On Monin–Obukhov Scaling in and Above the Atmospheric Surface Layer: The Complexities of Elevated Scintillometer Measurements

Abstract

In scintillometry Monin-Obukhov similarity theory (MOST) is used to calculate the surface sensible heat flux from the structure parameter of temperature (C T 2 ). In order to prevent saturation a scintillometer can be installed at an elevated level. However, in that case the observation level might be located outside the atmospheric surface layer (ASL) and thus the validity of MOST questioned. Therefore, we examine two concepts to determine the turbulent surface sensible heat flux from the structure parameter at elevated levels with data obtained at 60-m height on the Cabauw tower (the Netherlands). In the first concept (MOSTs) C T 2 is still scaled with the surface flux, whereas in the second (MOSTl) C T 2 is scaled with the local sensible heat flux. The C T 2 obtained from both concepts is compared with direct observations of C T 2 using a sonic anemometer/thermometer. In the afternoon (when the measurement height is located within the ASL) both concepts give results that are comparable to the directly observed values of C T 2 . In the morning (data outside the ASL), our data do not unequivocally support either of the two concepts. First, the peak in C T 2 that occurs when the measurement height is located in the entrainment zone disqualifies the use of MOST. Second, during the morning transition, local scaling shows the correct pattern (zero flux and a minimum in C T 2 ) but underestimates C T 2 by a factor of ten. Third, from the best linear fit a we found that the slope of MOSTl gave better results, whereas the offset is closer to zero for MOSTs. Further, the correlation between the direct observations and MOST-scaled results is low and similar for the two concepts. In the end, we conclude that MOST is not applicable for the morning hours when the observation level is above the ASL.

The Effect of Surface Heterogeneity on the Structure Parameters of Temperature and Specific Humidity: A Large-Eddy Simulation Case Study for the LITFASS-2003 Experiment

“…The measurement of the area-averaged fluxes at a regional scale is necessary both for a better understanding of the meteorological and hydrological processes as well as for the validation of parametrizations in numerical weather prediction (NWP) and climate models (De Bruin et al 1993;Li et al 2012;Beyrich et al 2012;Braam et al 2012). The grid resolution in NWP models is usually of the size of several km.…”

“…For heterogeneous surfaces, it is thus essential to measure surface fluxes that are representative of an area of the order of a few km 2 . Over homogeneous terrain, point measurements using the eddy-covariance technique are the traditional and most common means of determining the surface fluxes (Andreas 1991; Lee et al 2004;Braam et al 2012). Natural landscapes rarely provide horizontally homogeneous conditions such that the grid boxes in NWP models often contain different surface patches of farmland, settlements, water, forest, etc.…”

On the Discrepancy in Simultaneous Observations of the Structure Parameter of Temperature Using Scintillometers and Unmanned Aircraft

“…For stable conditions (z/L > 0) a = 1.39 and r 2 = 0.69 for the LASs, and a = 1.74 and r 2 = 0.58 for the sonics. These deviations for stable conditions illustrate the limited applicability of MOST at several tens of metres above the ground (see also Braam et al 2012a). Because most of the flight data were obtained during unstable conditions, we conclude that the method to normalize C 2 T to 50 m can be applied in the comparison of the C 2 T derived from the M 2 AV data and LAS data.…”

“…5 of Braam et al (2012a). The first step in this conversion consists of the application of Taylor's hypothesis to account for advection of the turbulent field by the mean wind (in the case of the virtual sonic) or the true air speed (in the case of the virtual aircraft).…”

Infrared and millimetre-wave scintillometry in the suburban environment – Part 1: Structure parameters

“…A fixed temporal separation of τ = 1 s was decided upon after investigation into suitable values in the inertial subrange. Calculation of C 2 T , C 2 q and C T q included the Schotanus et al (1983) correction for sonic temperature as discussed in Braam (2008) and Braam et al (2012). No corrections were made for spectral losses caused by the spatial separation of the sonic and IRGA and their finite path lengths, which can be expected to result in underestimations of about 5-7 % (Hill, 1991;Hartogensis et al, 2002).…”