Non-dimensional gradient functions for water vapor and carbon dioxide in the marine boundary layer

Sammanfattning: A better understanding of the exchange processes taking place over the oceans is of great importance since the oceans cover about 70 % of the Earth’s surface. With better knowledge the turbulent fluxes can be more accurate parameterized, which is essential in order to improve the weather- and climate models. In this study, the non-dimensional gradient functions for water vapor (Φq) and carbon dioxide (Φc) in the marine boundary layer have principally been studied. The quality of the instrumentation used in the study has also been evaluated. The study is mainly based on tower measurements of turbulent fluxes and vertical profiles of water vapor and carbon dioxide, taken from the Östergarnsholm Island located in the Baltic Sea. The measurements have been shown to represent open-sea conditions for most situations when the winds are coming from the east-south sector, even though the measurements are obtained over land. It was found that the best fitting non-dimensional gradient functions for water vapor during unstable conditions were Φq = 2(1–18z/L)–1/2 and Φq = 1.2(1–14z/L) –1/2 at the 10 and 26 m level on the tower, respectively. No unique relationship could be established for Φq during stable conditions. Φq showed a dependence with wind direction and could for winds coming from the sector 80°– 160° be described with the relationship Φq = 1.2 + 10.7z/L during stable conditions. For the wind sector 50°– 80° the relationship for Φq was found to be Φq = 1.8 + 7.1z/L during stable conditions. A high degree of scatter was apparent in the calculated values of Φc during both stable and unstable conditions and did not seem to show any Monin-Obukhov similarity behaviour. The results indicate that there might be measurement problems with the instruments measuring the turbulent fluxes of carbon dioxide, but further studies are needed in order to draw a firm conclusion about the quality of the instruments. The profile measurements of water vapor seem to work fine, but more studies of carbon dioxide are needed before a statement can be made regarding the quality of the profile measurements of carbon dioxide.