How Well Is the Lake-Land Breeze Circulation of Lake Victoria Captured in ECMWF Operational Analysis?

Abstract

Lake-breeze fronts (LBFs) are important drivers of cloudiness, deep moist convection, and heavy precipitation around large tropical lakes, yet their representation in global forecast systems remains poorly understood. This study evaluates the ability of the ECMWF Integrated Forecasting System (IFS) 9 km operational analyses to represent the lake-breeze system over the Lake Victoria basin in Uganda during December–February and June–August 2017–2022. A recently developed objective lake-breeze detection algorithm (OLBDA) is applied to hourly station data and IFS surface data to identify LBF passages. Results show that while the IFS captures the coherent progression of the LBF from early to late afternoon, the analysis reveals biases, including an overprediction of LBF occurrence by ~26% and an early arrival of 1–2 h at most stations. Among three commonly used parameters to delineate LBFs, that is, surface convergence, and gradients of surface specific humidity and boundary-layer height (BLH), the latter two emerge as the most reliable indicators of LBF structure and evolution. Composite and vertical cross-section analyses reveal physically consistent three-dimensional lake-breeze features in the IFS, yet contrasts occur between western and northern transects. The NW-SE transect shows stronger vertical motion and deeper inland penetration when compared to the North–South transect. The latter is likely related to the mean easterly low-level flow over the lake. The depth of the BLH in the late afternoon is 3 km. Overall, our results suggest that ECMWF IFS operational analyses can be reliably used to study lake-breeze systems in both observational and modeling contexts.