Diploma Thesis
Parameterization of the roughness sublayer above low and tall vegetation
Friederike Rütz (02/2008-06/2010)
Support: Thomas Foken, Katharina Köck (Staudt), Johannes Lüers
This work investigates the parameterisation of the roughness sublayer above low and tall vegetation. For investigation above tall canopies research was done on a 32 m flux measurement tower above a spruce forest canopy, with a mean height of 25 m, at the “Waldstein-Weidenbrunnen” research site (Fichtelgebirge, Germany). The measurements were done within the framework of the EGER (ExchanGE processes in mountainous Regions) project IOP2 (Intensive Observation Period) in 2008. Eddy-covariance measurements were done at the top of a 36 m turbulence tower. For investigations above low vegetation measurements were done within the framework of the COPS (Convective and Orographically induced Precipitation Study) experiment in 2007 with a flux measurement tower above a corn field with several measurement heights. The eddy-covariance data was measured at a turbulence measurement complex with changing heights due to the growing corn field. For parameterisation of the roughness sublayer the profile and eddy-covariance data of the COPS experiment was used to calculate half hourly values of the correction function for the roughness sublayer for wind speed and temperature. The resulting values for the correction functions were merged with results of the correction functions calculated from the profile and eddy-covariance measurements of the EGER data. Beside the already known dependency of the correction function for wind speed and temperature on the height within the roughness sublayer, a dependency on friction velocity was observed. A function fitted to the resulting scatterplot, firstly for the correction function for wind speed, shows an exponential shape. It implies different correction functions depending on friction velocity classes, whereas the exponential shape is true for the different correction functions but with differing empirical parameters. Half-hourly values for the correction function for temperature were calculated from the COPS and EGER data. A correction function was fitted only to the COPS data, due to the fact that the values for the correction function based on the EGER data showed unrealistic high values, clearly exceeding the value of 1. A function fitted to the calculated values of the correction function for temperature shows a linear shape. Even though half-hourly values for the correction functions could not be predicted for high friction velocity values (u* > 0.6 ms-1) the existing values for the correction function show a similar behaviour for comparable friction velocity classes as the correction function for the roughness sublayer for wind speed. Therefore a dependency of the correction function for temperature on the friction velocity is presumable, causing different correction functions, depending on the associated friction velocity classes, but with differing parameters for the y-axis intercept and the slope of the function. Altogether, more data has to be calculated to predict firstly reliable boundaries for friction velocity classes, especially for higher friction velocity values. Secondly to predict correction functions for the roughness sublayer depending on these boundaries, with either an exponential shape for wind speed or a linear shape for temperature, whereas the empirical parameters probably depend on defined friction velocity classes.