Exploring Scintillometry in the Stable Atmospheric Surface Layer

The main objective of this thesis is to investigate observation methods of heat and momentum exchange and key variables that characterise turbulence in the atmospheric stable surface layer (SSL), a layer defined as the lower part of the stable boundary layer (SBL) where surface fluxes do not change significantly with height. The SBL is often confined to a shallow layer above the surface and is often intermittent, i.e. quiescent periods with almost laminar flow are interchanged with turbulent bursts. These conditions complicate surface flux measurements considerably, since ideally these then need to take place close to the surface and over short flux averaging intervals. Scintillometers, unlike traditional flux measurement techniques such as eddy covariance (EC), can be operated just above the surface (< 1 m) and over short flux averaging intervals (< 1 minute). These features have led us to explore in more detail the applicability of scintillometers in the SSL.

Two types of scintillometers will be considered, notably the displaced-beam small-aperture scintillometer (DBSAS) and the large-aperture scintillometer (LAS) deployed in three field campaigns we contributed to as part of this thesis: RAPID in Idaho, USA, (1999), CASES-99 in Kansas, USA (1999) and BBC in Cabauw, the Netherlands (2001). In addition, an old data-set is analysed with LAS data gathered during the La Poza experiment in Sonora, Mexico (1996).

The DBSAS and the LAS are optical instruments that consist of a transmitter and a receiver. The receiver records intensity fluctuations of the light beam emitted by the transmitter, which are caused by refraction of the beam upon its passage through the turbulent surface layer. These intensity fluctuations are a measure of the structure parameter of temperature,C _T². The DBSAS obtains also the dissipation rate of turbulent kinetic energy,e, from the correlation between the two displaced beams. In itself, these quantities are important properties of turbulence. Moreover, when the flow is turbulent they are related to the turbulent fluxes of sensible heat, H , and momentum,t, usually expressed by the velocity scale u _* , by virtue of Monin-Obukhov similarity theory (MOST).

The DBSAS is the most suitable scintillometer to be used in the SBL, since it gives a measure of the mechanically induced turbulence (i.e.e), which is the only turbulence generating mechanism in stable conditions. For the LAS - that does not measuree- the mechanical turbulent transport is usually included using wind speed measurement and an estimate of the roughness length.

Several detailed aspects of the application of scintillometry and EC in obtaininge, C _T² , H andtare discussed. The most general aspects presented are the following. For CASES-99 and BBC we compared the DBSAS performance against EC in obtaininge, C _T² , H andtover a wide range of stable conditions and conclude that the DBSAS is superior in obtaining turbulence information over short intervals with remarkably little scatter, but that the derived parameters contain systematic errors. When corrected for the systematic errors (using ad-hoc solutions) the DBAS appears to provide accurate C _T² ,eand resulting H , andtfor short time intervals and close to the ground. In addition, for the BBC we also investigated the LAS and combinations of LAS and DBSAS to jointly solveeand C _T² for both stable and unstable conditions. Furthermore, for CASES-99 we derived new MOST relations foreand C _T² and show how these can be used to evaluate the MOST relations for dimensionless wind speed and temperature gradients. Also, alternative scaling parameters based oneand C _T² are introduced. Last, we investigated an important practical aspect of the scintillometer application, i.e. what effective height to use to calculate H when the beam-height of the instrument varies along the path. This is done based on a data-set from the La Poza experiment in Sonora, Mexico (1996).