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Determination of the surface sensible heat flux from the structure parameter of temperature at 60 m height

M Braam

Recently a scintillometer is installed at 60 m height at Cabauw to determine the surface sensible heat flux. Since 60 m is not located in the surface layer the entire day, the applicability of Monin Obukhov Similarity Theory (MOST) is questionable. Therefore, in this research two different theoretical concepts to determine the surface sensible heat flux from the structure parameter of temperature (CT2) obtained from a scintillometer at 60 m under unstable conditions are investigated. The first concept assumes that CT2 at 60 m has to be scaled with the 60 m heat flux, whereas the second concept assumes that CT2 at 60 m has to be scaled with the surface heat flux. To ensure comparable footprints of CT2 and the turbulent heat flux; CT2 is calculated from the sonic anemometer/thermometer observations at 60 m height instead of using scintillometer data. Data from sonic anemometer/thermometer systems at 3 m, 100 m and 180 m are investigated as well. By analyzing the fluxes at the four levels, we found that the 3 m temperature flux deviates from the for flux divergence corrected fluxes at the other levels. The observed structure parameter at 3 m follows the
one calculated with MOST, as expected because 3 m lies in the surface layer the entire day. The observations of 60 m show that neither of the two concepts is fully supported by the observations. The
moment of the morning transition is measured at the same time for the local flux and the local structure
parameter. Further, in the morning period the use of the surface sensible heat flux show the best result. On the other hand, in the afternoon when MOST has to be valid, the two methods differ a lot and the use of the local flux is the best option, which is caused by the deviation of the 3 m sensible heat flux. Therefore, we have done the same analysis with the surface flux derived from the 60 m flux corrected for temperature storage below 60 m. These results are the same as before for the moment of transition, however, in the
morning period the observed CT2 lies between the two of MOST and in the afternoon all the structure
parameters are comparable. Increasing of height, result in less correlation between observed and calculated CT2 and an increase in the underestimation of the calculated CT2. Perhaps, other processes than surface processes play a role as well.

Bibliografische gegevens

M Braam. Determination of the surface sensible heat flux from the structure parameter of temperature at 60 m height
31--2008, pp0

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