A field experiment has been carried out with the specific objective in mind to determine the value of von Kármán's constant in atmospheric boundary layer flow, and also to investigate its possible dependence on the surface Rossby number Ro0. Data from three field measurement campaigns at Marsta, Sweden, were used in the analysis. Depending on the ground cover: deep snow (40 cm), plowed field or 15–20 cm high wheat crop the surface roughness in the three campaigns was respectively: 1 mm, 7.5 mm and 24 mm. The corresponding range in Ro0 is roughly 2.5×106−6.5×107. In the experiment a turbulence instrument was used (the MIUU instrument) that has been carefully examined for aerodynamic error effects and duly corrected. Measurements of turbulent fluxes made with this instrument at ca. 2 and 6 m above the surface were used, together with wind profile measurements. Data from the measurements with the turbulence instruments were also used to correct the cup anemometers for overspeeding (found to be 5%). ... Abstract A field experiment has been carried out with the specific objective in mind to determine the value of von Kármán's constant in atmospheric boundary layer flow, and also to investigate its possible dependence on the surface Rossby number Ro0. Data from three field measurement campaigns at Marsta, Sweden, were used in the analysis. Depending on the ground cover: deep snow (40 cm), plowed field or 15–20 cm high wheat crop the surface roughness in the three campaigns was respectively: 1 mm, 7.5 mm and 24 mm. The corresponding range in Ro0 is roughly 2.5×106−6.5×107. In the experiment a turbulence instrument was used (the MIUU instrument) that has been carefully examined for aerodynamic error effects and duly corrected. Measurements of turbulent fluxes made with this instrument at ca. 2 and 6 m above the surface were used, together with wind profile measurements. Data from the measurements with the turbulence instruments were also used to correct the cup anemometers for overspeeding (found to be 5%). ...