A Forecast Evaluation of Planetary Boundary Layer Height over the Ocean

The planetary boundary layer (PBL) is the layer of the atmosphere closest to the Earth's surface through which fluxes of heat, moisture, and momentum take place between the surface and free atmosphere. As these fluxes affect the atmospheric circulation, it is necessary for weather forecasting systems to accurately capture them to provide skillful weather forecasts. A fundamental PBL property is its height, and the research presented investigates the forecast skill of PBL height over the oceans in the European Centre for Medium‐Range Weather Forecasts (ECMWF) Integrated Forecasting System (IFS). Using the ECMWF IFS and 1,959 dropsonde profiles from multiple observational campaigns, the observed PBL height was on average 744.2 m and the short‐range (3–15 hr) forecast had a root‐mean‐square error of 242.5 m. The merging of observations in the data assimilation procedure to produce the new forecast analysis reduced the root‐mean‐square error to 180.4 m.

These PBL heights at the dropsonde positions were also analyzed based on their geographic location, the observed 925‐hPa wind speed, and the PBL stability. The findings showed that the IFS had the largest errors in the midlatitudes, in weaker winds on forecast days 1–4, and during unstable PBL conditions. It was also found that the medium‐range forecasts are underdispersive, which is largely due to representativeness errors arising because the IFS represents the grid box average rather than the subgrid atmospheric variability. The results highlight an area of forecast uncertainty in current forecasting systems.