A Forecast Evaluation of Planetary Boundary Layer Height Over the Ocean. Issue 9 (6th May 2019)

• Record Type: Journal Article
• Authors: Lavers, David A.; Beljaars, Anton; Richardson, David S.; Rodwell, Mark J.; Pappenberger, Florian
• Is Part Of: Journal of geophysical research. Volume 124:Issue 9(2019)
• Page Start: 4975
• Page End: 4984
• Publication Date: 2019-05-06
• Subjects: Atmospheric physics -- Periodicals; Geophysics -- Periodicals; 551.5
• DOI: 10.1029/2019JD030454
• Languages: English
• ISSNs: 2169-897X

Abstract

Abstract: The planetary boundary layer (PBL) is the layer closest to the Earth's surface in which heat, moisture, and momentum fluxes occur between the surface and free atmosphere. As these fluxes affect the atmospheric flow, it is important for weather forecast systems to accurately characterize them to provide skillful forecasts. A key PBL property is its height, and this study assesses the PBL height forecast skill 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 flight missions, the average observed PBL height was 744.2 m and the short‐range (3–15 hr) forecast root‐mean‐square error was 242.5 m; the data assimilation step reduced the analysis root‐mean‐square error to 180.4 m. These data were also analyzed based on their location, the observed 925‐hPa wind speed, and the PBL stability, and results showed that the IFS had larger errors in the midlatitudes, under weaker winds on forecast days 1–4, and in unstable PBL conditions. The medium‐range forecasts are also underdispersive, which is considered to be largely due to representativeness errors, whereby the forecast model attempts to represent the grid box average rather than subgrid atmospheric variability. These results highlight an area of forecast uncertainty in current forecasting systems. Plain Language Summary: The planetary boundary layer (PBL) is the layer of the atmosphere closest to the Earth's surface. It has an important role in transporting heat and moisture into the atmosphere and, as such, can affect the atmospheric flow. In this study, we evaluate the skill of the European Centre for Medium‐Range Weather Forecasts Integrated Forecasting System to predict over the ocean the PBL height, a key PBL property, using unique dropsonde measurements from multiple flight missions. Results show that the average PBL height was 744.2 m and the short‐range forecasts had a root‐mean‐square error of 242.5 m. Conditioning on dropsonde location, the 925‐hPa observed wind speed, and the PBL stability, results showed that the forecasts had larger errors in the midlatitudes, under weaker winds on forecast days 1–4, and in unstable PBL conditions. The forecasts were also underdispersive, largely because the grid‐scale ensemble spread of PBL height is smaller than its error relative to point observations. These results highlight an area of forecast uncertainty in current forecasting systems. Key Points: Evaluate the planetary boundary layer (PBL) height over the ocean in ECMWF forecasts against observed dropsonde profiles At 1, 959 dropsonde locations, the average observed PBL height is 744.2 m and the short‐range forecast PBL height is 744.1 m The PBL height forecasts are underdispersive and larger forecast errors exist in the midlatitudes, in unstable PBLs, and in weak wind PBLs