the wind speeds show a small but sudden rise in the morning hours and a drop in the late afternoons. This pattern is absent in the summer months but re-emerges briefly during August which is the peak of the local summer monsoon season. Version 5 of the global climate reanalysis by the European Centre for Medium-Range Weather Forecasts (ERA5) underestimates the magnitude of wind ramps and misrepresents their diurnal timing, with the peak occurrence shifted. Land-atmosphere interactions are critical in shaping wind dynamics within the planetary boundary layer, particularly through their influence on the turbulent processes, that are pronounced in the surface layer where wind turbines are placed. According to widely adopted bulk parameterizations, the sensible heat flux (SHF) at the surface is parameterized in terms of wind speed (U) and temperature difference (DT) between the surface and the reference altitude. Numerous modifications pertaining to different scaling approaches have been proposed to this parameterization to improve the model accuracy under various conditions. In this study, we found that during night hours, the SHF downward to the land is strongly coupled with U by a linear relationship that is particularly pronounced in winter and absent during summer. Moreover, the periods of prominent nighttime correlation coincide precisely with the ramp‐free hours. After sunrise, the boundary layer undergoes a transition from forced to mixed convective regime leading to a brief period of linear coupling between the SHF and U from 12 AM to 4 PM. Unlike the nighttime regime that is a feature of winters and spans a duration of nearly 12 hours, the daytime regime is observed during summer over a shorter temporal scale. We find that the stable wind regime during night is captured in ERA5 successfully, while the daytime regimes need a significant improvement in representation.