Aspects of Atmospheric Turbulence Related to Scintillometry

The heat and water transport (evaporation of water from soil and vegetation) from the earth’s surface into the atmosphere is of interest for weather models. Moreover, evaporation is important for fog and cloud formation and in water budget studies.

Promovendus M (Miranda) Braam MSc
Promotor prof.dr. AAM (Bert) Holtslag
Copromotor AF (Arnold) Moene
Copromotor dr F Beyrich
Organisatie Wageningen University, Meteorology and Air Quality

wo 28 mei 2014 13:30 tot 15:00

Locatie Auditorium, building number 362
Generaal Foulkesweg 1
6703 BG Wageningen

The transport of the surface fluxes is turbulent: chaotic. This chaotic behaviour is visible when observing the transport of smoke. Nowadays scintillometers are often used to obtain the surface fluxes. Scintillometers emit a light beam that will be received with a detector. The light beam is refracted in several directions (it scintillates) due to the chaotic turbulent transport. This is also visible during a hot summer day when the asphalt seems to ‘dance’. Because the received fluctuations in the light beam are caused by turbulence, scintillometer can be used to obtain the area-averaged surface fluxes.

Miranda Braam stelling
The quest for the universal Monin Obukhov similarity function from micro meteoro- logical observations is like the quest for the holy grail.
Miranda Braam

The advantage of scintillometers is that the surface fluxes are already area-averaged. Area-averaged fluxes are needed because the surface is heterogeneous on the scale of the weather models and water management: from a vegetated surface more water evaporates (the air is heated less) than above built-up area.

Application in the early morning

The disadvantage of scintillometers is that the model that links the fluctuation in the light beam with the surface fluxes is not suitable above a heterogeneous area and in the morning when turbulence is weak. In this thesis, therefore, the applicability of the model is discussed. For applicaton in the early morning, we propose two variations of this model, and found that both did not work optimally. We tested the sensitivity of the model with respect to the measurement level and the amount of turbulence. Finally, we compare the area averaged signal of the scintillometer over heterogeneous terrain with those of an unmanned aircraft.

This thesis defines better the borders of the model and the use of scintillometers in general.